Mutations

Question 1

How do mutations in the number of chromosomes arise?

Answer 1

• Mutations in number of chromosomes can arise spontaneously by chromosome non- disjunction during meiosis.

Question 2

What is non-disjunction?

Answer 2

• Non disjunction is when chromosomes/ chromatids do not split/ separate equally during anaphase (so, cells produced won’t have the correct no of chromosomes.)

() - extra info for clarification.

Question 3

True or False

Chromosome non-disjunction can only occur in meiosis I.

Answer 3

• False.
• Chromosome non-disjunction can occur in either meiosis I or meiosis II.
• Meiosis I: either one/ all chromosomes don’t separate equally.
• Meiosis II: either either one/ all chromatids don’t separate equally.

Question 4

What are the 2 different effects that chromosome non-disjunction can have?

Answer 4

• Changes in whole sets of chromosomes - polyploidy.
• Changes in the number of individual chromosomes - aneuploidy.

Question 5

When does polyploidy occur?

Answer 5

• Polyploidy occurs when you have changes in whole sets of chromosomes.
• Changes in whole sets of chromsomes occur when organisms have three/ more sets of chromosomes rather than the usual two.

Question 6

Describe process that would lead to polyploidy (non-disjunction in meiosis I.)

Answer 6

• Each homologous pair is doubled (due to DNA replication in interphase.)
• Non disjunction in meiosis I (all chromosomes fail to separate equally.)
• Normal division in meiosis 2, chromatids separate equally.
• 4 gametes produced. 2, 2n, diploid gametes and 2 gametes with no chromosomes.

Question 7

What would happen if a 2n gamete (produced by non-disjunction) fuses with a normal n gamete?

Answer 7

• A triploid (3n) cell would be produced - rather than a diploid cell.

Question 8

How does aneuploidy actually occur?

Answer 8

• Changes in number of individual chromosomes.
• Individual homolgous pairs of chromosomes fail to separate in meiosis.
• Results in gamete having one more/ one fewer chromosome.
• When gamete with one more/ less chromosome fuses with normal gamete, the zygote will have more/ fewer chromosomes than in body cells.

Question 9

Describe process that would lead to aneuploidy (non-disjunction occuring in meiosis I.)

Answer 9

• Each homologous pair is doubled (due to DNA replication in interphase.)
• Non disjunction in meiosis I (one homolous pair of chromosomes does not separate equally.)
• Normal division in meiosis 2, chromatids separate equally.
• 4 gametes produced: two n+1 haploid gametes with one additional chromosome AND two n-1 haploid gametes with one missing chromosome.

Question 10

If chromosome non- disjunction occurs in meiosis I - leading to polyploidy, what issue happens during meiosis?

Answer 10

• All chromosomes fail to separate equally.

Question 11

If chromosome non- disjunction occurs in meiosis I - leading to aneuploidy, what issue happens during meiosis?

Answer 11

• One chromosome doesn’t separate equally (one cell has two of same homologous pair and other cell doesn’t contain that specific homologous pair.)

Question 12

If chromosome non- disjunction occurs in meiosis II (from one cell) - leading to polyploidy, what issue happens during meiosis?

Answer 12

• All the chromatids don’t separate equally.
• 4 gametes: n,n,no chromosomes, 2n.

Question 13

If chromosome non- disjunction occurs in meiosis II - leading to aneuploidy, what issue happens during meiosis?

Answer 13

• One chromatid doesn’t separate equally.
• 4 gametes –> n-1, n+1, n,n

Question 14

What will happen when n+1 gamete fuses with n (normal gamete?)

Answer 14

• Forms 2n+1 zygote (diploid.)
• Resultant zygote will have 1 extra chromosome.

Question 15

What do gene mutations involve?

Answer 15

• Gene mutations involve a change to the base sequence of chromosomes.

Question 16

What are the two different types of gene mutation (that we need to know for AS?)

Answer 16

1.) Base deletion
2.) Base substitution.

Question 17

How can gene mutations arise?

Answer 17

• Gene mutation can arise spontaneously during DNA replication.

Question 18

What do “mutagenic agents” do?

Answer 18

• Mutagenic agents can increase the rate of gene mutation.

Question 19

The genetic code is degenerate. What does this mean for gene mutations?

Answer 19

• Means that not all BASE SUBSTITUIONS cause a change in the sequence of encoded amino acids.

Question 20

Give 2 different examples of mutagenic agents/ what do they do?

Answer 20

1.) Radiation - disrupt structure of DNA.
2.) Carcinogens - interfere with transcription.

Question 21

What does a deletion mutation lead do?

Answer 21

• Causes frame shift to the right (all of the bases move along one.)
• This could result in a different polypeptide chain and a non-functioning protein.

() - extra info for clarification.

Question 22

True or False

Substitution mutation leads to frame-shift and has a huge impact.

Answer 22

• False.
• There is no frame-shift, only one codon is changed.
• May not have impact due to genetic code = degenerate (ie. codon may still code for the same amino acid.)

Question 23

Why does triploid/ tetraploid sets of chromosomes lead to organisms being sterile?

Answer 23

• Chromosomes won’t pair - so, no meiosis.

Question 24

(past paper Q)

Explain how mutation can result in the production of a non-functional protein receptor.

Answer 24

• Change in DNA base sequence.
• Change in primary structure.
• Alters position of hydrogen/ ionic/ disulphide bonds.
• Change in tertiary structure of receptor.