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10.5: Stages of Meiosis - Biology

10.5: Stages of Meiosis - Biology


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The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. In kind does not generally mean exactly the same.

As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. While many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitosis, many single-celled organisms and most multicellular organisms reproduce regularly using another method: meiosis. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.

Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.

Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.


Meiosis: Function and Stages of Meiosis

Meiosis is a form of nuclear division that is of funda­mental importance among sexually reproducing organisms.

An in-depth discussion of meiosis on a cel­lular as well as a genetic basis is beyond the scope of this book such discussions are normally treated at length in textbooks of genetics.

However, for the sake of completeness we will consider some of the ma­jor meiotic events and their implications. Meiosis occurs in eukaryotic organisms whose cells contain the diploid number (2n) of chromosomes.

Dip­loid implies “double” in the sense that the genetic in­formation present in any one chromosome can also be found in an identical (or somewhat modified) form in a second chromosome in the nucleus. The two chromo­somes forming such pairs are said to be homologous.

Human cells contain 46 chromosomes or 23 homologous pairs (i.e., in hu­mans w = 23). The 46 chromosomes of the zygote formed at fertilization are derived equally from the sperm cell and egg cell of the male and female par­ents.

Each of these gametes contributes one member of each pair of homologues. Once the zygote is formed, mitosis produces the billions of cells that ultimately make up the whole organism. Because sperm cells and egg cells contain only one member of each pair of homologues, they are said to be haploid. It is meiosis that produces haploid cells, the process being re­stricted to the reproductive tissues (i.e., ovaries and testes).

During meiosis, the replicated chromosomes of the nucleus are apportioned among four daughter nuclei, each nucleus acquiring half the number of chromo­somes of a diploid cell. Although the resulting cell nu­clei contain only half the diploid number of chromo­somes, the chromosome set is genetically complete, because each nucleus acquires one member of each pair of homologous chromosomes.

The homologous chromosomes are assorted randomly at anaphase, and this accounts in part for the genetic variation that characterizes sexually reproducing organisms. Addi­tional genetic variation occurs during the prophase of the first nuclear division by a process called crossing- over. The genetic implications of random assortment and crossing-over are principal subjects of genetic courses.

The various stages of meiosis may be summarized as follows:

1. Leptotene stage (leptonema):

The chromosomes become visible as condensation of the chromatin be­gins each chromosome can be seen to consist of two chromatids.

2. Zygotene stage (zygonema):

Homologous chromosomes are aligned side-by-side so that allelic genes (i.e., those encoding products of similar or identical function) are situated adjacent to one another. This phenomenon is called synapsis. The unit consisting of two synapsed and duplicated homologous chromo­somes is called a bivalent. As synapsis progresses, a protein framework joining adjacent, non-sister chro­matids of each tetrad is formed at one or more points in the narrow space separating the homologues.

It is in the region of these synaptonemal complexes that crossing-over occurs. Crossing-over or chiasma for­mation results from the cleavage by endonucleases of the DNA in corresponding positions of two non-sister chromatids, followed by the transposition and rejoin­ing of the free ends of homologous strands (see Fig. 20-23 for details). As a result of crossing-over, new combinations of genes are created in the homologous chromosomes.

3. Pachytene stage (pachynema):

During this stage the chromatids become increasingly distinct as con­densation continues.

4. Diplotene stage (diplonema):

The diplotene stage is characterized by the separation of the paired homologous chromosomes except at points where chi- asmata are formed.

Diakinesis brings prophase I to an end. During this stage chromosome condensation is completed.

In this phase, the spindle apparatus forms, much as it does in mitosis, and the bivalents align on the equato­rial plate. The centromeres of homologous chromo­somes attach to spindle fibers arising from opposite poles of the cell.

Homologous chromosomes (but not sister chromatids) of each tetrad separate from each other and move to opposite poles of the spindle.

Telophase I brings the first meiotic division to a con­clusion as the separated homologues aggregate at their respective poles so that two nuclear areas are distinguishable. In most organisms, a new nuclear en­velope is formed and some de-condensation of the chro­mosomes occurs.

Interkinesis (or Interphase):

Interkinesis is the period between the end of telo­phase I and the onset of prophase II. This period is usually quite short. The DNA of the two nuclei pro­duced by the first meiotic division does not engage in replication during interkinesis.

Meiotic Division II:

The events characterizing this phase are similar to mi­totic prophase, although each cell nucleus has only half the number of chromosomes as a cell in prophase I, that is, the nucleus is already haploid. Each chromo­some remains composed of the two sister chromatids formed prior to prophase I, except for segments that were interchanged during crossing-over.

