VJC 2021 H2 Bio 9744 P3_Answers

1 Victoria Junior College Biology Department 2021 Prelims H2 Paper 3 – Proposed Answers 1(a) (i) Circle the sex chromosomes in Fig 1.1. [1] Fig 1.1 (ii) Explain what is meant by ‘heterogametic sex”. [2] • The organism where the pair of sex chromosomes are different in shape/size; • Hence there are 2 types of gametes produced – one type containing X chromosome, the other containing Y chromosome; (b) Explain how the behavior of chromosome during meiosis supports Mendel’s law of segregation and independent assortment. [4] [Note: the basis is genes are found on chromosomes.] Law of segregation: • Separation of homologous chromosomes in Anaphase I /meiosis I and sister chromatids in Anaphase II/meiosis II; • results in each gamete containing only 1 allele of each gene; Law of independent assortment: • Arrangement of pairs of homologous c hromosomes across the equator in Metaphase I is random; • Subsequent segregation results in random combination of maternal and paternal chromosomes in the gametes; (c) Describe one way in which translation of the maternal mRNA in the egg can be prevented. [2] [Any 1 below] • Translational repressors/ sequence-specific RNA binding proteins bind to the 5’ UTR of mRNA; • and prevents ribosome binding and formation of translational initiation complex; Or • Binding of a complementary RNA strand to a specific region/critical region of the mRNA; • to block the binding to ribosome;

2 (d) (i) Suggest an advantage of nuclei of the early embryo sharing a common cytoplasm. [1] Any 1 below • Large protein molecules present in the cytoplasm can move directly into the nuclei without having to cross any cell membranes; • Sharing of a common pool of nutrients eg. nucleotides, ATP, enzymes, proteins; • Enable nuclei to respond to the same signal; • AVP; R: allows for nuclear divisions as the 3rd diagram (Fig 1.2) showed that there is no further division. There is only migration of nuclei to the periphery. (ii) Cellularisation occurs when cell membranes are formed around each nucleus to form cells. Explain the importance of cellularisation in the continued development of the embryo. [2] • Compartmentation which allows for setting up of specific environments; • Allowing differentiation/ specialisation of cells via development of tissue specific structures/differential gene expression; • Allows for more cells to be formed as cells undergo cell division/mitosis; • Cells can then differentiate/specialised via expression of different genes/development of tissue specific structures; € Using the information from Fig. 1.3 and 1.4, (i) account for the distribution of bicoid and nanos proteins. [2] • Bicoid and nanos mRNA are only found concentrated at the anterior and posterior ends of the embryo respectively; • When translated, the bicoid and nanos proteins will only be found in these regions respectively; (ii) suggest explanati