2007 a Level H2 Biology P2 Ans

2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME Section A / Alternatives ___ must be present ; end of point ( ) marks not dependent on it R reject 1. (a) A B C

(Viral) RNA; Reverse transcriptase; Capsomere of capsid / coat particles (molecule); R: Capsomere, capsid (alone, not molecule)

(b) Ref. to synthesis of glycoprotein; (Cell surface) membrane; Surrounds virus particle / (viral) RNA and protein; Buds / pinches off / evaginates; R: exocytosis (no membrane around Viruses). R: buds off via exocytosis. (c) (i) Block / bind to active site (of protease); Substrate unable to bind; Viral polyprotein not cut / structural / enveloped proteins not formed; So (newly formed) virus not infective / mature / functional / unable to infect further cells; (ii) Similar shape / 3D conformation to polyprotein / complementary shape to active site / similar change; Does not break down easily / not hydrolysed (so remains in active site); [Total: 11]

2. (a) Fluid: phospholipids / proteins free to move; phospholipids move within own layer / transversely; Mosaic: Proteins held / embedded in phospholipids bilayer; Scattered / random / studded / ref. to position; … (b) (i) Glucose: polar oxygen / H2O soluble / hydrophilic; Ions: charged / have a hydration shell; Unable to pass through hydrophobic region (w/o equal to question); Ref. to specificity of (transport) paths; Ref. to active transport; (ii) high levels of ATP close ATP sensitive potassium channels; build up of potassium ions (inside cell); depolarize membrane / generates a potential difference;

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2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME (iii) insulin is protein; too large to pass through membrane; refer to channel large enough to allow other insulin through would allow molecules through; because insulin in vesicles are released via exocytosis; … (c) Vesicles containing insulin; Move to (cell structure) membrane; (membrane of) vesicles fuses with (cell structure) membrane; vesicles contents empties / exocytosis;

3. (a) DNA unzips / DNA strands separate / ref. to H – bonds break; Both strands / each strand acts as a template; Each new (R: strand) DNA molecule contains one new and one old / original strand of DNA; (b) Ref. to shape of active site of DNA polymerase; Needs 3’ OH to initiate; Primers only add at 3’; (nucleotides only added at 3’ end of primers); (c) 1. DNA in bacteria circular / in a loop; 2. bacterial DNA naked / human DNA arranged in nucleosome / wrapped around histones / ref. to human DNA associated with proteins; 3. Ref. to introns in human DNA; 4. Ref. to telomeres in human chromosomes, centromeres in human chromosomes; 5. Ref. to human chromosomes in pairs; 6. Ref. to operons only in bacteria; (d) 1. Much more / larger DNA molecule / ref. very long; 2. Ref. to DNA packaging / ref. to nucleosomes / wrapped around histones; 3. More junk / non – coding DNA; 4. Ref. idea of decondensing; (ii) More than one replication site (on each chromosome); More DNA polymerase enzymes;

4. (a) (Inherited) mutation in DNA / nucleotide sequence / base sequence; Base substitution; Mutation in codon / triplet for a.a. / mutation in R groups; Base addition / deletion; Ref. to frameshift or described;

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2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME (b) (If addition / deletion resulting in) frameshift (of codons); Protein coded for no longer synthesized / non – functional; (if substitution has occurred) an amino acid being changed; Change 3D / tertiary structure of protein due to different R groups; Ref. to in mutation in binding site; (c) Both copies of a tumour suppressor gene must be mutated (before a person will develop cancer); Ref. to effect of mutation being masked /; ref. to (normal allele) dominant / remaining functional copy of gene; Sufficient quantities of protein synthesized to be effective; (d) Specific shape of binding area of p53 protein; R: active site Corresponding to / complementary to shape, part of DNA sequence; Ref. to idea different sequences of bases (have different shape); Ref. to major and minor grooves on DNA; (e) Mutation in a.a. in region away from binding area; May not affect shape or binding properties; Ref. idea some a.a. coded for by more than one codon / triplet of bases / ref. genetic code / some codons degeneration; So (mutation in base / codon) may not change a.a. / neutral;

5. (a) RRPP, RrPP, RrPp, RRPp; (b) Parental phenotype : Parental genotype : Gametes of parent : F1 genotype : F1 phenotype :

F1 phenotype F1 genotype Gametes of F1

: : :

Rose combed RRpp Rp

x

Pea combed rrPP ; rP

RrPp All walnut combed F1 Walnut combed RrPp RP Rp rP rp

RP

Rp

x

;

F1 Walnut combed RrPp RP Rp rP rp ; rP

rp

RP

RRPP (walnut)

RRPp (walnut)

RrPP (walnut)

RrPp (walnut)

Rp

RRPp (walnut)

RRpp (rose)

RrPp (walnut)

Rrpp (rose)

rP

RrPP (walnut)

RrPp (walnut)

rrPP (pea)

rrPp (pea)

rp

RrPp (walnut)

Rrpp (rose)

rrPp (pea)

rrpp (single) ;;

F2 phenotypic ratio: 9 walnut combed: 3 rose combed: 3 pea combed: 1 single combed all genotypes must be shown correct numbers for both marks.

