Introduction

Recall from our prior discussion on eukaryotic cell reproduction that one of the critical steps to ensure that each progeny (daughter cell) receives a full complement of DNA is that the DNA must first be copied or replicated during the S-phase. Let us take a look at how the process of DNA replication works in the cell.

Learning

After the structure of DNA was determined, it became obvious to scientists that each nucleotide strand in the double helix could serve as a template for the replication of a new partner strand. This is because we know that for every:

• (A)denine on a parent strand of DNA, we would place a (T)hymine on the opposite strand during the copy process.

• For every (G)uanine, we would place a (C)ytosine on the opposite strand.

(Recall Chargaff’s Rule!)

Knowing these rules, scientists were able to decipher the mechanism of DNA replication by breaking it down into the following steps:

1.  The double helix is ‘unwound’
   • This step is performed by a special enzyme called ‘Helicase’; its purpose is to ‘unzip’ the DNA double-helix to separate the two nucleotide strands.
2. Parent strands serve as ‘templates’.
   • Enzymes in the nucleus are then capable of utilizing free nucleotides to ‘place’ them in the proper order to create a new DNA strand using the original parent strands as a template. The enzyme responsible for reading the template strand and assembling a new chain by joining together nucleotides is called ‘DNA Polymerase’.
   • Note: both the parental strands of DNA serve as a template for the simultaneous production of two ‘new’ complementary strands of DNA.
3. New DNA chains assemble as a double helix
   • As DNA Polymerase builds a new nucleotide chain, the double helix structure reforms for both copies of the ‘new’ DNA molecules.

Semi-Conservative Replication

This model of DNA replication is called ‘semiconservative’; this is because after DNA replication, each of the 2 new DNA strands produced from the original DNA molecule consist of:

• 1 nucleotide chain from the original DNA molecule
• 1 newly formed nucleotide chain

Please note, that unless errors have occurred, each of the two new DNA molecules represents an identical copy of the original DNA molecule.  Why is this so important?

Recall the overall goal of Mitosis – to produce two identical cells from a single original ‘parent’ cell.  In order for this to be true, each new cell must receive an identical copy of DNA – hence the reason why the fidelity of the DNA replication process is so critical!

Summary

In this section, we have learned:

• DNA replication occurs in a semi-conservative manner.
• The DNA double helix is unwound by the enzyme ‘helicase’ and copied by the enzyme ‘DNA polymerase’
• DNA replication depends on Chargaff’s Rule for base pairing; A pairs with T, G pairs with C.
• Following DNA replication, the end product is two identical copies of DNA, each identical to the original parental DNA molecule.

Sources:

DNA Replication. (2021, March 6). Retrieved May 22, 2021, from https://bio.libretexts.org/@go/page/7023.