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Topic last updated: 02 Feb 2022

- Chemistry
- Biology
- 16+
- 75

Chemistry of life

13 of 14

DNA replication

The ability of the DNA molecule to make exact copies of itself is what makes it ideal as the molecule of inheritanceinheritance
The transfer of characteristics from parents to children through their genes. . DNA replication allows all of the cells of your body – all produced from the original cell formed when an ovum and a sperm met – to carry the same genetic information. The importance of enzymes such as DNA helicaseDNA helicase
An enzyme which unzips the hydrogen bonds between the two strands of DNA during DNA replication. , DNA polymeraseDNA polymerase
An enzyme involved in the production of a new nucleotide strand to form a new DNA double helix. and DNA ligaseDNA ligase
An enzyme which is involved in DNA replication by catalysing the formation of phosphodiester bonds. for life on earth cannot be overestimated. Look at the diagram below to see how this elegant process happens, and follow it through in the animation which follows.

Step 0: The DNA double helix

Step 1: The enzyme DNA helicaseDNA helicase
An enzyme which unzips the hydrogen bonds between the two strands of DNA during DNA replication. unwinds the DNA double helix by unzipping the hydrogen bondhydrogen bond
An intermolecular attractive force between hydrogen, when it is covalently bonded to a highly electronegative atom (fluorine, oxygen or nitrogen), and an oxygen, nitrogen or fluorine atom on another molecule. s between the base pairs. This prepares the molecule for replication, transcription or recombinationrecombination
The process by which maternal and paternal chromatids in a homologous pair of chromosomes are broken and rejoined in a random way by large multi-enzyme complexes, introducing variation during meiosis. .

Step 2: The original strands act as templates for the new DNA strands. The exposed bases attract free DNA nucleotidenucleotide
Monomer unit of the nucleic acids DNA and RNA. Each nucleotide is made up of three parts: a pentose sugar, a phosphate group and a nitrogenous base. s and new hydrogen bondhydrogen bond
An intermolecular attractive force between hydrogen, when it is covalently bonded to a highly electronegative atom (fluorine, oxygen or nitrogen), and an oxygen, nitrogen or fluorine atom on another molecule. s are formed between the matching base pairs. The enzymes DNA polymeraseDNA polymerase
An enzyme involved in the production of a new nucleotide strand to form a new DNA double helix. and DNA ligaseDNA ligase
An enzyme which is involved in DNA replication by catalysing the formation of phosphodiester bonds. control the process and ensure that the copies are exact.

Step 3: As the hydrogen bondhydrogen bond
An intermolecular attractive force between hydrogen, when it is covalently bonded to a highly electronegative atom (fluorine, oxygen or nitrogen), and an oxygen, nitrogen or fluorine atom on another molecule. s form within the structure, the molecule coil up into their typical double helix shape. At the end of the process, there are two new DNA double helices, each identical to the original.

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For some years after the structure of the DNA double helix had been worked out, scientists argued about how DNA replicated itself. There were two main theories:

Conservative replication, which assumed that the original DNA double helix remained intact with new strands forming on the outside
Semi-conservative replication, which assumed the original strand opens up and each new helix contains one old and one new strand of DNA

The arguments were finally settled by Matthew Meselson and Franklin Stahl in a series of experiments which are best seen in action:

1. Control

In this control experiment, two reference lines are created in the centrifuge ‒ one for a DNA double helix known to be synthesised using only 'heavy' nitrogen (¹⁵N), and one for a DNA double helix synthesised using only 'light' (normal) nitrogen (¹⁴N).

2. Conservative replication

If DNA replication is conservative, the result of placing 'heavy' ¹⁵N double helix of DNA in a medium containing only 'light' ¹⁴N atoms will be the original 'heavy' ¹⁵N DNA double helix and a new 'light' ¹⁴N DNA double helix.

3. Semi-conservative replication

If DNA replication is semi-conservative, the result of placing 'heavy' ¹⁵N double helix of DNA in a medium containing only 'light' ¹⁴N atoms will be two double helices of hybrid DNA. Each will contain one strand of 'heavy' ¹⁵N DNA and a new strand of 'light' ¹⁴N DNA.

If hybrid DNA is allowed to replicate again in the 'light' ¹⁴N medium, half of the DNA double helices will have only 'light' ¹⁴N. The other half will be hybrid helices with one strand of the original 'heavy' ¹⁵N DNA and a new strand of 'light' ¹⁴N DNA.

This classic experiment showed scientists exactly how DNA replicated

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Topic last updated: 02 Feb 2022

Chemistry of life

DNA replication

DNA and RNA

Types of RNA

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