Short Answer Questions - Molecular Biology
1. Describe the functions of Rb and p53.
Rb and p53 are tumour suppressors which prevent progression of the cell cycle from G1 phase to S phase.
2. The pathway for pyrimidine biosynthesis is a target for antineoplastic drugs. Name examples of these drugs and describe their effect on de novo pyrimidine biosynthesis.
Examples of antineoplastic drugs affecting pyrimidine biosynthesis are: (i) the antifolate methotrexate, which inhibits dihydrofolate reductase; (ii) the antipyrimidine 5-fluorouracil, which inhibits thymidylate synthase; (iii) hydroxyurea, which inhibits ribonucleotide reductase preventing formation of dUMP and dTMP.
3. The pathway for purine biosynthesis is a target for antineoplastic drugs. Name examples of these drugs and describe their effect on de novo purine biosynthesis.
Examples of antineoplastic drugs affecting purine biosynthesis are: (i) the antifolate methotrexate, which inhibits dihydrofolate reductase; (ii) the antipurine 6-mercaptopurine, which inhibits phosphoribosylpyrophosphate (PRPP) amidotransferase; (iii) hydroxyurea, which inhibits ribonucleotide reductase preventing formation of dGTP and dATP.
4. Which enzyme is deficient in Lesch–Nyhan syndrome?
Adenine phosphoribosyltransferase.
5. Describe base pairing in DNA. Which base pairing is the strongest bond?
Adenine pairs with thymine using two hydrogen bonds. Guanosine pairs with cytosine using three hydrogen bonds. So, guanosine/cytosine pairing is stronger.
6. What are telomeres?
Telomeres are protective caps found at both ends of a chromosome. They are composed (in vertebrates) of 6-nucleotide repeats: (TTAGGG).
7. Consult a table of the genetic code to determine the amino acid sequence coded by an mRNA with the sequence UUC-UAU-GUG-AGC.
Phe-Tyr-Val-Ser.
8. What is the functional difference between heterochromatin and euchromatin?
Heterochromatin is condensed, tightly packed and incapable of transcription. Euchromatin is less condensed and is accessible to molecules participating in the transcription of DNA to mRNA.
9. What is an Okazaki fragment?
Okazaki fragments are short lengths of DNA formed on the lagging strand during DNA replication in both eukaryotes and prokaryotes.
10. What is the function of DNA topoisomerase?
DNA topoisomerase relieves supercoiling in the DNA helix during replication. It does this by cutting the strands, relaxing the tension and rejoining the strands.
11. Which enzyme is deficient in xeroderma pigmentosa?
UV-specific endonuclease.
12. What is the codon which signals the start of transcription? It codes for different amino acids in eukaryotes and prokaryotes: what are they?
The start codon is AUG. In eukaryotes AUG codes for methionine (Met); in prokaryotes, it codes for N-formylmethionine (fMet).
13. What is the molecular cause of systemic lupus erythematosus (SLE)?
Patients with SLE produce autoantibodies to small nuclear ribonucleoproteins (snRNPs).
14. On ribosomes, what are the functions of the A site, P site and E site?
The incoming aminoacyl CoA targets the A site. The growing peptide develops on the P site. tRNA exits from the E site.
15. What are the Kozak and Shine–Dalgarno sequences?
They are initiation-enhancing sequences upstream of the start (AUG) codon. The Kozak consensus sequence occurs in eukaryotes. The Shine–Dalgarno consensus sequence occurs in prokaryotes.