The events occurring in this phase are similar to those in mitotic metaphase. The paired chromatids migrate to the center of the spindle and are attached there to the spindle’s microtubules.

The events occurring in this phase are similar to those in mitotic anaphase, but differ from those of anaphase I of meiosis. In anaphase II, sister chromatids sepa­rate from one another and are drawn to opposite poles of the spindle. (Recall that sister chromatids do not separate in anaphase I.)

The events occurring in this phase are similar to those in mitotic telophase. The separated chromosome groups are enclosed in a newly developing nuclear en­velope and begin to undergo decondensation. Meiosis produces four cells, each with the haploid number of chromosomes. In many higher animals and some plants, meiosis in the female reproductive tis­sues is accompanied by an uneven division of the cyto­plasm, in which case one of the two cells formed dur­ing telophase I is a nonfunctional polar body and may not enter prophase II (Fig. 20-24).

In some organisms (such as humans) the polar body completes meiosis, but the two smaller polar bodies produced during telo­phase II are similarly nonfunctional. The second mei­otic division of the larger cell produced during telo­phase I is also unequal and produces an additional polar body. During the production of spermatozoa in the male reproductive tissues, division of the cytoplasm is equal, but remarkable cytoplasmic differentiation of the four spherical haploid spermatids pro­duced by meiosis is required (Fig. 20-24) before functional, flagellated spermatozoa are produced.


10.5: Stages of Meiosis - Biology

Unit Three. The Continuity of Life

9.3. The Stages of Meiosis

Now, let’s look more closely at the process of meiosis. Meiosis consists of two rounds of cell division, called meiosis I and meiosis II, which produce four haploid cells. Just as in mitosis, the chromosomes have replicated before meiosis begins, during a period called interphase. The first of the two divisions of meiosis, called meiosis I (meiosis I is shown in the outer circle of the Key Biological Process illustration on the facing page), serves to separate the two versions of each chromosome (the homologous chromosomes or homologues) the second division, meiosis II (the inner circle), serves to separate the two replicas of each version, called sister chromatids. Thus when meiosis is complete, what started out as one diploid cell ends up as four haploid cells. Because there was one replication of DNA but two cell divisions, the process reduces the number of chromosomes by half.

Meiosis I is traditionally divided into four stages:

1. Prophase I. The two versions of each chromosome (the two homologues) pair up and exchange segments.

2. Metaphase I. The chromosomes align on a central plane.

3. Anaphase I. One homologue with its two sister chromatids still attached moves to a pole of the cell, and the other homologue moves to the opposite pole.

4. Telophase I. Individual chromosomes gather together at each of the two poles.

In prophase I, individual chromosomes first become visible, as viewed with a light microscope, as their DNA coils more and more tightly. Because the chromosomes (DNA) have replicated before the onset of meiosis, each of the threadlike chromosomes actually consists of two sister chromatids associated along their lengths (held together by cohesin proteins in a process called sister chromatid cohesion) and joined at their centromeres, just as in mitosis. However, now meiosis begins to differ from mitosis. During prophase I, the two homologous chromosomes line up side by side, physically touching one another, as you see in figure 9.5. It is at this point that a process called crossing over is initiated, in which DNA is exchanged between the two nonsister chromatids of homologous chromosomes. The chromosomes actually break in the same place on both nonsister chromatids and sections of chromosomes are swapped between the homologous chromosomes, producing a hybrid chromosome that is part maternal chromosome (the green sections) and part paternal chromosome (the purple sections). Two elements hold the homologous chromosomes together: (1) cohesion between sister chromatids and (2) crossovers between nonsister chromatids (homologues). Late in prophase, the nuclear envelope disperses.

In crossing over, the two homologues of each chromosome exchange portions. During the crossing over process, nonsister chromatids that are next to each other exchange chromosome arms or segments.

In metaphase I, the spindle apparatus forms, but because homologues are held close together by crossovers, spindle fibers can attach to only the outward-facing kinetochore of each centromere. For each pair of homologues, the orientation on the metaphase plate is random which homologue is oriented toward which pole is a matter of chance. Like shuffling a deck of cards, many combinations are possible—in fact, 2 raised to a power equal to the number of chromosome pairs. For example, in a hypothetical cell that has three chromosome pairs, there are eight possible orientations (2 3 ). Each orientation results in gametes with different combinations of parental chromosomes. This process is called independent assortment. The chromosomes in figure 9.6 line up along the metaphase plate, but whether the maternal chromosome (the green chromosomes) is on the right or left of the plate is completely random.

