A-LevelBiologyImmunity9700/42 · Feb/Mar 2019 · Q317 Marks
During an immune response, only B-lymphocytes with receptors that are specific to the antigens present are activated. Activation occurs when an antigen binds to a receptor of a B-lymphocyte. Activated B-lymphocytes grow in size and then divide by mitosis. Many further mitotic cell divisions occur, increasing the number of B-lymphocytes with receptors specific to the antigen. Eventually, cells produced in this process will develop into either plasma cells that secrete antibodies or memory B-cells. [Figure 3.1] is a summary of B-lymphocyte activation and the events that follow. The development of plasma cells and memory B-cells in this process depends on transcription factors.
- (a)Explain the role of transcription factors in gene expression in eukaryotic cells.[3]
- (b)Transcription factors are proteins. Genes that code for proteins can become mutated. Describe how different types of gene mutation can cause changes in the protein that is synthesised.[5]
- (c)BLIMP-1 is a transcription factor that is essential for the development of plasma cells and memory B-cells in the process shown in [Figure 3.1]. BLIMP-1 reduces the synthesis of c-Myc in B-lymphocytes. c-Myc is a protein that is required for the mitotic cell cycle to continue. Suggest and explain how a mutation in the gene coding for BLIMP-1 can prevent the development of plasma cells and memory B-cells.[3]
- (d)(i)Microarrays can be used to analyse the effect of transcription factors, such as BLIMP-1, on gene expression. Describe how a microarray is used in the study of gene expression.[4]
- (d)(ii)BCL6 is another transcription factor found in B-lymphocytes. The effect of BCL6 on gene expression was compared in two samples of B-lymphocytes. • Sample 1 consisted of B-lymphocytes that were producing BCL6. • Sample 2 consisted of B-lymphocytes that were not producing BCL6. Suggest why a microarray is suitable for identifying the function of the transcription factor BCL6 in these two samples.[2]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.
- [3] any three from: 1 binds to DNA ; 2 at, promoter/enhancer ; 3 allows, RNA polymerase / other transcription factors, to bind (to, DNA / gene / promoter) ; 4 regulates / initiates / inhibits, gene expression / transcription; 5 A switches genes, on / off (so that genes are expressed) at the correct time / in the correct context / in the correct cell type / in the correct order ;
- [5] any five from: 1 base substitution / mis-sense mutation; 2 changes, triplet / codon; 3 base, deletion / insertion; 4 (results in) frame shift / description; 5 change in, protein primary structure / amino acid sequence ; 6 protein folds incorrectly / changes tertiary or 3-D structure ; 7 changes protein function / prevents protein function / makes protein unstable; 8 idea of new STOP codon; 9 only, short/first part of /no, protein is produced ;
- [3] any three from: 1 BLIMP-1, is not synthesised / is non-functional / has changed function ; 2 expression of the c-Myc gene, is not reduced / continues ; 3 synthesis / concentration, of c-Myc (protein), is maintained / increases ; 4 (B-lymphocytes continue to) divide by mitosis / proliferate ; A clonal expansion 5 differentiation / specialisation, prevented ;
- [4] any four from: 1 mRNA extracted from cells (of interest); 2 (mRNA) used (as template) to synthesise cDNA; 3 cDNA is tagged with a fluorescent dye ; 4 (fluorescent) cDNA, binds to / hybridises with, probe ; 5 each probe is unique to a different gene / AW ; 6 fluorescence indicates gene is expressed ; 7 AVP; e.g. detail of probe / use of UV light
- [2] any two from: 1 BCL6/transcription factors, regulate expression of genes ; 2 (microarray) can detect expression of genes ; 3 idea of difference in, fluorescence / gene expression, between two samples ; 4 difference in gene expression indicates regulation by BCL6 ;
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