MEIOSIS:-the process used by sexually reproducing

organisms to create cells responsible for producing offspring.

-like does not beget like in meiosis!!

-cells have the haploid number of chromos(23 in humans; n=23) (autosomes, produced by mitosis, are identical diploid # = 46 in humans; 2n = 46)

-since they have half the chromosomes they arenot identical to the parent cell

-unlike mitosis, meiosis happens only in cells concerned with reproduction

-these cells are called the GAMETES (sperm and egg)

-the end result of meiosis are the sperm cells inmale organisms and egg cell in female organisms

http://scienceline.org/wp-content/uploads/2008/06/ashford_sperms_color.jpg http://powerstates.com/wp-content/uploads/egg-sperm.jpg

Meiosis requires 2 nuclear divisions (it goes through prophase, metaphase, anaphase and telophase 2 times)

Creates cells with a haploid number of chromosomes.

Interphase in meiosis is identical to interphase in mitosis

Metaphase, anaphase and telophase are also the same.

http://mtwow.org/meiosis.gif

Interphase

Prophase 1 with crossing over and tetrads

Gametes with haploid chromosome number

Meiosis 2 with no interphpase to double chromos

Meiosis differs from mitosis in prophase I.-2 pairs of sister chromatids will join at the

centromere forming a cluster of 4 sister chromatidscalled a tetrad.

When these chromatidslie close together, they will exchange pieces of chromosome (crossing over). This creates 4 unique chromatids.

tetrad

Meiosis = 2 nucleardivisions, mitosis =one nuclear division.

Meiosis cells are all genetically dissimilar, mitosis cells are all genetically identical.

Meiosis produces haploid cells. Mitosis produces diploid cells.

Meiosis has crossing over. Mitosis has no crossing over.

Meiosis = reproductionMitosis = growth, replacement and repair of worn out cells.

What is so important about meiosis???

-First, it creates genetically dissimilar chromatids during crossing over..this allows for variability within species. This can be the difference between survival and extinction.

…weeds that can resist herbicides and bacteria that won’t respond to Antibiotics are great examples of this variability!!

http://www.niaid.nih.gov/SiteCollectionImages/topics/antimicrobialresistance/1whatIsDrugResistance.gif

-Second, it ensures that the diploid chromosomenumber in species stays constant during reproduction by creating haploid cells that fertilize each other.

23 + 23 = 46 vs. 46 + 46 = 92

-chromosome number is very important in determining species of organism and function of that organism.

http://www.toonpool.com/user/997/files/chromosomes_kid_draw_on_wall_901815.jpg

homepages.uel.ac.uk/V.K.Sieber/human.htm

www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm

Human Chromosomes beforebeing separated and placed informal karyotype form. Can you count 23 pairs?

Normal human karyotype. Isthis a male or female?

Sex chromosomes are located on the 23rd pair . XX is a female and XY is a male

Meiosis can also be a determining factor inchromosomal disorders.

If non-disjunction (or failure of the sister chromatids to separate during anaphase of either meiosis I or II) occurs, then the resulting cell formed after fertilization will contain more or less chromosomes than the normal diploid number.

In some cases, this can result in abnormal conditions for the organism.

Some examples of Chromosomal Disorders are:-Down’s Syndrome—also called Trisomy 21

happens when non-disjunction occurs on the21st pair of chromosomes resulting in 3chromosomes being here.

-affects about 1/700 children born…most commonbirth defect in the US.

www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm

Non-disjunction can also occur in the sex chromosomes.--Klinefelters syndrome affects 1/2,000 males

XXY results in males with female sex characteristics

--Turners Syndrome affects 1/5,000 femalesXO results in females who never develop secondary sex characteristics

Turner’s Syndrome Klinefelter’s Syndrome