DNA replication

Why is DNA replication important

So each daughter cell receives the correct genetic information.

Why is DNA replication described as semi-conservative

Two identical molecules are produced each containing one original strand (acting as a template) and one new strand.

Why does DNA replication start at many sites along a chromosome

to reduce the time taken to replicate the chromosome.

First step of DNA replication

Remove histone proteins from DNA and activate DNA nucleotides by addition of 2 phosphate groups from ATP.

Second step of DNA replication

Gyrase unwinds the DNA and DNA helicase unzips the DNA by breaking the hydrogen bonds between the bases to separate the 2 DNA strands.

Third step of DNA replication

Two DNA polynucleotide strands are exposed and both act as templates.

Forth step of DNA replication

DNA polymerase binds to each template strand.

Fifth step of DNA replication

Free activated DNA nucleotides align via complementary base pairing to template strands and bind forming hydrogen bonds between the bases.

Sixth step of DNA replication

DNA polymerase catalyses the condensation reaction that joins the nucleotides together by the formation of a phosphodiester bond.

Seventh step of DNA replication

DNA is rewound and wrapped around histones to form chromosomes.

In which direction does DNA polymerase synthesise the new strand

From 5’ to 3’

Which end of DNA can DNA polymerase add nucleotides

3’ end

Leading strand

The DNA strand which is synthesised continuously

Lagging strand

The DNA strand that is synthesised in Okazaki fragments

Role of gyrase in DNA replication

Unwinds/uncoils DNA

Role of DNA helicase in DNA replication

Unzips DNA by breaking hydrogen bonds between the bases to separate the strands.

Role of DNA polymerase in DNA replication

Catalyses the condensation reaction that joins adjacent nucleotides together by phosphodiester.

Role of DNA ligase in DNA replication

Catalyses the condensation reaction that joins short DNA fragments together to form a complete strand of DNA in the synthesis of the lagging strand.

First step in the Meselson-Stahl experiment

E. coli bacteria were grown on a medium containing the normal ¹⁴N isotope of nitrogen.

Second step in the Meselson-Stahl experiment

The E. coli were then switched to a medium containing the heavy nitrogen isotope ¹⁵N and grown for many generations until most of the DNA was labelled with ¹⁵N.

Third step in the Meselson-Stahl experiment

The bacteria were then transferred back to a medium containing the normal ¹⁴N isotope and were allowed to reproduce for 1,2 or 3 generations.

Forth step in the Meselson-Stahl experiment

DNA was extracted from the bacteria after each generation.

Fifth step in the Meselson-Stahl experiment

DNA was analysed by centrifugation through a caesium chloride density gradient to separate the DNA molecules according to density.

How were the results obtained from the Meselson-Stahl experiment consistent with semi-conservative replication

After the first generation they obtained DNA of intermediate density, which is not possible with conservative replication.

Why is it important that DNA replication is accurate

Mutations may occur and the daughter cells will not have the correct genetic information required to synthesise proteins.

Mutation

A random change in the DNA base sequence.

What occurs if a mutation occurs in a gene

It produces a new allele and may alter the amino acid sequence of the polypeptide and so alter its function.

During DNA replication, how is the mutation rate reduced

If a mismatched base is inserted the DNA polymerase removes it and re-inserts the correct base.

After DNA replication, how is the mutation rate reduced

DNA mismatch repair mechanisms operate to check the DNA for errors and repair the new DNA strand.