3.3- 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.

n (not 2n) humans-23

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

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 - 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

in interphase before meiosis

havles the chromosome number

pairing of homologous chromosomes and crossing over followed by condensation

befor meiosis so that all chromosmes consist of two sister chromatids

> 8 million

meiosis and allele segregation meiosis and independant assortment meiosis and crossing over

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

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

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