Figure 9.6. Independent assortment.

Independent assortment occurs because the orientation of chromosomes on the metaphase plate is random. Shown here are four possible orientations of chromosomes in a hypothetical cell. Each of the many possible orientations results in gametes with different combinations of parental chromosomes.

In anaphase I, the spindle attachment is complete, and homologues are pulled apart and move toward opposite poles. Sister chromatids are not separated at this stage. Because the orientation along the spindle equator is random, the chromosome that a pole receives from each pair of homologues is also random with respect to all chromosome pairs. At the end of anaphase I, each pole has half as many chromosomes as were present in the cell when meiosis began. Remember that the chromosomes replicated and thus contained two sister chromatids before the start of meiosis, but sister chromatids are not counted as separate chromosomes. As in mitosis, count the number of centromeres to determine the number of chromosomes.

In telophase I, the chromosomes gather at their respective poles to form two chromosome clusters. After an interval of variable length, meiosis II occurs in which the sister chromatids are separated as in mitosis. Meiosis can be thought of as two consecutive cycles, as shown in the Key Biological Process illustration on the previous page. The outer cycle contains the phases of meiosis I and the inner cycle contains the phases of meiosis II, discussed next.

After a brief interphase, in which no DNA synthesis occurs, the second meiotic division begins. Meiosis II is simply a mitotic division involving the products of meiosis I, except that the sister chromatids are not genetically identical, as they are in mitosis, because of crossing over. You can see this by looking at figure 9.7, where some of the arms of the sister chromatids contain two different colors. At the end of anaphase I, each pole has a haploid complement of chromosomes, each of which is still composed of two sister chromatids attached at the centromere. Like meiosis I, meiosis II is divided into four stages:

1. Prophase II. At the two poles of the cell, the clusters of chromosomes enter a brief prophase II, where a new spindle forms.

2. Metaphase II. In metaphase II, spindle fibers bind to both sides of the centromeres and the chromosomes line up along a central plane.

3. Anaphase II. The spindle fibers shorten, splitting the centromeres and moving the sister chromatids to opposite poles.

4. Telophase II. Finally, the nuclear envelope re-forms around the four sets of daughter chromosomes.

The main outcome of the four stages of meiosis II— prophase II, metaphase II, anaphase II, and telophase II—is to separate the sister chromatids. The final result of this division is four cells containing haploid sets of chromosomes. No two are alike because of the crossing over in prophase I. The nuclei are then reorganized, and nuclear envelopes form around each haploid set of chromosomes. The cells that contain these haploid nuclei may develop directly into gametes, as they do in most animals. Alternatively, they may themselves divide mitotically, as they do in plants, fungi, and many protists, eventually producing greater numbers of gametes or, as in the case of some plants and insects, adult haploid individuals.

The Important Role of Crossing Over

If you think about it, the key to meiosis is that the sister chromatids of each chromosome are not separated from each other in the first division. Why not? What prevents microtubules from attaching to them and pulling them to opposite poles of the cell, just as eventually happens later in the second meiotic division? The answer is the crossing over that occurred early in the first division. By exchanging segments, the two homologues are tied together by strands of DNA. It is because microtubules can gain access to only one side of each homologue that they cannot pull the two sister chromatids apart! Imagine two people dancing closely—you can tie a rope to the back of each person’s belt, but you cannot tie a second rope to their belt buckles because the two dancers are facing each other and are very close. In just the same way, microtubules cannot attach to the inner sides of the homologues because crossing over holds the homologous chromosomes together like dancing partners.

Key Learning Outcome 9.3. During meiosis I, homologous chromosomes move to opposite poles of the cell. At the end of meiosis II, each of the four haploid cells contains one copy of every chromosome in the set, rather than two. Because of crossing over, no two cells are the same.

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The Cell Cycle, Mitosis & Meiosis

You think you’ve got a lot of chores? Our bodies are tirelessly undergoing cell division, replacing the skin cells that we’re constantly shedding to make sure we don’t disappear in a poof of smoke. There are two types of cell division - mitosis, which produces genetically identical cells for growth and repair and meiosis which produces genetically unique cells for sexual reproduction.

Mitosis and the Cell Cycle

Mitosis is a type of cell division where cells produce identical copies of themselves and is used for growth and repair and asexual reproduction. It differs from meiosis, which is the type of cell division used to produce gametes.

