8: DNA Structure and Replication - Biology

8: DNA Structure and Replication - Biology

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8: DNA Structure and Replication

8: DNA Structure and Replication - Biology

In this outcome, we’ll learn more about the precise structure of DNA and how it replicates. Watch this video for a quick introduction to this topic:

Learning Outcomes

  • Outline the basic steps in DNA replication
  • Identify the major enzymes that play a role in DNA replication
  • Identify the key proofreading processes in DNA replication

8: DNA Structure and Replication - Biology

1. DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

• Skill: Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.

• Application: Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction.

  • 5’ terminal: at one end of each DNA strand is a phosphate group linked to carton atom 5 of deoxyribose
  • 3’ terminal: at one end of each DNA is a hydroxyl group attached to carbon atom 3 of deoxyribose
  • adjacent nucleotides are linked by a covalent bond between the phosphate group, attached at the 5’ end of one nucleotide and carbon atom 3’ of the other nucleotide
  • between purines and pyrimidines
  • purines: adenine and guanine have two rings in their molecules
  • pyrimidines: cytosine and thymine have a single ring in their molecules

  • 8 histone proteins (4 types, 2 of each type) inside each nucleosome
  • 1 histone protein outside each nucleosome, which functions to organize and hold the nucleosome together
  • a structure for coiling DNA by combining it with histone proteins
  • DNA is wrapped twice around each nucleosome

• Skill: Utilization of molecular visualization software to analyse the association between protein and DNA within a nucleosome.

unique or single-copy genes include e xons and introns

  • exons code for mature mRNA which codes for polypeptides
  • introns are transcribed into RNA, but then removed by enzymes which splice together exons into mature mRNA, which codes for polypeptides

genes for other RNA types do not code for proteins

some sections of DNA act as regulators of gene expression

telomeres at the ends of chromosomes do not code for proteins

highly repetitive sequences serve no known function

  • also known as satellite DNA, constitute 5-45% of the genome
  • sequences are 5-300 base pairs per repeat, and my be repeated up to 10,000 times per genome
  • the function of repetitive DNA is not known
  • since repetitive sequences vary from person to person, they are useful in DNA profiling, which allows for DNA fingerprinting to identify samples from individuals

• The regions of DNA that do not code for proteins should be limited to regulators of gene expression, introns, telomeres and genes for tRNAs.

  • DNA sequencing of the human genome reveals that 98.5% does not code for proteins, rRNA or tRNA
  • about a quarter of the human genome codes for introns and gene-related regulatory sequences

• Application: Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples for base sequencing.

  • many copies of sample DNA + dideoxyribonucleic acid (dDNA) nucleotides, each with a unique florescence mixed in solution
  • dDNA stop replication when incorporated into DNA
  • gel electrophoresis separates DNA fragments by size
  • images of florescent fragments allow sequencing based on size

• Application: Tandem repeats are used in DNA profiling.

  • each nucleotide contains a deoxyribose molecule with 5 carbons, with a nitrogenous base attached to carbon 1 and a phosphate attached to carbon 5
  • a DNA strand is built with a repeating deoxyribose - phosphate - deoxyribose - phosphate backbone
  • thus, one end of the DNA molecule has a free 5’ phosphate and the other has a free 3’ carbon replication only adds DNA nucleotides at the 3’ end

5. DNA replication is carried out by a complex system of enzymes and is continuous on the leading strand and discontinuous on the lagging strand.

• Details of DNA replication differ between prokaryotes and eukaryotes. Only the prokaryotic system is expected.

• The proteins and enzymes involved in DNA replication should include helicase, DNA gyrase, single strand binding proteins, RNA primase and DNA polymerases I and III.

Basics of DNA Replication

Figure 4. The three suggested models of DNA replication. Grey indicates the original DNA strands, and blue indicates newly synthesized DNA.

The elucidation of the structure of the double helix provided a hint as to how DNA divides and makes copies of itself. This model suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied. What was not clear was how the replication took place. There were three models suggested: conservative, semi-conservative, and dispersive (see Figure 4).

In conservative replication, the parental DNA remains together, and the newly formed daughter strands are together. The semi-conservative method suggests that each of the two parental DNA strands act as a template for new DNA to be synthesized after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. In the dispersive model, both copies of DNA have double-stranded segments of parental DNA and newly synthesized DNA interspersed.

Meselson and Stahl were interested in understanding how DNA replicates. They grew E. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15 N) that gets incorporated into nitrogenous bases, and eventually into the DNA (Figure 5).

