DNA

Question 1

DNA replication:

Answer 1

Unwind/ untwist - gyrase
Unzip- DNA helicase
H bonds break
Both strands act as template
Free DNA nucleotides activated with 2 phosphates
Complementary base pairing
C to G (3H bonds)/ T to A (2H bonds)/ purine (C, T, U) to pyrimidine (G, A)
H bonds reform
Sugar-phosphate backbone forms using covalent/ phosphodiester bond
Semi-conservative replication
DNA polymerase

Question 2

Twenty different amino acids are commonly used for protein synthesis. In theory, this would need only 20 different base combinations. Explain the uses of the remaining 44 combinations. (2mks) f215 june 14 q4aiii

Answer 2

Several, triplet(s) / codon(s) , code for one amino acid ;
(some are used as) start / stop / termination ;
idea that mutation may , not result in change in amino acid / have a neutral effect / result in silent mutation;
[IGNORE degenerate]

Question 3

Describe how a nucleotide base sequence in a gene is used to synthesise a polypeptide. (7mks) f215 june 14 q4

Answer 3

transcription
1 DNA / gene , copied / transcribed , into mRNA ;
2 free / activated , (RNA) nucleotides / (RNA) nucleoside triphosphates ;
3 (line up by) complementary base-pairing / described ;
4 (to) one / template [non-coding] / reference / sense , (DNA) strand ;
5 (catalysed by) RNA polymerase ;
translation
6 (mRNA moves to) ribosomes ; [translation occurs at ribosomes]
7 tRNA (molecules) bind to mRNA ;
8 anticodon(s) , match / pair with / bind to , codons ;
9 specific / correct , amino acid attached to tRNA ;
10 formation of peptide bond between amino acids;
[tRNA anticodons bind to mRNA codons = 2 marks]

Question 4

State the role of a gene. (F212 June 14 5di)

Answer 4

code for (one or more) polypeptide(s); [protein]

Question 5

Explain how the structure of DNA allows replication. (5mks) (f212 June 14 5dii)

Answer 5

1) double stranded; [double helix or two, polynucleotides / strands / chains or antiparallel strands]
2) each / both (strands) act as template;
3) hydrogen bonds, easily, break/ form, between
bases; [WEAK H bonds between bases break]
4) complementary (specified) base, pairing;
5) purine (only able to) bind to pyrimidine;
6) (due to) different sizes of purines and pyrimidines;
7) hydrogen bonding different between A & T and C & G or 3 H bonds between C & G and 2 H bonds between A & T; [A=T and C≡G without reference to H bonds]

Question 6

State two ways in which a diagram of part of an RNA molecule would appear different from the DNA molecule shown in Fig. 5.1. F212 june13 5aiv

Answer 6

(usually) single stranded / would not have 2 strands; [only one backbone]
uracil / U, instead of thymine / T;

Question 7

Explain why DNA replication is considered to be semi-conservative. F212 june13 5bi

Answer 7

1 one strand, from original DNA and one strand newly formed; [one old and one new strand]
2 (original) strand / polynucleotide , acts as template (for new strand); [each strand is copied]

Question 8

Explain why complementary base-pairing is important in DNA replication. F212 june13 5bii

Answer 8

1 (DNA) can be replicated without error/ same sequence of nucleotides is produced; [formation of identical DNA; same / correct , order / sequence, of bases]
2 reduces occurrence of mutation; [prevents mutation]
3 allows (re-)formation of , hydrogen/ H , bonds;

Question 9

The technique of centrifugation used by Meselson and Stahl involves:
•mixing the DNA sample with concentrated sugar solution
•placing the mixture of DNA and sugar solution in test-tubes
•spinning the test-tubes at a very high speed.
Suggest three precautions that Meselson and Stahl would have taken in order to ensure that the centrifugation part of their investigation produced valid results. F212 june13 5d

Answer 9

1 same concentration of sugar (solution in each tube);
2 same volume of, mixture / solution / sugar solution (in each tube);
3 spin (all tubes) at same, speed / acceleration; [tubes spun at constant speed]
4 spin (all tubes) for same (length of) time;