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DNA replication

DNA replication is the process leading to the creation of two copies of DNA from one copy.

Semi-conservative replication limits the number of mutations (in this case, replication errors) resulting from the replication process.

This is achieved by separating the two strands of the parent DNA and then assembling complementary strands for each of them.

As a result, the new DNA molecules each consist of one old and one new strand (hence the name semi-conservative).

In semi-conservative replication, each of the two new DNA molecules contain one strand of the old DNA molecule.
In semi-conservative replication, each of the two new DNA molecules contain one strand of the old DNA molecule.

Having one parent strand act as a template minimises the number of errors in the newly synthesised strand.

DNA replication is highly accurate, so entire genes may be identical between two distantly related species.

At the start of DNA replication, the enzyme DNA helicase separates the two strands of DNA.

The enzyme breaks the hydrogen bonds between nitrogenous bases. This exposes the bases so that they can be used as templates to build new strands.

The enzyme DNA polymerase creates the new strand. It selects nucleotides that are complementary to the bases on the template and joins them to the newly forming strand.

Only small sections of DNA are unwound and separated at one time.

The DNA replication process starts at specific sequences on the DNA called origins of replication. The area at which replication is occurring is the replication fork.

A simplified diagram of DNA replication.
A simplified diagram of DNA replication.

DNA polymerase can only assemble a new strand in the 5' to 3' direction. In other words, the enzyme can only add new nucleotides to the 3' end of a strand being assembled.

Because two complementary strands run in opposite directions, the template (or parent) DNA strand can only be read in the 3' to 5' direction.

The directionality arises because the 3' carbon of one deoxyribose is linked to the 5' carbon of another deoxyribose.

In a double strand, the 3' end on one strand is adjacent to the 5' end on the other strand. The two strands must be read in opposite directions.

Synthesis of a new DNA strand always goes in the 5' to 3' direction (labelled in blue).
Synthesis of a new DNA strand always goes in the 5' to 3' direction (labelled in blue).

DNA can only be replicated from the 5' end of a new strand to the 3' end. New nucleotides must be added to the 3' end of a strand.

The two anti-parallel parent strands have different directions.

The leading strand is exposed in the 3' to 5' direction. The complementary strand can form continuously along the leading strand as the DNA 'unzips', since nucleotides are free to join the 3' end of the strand that is forming.

The lagging strand is exposed in the 5' to 3' direction. The new strand cannot form in the 3' to 5' direction so instead it is made in short sections.

This process is complex and so production of this new strand lags behind.

The segments formed by this discontinuous process are called Okazaki fragments.

They are joined together by an enzyme called DNA ligase.

The strand complimentary to the lagging parent strand
The strand complimentary to the lagging parent strand

Partly due to the Meselson-Stahl experiment in 1958, semi-conservative replication is now considered to be the best description of DNA replication.

At the time, there were two alternative models of DNA replication:

Under conservative replication, the original DNA is conserved (left intact) and the daughter strand is an entirely new copy of the parent DNA.

Under dispersive replication, double-stranded sections of parent DNA are divided among the daughter molecules.

The Meselson-Stahl experiment suggests that DNA replication is semi-conservative rather than conservative or dispersive.
The Meselson-Stahl experiment suggests that DNA replication is semi-conservative rather than conservative or dispersive.

The Meselson-Stahl experiment in 1958 provided experimental evidence for semi-conservative replication.

Initially, bacteria (E. coli) were grown in an environment containing only the rare nitrogen isotope $$\ce{^{15}N}$$.

DNA containing $$\ce{^{15}N}$$ is functional but heavier than normal DNA.

After several generations of replication, the nitrogen in the bacterial DNA was almost entirely $$\ce{^{15}N}$$.

Then a batch of heavy bacteria was allowed to replicate in an environment of the more common nitrogen isotope ($$\ce{^{14}N}$$).

After each replication, the DNA of a sample from the batch was put into a centrifuge to separate the DNA molecules by their density $$(=\text{mass}\div\text{volume})$$.

After the first replication, each DNA molecule contained 50% $$\ce{^{15}N}$$ and 50% $$\ce{^1^4N}$$. All the DNA molecules were $$\ce{^1^4N}$$/$$\ce{^1^5N}$$ hybrids.

This excluded the conservative hypothesis. If it had been correct, half of the DNA molecules would contain only $$\ce{^{15}N}$$ and the other would contain only $$\ce{^{14}N}$$.

After the second replication, half the strands were made up of 50% $$\ce{^1^5N}$$ and 50% $$\ce{^1^4N}$$ and the other half entirely $$\ce{^1^4N}$$.

Dispersive replication would have predicted that all DNA consists of 25% $$\ce{^{15}N}$$ and 75% $$\ce{^{14}N}$$ but this was not the case.

The experiment lent support to semi-conservative replication.

Percentage of DNA
Generation Purely $$\ce{^1^4N}$$ $$\ce{^1^4N}$$/$$\ce{^1^5N}$$ hybrid Purely $$\ce{^1^5N}$$
0 0% 0% 100%
1 0% 100% 0%
2 50% 50% 0%