Biology for CAPE Original material Cambridge University Press 2011 1

5 Answers to end-of-chapter questions 1 B [1]

2 C [1]

3 B [1]

4 C [1]

5 C [1]

6 C [1]

7 C [1]

8 B [1]

9 D [1]

Structured questions 10 a Purines: adenine and guanine [1]

Pyrimidines: thymine and cytosine [1] b

6 labels [3] 45 labels [2]

Note that A pairs with T and C with G 23 labels [1] ii Hydrogen bonds [1] iii Shown on diagram [2] c i Refers to the direction of the two strands

One end of the DNA molecule is a phosphate labelled 5' (5 prime) At the other end is a hydroxyl group labelled 3'. This DNA strand runs in the 5' 3' direction It is the polar hydroxyl group that relates to the term polarity The 2nd strand runs in the 3' 5' direction The strands are said to be antiparallel Well described [2]

S P C or G A or T

G or C

T or A

3 3 3'

5' 3'

5'

Biology for CAPE Original material Cambridge University Press 2011 2

ii Shown on diagram above [1] iii The two strands are held together by hydrogen bonds between the bases [1] d Chromosomes: when cell division begins, DNA coils around the proteins

forming visible structures called chromosomes. A pair of chromatids joined by a centromere form the chromosome. It therefore consists of a pair of sister chromatids [2]

Chromatin: is a mass of uncoiled DNA and associated proteins called histones. Chromatin appears to be loosely coiled, long thin threads spread throughout the nucleus. [1]

11 a Replication: molecule can be copied perfectly over and over again so it can be

passed down from cell to cell and generation to generation Expression: production of proteins from coded information in the DNA [2]

b

Well drawn diagram which shows correct base pairing [3] [note: 2 rows dots between a and T; 3 rows between C and G]

c To have a complete set of the genetic information / all nuclei are

genetically identical / same genetic information So that the proteins coded for by the DNA remain the same from

generation to generation / the functioning of cell would remain the same

No changes to genetic information / mutation If there are changes, proteins would change / cells would not

function effectively Each point 1 mark [max 2] d i Yes:

Their hypothesis suggested that the resulting DNA molecules would be made up of one old strand and one new strand

After one generation / the 2nd generation, one intermediate band (14N/15N) would be seen

This is because the new DNA strands would be made up of an old strand (15N) and a new strand (14N) / two isotopes in one molecule

In the 3rd generation, 2 bands would be seen; an intermediate band and a light band

This is because the parent strands from the 2nd generation both have a strand with 15N and light band, 14N. These Well explained strands would separate to act as a template, giving rise to two using three pieces bands of evidence [3]

ii Semi-conservative [1]

Biology for CAPE Original material Cambridge University Press 2011 3

iii

[2] e

Percentage of base present DNA

sample Cytosine Adenine Guanine Thymine

Strand 1 24 38 26 12 Strand 2 26 12 24 38 [2]

Essay questions 12 a

b i Used in transcription the production of an mRNA molecule

that has a complementary base sequence to one strand of a length of DNA

The RNA polymerase proceeds to read one strand / reference / coding strand of the DNA

As the RNA polymerase travels along the DNA strand, it assembles ribonucleotides into a strand of RNA

It catalyses the condensation reactions that take place to form phosphodiester bonds between the adjacent nucleotides

Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing

Thus for each C encountered on the DNA strand, a G is inserted in the RNA; for each G, a C; for each T, an A and for A, a U 67 points [3]

mRNA tRNA essentially linear folds into a clover-leaf

shape number of nucleotides varies / length

of a gene about 80 nucleotides long

carries the code for proteins in form of codons, in form of 3 adjacent nucleotides coding for an amino acid

consists of a single anticodon

no place of attachment for amino acid

attaches to an amino acid and carries it to ribosome Any 2 points [2]

Biology for CAPE Original material Cambridge University Press 2011 4

The enzyme also checks that the bases have been correctly 45 points [2] paired up 23 points [1]

ii Used in translation of mRNA the process by which the code for making a protein, now carried by the mRNA molecule, is used to line up amino acids in a particular sequence and link them together to form a polypeptide

It has an anticodon complementary to the codon on mRNA It works as an adaptor that matches the amino acids to their

codons Each amino acid has its own tRNA Specific enzymes / tRNA transferases for each amino acid load

the specific amino acid on the tRNA The tRNAamino acid complex then moves to the ribosomes It binds to the complementary codon Allows for correct sequence of amino acids along the growing 34 points [2]

polypeptide 2 points [1] iii Used in translation in protein synthesis

Composed of large and small subunits Function is to hold in position mRNA, tRNA and enzymes

controlling the process Holds in the small subunit in a cleft six nucleotides or two

codons at a time / provides a structural site for mRNA In large subunit, two tRNAamino acid complexes are held When the peptide linkage is formed between the amino acids, 34 points [2]

the ribosome moves down one codon 2 points [1] c Two strands present held by hydrogen bonds

Therefore, bonds can break to separate strands and allow each strand to act as a template

Semiconservative method of replication Free nucleotides pair with each strand Complementary base paring / purine to pyrimidine / AT; CG 67 points [3] Results in two DNA molecules that are identical to each other 45 points [2] Each consists of one strand from parent and one new strand 23 points [1]

d Inhibit synthesis of new DNA strands so cells would not replicate.

