meiosis & genetic diversity Flashcards
increasing variation during meiosis, mutations, selection
differences between mitosis and meiosis
- mitosis only has 1 nuclear divisions, meiosis has 2 nuclear division
- mitosis produces 2 diploid cells, meiosis produces 4 haploid cells
- mitosis produces genetically identical cells, meiosis introduces genetic variation
define ‘haploid’
(& give an example)
when a cell has 1 copy of each chromosome
eg. gametes
define ‘diploid’
(& give an example)
- when a cell contains 2 copies of each chromosome
- eg. body cells
define ‘chromosome’
2 sister chromatids joined by a centromere due to DNA replication
define ‘homologous pair’
a pair of matching chromosomes –> each chromosome contains the same gene but diferent alleles
the stages of meiosis
- interphase
- meiosis I
- meiosis II
- cytokinesis
interphase
G1 –> protein synthesis
S –> DNA replication
G2 –> organelle synthesis
which two mechanisms occur during meiosis to increase variation?
- the independant segregation of homologous pairs of chromosomes
- crossing over
what stage of meiosis would ‘crossing over’ or ‘independant segregation’ occur in?
meiosis I
the independant segregation of homologous pairs of chromosomes
- h. pairs line up opposite each other at equator
- maternal & paternal chromosomes randomly lie on either side of the equator
- h. pairs seperated –> combination of maternal & paternal chromosomes in gametes = random
how does ‘independant segregation’ increase variation?
creates large no. of possible combos of chromosomes in daughter cells
formula for ‘independant segregation’
2^n
(where n = no. of homologous pairs)
what is ‘crossing over’?
exchange of alleles between chromatids
the process of crossing over
- the chromatids of each pair twist around each other
- this puts tension on the chromatids, causing parts of them to break
- the broken part of the chromatid recombines w the chromatid of its h. partner
> results in new combinations of alleles
define ‘mutation’
a change in the base seq. of DNA
when do mutations occur?
randomly during DNA replication (interphase)
what increases the chances of mutations?
exposure to mutagenic agents eg. ionising radiation, certain chemicals
what are the 2 types of mutations?
- gene mutations
- chromosome mutations
the two types of gene mutations
- base substitution
- base deletion
base substitution
- when a base in the og DNA seq is replaced eg. AGT –> AGG
base deletion
one base is removed & the rest of the seq. shifts along
why is base substitution less likely than base deletion to produce a new amino acid?
- genetic code = degenerate
- new codon may still code for same a.a
what is a chromosome mutation ?
a change in the no. of chromosomes due to chromosome non-disjunction
describe the two types of chromosome mutations
- 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
process of ‘chromosome mutation’
- change in no. chromosomes
- due to non-disjunction
- in meiosis
- chromosomes not seperated
can mutations spread?
- 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
define ‘genetic diversity’
the no. of diff. alleles of genes in a population
define ‘alleles’
an alternative form of a gene
advantage of greater genetic diversity in a population
species more likely to adapt to environmental changes
the 3 ways that genetic diversity might be affected
- ‘genetic bottlenecks’ –> even causes sudden population crash –> less allele variation –> lower g.d
- ‘founder effect’ –> new region colonised –> lower g.d
- ‘selective breeding’–> less allele variation –> lower g.d
process of ‘natural selection’
- new alleles for gene created by mutations OR initially few with advantageous allele
- animals with advantageous allele survive & have more offspring
- offspring inherit advantageous allele
- takes many generations for advantageous allele to become common allele
types of adaptations
- behavioural
- anatomical
- physiological
directional selection
- 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)
stabilising selection
- 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