Figure 5. Meselson and Stahl experimented with E. coli grown first in heavy nitrogen ( 15 N) then in 14 N. DNA grown in 15 N (red band) is heavier than DNA grown in 14 N (orange band), and sediments to a lower level in cesium chloride solution in an ultracentrifuge. When DNA grown in 15 N is switched to media containing 14 N, after one round of cell division the DNA sediments halfway between the 15 N and 14 N levels, indicating that it now contains fifty percent 14 N. In subsequent cell divisions, an increasing amount of DNA contains 14 N only. This data supports the semi-conservative replication model. (credit: modification of work by Mariana Ruiz Villareal)

The E. coli culture was then shifted into medium containing 14 N and allowed to grow for one generation. The cells were harvested and the DNA was isolated. The DNA was centrifuged at high speeds in an ultracentrifuge. Some cells were allowed to grow for one more life cycle in 14 N and spun again. During the density gradient centrifugation, the DNA is loaded into a gradient (typically a salt such as cesium chloride or sucrose) and spun at high speeds of 50,000 to 60,000 rpm. Under these circumstances, the DNA will form a band according to its density in the gradient. DNA grown in 15 N will band at a higher density position than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N after they had been shifted from 15 N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15 N and 14 N. This suggested either a semi-conservative or dispersive mode of replication. The DNA harvested from cells grown for two generations in 14 N formed two bands: one DNA band was at the intermediate position between 15 N and 14 N, and the other corresponded to the band of 14 N DNA. These results could only be explained if DNA replicates in a semi-conservative manner. Therefore, the other two modes were ruled out.

During DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. When two daughter DNA copies are formed, they have the same sequence and are divided equally into the two daughter cells.

Discovery of DNA

As microscopes started to become more sophisticated and provide greater magnification, the role of the nucleus in cell division became fairly clear. On the other hand, there was the common understanding of heredity as the ‘mixing’ of maternal and paternal characteristics, since the fusion of two nuclei during fertilization had been observed.

However, the discovery of DNA as the genetic material probably began with the work of Gregor Mendel. When his experiments were rediscovered, an important implication came to light. His results could only be explained through the inheritance of discrete particles, rather than through the diffuse mixing of traits. While Mendel called them factors, with the advent of chemistry into biological sciences, a hunt for the molecular basis of heredity began.

Chemical Isolation of DNA

DNA was first chemically isolated and purified by Johann Friedrich Miescher who was studying immunology. Specifically, he was trying to understand the biochemistry of white blood cells. After isolating the nuclei from the cytoplasm, he discovered that when acid was added to these extracts, stringy white clumps that looked like a tufts of wool, separated from the solution. Unlike proteins, these precipitates went back into solution upon the addition of an alkali. This led Miescher to conclude that the macromolecule was acidic in nature. When further experiments showed that the molecule was neither a lipid nor a protein, he realized that he had isolated a new class of molecules. Since it was derived from the nucleus, he named this substance nuclein.

The work of Albrecht Kossel shed more light on the chemical nature of this substance when he showed that nuclein (or nucleic acid as it was beginning to be called) was made of carbohydrates, phosphates, and nitrogenous bases. Kossel also made the important discovery connecting the biochemical study of nucleic acids with the microscopic analysis of dividing cells. He linked this acidic substance with chromosomes that could be observed visually and confirmed that this class of molecules was nearly completely present only in the nucleus. The other important discovery of Kossel’s was to link nucleic acids with an increase in protoplasm, and cell division, thereby strengthening its connection with heredity and reproduction.

Genes and DNA

By the turn of the twentieth century, molecular biology experienced a number of seminal discoveries that brought about an enhanced understanding of the chemical basis of life and cell division. In 1944, experiments by three scientists, (Avery, McCarty and McLeod) provided strong evidence that nucleic acids, specifically DNA, was probably the genetic material. A few years later, Chargaff’s experiments showed that the number of purine bases in every DNA molecule equaled the number of pyrimidine bases. In 1952, an elegant experiment by Alfred Hershey and Martha Chase confirmed DNA as the genetic material.

By this time, advances in X-Ray crystallography had allowed the crystallization of DNA and study of its diffraction patterns. Finally, these molecules could be visualized with greater granularity. The data generated by Rosalind Franklin allowed James Watson and Francis Crick to then propose the double-stranded helical model for DNA, with a sugar-phosphate backbone. They incorporated Chargaff’s rules for purine and pyrimidine quantities by showing that every purine base formed specific hydrogen bond linkages with another pyrimidine base. They understood even as they proposed this structure that they had provided a mechanism for DNA duplication.

