Tut 10 - Mutations (Teacher)

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 1 Full Name: Civics group: 21S Index no.: Date: Core Idea 2: Genetics & Inheritance Gene mutation & Chromosomal aberration Tutorial 10 MCQ ANSWERS 1 2 3 4 5 6 7 A D C D B D C 1 A point mutation is a change to a single nucleotide and can occur anywhere in a gene. Which statements are true? 1 A point mutation in an exon can result in a different amino acid sequence. 2 A point mutation in an exon can produce a shorter protein if a stop codon produced. 3 A point mutation in an intron can alter the binding site of a splicing enzyme. A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only 2 A mutation results in the substitution of thymine for cytosine in the base sequence ATC in a section of a template DNA strand. What are the base sequences on the tRNA corresponding to the new triplet? template DNA: ATC mutant DNA: ACC mRNA: UGG tRNA: ACC (I,e same as mutant template DNA, except U instead of T) A ATC B TAG C UGG D ACC

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 2 3 The diagram shows part of the normal sequence of an mRNA molecule. 5’ – CCA AGU GGU CCG CUA AAA UGG C – 3’ Template DNA: GGT TCA CCA GGC GAT TTT ACC G Polypeptide: gly - ser – pro – gly - ??? Mutant DNA: GGT TCA CCA GGC ATT TTA CCG Polypeptide: gly - ser – pro – gly – ile – leu – pro Mutation is a base deletion of 13th nucleotide guanine. A mutation in the DNA resulted in a polypeptide beginning with the following sequence. glycine – serine – proline – glycine – isoleucine – leucine – proline The DNA triplets for some amino acids are Glycine Isoleucine Leucine Proline Serine CGA ATA TTA CCA TCA GGT ATT CTT CCG TCG GGC CTC Which mutation has occurred in the DNA molecule? A A reversal in the order of nucleotide B An addition of an extra nucleotide C The loss of a nucleotide D The replacement of one nucleotide by a different nucleotide 4 Two enzymes X and Y, are each encoded by different alleles of the same gene . The amino acid seqwuences of the two enzymes differ between positions 87 and 91 of the polypeptides. The amino acid sequences of enzymes X and Y, and the corresponding DNA sequence of enzyme X from position 86 to position 93 of the polupeptides, are shown in the table below.

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 3 The actual mRNA codons for the amino acids in these positions for enzymes X and Y, are shown in the table below. What could account for the difference in amino acid sequence of enzymes X and Y? A A single frame shift by deletion in the DNA code at position 87. B Frame shift mutations in the DNA codes at position 87 and position 90. C A change in the sequences of the second and third nucleotides at positions 87 and 88 of the DNA codes and frame shifts at positions 89 and 91. D A deletion in the DNA code at position 87 and an insertion into the DNA code at position 92. DNA for enzyme X TTT TCA GGT AGT GAA TTA CGA CGA mRNA for enzyme X AAA AGU CCA UCA CUU AAU GCU GCU (A at position 87 is deleted) mRNA for enzyme Y AAA GUC CAU CAC UUA AUG GCU GCU (G at position 92 is inserted) 5 The diagram shows the banding pattern of two human chromosomes. P is a normal chromosome. What accounts for chromosome Q? A crossing over between sister chromatids (False as the banding patterns are different for P & Q. Crossing over does not affect banding pattern as it involves exchange of equivalent portions of non -sister chromatids of a homologous pair (both of which have the same gene loci and banding pattern)..) B inversion of part of the chromosome (True. See red box for comparison.) C deletion of part of the chromosome (False as length of chromosomes are the same. For deletion, we expect Q to be shorter than P.) D translocation of part of another chromosome (False as length of chromosomes are the same. For translocation, we expect Q to be longer the P.)

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 4 6 2019/9744/1/16 A small proportion of men have the genotype XYY. Such genotypes are usually the consequence of non -disjunction occurring during meiosis. Non-disjunction results from a failure of chromosomes to separate correctly. In which gamete and at which stage of meiosis must this non-disjunction occur? A An egg produced by non-disjunction in meiosis I B An egg produced by non-disjunction in meiosis II C A sperm produced by non-disjunction in meiosis I D A sperm produced by non-disjunction in meiosis II Explanation: - Men with genotype XYY means they got YY from father’s sperm & X from mother’s egg (since mother does not have Y chromosome) - To get an aberrant sperm with 2 Y chromosomes, this means that the sister chromatids of the duplicated Y chromosome failed to separate during Anaphase II. [See figure above]

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 5 7 A Robertsonian translocation is a type of chromosomal translocation in which the long arms of two chromosomes fuse together. Fig. A shows this event occurring between chromosomes 14 and 21. Fig. A An individual who inherits the translocated chromosome in Fig. A will either have Down’s syndrome or be a carrier of the disorder. A couple has a child. The mother is a carrier and the father is genetically normal. The genetic material with respect to chromosomes 14 and 21 in the somatic cells of the parents are shown in Fig. B. Fig. B The child is born with Down’s syndrome .  implied child has the translocated chromosome.

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 6 Which of the following show s the correct genetic material with respect to chromosomes 14 and 21 in the zygote of the child? ANS: C A B C D STRUCTURED QUESTIONS QUESTION 1 [9747 / 2008 / Nov / P2Q2] Sickle cell anaemia is most commonly caused by the haemoglobin variant HbS. In HbS the amino acid valine takes the place of glutamic acid at the sixth amino acid position of the beta globin polypeptide chain. Table 1.1 shows the details of this change. Table 1.1 (a) State the type of mutation responsible for this change in the amino acid sequence. …………………………………...……………..………………………………………......[1] 1 (single base / base) substitution ; REJECT: point mutation, missense mutation. Reasons: Point mutation describes the change in one base which could be brought about by a single base substitution, deletion or addition. The term “point mutation” is considered too vague in comparison to “base substitution”. The information in Table 1.1 showed that there is substitution of 1 base from A to T. Take note that the actual DNA mutation that leads to sickle-cell anaemia is CTT  CAT (Thymine is replaced by Adenine)

St Andrew’s Junior College Genetics & Inheritance / Gene mutation & Chromosomal aberration 2021 H1 Biology 7 Missense mutation describes the effect of mutation on amino acid sequence (at the protein level) i.e. the change of one amino acid for another , to form a non - functional protein . A m issense mutation is not responsible for the change in amino acid sequence which is caused by changes at DNA level. (b) Explain the significance of the change in amino acid to the properties of haemoglobin. …………………………………...……………………………………..………………......[3] 1 Valine replaced glutamic acid in the polypeptid e; Valine has non-polar / hydrop