If DNA replicated, more cells would be produced to form a tumour Inhibit the production of one of the nucleotides so that

complementary basepairing in replication does not occur Disrupt production of DNA and RNA so that replication,

transcription and translation do not occur 1 point each[max 3] 13 a b Transcription DNA helicase breaks hydrogen bonds between DNA strands

DNA strands separate / DNA unzips

DNA RNA made up of two strands made up of one strand sugar is deoxyribose sugar is ribose base thymine present base uracil present molecule is large molecule is shorter

Any 2 points [2] [max 4]

Biology for CAPE Original material Cambridge University Press 2011 5

Exposing required gene / sequence of nucleotides One strand / sense / coding / reference strand is used as template Complementary base pairing: CG; GC; TA; AU RNA polymerase catalyses the condensation reactions that take place to form

phosphodiester bonds between the adjacent nucleotides, forming mRNA The enzyme also checks that the bases have been correctly paired up 78 points [4] RNA polymerase continues moving along the sense strand of DNA 56 points [3]

until it reaches the end of the gene 34 points [2] mRNA then passes through the nuclear pore to the ribosomes 12 points [1]

Translation

Occurs in the ribosome Codon carried on the transcribed mRNA mRNA held in the small subunit / in a cleft 6 nucleotides or two codons

at a time / provides a structural site for mRNA Codon binds with complementary anticodon The specific tRNA for the complementary codon attaches the specific

amino acid, catalysed by a tRNA transferase In large subunit, two tRNAamino acid complexes are held Process is made up of initiation, elongation and termination Initiation involves the tRNA with an anticodon that is complementary

to the first codon, AUG The tRNA with the appropriate amino acid, methionine, binds to the

codon by hydrogen bonding The next tRNA with the specific amino acid dictated by the codon

arrives in the large subunit It binds with the complementary codon A peptide bond is formed between the two amino acids in the ribosomes

resulting in elongation of the strand The ribosome moves one codon and another specific amino acid is

added 78 points [4] This continues until a stop signal is encountered / termination 56 points [3] The polypeptide is released 34 points [2] The ribosome subunits separate 12 points [1]

c 14 a i Unwinds DNA by breaking hydrogen bonds [1] ii Proceeds along template strand / the single stranded 5' 3'

direction

Transcription Translation synthesis of mRNA from

DNA synthesis of proteins from

mRNA occurs in nucleus occurs in cytoplasm /

ribosomes linking of nucleotides linking of amino acids phosphodiester bonds formed involves producing codons catalysed by RNA

polymerase

peptide bonds formed involves joining of codons

and anticodons catalysed by various

enzymes in ribosomes Any 3 points

[max 3]

Biology for CAPE Original material Cambridge University Press 2011 6

Obtaining complementary nucleotides/ AT; CG) Catalyses the condensation between two nucleotides to form a

phosphodiester bond 1 point 1 mark [max 3] iii Joins pieces of replicated DNA / lagging strands [1]

With a phosphodiester bond [1] b So that the resulting DNA molecules would be identical to parent

DNA [1] To carry the exact genetic information / allow for exact replication [1]

c DNA is double-stranded and antiparallel / one strand runs in a 5' 3' direction, the other in a 3' 5' direction

Replication is catalysed by DNA polymerase DNA polymerase synthesises the new DNA strand only in a 5' 3'

direction This means that it has to jump ahead in one of the strands and

synthesise the DNA backwards in pieces One strand would be therefore produced continuously / leading and

the other in pieces / lagging Any 2 points [2] d e Tumour formation

Cells may activate the enzyme telomerase which prevents the telomeres from getting shorter [1]

Cells can continue to divide uncontrollably [1] Aging When the telomere gets too short, cells stop dividing / no

replacement of cells [1]

Transcription Replication 1 DNA strand copied / sense

strand 2 DNA strands copied

RNA polymerase used DNA polymerase used mRNA / codons produced DNA molecules / sister

chromatids base uracil instead of thymine

used only a short piece of DNA /

gene unzips

base thymine instead of uracil used

entire DNA unzips

Any 2 points [2]