Lecture 11 - Mutation & Its Effect on Phenotypes

Question 1

What are the two types of cells that mutations can occur in?

Answer 1

• somatic cells of a multicellular organism

- germ-line cells, e.g. sperm or eggs

Question 2

Where/how does somatic cell mutations occur?

Answer 2

in non-reproductive cells e.g. liver, intestine, skin, etc. and are passed to the new cells through mitosis, creating a clone of cells having the mutant gene

Question 3

What are the characteristics of somatic mutations?

Answer 3

• can arise spontaneously or be induced by a chemical or physical mutagen
• They may be transmitted to daughter cells after mitotic cell division
• they are NOT inherited by the offspring of a parent carrying the somatic mutation
• in most cases, a somatic mutation has little or no affect on the individual, but some somatic mutations can give rise to cancer

Question 4

Where does a germ-line mutation occur?

Answer 4

• in the cells of the ovary or testis that give rise to gametes
• Passed to approx. half the members of the next generation through meiosis and sexual reproduction

Question 5

What are the three types of mutation?

Answer 5

• chormosomal mutations that affect the number (e.g. aneuploidy: monosomy or trisomy) or structural aberrations (e.g. translocations, insertions, deletions, etc.)
• transposition of genes, e.g., via transposable elements
• point mutations: alterations involving one or a few changes in the DNA, e.g. a substitution of one base pair for another, or insertion or deletion of one or more base pairs

Question 6

What leads to the gain or loss of chromosomes during meiosis?

Answer 6

nondysjunction

Question 7

What does a base substitution due to transitions mean?

Answer 7

A -> G, T -> C, and vice versa

Question 8

What does a base substitution due to transversions mean?

Answer 8

A -> C or T, G -> C or T, T -> G or A, C -> G or A

Question 9

What mutation results in no change in the amino acid sequence of the polypeptide?

Answer 9

Silent mutations

Question 10

What mutation can have the following various affects on phenotypes?

• Neutral (alters amino acid sequence, but has no affect on function of the polypeptide)
• Radical leading to “loss-of-function” (complete or partial absence of polypeptide function)
• Radical leading to “gain-of-function” (completely new function for the polypeptide, or inappropriate spatial and/or temporal pattern of its expression)

Answer 10

Missense mutations

Question 11

What type of mutation stops the codon, usually resulting in a truncated polypeptide?

Answer 11

Nonsense mutation

Question 12

What mutation leads to a neutral or radical affect on phenotype owing to either a missense or nonsense mutation?

Answer 12

Frameshift mutation

Question 13

What kind of mutation leads to a second mutation that restores the function of the polypeptide?

Answer 13

Reverse mutations

Question 14

What are the two suppressor mutations, where the first mutation is surppressed by a second mutation?

Answer 14

• Intragenic suppressor mutation (in same gene)

- Intergenic suppressor mutation (in a different gene)

Question 15

How do mutations happen?

Answer 15

• spontaneously

- induced by physical and chemical agents

Question 16

What are the three types of spontaneous mutations?

Answer 16

• Tautomeric shifts (base tautomers) during replication
• DNA strand-slippage during DNA replication
• Misalignment of homologous chromosomes during crossing-over (recombination) at meiosis I

Question 17

What does base tautomers cause?

Answer 17

incorrect base-pairing during DNA replication