In order to visualize this molecule, they built a three-dimensional model of a double helical DNA, using aluminum templates. The image above shows the template of the base Thymine, with accurate bond angles and lengths.

The final model built by Watson and Crick (as seen above) is now on display at the National Science Museum in London.

1. Which of these statements about DNA is NOT true?
A. In eukaryotes, DNA is present exclusively in the nucleus
B. DNA is the genetic material for some viruses
C. DNA replication is semi-conservative
D. None of the above

2. Which of these scientists designed an experiment to show that DNA replication was semi-conservative?
A. Meselson
B. James Watson
C. Linus Pauling
D. All of the above

3. Why was the rediscovery of Mendel’s experiments important for the development of molecular biology?
A. Mendel’s experiments suggested that DNA was the hereditary material
B. Mendel’s laws of inheritance suggested that there were discrete biochemical particles involved in heredity
C. Mendel’s experiments with pea plants gave molecular biologists a useful model organism
D. All of the above

DNA Replication

For freshman, DNA replication is only covered in basic terms, where students are told that the process is semi-conservative and leads to the production of two new identical strands.

AP Biology students are required to learn the steps of DNA replication and the roles that enzymes like DNA polymerase, helicase, and ligase, play in the process. They must grapple with the concept that the two sides are not copied in the same way due to the fact that DNA polymerase can only travel in the 3′ to 5′ direction. This means that one side, the leading strand is copied continuously, while the lagging strand is copied discontinuously and creates Okazaki fragments that must be bound together later.

This worksheet was designed for students to help them learn or study the steps in involved in DNA replication and the enzymes needed for the process. This document could also be used for assessments, though the focus is mainly the steps and vocabulary associated with replication.The image was created from a Wikipedia Image of Replication where I added boxes for labeling. I made two versions, one with a word bank and one without a wordbank.

Grade Level: 10-12
Time Required: 10-15 minutes

HS-LS1-1 Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins which carry out the essential functions of life through systems of specialized cells

HS-LS3-2 Make and defend a claim based on evidence that inheritable genetic variations may result from: (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.

DNA Replication

In mitosis, the cell splits apart to form two identical, same cells. That means that it has the same #"DNA"# and number of chromosomes as the previous cell. So, mitosis's main function is literally #"DNA"# replication.


Base pair in DNA replication is a way that the chromosomes have to double check to make sure that the duplication is exact.


Base pair in DNA replication is a way that the chromosomes have to double check to make sure that the duplication is exact.

The replication is termed semiconservative since each new cell contains one strand of original DNA and one newly synthesized strand of DNA. The original polynucleotide strand of DNA serves as a template to guide the synthesis of the new complementary polynucleotide of DNA. A template is a guide that may be used for example, by a carpenter to cut intricate designs in wood.

3 Phases of DNA Replication Process (With Diagram)

Read this article to learn about the three phases of DNA replication process.

The three phases of replication process are: (1) Initiation (2) Elongation and (3) Termination.

Replication in prokaryotes and eukaryotes occurs by very similar mechanisms, and thus most of the information presented here for bacterial replication applies to eukaryotic cells as well.

It is composed of three phases which are listed below:

It involves recognition of the positions on a DNA molecule where replication will begin.

It includes the events occurring at the replication fork, where the parent poly-nucleotides are copied.

It is less understood. It occurs when the parent molecule has been completely replicated.

(a) Initiation:

In a cell, DNA replication begins at specific locations in the genome, called “origins”. In case of E. coli the origin of replication is a sequence of approximately 245 base pairs (bp) called oriC. Origins contain DNA sequences recognized by replication initiator proteins (e.g. DnaA in E. coli and the Origin Recognition Complex in yeast), these proteins bind to start the process of replication. The initiator proteins recruit other proteins to separate the two strands and initiate replication forks.

Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. The replication fork is a structure which forms when DNA is being replicated. It is created through the action of helicase, which breaks the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching “prongs”, each one made up of a single strand of DNA.

In bacteria, which have a single origin of replication on their circular chromosome, this process eventually creates a “theta structure” (resembling the Greek letter theta: 8). In contrast, eukaryotes have longer linear chromosomes and initiate replication at multiple origins and whose replication forks progress for shorter distances. For example yeast has about 322 origins, which corresponds to 1 origin per 36 kb of DNA, and humans have some 20,000 origins, or 1 origin for every 150 kb of DNA. Once initiated, two replication forks can emerge from the origin and progress in opposite direction along the DNA. Replication is therefore bidirectional with most genomes (Fig. 3.4).

