RI 2020 A level H2 P3

2020 H2 A level Paper 3 2020 H2 Biology A Level Paper 2 [Turn over Section A Answer all questions. 1 Animals that possess hair are classified within the phylum Chordata and class Mammalia. (a) Define biological classification and explain how classification relates to phylogeny.[3] 1. Biological classification groups organisms based on overall morphological similarities and usually does not consider their evolutionary history; 2. It uses a naming system where each organism is given a binomial name* and grouped into a domain, kingdom, phylum, class, order, family, genus and species in a hierarchical manner; 3. Phylogeny also groups organisms but they are grouped based on their evolutionary history / ancestor-descendent relationships using molecular data and; 4. It does not rank organisms but instead assigns each organism a position on a phylogenetic tree* based on its evolutionary relationship with other organisms on the tree; (b) (i) Epithelial stem cells and melanocyte stem cells are examples of a particular type of stem cell. State two key features of this type of stem cell. [2] 1. They are multipotent* stem cells that can differentiate into a limited range of specialized cell types in the presence of the correct molecular signals; 2. They are able to undergo extensive proliferation and self-renewal by mitosis* in the presence of the correct molecular signals and hence give rise to a constant pool of cells; (ii) Discuss how the presence of stem cells within the skin and the presence of hairs covering the skin influence the harmful effects of high levels of ultraviolet light on the health of mammals. [4] 1. The presence of hairs covering the skin play a protective role as they reduce exposure of the skin by blocking the harmful ultraviolet radiation; 2. Since stem cells are near the skin surface , the high levels of ultra violet light can damage the DNA is stem cells as; 3. the ionizing radiation can cause the DNA is the stem cells in the skin to mutate and if these mutations are in genes that regulate cell cycle checkpoints/proto oncogenes/ tumour suppressor genes; 4. it could lead to uncontrolled cell division/ tumour formation and increase the chances of an individual getting cancer; 5. especially because stem cells already have telomerase activity;

2 2020 H2 Biology A Level Paper 3 (c) (i) The shape of MC1R is characteristic of a particular group of transmembrane receptor proteins. Name this group of proteins.[1] 7 pass transmembrane G-protein coupled receptor *

3 2020 H2 Biology A level Paper 3 [Turn over (ii) In cats with agouti hairs, ASIP is secreted at intervals during hair growth. With reference to Table 1.1 and the information provided, explain how this interrupted pattern of ASIP secretion leads to striped agouti hairs in cats.[4] 1. During intervals when ASIP is not secreted, α-MSH binds to MC1R (a GPCR), causing the MC1R to undergo a conformation change in its intracellular domain allowing G-protein to bind to it; and 2. G-protein is activated when it displaces its attached GDP for GTP; 3. Activated G-protein will translocate along membrane and bind to enzyme adenylyl cyclase and phosphorylate it, thus activating it; 4. Adenylyl cyclase will catalyze conversion of ATP to cAMP, which will trigger a phosphorylation cascade that will cause black eumelanin pigment to be produced, making the hair black; 5. However, during intervals when ASIP is secreted, ASIP will prevent α-MSH from binding to MC1R/ ASIP will preferentially bind to MC1R; 6. Thus MC1R will not undergo a conformational change that allows G-protein to bind to it and so the G protein will not be activated, adenylyl cyclase will not activated, cAMP is not produced and so yellow phaeomelanin pigment is produced making the hair yellow; Hence the interrupted pattern of ASIP secretion, will lead to the striped yellow and black agouti hairs in cats.

4 2020 H2 Biology A Level Paper 3 Table 1.2 (d) (i) Use Table 1.2 to identify: a recessive allele: 2 a dominant allele : 15 an allele containing a frameshift mutation 2 [3] (ii) With reference to Table 1.1, Table 1.2 and the information provided, explain why melanic (black) individuals of F. catus have hairs that do not contain any phaeomelanin. [4] 1. Melanic F.catus has 2 copies of 2 and hence has 2 copies of the mutated ASIP alleles; 2. A 2 base pair deletion in the gene will result in a frame shift in the DNA resulting in a change the sequence of codons* in the mRNA sequence; 3. which will cause a change in the sequence of amino acids in the polypeptide which in turn will affect the R group interactions between the amino acids in the tertiary structure of the ASIP protein, resulting in a change in its 3D conformation; A: truncated non-functional polypeptide 4. As a result, the non-functional ASIP (ligand) cannot bind to the MC1R (receptor) on the melanocyte cell as it is no longer complementary* in shape to it; 5. Thus and cAMP (second messenger) production will continue and black eumelanin is produced in the melanocyte cell; Thus the melanic individuals have hairs that do not contain any phaeomelanin; (iii) Scientists have concluded that the black fur colour phenotype found in several cat species has evolved more than once. Use information in Table 1.2 to justify this conclusion.[1] Since cats with the same phenotype, that is, having black fur, have 2 different mutations in 2 different gene loci, either in the ASIP locus or the MC1R locus, it shows that black fur colour phenotype in cat species evolved more than once;

5 2020 H2 Biology A level Paper 3 [Turn over (e) (i) Explain how temperature variation of the skin contributes to the phenotype of the cat in Fig. 1.1.[2] 1. Since the ears, face, paws and tail are the cooler parts of the body of the cat, the temperature sensitive tyrosinase enzyme does not denature in those areas and hence black pigment can be produced in the hairs in those areas; 2. The other parts of the body are warmer above 30°C and the temperature sensitive tyrosinase enzyme starts to denature, lowering enzyme activity and hence reducing the production of pigment in the hairs there; (ii) Draw a genetic diagram to show the predicted coat colours of the offspring of this cross. Insert the symbols A and a for the alleles of the ASIP gene and the symbols H and h for the alleles of the tyrosinase gene in the key to symbols table, before completing the genetic diagram. [4] key to symbols ∆2 allele of the ASIP gene a normal allele of the ASIP gene A Himalayan allele of the tyrosinase gene h non-Himalayan allele of the tyrosinase gene H genetic diagram showing predicted coat colours: Parental phenotype Himalayan cat with X Agouti, Non-Himalayan cat black extremities Parental genotype aahh AaHh Gametes Gametes Offspring genotype: Offspring phenotype: Offspring phenotypic: 1 : 1 : 1 : 1 ratio ah ah Ah aH AH ah ah aH Ah AH AaHh Aahh aaHh aahh Agouti, Non- Himalayan cat Himalayan cat with Agouti extremities Black, Non- Himalayan cat Himalayan cat with black extremities AaHh Aahh aaHh aahh Agouti, Non- Himalayan ca