Lecture 9

Question 1

Explain why the following are problems for the cell during DNA replication and explain how the cells solves these problems

Answer 1

Unwinding the DNA strands
- this is an issue because it doesn’t stay separated. This is then solved by gyrases
Synthesising DNA accurately
- mistakes are bound to happen as this process is done quite quickly, DNA polymerase have a ‘proof reading’ ability that allows them to be very accurate. If the nucleotide is not accurate, it is removed and replaced.
Primase begins the processes of DNA replication.
Replicating antiparallel strands
- there are problems in replication of a doubled - stranded DNA molecule as both strands must be copied and two new copies must be synthesised in opposite directions.
Solution - the lagging strands synthesis discontinuously

Question 2

Compare and contrast leading and lagging strands

Answer 2

Leading strand
- is synthesised continuously
Lagging strand
- is synthesised discontinuously

Question 3

List important enzymes and other molecules involved in replicating each of the two strands of the double stranded DNA molecule and explain their functions

Answer 3

Helicases enzyme unwinds the DNA strand
Topoisomerase - prevents tension at replication fork whilst DNA is unwinded
Single stranded protein - stops DNA sequence from rewinding
Primase - attaches a small sequence of mRNA to the 3’ end of the template
DNA polymerase I - proofreader of the two strands, before adding the next nucleotide in the chain they need to check the base-pair is properly paired
DNA polymerase II - for the repairing of inter-strand cross-links
Primer - a short nucleic acid polymer that provides the DNA polymerase with something to proof read
Polymerase III - checks the last nucleotide on the RNA primer and continuous with synthesis of a new strand of DNA
Leading is built throughout this process continuously
Lagging strand is built discontinuously.
Once this process is finished the stranded winded up and ligased and the Okaski fragments are formed