Outline the action of enzymes.

Explain the roles of specific enzymes in prokaryote DNA replication.

Many genetic diseases are due to recessive alleles of autosomal genes that code for an enzyme. Using a Punnett grid, explain how parents who do not show signs of such a disease can produce a child with the disease.

Catalyse/speed up reactions

Substrate-specific

Lower the activation energy «of a chemical reaction»

Substrate collides with/binds to active site

Enzyme–substrate complex formed
OR
transition state formed
OR
bonds in substrate weakened

«DNA» gyrase/topoisomerase «II» prepares for uncoiling/relieves strains «in the double helix»

Helicase uncoils/unwinds the DNA/double helix

Helicase separates/unzips/breaks hydrogen bonds between the two strands of DNA

«DNA» primase adds an RNA primer/short length of RNA  Accept RNA primase.

DNA polymerase III adds «DNA» nucleotides/replicates DNA/synthesizes complementary strand in a 5’ to 3’ direction

DNA polymerase III starts replication/adding nucleotides at the primer

DNA polymerase I removes the primer
OR
replaces RNA with DNA

«DNA» ligase seals the nicks
OR
links sections of replicated DNA
OR
links Okazaki fragments

DNA polymerase I/DNA polymerase III proofreads for mistakes

Key or text giving alleles with upper case for dominant allele and lower case for recessive allele/allele causing disease

Reject key showing a sex linked gene such as hemophilia.
Reject if X or Y chromosomes are shown with the alleles.
Accept Aa or any other upper and lower case letters.

Punnett grid showing that both parents can pass on either a dominant or a recessive allele in their gamete

For example row and column headings with A and a.
This mark can be awarded if X or Y chromosomes are shown but each parent has one recessive and one dominant allele as if for autosomal inheritance.

Four possible genotypes for child correctly shown on grid

AA, Aa, aA and aa for example.
This mark can be awarded if X or Y chromosomes are shown but the genotypes are correct for autosomal inheritance.

Double/homozygous recessive shown having the disease

Cannot be awarded with sex linkage.

25 % or 0.25 or 1/4 chance of inheriting the disease

This mark can be awarded if X or Y chromosomes are shown but the ratio is correct for autosomal inheritance.

This was generally well answered with most candidates able to give enough of the important features of enzyme action to score well. One mistake seen in a number of responses was to state that the active site is on the substrate rather than on the enzyme.

Knowledgeable candidates had no difficulty in scoring full marks by giving an accurate description of the role of enzymes in DNA replication. It was not necessary to focus on the leading and lagging strands as the action of the various enzymes is largely the same, though of course primers are repeatedly added to the lagging strand and then replaced. Some candidates were obviously concerned that they were being asked about prokaryote DNA replication. This is of course the type of DNA replication that is specified by the programme and has been for many years. It is worth making sure that candidates know that they have learned about this rather than eukaryote replication.

This part was very well answered with many candidates scoring full marks. There were a few errors in notation with different letters of the alphabet used for alleles of the same gene or X and Y chromosomes indicating confusion between autosomal and sex-linked genes.