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2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME (c) RRPP x RRpp;; RrPP x RRpp;; RRPP

x Rrpp;;

6. (a) P: Nucleus / euchromatin; Q: Nuclear envelope / membrane; R: Cell (structure) / plasma membrane; (b) Site of cellular respiration / OP / aerobic respiration; Synthesis / production / release of ATP; Ref. specific use of ATP e.g. sucrose loading / active transport / vesicle transport / mitosis;; Ref. ATP produced by mitochondria released into cytosol while ATP in chloroplasts not; (c) (i) 1. Double membrane; 2. highly folded / large surface area of inner membrane; 3. Ref. ETC / proton pumps; 4. stalked particles / ATP synthase / synthetase; 5. ref. (70S) ribosomes; 6. circular DNA; (ii) 1. Three membranes / double outer membranes in chloroplasts; 2. ref. position of stalked particles / chloroplasts (inside out); 3. Starch grains; 4. stack of thylakoid membranes / grana; 5. ref. to thylakoids once; 6. No cristae in chloroplasts; 7. contain pigments / chlorophyll, A: other photosynthetic pigments; (d) OP: final (and proton) acceptor / combines with electron and proton to form water; PP: (waste) product / from photosynthesis; 7. (a) Kingdom Animal Chordata Phylum Aves Class Passeriformes Order Family Dicruridae Genus Dicrurus species Dicrurus paradiseus 6 correct = 4 marks, 5 correct = 3 marks, 4 correct = 2 marks, 3 correct = 1 mark (b) Ref. inherited variation particular in alleles; ref. natural selection; different in selection pressures in different habitats; ref. to survive to pass on alleles; geographical isolation ref. allopatric; idea that broad leaves woodland not continuous; ref. islands; prevents interbreeding / gene flow; ref. to genetic drift / founder effect; (c)

Similar (morphological / anatomical / physiological) features; Capable of interbreeding; to produce / fertile offspring, not to be replace by “viable”; Section B

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2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME 8. (a) Describe the main structural features of cellulose and collagen. 1. a polysaccharide; 2. (chain of ) beta / β glucose; 3. 1 – 4 links; 4. ref. to microfibrils; 5. ref. to bundles of microfibrils / macrofibrils / fibres;

[8]

6. a fibrous protein; 7. (chain of) amino acids; 8. ref. to peptide bonds; 9. ref. to each chain helical; 10. triple helix / tropocollagen; 11. ref. to role / presence of glycine; 12. ref. to staggered ends; 13. ref. to H bonds; A: once in either (b) 14 a polysaccharide; 15 (chain of) α / alpha glucose; 16 1 -4 with (some) 1 – 6 links / branches; 17 insoluble; 18 (energy) store; 19 stored / produced in muscle / liver; glucagon 20 a globular protein; 21 (chain of) amino acids; 22 ref. to peptide bonds; 23 soluble / in plasma; 24 produced by α / alpha cells in pancreas / islets of Langerhans; 25 a hormone / regulates / raises blood glucose; (c) Suggest why plant cells mainly store carbohydrates and animal cells mainly store lipids. [4] 26 ref. energy reserves; 27 ref. lipids buoyancy; 28 ref. lipids (thermal) insulation; 29 ref. lipids more energy per unit mass / higher calorific value; 30 ref. to metabolic water; 31 ref. protection; 9. (a) 1. length of DNA; 2. gene / Lac 1, coding for repressor / inhibitor protein; 3. ref. to cap (catabolic activator protein) binding site next to promoter; 4. promoter (gene); 5. RNA polymerase binding site; 6. ref. to operator; 7. binding site for repressor / inhibitor protein; 8. ref. to (3) structural genes; 9. Lac Z (galactosidase); 10. Lac Y (lactose permease); 11. Lac A (lactose transacetylase);

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2007 GCE A LEVEL EXAMINATION BIOLOGY PAPER 2 MARK SCHEME 12. ref. to termination sequence; (b) 13. lactose enters (bacterial_ cell; 14. ref. to method of entry of lactose / qualified leaky membrane / small amount in; 15. allolactose / lactose binds to repressor protein; 16. results in a change in shape of repressor protein; 17. repressor protein no longer attached (to DNA); 18. exposes operator site 19. allows RNA polymerase activity; 20. transcription (of genes coding for proteins required to utilize lactose); 21. leading to synthesis of proteins; 22. ref. named enzymes / protein; (c) 23. gene switching / majority of genes switched off / for regulation of gene expression; 24. so that bacterium only produces enzymes required, A: converse; 25. inefficient / ref. waste of energy / resources; 26. ref. to selective advantage / disadvantage; 27. ref. group genes together for control; 28. ref. to bacteria able to use a variety of sugars / substrates;

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