2.6.3 meiosis Flashcards
haploid
having only 1 set of chromosomes - represented by symbol ‘n’
homologous chromosomes
matching chromosomes - contain same gene at same places (loci)
–> may contain diff. alleles for some of genes
meiosis
type of nuclear division resulting in formation of cells containing half number of chromosomes in parent cell
how does sexual reproduction increase genetic variation
- involves combining of genetic material from 2 (usually) unrelated individuals of same species
- via process of fertilisation
how does genetic variation within a population increase its chances of survival during environmental changes
some individuals will have characteristics which enable them to be better adapted to the change
what’s necessary for sexual reproduction to occur
- in many organisms, the body cells are diploid
- they must produce haploid gametes
why are haploid gametes required for sexual reproduction
when 2 gamete nuclei fuse during fertilisation, a diploid zygote is produced & the normal chromosomes number is maintained
where does meiosis occur
- in diploid germ cells to produce haploid gametes
where are the diploid cells undergoing meiosis found
specialised organisms called gonads - testes/ovaries
how many chromosomes are in your body cells
46
what are homologous chromosomes & what do they contain
- matching pairs of chromosomes
- one maternal & one paternal –> same genes at same places on chromosome
what occurred prior to meiosis
- the S phase of interphase:
- each chromosome was duplicated as its DNA replicated
- each chromosome now consists of 2 sister chromatids
how many divisions in meiosis/stages
2
- 4 stages in each (8 total)
stages in 1st meiotic division
prophase 1
metaphase 1
anaphase 1
telophase 1
what may the cell enter before the 2nd meiotic division
short interphase
stages in 2nd meiotic division
prophase 2
metaphase 2
anaphase 2
telophase 2
what does the 2nd meiotic division take place in
a plane at right angles to meiosis 1
what may occur at the end of the 2nd division
cytokinesis
events during stage 1 - prophase 1
- chromatin condenses & each chromosome supercoils
- nuclear envelope breaks down
- spindle threads of tubulin protein form from centriole (animal cells)
- chromosomes come together in homologous pairs
- each member of pair consists of 2 chromatids
- crossing over occurs –> non-sister chromatids wrap around each other & may swap sections = alleles shuffled
events during stage 2 - metaphase 1
- pairs of homologous chromosomes (crossed over state) attach along equator of spindle
- each attaches to spindle thread by centromere
- homologous pairs arranged randomly (members of each pair face opposite poles) = independent assortment
- way the line up determines how they’ll segregate independently when pulling apart in anaphase
events during stage 3 - anaphase 1
- members of each pair of homologous chromosomes pulled apart by motor proteins (drag along tubulin threads of spindle)
- centromeres don’t divide & each chromosome = 2 chromatids
- crossed-over areas separate = swapped areas of chromosome & allele shuffling
events during stage 4 - telophase 1
- (most animal cells) 2 new nuclear envelopes form around each set of chromosomes & cell divides (cytokinesis) –> then, short interphase & chromosomes uncoil
- each new nucleus contains half original no. of chromosomes but each chromosomes = 2 chromatids
- (most plant cells) cell goes straight from anaphase 1 to prophase 2
events during stage 1 - prophase 2
- (if nuclear envelopes reformed) break down again
- chromosomes coil & condense –> each one = 2 chromatids
- chromatids of each chromosome are not identical (due to crossing over)
- spindles form
events during stage 2 - metaphase 2
- chromosomes attach (centromere) to equator of spindle
- chromatids of each chromosome = randomly arranged
- way they’re arranged determines how chromatids separate in anaphase