Initiation of DNA Replication in Microorganisms (E. coli):

We know substantially more about DNA synthesis in prokaryotes than in eukaryotes. As we have discussed that oriC of E.coli spans 245 bp of DNA. Sequence analysis of this segment shows that it contains two short repeat motifs, one of nine nucleotides and the other of 13 nucleotides. The five copies of nine nucleotide repeat motif are presented dispersedly throughout oriC.

These regions are the binding site for a protein called DnaA. As there are five copies of the binding sequences, it might be imagined that five copies of DnaA attach to the origin, but in fact bound DnaA proteins cooperate with unbound molecules until some 30 copies are associated with the origin. Attachment occurs only when the DNA is negatively super-coiled, as is the normal situation for the E. coli chromosome.

The result of DnaA binding is that the double helix opens up (melts) within the tandem array of three AT-rich, 13 nucleotide repeats located at one end of the oriC sequence. The exact mechanism is unknown but DnaA does not appear to possess the enzymatic activity needed to break base pairs, and it is therefore assumed that the helix is melted by torsional stresses introduced by attachment of the DnaA proteins.

An attractive model imagines that the DnaA proteins form a barrel-like structure around which the helix is wound. Melting the helix is promoted by HU, the most abundant of the DNA packaging proteins of E. coli. Melting of the helix initiates a series of events that construct a new replication fork at either end of the open region. The first step is the attachment of a pre-priming complex at each of these two positions.

Each pre-priming complex initially comprises 12 proteins, six copies of DnaB, and six copies of DnaC, but DnaC has a transitory role and is released from the complex soon after it is formed, its function probably being simply to aid the attachment of DnaB.

The latter is a helicase, an enzyme which can break base pairs. DnaB begins to increase the single-stranded region within the origin, enabling the enzymes involved in the elongation phase of replication in E. coli as the replication forks now start to progress away from the origin and DNA copying begins.

Initiation of DNA Replication in Yeast:

Origins identified in yeast are called autonomously replicating sequences, or ARSs. Atypical yeast origin is shorter than E. coli oriC, being usually less than 200 bp in length. In it four sub-domains are recognized.

Two of these sub-domains A and B1 – make up the origin recognition sequence, a stretch of some 40 bp in total that is the binding site for the Origin recognition complex (ORC), a set of six proteins that attach to the origin.

ORCs have been described as yeast versions of the E. coli DnaA proteins, but this interpretation is probably not strictly correct because ORCs appear to remain attached to yeast origins throughout the cell cycle. Rather these are genuine initiator proteins. It is more likely that ORCs are involved in the regulation of genome replication, acting as mediators between replication origins and the regulatory signals that coordinate the initiation of DNA replication with the cell cycle.

There are similar sequences in yeast to that of oriC of E. coli. This leads us to the two other conserved sequences in the typical yeast origin, sub-domains B2 and B3. Our current understanding suggests that these two sub-domains function in a manner similar to the E. coli origin. Sub-domain B2 appears to correspond to the 13-nucleotide repeat array of the E. coli origin, being the position at which the two strands of the helix are first separated.

This melting is induced by torsional stress introduced by attachment of a DNA-binding protein, ARS binding factor 1 (ABF1), which attaches to sub-domain B3. As in E. coli, melting of the helix within a yeast replication origin is followed by attachment of the helicase and other replication enzymes to the DNA, completing the initiation process and enabling the replication forks to begin their progress along the DNA. Replications origins in higher eukaryotes have not been much understood.

(b) Elongation:

Once replication has been initiated the replication forks progress along the DNA and participate in the synthesis of new strand. At the chemical level, the template dependent synthesis of DNA is very similar to the template-dependent synthesis of RNA that occurs during transcription, but the two processes are quite different.

1. Discontinuous strand synthesis and the priming problem- During DNA replication both strands of the double helix must be copied. However, DNA polymerase enzymes are only able to synthesize DNA in the 5′ 3′ direction. This means that one strand of the parent double helix, called the leading strand, can be copied out in a continuous manner, but replication of the lagging strand has to be carried out in a discontinuous fashion, resulting in a series of short segments that must be ligated together to produce the intact daughter strand.

