Meiosis

Meiosis

Is the production of gameteswhen mitosis occurs two new cells

each have a complete set of chromosomes (a diploid cell)

at the end of meiosis gametes are produced, four cells -each with half a set of chromosomes - haploid cells

Fertilization involves restoring the diploid condition by combining the haploid gametes into a diploid, fertilized zygote

one set of chromosomes from each parent

How it works...

The chromosomes are duplicated in the S phase of interphase (just like mitosis)

In Meiosis there are 2 cell divisionsIn mitosis the chromosomes line up

individually, but in meiosis they line up in homologous pairs, next to each other

Gametes

Sex chromosomes (X & Y)Autosomes ( non-sex chromsomes)Somatic cells have 2 sets of

autosomesGametes have one set of sex

chromosomes and one set of autosomes

Steps are similar to mitosis

Duplicated chromosomes (chromatids) are present after interphase

the chromatids become paired one of each pair is contributed by the

organism’s mother, the other by the father because the pairing is of chromatids (2n)

there are actually 4 chromosomes presnet in what is called a tetrad (p 196)

The number of tetrads is the haploid number of chromosomes (humans have 46 chromosomes, so in prophase 1 there are 92 chromatids and 23 tetrads)

combination process is called synapsisSometimes during synapsis, genetic

material is exchanged between chromosomes in process called crossing over

all genes located on a particular linkage group and tend to be inherited together

except for crossing over

Meiosis 1

Tetrads line up in metaphase 1 and sister chromatids orient toward opposite poles

Anaphase 1 they separate and move toward poles but chromatids still connected by centromere!

At end of telophase 1 there are two doubled chromosomes at each end of cell.

Meiosis 2

No further duplication of DNAMetaphase 2 chromatids line up

againOne chromatids of each pair lines up

with each “pole”Centromeres split in anaphase 2 and

chromatids move to polesAt tend of telophase there is one

chromatid at each end (haploid)

Because homologous pairs line up in Metaphase 1 there is a 50/50 chance of which one of each pair will go to each pole

in humans with 23 chromosomes any couple would have 223 x 223

chance of different possible children (about 1 in 70 trillion and this doesn’t account for crossing over!)

Meiosis

2 cell divisions that result in four haploid daughter cells