3.3- meiosis Flashcards
meiosis
cell divison that halves the chromosome number so that each daugther cell has one haploid set
used to gametes in animals and pollen and eggs in plants.
how many chromosmes do gametes have
n (not 2n)
humans-23
meiosis process
before meiosis I the chromosomes replicate
Meisosis I
- homologous chromosomes are seperated
meiosis II
- each chromosme is split to sepeerrate chromatids
the cell divides twice but DNA replicates only one
at meiosis end cells have half the chromosome number
there are four cells which have different combinations of chromosomes amd alleles due to independant assortment and crossing over
The final outcome of meiosis is the production of four haploid daughter cells
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
meiosis is called a reduction division where homologous chromosomes seperate into gametes each gamete has half the number of chromosomes
the chromosome number is restored to 2n when two gametes fuse at fertilisation
meiosis I
Meisosis I
- homologous chromosomes are seperated
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
meiosis II
meiosis II
- each chromosme is split to sepeerrate chromatids
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
chromosomes replicate when?
in interphase before meiosis
seperation of homologous chromosome pairs in meiosis I does what
havles the chromosome number
the early stages of meisos involve what
pairing of homologous chromosomes and crossing over followed by condensation
when is DNA replicated
befor meiosis so that all chromosmes consist of two sister chromatids
a human can produce how many gametes
> 8 million
what are the three ways meiosis creates genetic variation
meiosis and allele segregation
meiosis and independant assortment
meiosis and crossing over
meiosis and allele segregation
each pair of alleles is sorted into different gametes and therefore different offspring
- because each allele is carried on seperate homologous chromosomes that seperated during meiosis
meiosis and independant assortment
chromosomes are sorted independantly of eachother
during meiosis alleles on one pair of homologous chromosomes seperate independantly from allele pairs on othe chromosomes.
these alleles will be inherited in the offspring in predictable ratios determind by the genotype of the parents
meiosis and crossing over
describe polyploidy
Polyploidy is a condition whereby an organism has more than two complete sets of chromosomes in all cells (i.e. > diploid)
Polyploidy is far more common in plant species which lack separate sexes and are capable of self-pollination
Polyploidy is very rare in animal species due to the consequences of having extra allele copies of every gene
