Question 23M.2.HL.TZ1.7
| Date | May 2023 | Marks available | [Maximum mark: 15] | Reference code | 23M.2.HL.TZ1.7 |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Describe, Explain, Outline | Question number | 7 | Adapted from | N/A |
All living organisms contain chromosomes. In addition to acting as stores of genetic information, these chromosomes are involved in a range of active processes during the life of a cell and of an organism.
Outline the changes to chromosomes that occur during prophase in the first division of meiosis.
[4]
- pairing/synapsis of homologous chromosomes / homologous chromosomes form bivalents;
- crossing over / chromatid breaks then rejoins to non-sister chromatid;
- exchange of DNA/alleles/genetic information between chromatids/chromosomes;
- recombination / new combinations of alleles/genes generated;
- condensation/shortening/thickening/supercoiling of chromatids/chromosomes;
- formation of a chiasma where crossing over occurred;
Answers were very varied, with some excellent accounts of first prophase in meiosis and some muddled ones. A common error was to think that a chiasma forms and then crossing over takes place, whereas the actual sequence of events is that crossing over leads to chiasma formation.
Describe the processes that are carried out by enzymes that bind to DNA.
[7]
replication
a. helicase unwinds the double helix/DNA;
b. helicase breaks hydrogen bonds between/separates/unzips DNA strands;
c. (DNA) gyrase/topoisomerase releases tensions in DNA as it unwinds;
d. (DNA) primase adds RNA primers (where DNA polymerase can bind);
e. DNA polymerase (III) replicates DNA/adds nucleotides (to make new strand);
f. DNA polymerase I replaces RNA (primers) with DNA;
g. DNA ligase seals nicks/joins sugar-phosphate backbones/joins (Okazaki)fragments;
transcription
h. RNA polymerase used for transcription;
i. RNA polymerase unwinds / separates DNA strands / binds to the promoter;
j. RNA polymerase copies DNA base sequence of a gene/makes mRNA;
k. restriction enzymes/endonucleases cut DNA at specific base sequences;
l. telomerase adds nucleotides to the ends of chromosomes/makes telomeres;
This question had the highest discrimination index on the exam, with well-prepared candidates having no difficulty in describing the roles of enzymes in replication and transcription. Some weak answers failed to name any enzymes. The weakest answers tended to confuse the processes of replication, transcription, and translation.
Explain the effects that the environment can have on DNA in living organisms.
[4]
Mutation
a. (environment can cause) mutation;
b. mutations are base sequence changes;
c. radiation/UV/gamma rays can cause mutations/changes to base sequences;
d. mutagenic/carcinogenic chemicals can cause mutations / mustard gas/another example;
Epigenetics
e. (environment) can cause changes to gene expression;
f. methylation (patterns) in DNA changed (in response to environmental factors);
g. methylation inhibits (gene transcription) / acetylation promotes (gene transcription);
h. body temperature/stress/diet (can affect gene expression);
For mpd do not allow ‘mutagen’ instead of ‘mutagenic chemical’ as it includes forms of radiation as well as chemicals.
Allow smoking and asbestos as examples of mutagens /carcinogens in mpd.
There were some excellent answers to this question with both mutation and epigenetics included. Some candidates failed to write about either of these processes and instead focussed on the environment causing evolution by natural selection. Marks were not awarded for this, because these effects of the environment are indirect.