Mitosis occurs as part of the cell cycle which consists of four distinct phases. First, interphase takes place which is made up of three growth phases (called G1 phase, S phase and G2 phase), followed by mitosis.

Gap Phase 1 (G1) - cell grows bigger and replicates its organelles. A high amount of protein synthesis is taking place in order to build new organelles.

Synthesis Phase (S) - the cell replicates its DNA

Gap Phase 2 (G2) - the cell keeps growing until all of the organelles have duplicated.

There are two ‘checkpoints’ in the cell cycle - one before S phase and one straight after S phase. During these checkpoints, the cell is checking its DNA for errors. This minimises the chances of duplicating any mutated DNA into the replicated cell.

The Stages of Mitosis

Mitosis can be divided into a series of stages depending on what’s going on with the chromosomes in the cell. You can use the acronym IPMAT to help you remember the order.

Interphase - the cell prepares for mitosis by growing larger, replicating its organelles and synthesising new DNA (see above). Once the DNA has replicated, each chromosome now consists of two sister chromatids, connected by a structure called the centromere. The mitochondria produce more ATP which will provide the energy for cell division.

Prophase - the chromosomes condense (they become shorter and fatter) and the nuclear envelope disintegrates. The centrioles move to opposite poles of the cell and form spindle fibres.

Metaphase - the chromosomes line up along the middle of the cell. They attach to the spindle fibre by their centromere.

Anaphase - the centromere splits and the chromatids are pulled to opposite poles of the cell.

Telophase & cytokinesis - the two groups of chromsomes decondense (they become long and thin) and a nuclear envelope reforms around them, forming two new nuclei. The cytoplasm divides (cytokinesis) and the plasma membrane pinches off to form two new, genetically-identical cells.

Mitotic Index

The mitotic index is a measure of the proportion of cells which are undergoing mitosis. You may be asked to calculate it in the exam. To do this, you need to count the number of cells with visible chromosomes and divide this by the total number of cells.

Meiosis

Meiosis is the type of cell division which produces gametes for sexual reproduction. Unlike mitosis, the daughter cells are genetically different from the parent cell and contain just half the number of chromosomes (i.e. they are haploid). When two haploid gametes join during fertilisation, a diploid cell called a zygote is formed. Meiosis involves two rounds of cell division which are referred to as meiosis I and meiosis II. It takes place in the following stages:

Interphase: the DNA replicates so there are now two identical copies of each chromosome (referred to as chromatids).

Prophase I: chromatids condense and arrange themselves into homologous pairs (called bivalents). Crossing over occurs (see below). The nuclear envelope disintegrates and spindle fibres form.

Metaphase I: homologous chromosomes line up along the equator and attach to the spindle fibre by their centromeres.

Anaphase I: homologous chromosomes are separated

Telophase I: chromosomes reach opposite poles of the cell. Nuclear envelope reforms around the chromosomes. Cytokinesis results in the formation of two daughter cells.

Prophase II: chromosomes condense, nuclear envelope disintegrates and spindle fibres form.

Metaphase II: chromosomes attach to the spindle fibre by their centromeres.

Anaphase II: sister chromatids are separated.

Telophase II: chromatids reach opposite poles of the cell. Nuclear envelope reforms and cytokinesis takes places. Four genetically unique daughter cells are produced.

Meiosis increases genetic variation

From an evolutionary point of view, it is important that organisms produce offspring that show as much genetic variation as possible. Imagine if a mother duck gave birth to a group of ducklings that were all had very similar genes - these ducklings will all be equally vulnerable to the same diseases and other threats to their survival. Meiosis increases genetic variation in two ways - crossing over and independent assortment.

Crossing Over

During prophase I of meiosis, a process called crossing over occurs. This is when the homologous chromosomes move towards each other and exchange genetic material. When the pair of chromosomes have come together, we call this a bivalent. A chromatid from the maternal chromosome becomes twisted around the paternal chromosome and they connect through a structure called the chiasmata. Pieces of chromosomes are exchanged and the chromatids separate, forming chromosomes with different combinations of alleles.


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Meiosis Stages

You have already know about prophase-I of the meiosis stages. Now you are going to have a discussion about the other meiosis stages.

Metaphase-I: –

The important events of this phase are: (i) in animal cell, spindle is formed between the two centrioles, apart of 180°. (ii) homologous pair of chromosomes are arranged on the equatorial plane in such a way that their centromeres remain on the two sides of the metaphase plate directed towards the opposite poles, while their chromatids remain at the metaphase plate. (iii) the repulsive force between the homologous chromosomes increase which tend to separate them.

