2.3.5 DNA Replication

Question 1

Semi-Conservative Replication of DNS

Answer 1

Before a (parent) cell divides, it needs to copy the DNA contained within it
- doubling the DNA ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA
The DNA is copied via a process known as semi-conservative replication (semi=half)
- the process is called this because in each new DNA molecule produced, one half of the polynucleotide DNA strand (half of the new DNA molecule) if from the original DNA molecule being copied
- the other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
- therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand

Question 2

The Importance of Retaining One Original DNA Strand

Answer 2

Retaining one original DNA strand ensures there is genetic continuity (i.e. genetic information is conserved) between generations of cells
In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
This is important because cells in our body are replaced regularly and therefore we need new cells to be able to do the same roles as the old ones
- replication of DNA and cell divisions also occurs during growth

Question 3

Semi-Conservative Replication Process

Answer 3

DNA replication occurs in preparation for mitosis, when a parent cell divides to produce two genetically identical daughter cells
- as each daughter cell contains the same number of chromosomes as the parent cell, the number of DNA molecules in the parent cell must be doubled before mitosis takes place
DNA replication occurs during the S phase of the cell cycle (which occur during interphase, when a cell is not diving)
The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands
Each of these single polynucleotide DNA strands act as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base paring
The new nucleotides are then joined together by the enzyme DNA polymerase which catalysed condensation reactions to form a new strand
The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules

Question 4

DNA Polymerase

Answer 4

In the nucleus, there are free nucleotides which contain three phosphate groups
- these nucleotides are known as nucleoside triphosphate or ‘activated nucleotides’
- the extra phosphates activate the nucleotides, enabling them to take part in DNA replication
The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands
The enzyme DNA polymerase synthesises new DNA strands from the two template strands
It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides
Hydrogen bonds then form between the complementary base pairs of the template and the new DNA strand

Question 5

Mutations

Answer 5

The replicated DNA molecules must be an exact copy of the parent DNA molecule, therefore the formation of the complementary strands must be a highly accurate process
Although the process is astonishingly accurate considering it is happening constantly in cells at a considerable speed, occasional mistakes occur in the form of:
- bases being inserted into the complementary strand in the wrong order
- and extra base being inserted by accident
- a base being left out by accident
These mistakes in the process of semi-conservative replication of DNA result in the occur each of random, spontaneous mutations
- i.e. errors in the genetic code