meiosis & genetic diversity

Question 1

differences between mitosis and meiosis

Answer 1

1. mitosis only has 1 nuclear division, meiosis has 2 nuclear divisions
2. mitosis produces 2 diploid cells, meiosis produces 4 haploid cells
3. mitosis produces genetically identical cells, meiosis introduces genetic variation

Question 2

define ‘haploid’

(& give an example)

Answer 2

when a cell has 1 copy of each chromosome
eg. gametes

Question 3

define ‘diploid’

(& give an example)

Answer 3

• when a cell contains 2 copies of each chromosome
• eg. body cells

Question 4

define ‘chromosome’ (structure)

Answer 4

2 sister chromatids joined by a centromere due to DNA replication

Question 5

define ‘homologous pair’

Answer 5

a pair of matching chromosomes –> each chromosome contains the same gene but diferent alleles

Question 6

the stages of meiosis

Answer 6

1. interphase
2. meiosis I
3. meiosis II
4. cytokinesis

Question 7

interphase

Answer 7

G1 –> protein synthesis
S –> DNA replication
G2 –> organelle synthesis

Question 8

which two mechanisms occur during meiosis to increase variation?

Answer 8

• the independant segregation of homologous pairs of chromosomes
• crossing over

Question 9

what stage of meiosis would ‘crossing over’ or ‘independant segregation’ occur in?

Answer 9

meiosis I

Question 10

the independant segregation of homologous pairs of chromosomes

Answer 10

1. h. pairs line up opposite each other at equator
2. maternal & paternal chromosomes randomly lie on either side of the equator
3. h. pairs seperated –> combination of maternal & paternal chromosomes in gametes = random

Question 11

how does ‘independant segregation’ increase variation?

Answer 11

creates large no. of possible combos of chromosomes in daughter cells

Question 12

formula for ‘independant segregation’

Answer 12

2^n
(where n = no. of homologous pairs)

Question 13

what is ‘crossing over’?

Answer 13

exchange of alleles between chromatids

Question 14

the process of crossing over

Answer 14

1. the chromatids of each pair twist around each other
2. this puts tension on the chromatids, causing parts of them to break
3. the broken part of the chromatid recombines w the chromatid of its h. partner
   > results in new combinations of alleles

Question 15

define ‘mutation’

Answer 15

a change in the base seq. of DNA

Question 16

when do mutations occur?

Answer 16

randomly during DNA replication (interphase)

Question 17

what increases the chances of mutations?

Answer 17

exposure to mutagenic agents eg. ionising radiation, certain chemicals

Question 18

what are the 2 types of mutations?

Answer 18

1. gene mutations
2. chromosome mutations

Question 19

the two types of gene mutations

Answer 19

• base substitution
• base deletion

Question 20

base substitution

Answer 20

• when a base in the og DNA seq is replaced eg. AGT –> AGG

Question 21

base deletion

Answer 21

one base is removed & the rest of the seq. shifts along

Question 22

why is base substitution less likely than base deletion to produce a new amino acid?

Answer 22

• genetic code = degenerate
• new codon may still code for same a.a

Question 23

what is a chromosome mutation ?

Answer 23

a change in the no. of chromosomes due to chromosome non-disjunction

Question 24

describe the two types of chromosome mutations

Answer 24

• polyploidy = changes in whole sets of chromosomes, mainly in plants (n, 2n, 3n)
• aneuploidy = individual homologous pair may fail to seperate, gamete has +1/-1 chromosomes (n+1, n-1)

aneuploidy is cause of Down’s Syndrome

Question 25

process of 'chromosome mutation'

Answer 25

* change in no. chromosomes * due to **non-disjunction** * in meiosis * chromosomes not seperated

Question 26

can mutations spread?

Answer 26

* **NO** * mutations = random * only the **rate** of mutation is affected by the environment * diff. species **don't** interbreed * so mutations can't be passed from 1 species to another

Question 27

*define* 'genetic diversity'

Answer 27

the no. of diff. alleles of genes in a population

Question 28

*define* '**al**leles'

Answer 28

an **al**ternative form of a gene

Question 29

advantage of greater genetic diversity in a population

Answer 29

species more likely to adapt to environmental changes

Question 30

the 3 ways that genetic diversity might be affected

Answer 30

1. **'genetic bottlenecks'** --> even causes sudden population crash --> less allele variation --> lower g.d 2. **'founder effect'** --> new region colonised --> lower g.d 3. **'selective breeding'**--> less allele variation --> lower g.d

Question 31

process of **'natural selection'**

Answer 31

1. new alleles for gene created by **mutations** OR initially **few** with advantageous allele 2. animals with advantageous allele **survive** & have more offspring 3. offspring **inherit** advantageous allele 4. takes many generations for advantageous allele to become **common** allele

Question 32

types of adaptations

Answer 32

* behavioural * anatomical * physiological

Question 33

directional selection

Answer 33

* one of the extremes of the phenotypes range has the selective advantage * occurs when there's a **change** in the environment * the modal trait **changes** | (eg. antibiotic resistant bacteria)

Question 34

stabilising selection

Answer 34

* the modal trait has the selective advantage * occurs when there's **no change** in the environment * the modal trait **remains the same** * s.d decreases as individuals with the extreme trait decreases