Anaphase-I: –

Important events of this meiosis stages are as follows:
i) each of the homologous chromosomes with their two chromatids and undivided centromere moves to the respective poles by the process of disjunction.

ii) Like mitosis the anaphasic movement of chromosomes is initiated by the coordinated effect of the contraction of chromosome fibres and elongation of the contraction of chromosomal fibres and elongation of the continuous fibres also.
iii) The chromosomes are distinctly detached into chromatids which are united at the centromere.
iv) Chromosomes at each pole is reduced to half of the meiocyte, that is the haploid because of disjunction.
v) One of the two chromatids remains unchanged while the other one is changed because of the crossing over.

Telophase-I

In the meiosis stages the important events of Telophase-I are as follows: -
i) Chromosomes reach the two poles and nuclear membrane is reorganised here.
ii) Chromosomes become despiralised, nuclear reticulum and nucleolus reappears.
iii) On the opposite poles two daughter nuclei are formed with haploid number of chromosomes after the reduction of numbered of chromosome.
iv) The threads of chromatin disappear due to hydrated condition of the nucleus.

Cytokinesis-I

Important events of cytokinesis-I are as follows:
i) In this new series stages of cytokinesis-I is same like of mitosis.
ii) The daughter cells have half the chromosome number of the parent cell.
iii) The two daughter cells quickly pass on to the second meiotic division after a very short resting time.
Interkinnesis or second interphase: – it is very short period between the first and second meiotic division without any significant changes.

Second meiotic division: –

in this meiosis stages the two haploid cells formed as a result of past meiotic division undergoes the second meiotic division to produce for identical of Floyd cells. The different stages of second meiotic division are described as follows:

Prophase-II

In this stage the chromosomes are visible again because of dehydration. The chromosomes do not exist in pairs but is made up of chromatids and a centromere. Chromatids are remaining free from each other. The spindle fibres appears at a right angles to the spindle of fast meiotic division towards the end of this stage. The chromosome defied and forms the centriolar spindle in the case of animals cell. The nucleolus and nuclear membrane also disappear.

Metaphase-II

In this meiosis stages the chromosomes are arranged along the equatorial line of the spindle fibres. The centromeres remain on the metaphase plate, while the chromatids are extent towards the poles. These centromeres are attached in the spindle fibres.

Anaphase-II

In this stage the chromosome divide longitudinally with each half having single Chromatid and half of the centromere. The separated chromatids more to the opposite poles due to the shortening of chromosomal microtubules and thus anaphasic movement of chromosomes are initiated.

Telophase-II

In this case of meiosis stages the daughter chromosomes reach the opposite poles. The endoplasmic reticulum resynthesizes the nuclear membrane. The nucleolus organiser synthesise r-RNA and ribosomal protein and forms the nucleus. Four daughter nuclei with haploid number of chromosomes are formed and the nucleus becomes hydrated and the chromosomes disappear.

Cytokinesis-II

The second meiotic division products are always four haploid cells with their chromosomes composter of both parental combination and new recombination character. They become the functional gametes in higher organisms as well as undergo sexual union restore the original diploid chromosome status.
So, meiosis stages maintain the chromosome number in particular species through subsequent generations. It is an essential process for the preparation of sexually reproducing species.


In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present . This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.


The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles (Figure 1). Non-kinetochore microtubules elongate the cell.

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell. During meiosis II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

Figure 1 In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

The Process of Meiosis

Sexual reproduction requires fertilization, the union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

Most animals and plants are diploid, containing two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent all together, they are considered a full set of chromosomes. Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. All animals and most plants produce eggs and sperm, or gametes. Some plants and all fungi produce spores.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.

Meiosis I

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G2 phase, also called the second gap phase, is the third and final phase of interphase in this phase, the cell undergoes the final preparations for meiosis.

During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the first meiotic phase.

Prophase I

Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) ([link]).

In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover—or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata ([link]) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

Prometaphase I

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes' kinetochores. With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

Metaphase I

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.

This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate the possible number of alignments therefore equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition ([link]).

To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

Anaphase I

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart ([link]).

Telophase I and Cytokinesis

In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.

Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation.

Meiosis II

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

Prophase II

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

Prometaphase II

The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in [link].

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes ([link]). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.

The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.

When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.

Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical (like in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

The Mystery of the Evolution of Meiosis Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble hypothesizing and testing how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved.

Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday 1 summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.

There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues 2 compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.

Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.

