Question 19M.2.SL.TZ1.7a
| Date | May 2019 | Marks available | [Maximum mark: 4] | Reference code | 19M.2.SL.TZ1.7a |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Outline | Question number | a | Adapted from | N/A |
Outline the roles of helicase and ligase in DNA replication.
[4]
helicase:
a. unwinds/uncoils the DNA «double helix» ✔
b. breaks hydrogen bonds «between bases» ✔
c. separates the «two» strands/unzips the DNA/creates replication fork ✔
ligase:
d. seals nicks/forms a continuous «sugar-phosphate» backbone/strand ✔
e. makes sugar-phosphate bonds/covalent bonds between adjacent nucleotides ✔
f. after «RNA» primers are removed/where an «RNA» primer was replaced by DNA ✔
g. «helps to» join Okazaki fragments ✔
This was generally well answered, with most candidates knowing at least something of the roles of these two enzymes. Most candidates knew that ligase connects Okazaki fragments but some claimed that it creates hydrogen bonds between nucleotides on template and the new strand. Many candidates did not distinguish between unwinding of DNA and separating the strands. Two details that should be more widely known are that helicase separates the two strands of a DNA molecule by encouraging the breakage of hydrogen bonds between bases and that ligase seals nicks by making sugar phosphate bonds.