These short segments of poly-nucleotides are called as Okazaki fragments. These fragments were first isolated from E. coli bacteria in 1969. Okazaki fragments are 1000-2000 nucleotides in length, but in eukaryotes the equivalent fragments appear to be much shorter, perhaps less than 200 nucleotides in length.

2. The another feature of DNA replication is that DNA polymerase cannot initiate DNA synthesis on a molecule that is entirely single stranded: there must be short single stranded region to provide a 3′ end onto which the enzyme can add new nucleotides. Nucleotides are added at a rate of 50,000 bases per minute. The choice of nucleotide is determined by complementary nature.

At this rate chances of error are one in one thousand base pair replicated. However, actual rate is quite low (one in one billion). This is equal to about one error per genome per one thousand bacterial replication cycles. This error is further corrected by proofreading (Removal of mismatch nucleotide by DNA polymerase III). This means that primers are needed, one to initiate complementary strand synthesis on the leading polynucleotide, and one for every segment of discontinuous DNA synthesized on the lagging strand.

As DNA polymerase cannot deal with an entirely single stranded template, RNA polymerases have no difficulty in this respect, so the primers for DNA replication are made of RNA. In bacteria, primers are synthesized by primase, a special RNA polymerase with each primer being 4-15 nucleotides in length and most starting with the sequence 5′-AG-3′. Once the primer has been completed, strand synthesis is continued by DNA polymerase III.

In eukaryotes the situation is more complex because the primase is tightly bound to DNA polymerase a, and cooperates with this enzyme in synthesis of the first few nucleotides of a new polynucleotide. This primase synthesizes an RNA primer of 8-12 nucleotides, and then hands over to DNA polymerase a, which extends the RNA primer by adding about 20 nucleotides of DNA. After completion of DNA-RNA primer, DNA synthesis is continued by the main replicative enzyme, DNA polymerase 5.

Priming needs to occur just once on the leading strand, within the replication origin, because once primed, the leading-strand copy is synthesized continuously until replication is completed. On the lagging strand, priming is a repeated process that must occur every time a new Okazaki fragment is initiated.

3. Replication fork elongation-As with the attachment of DnaB helicase, followed by extension of the melted region of the replication origin, the initiation phase ends. After the helicase has bound to the origin to form pre-priming complex, the primase is involved, resulting in the primosome, which initiates replication of the leading strand. It does this by synthesizing the RNA primer that DNA polymerase III needs in order to begin copying the template. More than one helicase is known and this enzyme is involved in various processes, such as transcription, recombination besides replications.

4. Complementary strands of a DNA tend to become duplex. During the process of replication, these sticky single stranded DNA are prevented to become duplex by special proteins called as single strand binding proteins (SSBs). Once 1000-2000 nucleotides are added in the leading strand, synthesis of lagging strand or Okazaki fragments began. This also requires an RNA primer and DNA polymerase III similar to leading strand.

5. DNA polymerase I is involved in removing the RNA primer from Okazaki fragments, having 5′ → 3′ exo-nuclease activity. Gap created by primer is filled by adding nucleotides at 3′ end. The nick between two Okazaki fragments is sealed by DNA ligase by the formation of phosphodiester bonds (Fig. 3.5).

(c) Termination:

Because bacteria have circular chromosomes, termination of replication occurs when the two replication forks meet each other on the opposite end of the parental chromosome. E coli regulate this process through the use of termination sequences which, when bound by the Tus protein, enable only one direction of replication fork to pass through.

As a result, the replication forks are constrained to always meet within the termination region of the chromosome. Eukaryotes initiate DNA replication at multiple points in the chromosome, so replication forks meet and terminate at many points in the chromosome these are not known to be regulated in any particular manner.

8: DNA Structure and Replication - Biology

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DNA, abbreviation of deoxyribonucleic acid, organic chemical of complex molecular structure that is found in all prokaryotic and eukaryotic cells and in many viruses. DNA codes genetic information for the transmission of inherited traits.

What does DNA do?

Deoxyribonucleic acid (DNA) is an organic chemical that contains genetic information and instructions for protein synthesis. It is found in most cells of every organism. DNA is a key part of reproduction in which genetic heredity occurs through the passing down of DNA from parent or parents to offspring.

What is DNA made of?

DNA is made of nucleotides. A nucleotide has two components: a backbone, made from the sugar deoxyribose and phosphate groups, and nitrogenous bases, known as cytosine, thymine, adenine, and guanine. Genetic code is formed through different arrangements of the bases.

Who discovered the structure of DNA?

