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meiosis 减数分裂 （formation of gametes） (facts (preceded by…
meiosis
减数分裂
（formation of gametes）
facts
preceded by interphase（chromosome replication）
先于间期
2 meiotic divisions
original cell is diploid
4 daughter cells produced that are monoploid
daughter cells contain half number of chromosomes as original cell
produces gametes（eggs&sperm）配子
occurs in the testes in males&ovaries in females
（睾丸&卵巢）
fundamental basis of sexual reproduction
replication of chromosomes
（replication occurs in interphase）
replicated copies - sister chromatids
hold together at centromere
homologs - same genes&different
alleles
等位基因
（homologs separate in meiosis 1）
sister chromatids - same genes&alleles
（sister chromatids separate in meiosis 2）
meiosis form haploid gametes
meiosis must reduce the chromosome number by half fertilization then restores the 2n number
tetrads form in prophase 1
synapsis 联会（染色体互换，同源重组）
crossing-over
homologous chromosomes in a
tetrad
四分染色体 cross over each other
pieces of genes are exchanged
produces genetic recombination in the offspring 后代
chiasma 交叉
sister chromatids may carry NON identical genetic information as a result of crossing over
results of meiosis
gametes form
4 haploid cells with one copy of each chromosome
1 allele of each gene
different combinations of alleles for different genes along the chromosome
gametogenesis
oogenesis
occurs in the ovaries
2 divisions produce 3 polar bodies that DIE and
1 egg
（DIE because of unequal division of cytoplasm）
immature egg called oocyte 卵母细胞
one oocyte matures into an egg/ovum every 28 days
fertilization may occurs at meiosis 2
spermatogenesis
occurs in the testes
2 divisions produce
4 spermatids
spermatids mature into sperm 精液
men produce 250,000,000 sperm per day
comparison of divisions
number of divisions
mitosis
1
meiosis
2
number of daughter cells
mitosis
2
meiosis
4
genetically identical?基因相同
mitosis
YES
meiosis
NO
chromosome
mitosis
same as parent
meiosis
half of parent
where
mitosis
somatic cells 体细胞
meiosis
germ cells 生殖细胞
when
mitosis
throughout life
meiosis
at sexual maturity
role
mitosis
growth&repair
meiosis
sexual reproduction
order
IPMAT
chromosomes
indistinct 模糊的
present
meet
apart
two