3.4.3 Genetic diversity can arise as a result of mutation or during meiosis

Question 1

What is a gene mutation?

Answer 1

• A random change in the DNA base/nucleotides sequence.
• results in the formation of a new allele

Question 2

Where can mutations occur?

Answer 2

• somatic (body cells)
• germ line (gametes). Germ line mutations cause discontinuous variation

Question 3

What is the difference between gene (point) mutations and chromosome mutations?

Answer 3

Gene mutations change one or more nucleotide bases whereas chromosome mutations result in changes in whole chromosomes

Question 4

What are the 3 types of gene mutation?

Answer 4

• addition mutations (bases are added to the sequence)
• deletion mutations (bases are removed from the sequence)
• substitution mutations (bases are swapped in the sequence)

Question 5

What is a non-sense mutation

Answer 5

• non-sense mutation: causes a stop condon to be coded for. Leads to truncated protein that can’t function properly.

Question 6

Explain a mis-sense mutation

Answer 6

• mis-sense mutation: causes different amino acid coded for
• so changes the sequence of amino acids (primary structure)
• which changes types of bonds formed
• which changes way polypeptide folds (tertiary structure)
• so changes function of protein

Question 7

Explain a silent mutation

Answer 7

• has no effect because the same amino acid is still coded for
• because genetic code is degenerate

Question 8

What is a frameshift mutation?

Answer 8

• caused by addition and deletion mutations
• reading frame is changed, changing the entire amino acid sequence coded for after the mutation

Question 9

What are the causes of mutations?

Answer 9

• spontaneously/randomly during DNA replication
• mutenic agents (‘mutangens’) can increase mutation rate
• including: high energy radiation and chemicals that alter DNA structure or interfere with transcription

Question 10

What are the advantages of mutations?

Answer 10

Can create genetic diversity which is required for natural selection and speciation

Question 11

What are the disadvantages of mutations?

Answer 11

• can create less advantageous alleles
• can cause cancer by disrupting cell division

Question 12

What 2 types of genes control cell division?

Answer 12

• proto-oncogenes stimulate cell division
• tumour suppressor genes slow/inhibit cell division

Question 13

What happens if proto-oncogenes mutate?

Answer 13

• mutate into oncogenes
• results in receptors on cell surface membrane being permanently activated so cell division is permanently switched on
  OR
• the oncogene itself codes for growth factors
• these are then produced in excess, continually stimulating cell division

Question 14

What happens if a tumour suppressor gene mutates?

Answer 14

• it becomes inactivated
• can’t inhibit cell division any longer
• so the cell divides constantly

Question 15

Meiosis overview

Answer 15

• produces daughter cells that are genetically different to each other
• involves two nuclear divisions
• creates 4 haploid daughter cells (gametes) from a single diploid parent cell
• can fuse with another haploid gamete during random fertilisation

Question 16

What are haploid and diploid cells

Answer 16

Haploid - only one set of chromosomes. n
Diploid - two copies of each chromosomes (homologous). 2n

Question 17

How is genetic variation introduced?

Answer 17

• independent segregation of homologous chromosomes
• crossing over between homologous chromosomes
• random fertilisation

Question 18

Describe independent segregation

Answer 18

• homologous pairs of chromosomes line up opposite each other at the equator of the cell
• Random which side of the equator the maternal and paternal chromosomes lie
• pairs separated so one of each homologous pair ends up in daughter cell

Question 19

How do you calculate the number of possible chromosomes combinations in the daughter cells/gametes? A zygote?

Answer 19

• 2^n
• n = number of homologous pairs
• 2^23 in humans
• when you take into account random fertilisation… (zygote)
• (2^n)^2
• in humans (2^23)^2
• doesn’t take into account crossing over

Question 20

Describe crossing over

Answer 20

• homologous pairs of chromosomes associate
• parts of chromatid become twisted around each other, chiasma forms
• sections/lengths of chromatids with different alleles exchanged
• new combination of alleles on the chromatids

Question 21

What mechanism is responsible for chromosomes mutations?

Answer 21

• non disjunction
• due to individual homologous chromosomes not separating during meiosis

Question 22

What are the two outcomes of chromosome mutations?

Answer 22

• changes in number if individual chromosomes - organisms have an extra copy of an individual chromosome
• polyploidy: changes in whole sets of chromosomes- when organisms have 3 or more sets of chromosomes

Question 23

When do gene mutations occur? When do chromosomes mutations occur?

Answer 23

• gene - during DNA replication
• chromosome - during meiosis

Question 24

Describe chromosome mutations

Answer 24

• mechanism: non-disjunction
• due to individual homologous chromosomes not separating during meiosis
• organisms have extra copy of an INDIVIDUAL chromosome (downs syndrome)
• OR polyploidy- changes in whole sets of chromosomes
• organisms have 3 or more sets of chromosomes

Question 25

Why does a mutation in a gamete affect all body cells?

Answer 25

• mutation in gamete that formed zygote
• all cells derived from zygote by mitosis
• mitosis produces genetically identical cells

Question 26

Why might a gene mutation not have an effect?

Answer 26

• genetic code is degenerate
• new amino acid is coded for but this does not affect tertiary structure
• new allele is recessive so has no effect on phenotype