Question
RNA molecules play numerous roles in gene expression and so contribute to variation.
a. Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA. [4]
b. Explain transcription. [7]
c. Distinguish between continuous and discrete variation, using examples.[4]
▶️Answer/Explanation
Markscheme
a. ribose drawn as pentagon and labelled sugar/ribose;
b. base drawn with correct link to (C1 of) ribose and labelled base/nitrogenous base;
c. phosphate drawn with correct link to (C5 of) ribose and labelled P/phosphate;
d. two (or more) ribonucleotides drawn with correct link (C3 to C5)
a. synthesis of RNA/mRNA / transcription of DNA to RNA;
b. RNA nucleotides linked together to form a strand/chain;
c. RNA strand assembled on DNA template/antisense strand / copy made of sense strand;
d. RNA polymerase carries out transcription/links RNA nucleotides;
e. uncoiling/separation of DNA strands;
f. 5’ end of nucleotides linked to 3’ end of (growing RNA) strand;
g. complementary base pairing (is the basis of copying the base sequence);
h. uracil instead of thymine in RNA;
i. starts at/RNA polymerase binds to a promoter;
j. regulated by transcription factors/DNA binding proteins/nucleosomes;
Annotated diagrams can be used.
Question
DNA methylation has a critical role in gene regulation by affecting transcription. Samples were taken from two colon cancer tumours (T1 and T2) and two normal colon samples (N1 and N2). A particular gene was implicated as a possible cause of cancer. The promoter of this gene was cloned (A–J). The data show the DNA methylation patterns from these samples. The numbers (32–269) represent different markers in the promoter.
a.i. Identify the stage of mitosis labelled X in the image, giving a reason.
[1]
a.ii. Outline what is indicated by the mitotic index of tissue taken from a tumour. [2]
a.iii. DNA has regions that do not code for proteins. State two functions of these regions.
1.
2.[2]
b.i. Outline the difference in methylation pattern between tumorous and normal tissue samples. [2]
b.ii. Suggest a way methylation may affect tumour cell genes. [1]
▶️Answer/Explanation
Markscheme
telophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔
a. mitotic index is an indication of the ratio/percentage of cells undergoing mitosis/cell division ✔
b. cancer cells «generally» divide much more than normal «somatic» cells ✔
c. a high/elevated mitotic index in tumours / possible diagnosis of cancer /measure of how aggressive/fast growing the tumour is ✔
a. promoters / operators / regulation of gene expression/transcription ✔
b. telomeres/give protection to the end of chromosomes «during cell division» ✔
c. genes for tRNA/rRNA production ✔
d. other valid function for non-coding sequence ✔
Do not accept stop codon, accept centromeres (connecting sister chromatids).
a. «overall» much more methylation in the colon tumour samples than normal ✔
b. tumour and normal samples the markers 258 and 269 similar degree of methylation/fewer differences ✔
c. degree of methylation on certain markers may correlate with the presence of cancer / correct example of a marker only methylated in tumour cells eg marker 32 ✔
a. «DNA» methylation may inhibit transcription of genes that would prevent cancer/tumor formation ✔
b. «DNA» methylation may increase mitosis/cell division leading to tumor formation ✔
Do not accept discussion of histone methylation.