Section Summary

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set instead of the two sets of chromosomes in the parent cell. The main differences between the processes occur in the first division of meiosis, in which homologous chromosomes are paired and exchange non-sister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is more similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover.


Meiosis

19 comments:

The steps of Meiosis are Interphase, Prophase 1, Metaphase 1, Anaphase 1, telophase 1, Prophase II, Metaphase II, Anaphase II, Telophase II . In other organisms the cells go from the late anaphase of meiosis 1 to metaphase of meiosis II. The second division in Meiosis is simply a mitotic division of the products of meisosis I. It adds a major source of variation among organisms. Variation is important toa species because it is the raw material that forms the basis for evolution.

It is important for living things because it allows the cells to divide and multiply. The new cells have the same number and kind of chromosomes as the origional ones. If we didn't have meiosis then our cells couldn't multiply and we couldn't grow.

The phases of meiois are Interphase, Prophase 1, metaphase 1, anaphase 1, telophase 1. Without metaphase all of the children we have would have defects. The world would be messed up if we didnt have meiosis. By the way this is Jackson my screen name is SMAX

Interphase, Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Prophase 2, Metaphase 2, Anaphase 2, Telophase 2. Meosis is a kind of cell division, it occurs in the specialized body cells of each parent that produce gametes.This allows children to have the same number of chromosomes as their parents.

meiosis is a type of cell division where one body cell produces four gametes, and each containging half the number of chromosomes as a parent's body cell. the steps of meiosis are interphase, phrophas, metaphase, anaphase, and telophase. meiosis is important to living organisms because its what makes new organisms like repoduction.

The steps are interphase, prophase 1, metaphase, anaphase 1, telophase 1, prophase 2, metaphase 2, anaphase 2, and telophase 2. Meiosis is a kind of cell division which produces gametes containing half the number of chromosomes as a parent's body cell. Meisosis occurs in teh specialized body cells of each parent that produce gametes.

the steps are interphase prophase 1 metaphase 1 anaphase 1 telophase1 prophase 2 metaphase 2 anaphase2 telophase2. meiosis is what makes us. it keeps the world going round

The phases of meiosis are Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphse II, Anaphase II, and Telophase II. Meiosis is important because it helps with genetic recombination which causes the gametes to cross and combine. It also explains Mendel's results by showing that Mendel's obsevation of each parent giving one allele at random to offspiring at random.

The phases of meiosis are Interphase, Prophase I, Metaphase II, Anaphase I, TelpphaseI, Prophase II, Metaphase II, AnaphaseII, and Telophase II. This phase is very important becasue this is the phase when the gametes from the parents are produced.

The phases of Meiosis are Interphase, Phropahse 1, Metaphase 1, Anaphase 1, and Telophase 1. Meiosis provides for genetic variation. We need meiosis so we have differnt characteristics.

There are nine steps to Meiosis (1-2). Meiosis 1 is Interphase, Prophase1, Metaphase1, Anaphase1, and Telophase1. Meiosis 2 starts with Prophase2, Metaphase2, Anaphase, and ends with Telophase2.

Meiosis is important because it occurs in the specialized body cells of each parent that produce gametes. Its important in the reproduction of life.

Meiosis has 9 steps. They are Interphase, Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Prophase 1, Metaphase 2, Anaphase 2, and Telophase 2. Meiosis is important to organisms because it creates lots of variation between all the organisms throughout the environment. The variation makes the raw material that is used.

The steps of meiosis are Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, and Telophase II. Meiosis consists of two separate divisions, known as meiosis I and meiosis II. Meiosis I begins with one diploid cell. By the end of meiosis II, there are four haploid cells. These haploids cells are called sex cells or gametes. When a male gametes and female gametes join they create a kid. Meiosis occurs in the specialized body cells of each parent that produce gametes.

the steps of Meiosis are interphase,prophaseI,metaphaseI,anaphaseI,telophaseI
cells of Meoisis is useful for reproduction, helps to parent cells.

Meiosis is the proses for a cell to divid. the steps are Interphase, Prophase, Metaphase, Anaphase, Telophase. It is in portent for thing to grow.

The steps of Meosis are.
(In meosis I)
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
(In meosis II)
Prophase II
Metaphase II
Anaphase II
Telophase II

The first generation of meiosis is Interphase, Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Prophase 2, Metaphase 2, Anaphase 2, and Telophase 2. It makes everybody different when this process happens. Mendel's laws now explain the theory of heredity.


Watch the video: Meiosis. Genetics. Biology. FuseSchool (October 2022).