Meiosis

Question 1

Reproducing sexually by seeds leads to variable yields, why

Answer 1

1.meiosis/crossing over
2.fusion of genetically different gametes/random fertilisation

Question 2

Advantage of growing plants from tissue culture than seeds

Answer 2

Will be genetically identical due to mitosis
So have desired features

Question 3

Describe what happens to chromosomes in meiosis

Answer 3

1. Chromosomes shorten/thicken/condense;
2. Chromosomes associate in homologous pairs / formation of bivalents
3. Crossing-over / chiasma formation;
4. Join to spindle (fibres) / moved by spindle
5. (At) equator/middle of cell; (*)
6. (join via) centromere
7. (Homologous) chromosomes move to opposite poles / chromosomes separate/ move apart
8. (Pairs of) chromatids separated in 2nd division; max 6
   (x) OR “ independent assortment”

Question 4

Describe how meiosis causes genetic variation in offspring formed by sexual reproduction and why this is good

Answer 4

1. Crossing-over;
2. Independent/random assortment/orientation/segregation of (homologous) chromosomes in meiosis I;
3. Independent/random assortment/orientation/segregation of chromatids in meiosis II;
   Any three from:
4. Different ADAPTIONS / some better adapted;
5. Some survive / example described;
6. To reproduce;
7. Pass on gene/allele;
8. Allows for CHANGING /different environment

Question 5

What are homologous chromosomes

Answer 5

• Homologous chromosomes are:
• The same size, shape and contain the same genes (but may have different alleles)
• They are very similar, but are not identical.
• Maternal Paternal

Question 6

Mutagenic agents

Answer 6

Mutagenic agents
increase the likelihood/rate of gene mutations
e.g. Mustard gas/ benzopyrene in tobacco smoke
e.g. X-rays/gamma rays

Question 7

Nonsense substitution

Answer 7

1. Nonsense - the mutation results in one of the 3 stop codons

Question 8

Mis-sense substitution

Answer 8

2. Mis-sense – results in a different amino acid sequence being coded for. Eg. GTC changes to GAC (find out which amino acids these code for)

Question 9

Silent mutation

Answer 9

3. Silent mutation – although it is a different codon, the same amino acid is coded for. Eg. GTC to GTT (can you find any other examples)

Question 10

Mutations may have no effect as….

Answer 10

A functional protein is still made

Question 11

Mutations may be damaging –

Answer 11

the protein is not functional

Question 12

Mutations may be beneficial –

Answer 12

rare but possible e.g. HIV immunity or lactose digestion