DNA replication is a crucial part of the cell cycle. It happens before the cell splits into two during mitosis and meiosis. This ensures that the daughter cells have the right amount of genetic material. But why do cells divide in the first place? Mitosis helps with tissue growth, repair, and asexual reproduction. Meiosis is essential for sexual reproduction and making gametic cells. In summary, DNA replication is necessary for cell division, which is vital for growth and reproduction.

DNA replication

The phases of the cell cycle.

Semiconservative DNA replication steps

Semiconservative replication is a crucial process where each strand of the original DNA molecule is used as a template for creating a new DNA strand. Accurate execution of the steps is essential to prevent mutated DNA in the daughter cells. It's important to remember that the DNA double helix is anti-parallel.

The steps for semiconservative DNA replication are as follows. The enzyme DNA polymerase catalyses the formation of phosphodiester bonds. It can only create new DNA strands in the 5 'to 3' This newly synthesized strand is called the leading strand. It's replicated continuously by DNA polymerase, which travels towards the replication fork.

What are the DNA replication enzymes?

Semiconservative DNA replication relies on the action of enzymes. The 3 main enzymes involved are:

DNA helicase DNA polymerase DNA ligase

DNA helicase

DNA helicase is involved in the early steps of DNA replication. It breaks the hydrogen bonds between the complementary base pairs to expose the bases on the original strand of DNA. This allows free DNA nucleotides to attach to their complementary pair.

DNA polymerase

DNA polymerase catalyses the formation of new phosphodiester bonds between the free nucleotides in condensation reactions. This creates the new polynucleotide strand of DNA.

DNA ligase

Evidence for semiconservative DNA replication

There are two models of DNA replication: conservative and semiconservative. The conservative model proposes that after one round of replication, there is an original DNA molecule and a completely new DNA molecule made of new nucleotides. On the other hand, the semervative model suggests that one round of both DNA molecules have one original strand and one new strand DNA. As we discussed earlier the semiconservative model is the one that has been widely accepted.

Meselson and Stahl experiment

In the 1950s, two scientists named Matthew Meselson and Franklin Stahl performed an experiment that led to the semiconservative model becoming widely accepted in the scientific community.

So how did they do this? The DNA nucleotides contain nitrogen within the organic bases and Meselson and Stahl knew there were 2 isotopes of nitrogen: N15 and N14, with N15 being the heavier isotopes.

The scientists began by culturing E. coli in a medium containing only N15, which led to the bacteria taking up the nitrogen and incorporating it into their DNA nucleotides. This effectively labelled the bacteria with N15.

Generation 0: 1 single band. This indicates the bacteria only contained N15.Generation 1: 1 single band in an intermediate position relative to Generation 0 and the N14 control. This indicates that the DNA molecule is made of both N15 and N14 and thus has an intermediate density. The semiconservative DNA replication model predicted this outcome. Generation 2: 2 bands with 1 band in the intermediate position which contains both N15 and N14 (like Generation 1) and the other band positioned higher, which contains only N14. This band is positioned higher than N14 has a lower density than N15.

Illustration of the findings of the Meselson and Stahl experiment.

The evidence from Meselson and Stahl's experiment demonstrates that each DNA strand acts as a template for a new strand and that, after each round of replication, the resulting DNA molecule contains both an original and a new strand. As a result, the scientists concluded that DNA replicates in a semiconservative manner.

DNA Replication - Key takeaways DNA replication happens before cell division during the S phase and is important for ensuring each daughter cell contains the correct amount of genetic information. Semiconservative DNA replication state that the new DNA molecule will contain one original DNA strand and one new DNA strand. This was proved correct by Meselson and Stahl in the 1950s. The main enzymes involved in DNA replication are DNA helicase, DNA polymerase and DNA ligase.

DNA replication

What is DNA replication?

DNA replication is the copying of the DNA found within the nucleus before cell division. This process happens during the S phase of the cell cycle.

Why is DNA replication important?

DNA replication is important because it ensures that the resulting daughter cells contain the correct amount of genetic material. DNA replication is also a necessary step for cell division, and cell division is highly important for the growth and repair of tissues, asexual reproduction and sexual reproduction. 

What are the steps of DNA replication?

DNA helicase unzips the double helix by breaking the hydrogen bonds. Free DNA nucleotides will match with their complementary base pair on the now-exposed DNA strands. DNA polymerase forms phosphodiester bonds between adjacent nucleotides to form the new polynucleotide strand.

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