DP Biology (first assessment 2025)

Test builder »

Question 23M.2.HL.TZ1.7

Select a Test
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
7.
[Maximum mark: 15]
23M.2.HL.TZ1.7

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.

(a)

Outline the changes to chromosomes that occur during prophase in the first division of meiosis.

[4]

Markscheme
  1. pairing/synapsis of homologous chromosomes / homologous chromosomes form bivalents;
  2. crossing over / chromatid breaks then rejoins to non-sister chromatid;
  3. exchange of DNA/alleles/genetic information between chromatids/chromosomes;
  4. recombination / new combinations of alleles/genes generated;
  5. condensation/shortening/thickening/supercoiling of chromatids/chromosomes;
  6. formation of a chiasma where crossing over occurred;
(b)

Describe the processes that are carried out by enzymes that bind to DNA.

[7]

Markscheme

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;

(c)

Explain the effects that the environment can have on DNA in living organisms.

[4]

Markscheme

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.

Syllabus sections