The discovery of DNA’s double-helix structure is credited to the researchers James Watson and Francis Crick, who, with fellow researcher Maurice Wilkins, received a Nobel Prize in 1962 for their work. Many believe that Rosalind Franklin should also be given credit, since she made the revolutionary photo of DNA’s double-helix structure, which was used as evidence without her permission.

Can you edit DNA?

Gene editing today is mostly done through a technique called Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), adopted from a bacterial mechanism that can cut out specific sections in DNA. One use of CRISPR is the creation of genetically modified organism (GMO) crops.

What is a DNA computer?

DNA computing is a proposed computer architecture that would use the self-binding nature of DNA to perform calculations. Unlike classical computing, DNA computing would allow multiple parallel processes and calculations to occur at the same time.

A brief treatment of DNA follows. For full treatment, see genetics: DNA and the genetic code.

The chemical DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. In 1953 James Watson and Francis Crick, aided by the work of biophysicists Rosalind Franklin and Maurice Wilkins, determined that the structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands wound around each other. The breakthrough led to significant advances in scientists’ understanding of DNA replication and hereditary control of cellular activities.

Each strand of a DNA molecule is composed of a long chain of monomer nucleotides. The nucleotides of DNA consist of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases: two purines ( adenine and guanine) and two pyrimidines ( cytosine and thymine). The nucleotides are joined together by covalent bonds between the phosphate of one nucleotide and the sugar of the next, forming a phosphate-sugar backbone from which the nitrogenous bases protrude. One strand is held to another by hydrogen bonds between the bases the sequencing of this bonding is specific—i.e., adenine bonds only with thymine, and cytosine only with guanine.

The configuration of the DNA molecule is highly stable, allowing it to act as a template for the replication of new DNA molecules, as well as for the production (transcription) of the related RNA (ribonucleic acid) molecule. A segment of DNA that codes for the cell’s synthesis of a specific protein is called a gene.

DNA replicates by separating into two single strands, each of which serves as a template for a new strand. The new strands are copied by the same principle of hydrogen-bond pairing between bases that exists in the double helix. Two new double-stranded molecules of DNA are produced, each containing one of the original strands and one new strand. This “semiconservative” replication is the key to the stable inheritance of genetic traits.

Within a cell, DNA is organized into dense protein-DNA complexes called chromosomes. In eukaryotes, the chromosomes are located in the nucleus, although DNA also is found in mitochondria and chloroplasts. In prokaryotes, which do not have a membrane-bound nucleus, the DNA is found as a single circular chromosome in the cytoplasm. Some prokaryotes, such as bacteria, and a few eukaryotes have extrachromosomal DNA known as plasmids, which are autonomous, self-replicating genetic material. Plasmids have been used extensively in recombinant DNA technology to study gene expression.

The genetic material of viruses may be single- or double-stranded DNA or RNA. Retroviruses carry their genetic material as single-stranded RNA and produce the enzyme reverse transcriptase, which can generate DNA from the RNA strand. Four-stranded DNA complexes known as G-quadruplexes have been observed in guanine-rich areas of the human genome.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.

References and Resources

Guided Paper

Meselson, M. and Stahl, F.W. (1958). The replication of DNA in Escherichia coli. Proceedings of the National Academy of Sciences U.S.A., 44: 672–682.


  • Matthew Meselson’s letter to James Watson from November 8, 1957, describing the results of their experiments on DNA replication. Download.
  • Meselson, M., Stahl, F.W., and Vinograd, J. (1957). Equilibrium sedimentation of macromolecules in density gradients. Proceedings of the National Academy of Sciences U.S.A., 43: 581–588.

This paper describes the use of the centrifuge and density gradient to analyze biological molecules, a technique that was used in their 1958 paper but also very broadly used for many applications in biology. See also Dig Deeper 3 .

An outstanding resource for those wanting a detailed, accurate description of the Meselson–Stahl experiment.


  • White Board Video on the Semi-Conservative Model of DNA and the Meselson–Stahl Experiment by iBiology.

A nice 7:30 min video describing the Meselson–Stahl experiment and its conclusions.

This film documents the discovery of the structure and replication of DNA including interviews with James Watson who, along with Crick, proposed the double helix model of DNA.

This activity is often used in conjunction with the short film The Double Helix. It introduces students to Meselson and Stahl experiment and helps them understand the concepts generated via those experimental results.

This collection of resources from HHMI Biointeractive addresses many of the major concepts surrounding DNA and its production, reading, and replication.

Watch the video: DNA Structure and Replication: Crash Course Biology #10 (October 2022).