2018 H2 Bio Paper 3_updated 2023

2018 H2 A level Paper 3 Section A Answer all questions in this section. 1 Gene expression in eukaryotes can be regulated at different levels. Long -term changes in gene expression that are passed on to daughter cells are called epigenetic changes. An epigenetic change does not alter the DNA nucleotide sequence. (a) (i) Describe the patterns shown in Table 1.1. [2] 1. As the degree of differentiation / specialization increases from blood stem cell from bone marrow to mature B lymphocyte from lymph gland, 2. the degree of DNA methylation decreases from 1.00 to 0.61 arbitrary units(AU), 3. and the degree of gene expression increases from 1.0 to 5.5AU; (ii) Explain how DNA methylation changes gene expression. [3] 1. addition of methyl group to selected cytosine* nucleotides in on DNA (e.g. a CG sequence); 2. l eading to recruitment of histone deacetylase* and chromatin remodeling complexes* to condense chromatin (decrease accessibility of promoter to general transcription factors and RNA polymerase) 3. R educes the accessiblity of promoter, hence prevents the binding of general transcription factors * and RNA polymerase* to promoter *, preventing formation of transcription initiation complex* 4. Preventing transcription of the gene; (iii) Table 1.2 shows similar data for cancerous B lymphocytes. Compare the data for the cancerous B lymphocytes in Table 1.2 with the data in Table 1.1 and discuss what this suggests about the nature of the cancerous B lymphocytes. [4] 1. Both the degree of DNA methylation (0.95AU) and gene expression (1.5 AU) for cancerous B Lymphocytes are in between these values blood st em cell from bone marrow and immature B lymphocyte from bone marrow. 2. High levels of DNA methylation, which results in high levels of condensation of chromatin in the genome. 3. Hence cancerous B lymphocytes is expected to largely undifferentiated, similar to that of the blood stem cell. 4. A nd that many tissue- specific genes and proteins, e.g antibodies are not likely to expressed. (b) Epigenetic changes may be important in the development and treatment of cancer. Fig. 1.2(a) represents the balance between two types of genes controlling cell division in a healthy cell. If genes of type X become overexpressed or genes of type Y become under expressed, the balance is tipped towards a cell becoming cancerous, as shown in Fig. 1.2(b).

Epigenetic DNA methylation pattern changes as cell lines age. For example, the promoter region of the p53 tumour suppressor gene has a tendency to become methylated in the cells of older people. (i) Describe and explain the potential consequences of the cell and individual person of methylation of the promoter region of the p53 tumour suppressor gene. [4] 1. Methylation at promoter region of p53 tumour suppressor gene results in condensation of chromatin in the promoter region. 2. Hence general transcription factors(GTF)*