3.4.3 - genetic diversity, mutation and meiosis

Question 1

what does a diploid number of chromosomes mean?

Answer 1

means each cell contains two of each chromosomes - one from the mum and one from the dad

in humans this means 23 pairs of homologous chromosomes

Question 2

what are homologous chromosomes like?

Answer 2

both in the pair are the same size and have the same genes, however they may have different versions of these genes (alleles)
alleles coding for the same characteristic will be found at the same locus on each chromosome.

Question 3

do gametes have haploid or diploid number of chromosomes?

Answer 3

haploid (23 individual chromosomes)

Question 4

what is the result when two gametes fertilise?

Answer 4

a zygote

Question 5

when does meiosis take place?

Answer 5

in the reproductive organs of eukaryotic organisms

Question 6

what is a gene mutation?

Answer 6

involves a change in the DNA base sequence of chromosomes

Question 7

what is a substitution gene mutation?

Answer 7

where one base is substituted with a different one

Question 8

what is a deletion gene mutation?

Answer 8

where a base is deleted (permanently removed)

Question 9

what are mutagenic agents and examples?

Answer 9

things that can cause an increase in the rate of gene mutations including:
-UV radiation
-ionising radiation
-some chemicals
-some virus’

Question 10

what are chromosome mutations?

Answer 10

(when meiosis works properly, all 4 daughter cells will end up with 23 whole chromosomes)

sometimes when meiosis goes wrong and the cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes
(e.g. two cells may have 23 chromosomes, one of each from 1-23, but one of the cells may have 2 number 6’s and the other having no number 6)

Question 11

what are the two types of gene mutation?

Answer 11

1. base deletion
2. base substitution

Question 12

when do gene mutations happen?

Answer 12

they can happen spontaneously during DNA replication

Question 13

what is the effect of a substitution mutation?

Answer 13

due to the degenerate nature of the genetic code, not all base substitutions result in a change in sequence of amino acids

Question 14

what is the effect of deletion mutations?

Answer 14

deletions will always lead to changes in the amino acid sequence as it changes the number of bases present which will cause a shift in all the base triplets after it

Question 15

what is the effect of chromosome mutations?

Answer 15

can lead to inherited conditions because the errors have happened in gametes
e.g. downs syndrome is caused by some having an extra chromosome 21

Question 16

what is an example of a type of chromosome mutation?

Answer 16

chromosome non-disjunction

this is a failure of the chromosomes to separate properly

Question 17

what is the result of meiosis?

Answer 17

four genetically varied haploid daughter cells

Question 18

fully describe the process of meiosis.

Answer 18

starting cell = diploid with 23 pairs of homologous chromosomes

1. interphase - DNA and organelle replication
2. meiosis 1

prophase 1 - chromosomes condense and become visible, nuclear membrane breaks down, spindle fibres assemble into spindle apparatus
homologous chromosomes link together forming chiasmata (crossing over can happen here)

metaphase 1 -
homologous chromosomes line up in pairs on the equator of the cell

anaphase 1 -
spindles condense and chromosomes move towards opposite poles, chiasmata is broken

telophase 1 -
chromosomes have reached opposite poles of the cell, nuclear membranes reform, chromosomes uncoil, cytokinesis

result - 2 haploid cells (daughter cells containing a single member of each chromosomes consisting of 2 sister chromatids)

2. meiosis 2:

prophase 2 -
chromosomes condense and become visible again, nuclear membrane breaks down, spindle fibres start to develop

metaphase 2 -
chromosomes line up on the equator of the cell

anaphase 2 -
centromeres of each chromosomes divide as spindles condense, chromatids are pulled to opposite poles

telophase 2 -
chromatids have reached opposite pole (now known as chromosomes), nuclear membrane reforms, cytokinesis

4 GENETICALLY VARIED HAPLOID CELL

Question 19

what is crossing over of chromatids?

Answer 19

-during prophase 1, homologous chromosomes form chiasmata
-the chromatids twist around each other and bits of chromatids swap over
-they contain the same genes but have a different combination of alleles

Question 21

how can crossing over of chromatids lead to genetic variation?

Answer 21

means each of the 4 daughter cells formed after meiosis 2 contain chromatids with different alleles

Question 22

what is independent segregation of chromosomes?

Answer 22

when homologous pairs are separated in anaphase 1, it is completely random which chromosome from each pair ends up in which daughter cell

Question 23

how does independent segregation of chromosomes lead to genetic variation?

Answer 23

the four daughter cells produced have completely different combinations of maternal and paternal chromosomes