Mutatio: Unit 4, Topic 1

Question 1

What is a mutation?

Answer 1

• a change in a gene or chromosome
  Mutagen: any agent capable of inducing mutation

the rate at which mutations occur can be increase by physical or chemical mutagens

Question 2

What is spontaneous mutations during DNA replication?

Answer 2

• occurs in the absence of exposure to mutagens.
• when DNA is unwound for replication and is exposed to vulnerable damage.

Point mutation:
- a mutation that affects a single base-pair position within a gene.

Substitute mutation: a type of point mutation where another replaces one nucleotide.

frameshift mutation: either insertion mutation (one or more nucleotides added to original sequence) OR deletion mutation (loss of one or more nucleotides) the starting point for the reading of codons in translation is shifted and consequently affects the resulting protein.

Failed repair mechanism:
- after replication and before cell division, DNA is proofread and errors are detected and repaired -> a mutation is not repaired or it is improperly repaired, the mutation becomes part of the DNA sequence.

Question 3

What is mutations during cell division (non-disjunction)

Answer 3

• the failure of homologous chromosomes or sister chromatids to separate during meiotic (sometimes in mitosis) cell division.
• the chromosomes go to one daughter cell and none to the other
• the unequal separation can produce daughter cells with unexpected chromosome numbers (aneuploids)
• trisomy 21 (down syndrome) OR monosomy X (turner syndrome)

Question 4

What are mutations by mutagen?

Answer 4

• environmental mutagens are either chemical or physical
• induce genetic mutations or increase mutation rates

Examples of chemical mutagens:
- acridine orange (bases in DNA are added or removed)
- Mustard gas (guanine in DNA is replaced by other bases)

Examples of physical mutagens:
- ionising radiation (radiation that is strong enough to break chemical bonds in molecules)
- UV lights, X-rays, gamma rays, nuclear radiation.

Question 5

What is nondisjunction and how does it lead to aneuploidy?

Answer 5

BACKGROUND INFO:
somatic cells (any cell except sex cells): diploid (contain two sets of chromosomes - one set from each parent)

nondisjunction:
- when both sets of chromosomes go into the same cell during meiosis I.
- sister chromatids fail to segregate into separate cells in Meiosis II.

Aneuploidy: unconventional number of chromosomes (less or more)

OUTCOMES OF ANEUPLOIDY:
Trisomy (3 Chromosomes):
- gamete containing two homologous chromosomes + normal gamete
EXAMPLES OF TRISOMY:
- autosomal (non-sex chromosome) trisomy: Trisomy 21 (aka Down syndrome) - one extra non-sex chromosome
- sex chromosome trisomy: Klinefelter syndrome - XXY for sex chromosomes

Monosomy (1 chromosome):
- gamete without a particular chromosome [0]

Question 6

What is a Karyotype?

Answer 6

• organised profile of an individual chromosome used to identify genetic abnormalities

NORMAL HUMAN KARYOTYPE:
- 23 pairs of chromosomes (22 autosomes, 1 sex chromosome.

Ploidy:
- the number of complete sets of chromosomes in a cell
Diploid: 2 sets of chromosomes
Haploid: 1 set of chromosomes

Trisomy 21:
- (aka Down syndrome) - one extra non-sex chromosome,
Monosomy X
- three 21 chromosomes.

Monosomy X
- missing or damaged X chromosome, (Turner syndrome) - only in females
- gamete without a particular chromosome [0]

trisomy 18
- three 18 chromosomes
- Edwards syndrome

Question 7

Explain inherited mutations

Answer 7

• most organism that inherit mutations are at a disadvantage
• the premature death of organisms with harmful mutations, before reproductive age, prevents these mutations from accumulating in populations.
• useful mutations are rare, and transmission into future generations is even more rare.
• mutations that affect gametes can be inherited
• mutations that occur in somatic cells CAN NOT be inherited
• alleles can arise by mutation (hence changing the genotype)