MeiosisStages of meiosis

by Qwabe Simphiwe

What is meiosis?

Meiosis is a type of cell division. Occurs in gamete cell. There is a single DNA replication, followed by 2 nuclear

divisions. Each of the resulting daughter cells has one half of the

number of chromosomes as the parent cell.

From Diploid (2n) Haploid (n)

MeiosisAlternating stageschromosome number must be reduced

diploid46 haploid232n n

humans: 46 23meiosis reduces chromosome numbermakes gametes

fertilization restores chromosome number

haploid diploidn 2n

Chromosome Number• Organisms produce gametes (sex cells) that contain one of each kind of

chromosome.

• A cell with only one of each kind of chromosome is called haploid (n).

• Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid.

• Each species has a specific number of chromosomes.

• Humans have 23 pairs (46 total)

• Fruit Flies have 4 pairs (8 total)

• Dogs have 39 pairs (78 total)

Human Chromosomes

• Humans have 23 pairs of chromosomes

(46 total)• 22 pairs of autosomes

• 1 pair of sex chromosomes

• Half of each pair came from one parent and half

came from the other parent

Meiosis Phases

• Meiosis I– Prophase I– Metaphase I– Anaphase I– Telophase I

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase II

THE OVERVIEWOF MEIOSIS

STAGES OF MEIOSIS

Requires two divisions Meiosis I - the chromosomes of each homologous pair

separate Meiosis II - the sister chromatids of each duplicated

chromosome separate

MEIOSIS 1:EARLY PROPHASE1

Chromosomes condense. Spindle forms. Nuclear envelope fragments.

MEIOSIS 1: Late prophase

Homologs pair. Crossing over occurs

PROPHASE 1: CROSSING OVER

Homologous chromosomes in a tetrad cross over each other

Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring

Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

MEIOSIS1: METAPHASE 1

• Homologous pairs of chromosomes align along the equator of the cell

MEIOSIS 1:ANAPHASE 1

Homologs separate and move to opposite poles.

Sister chromatids remain attached at their centromeres.

MEIOSIS 1: TELOPHASE 1

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

MEIOSIS 2:

• Meiosis II produces gametes with

• one copy of each chromosome and thus one copy of each gene.

PROPHASE2

Nuclear envelope fragments.

Spindle forms.

Metaphase 2

Chromosomes align

along equator of cell.

Anaphase 2

Sister chromatids separate and move to opposite poles.

TELOPHASE 2• Nuclear envelope assembles.

• Chromosomes decondense.

• Spindle disappears.

• Cytokinesis divides cell into two.

Results of Meiosis• Gametes (egg & sperm)

form

• Four haploid cells with one copy of each chromosome

• One allele of each gene

• Different combinations of alleles for different genes along the chromosome

references

Precious on May 28, 2013 . Meiosis in eukaryotic cells

http://www.slideshare.net/Pheshu/meiosis-in-eukaryotic-cells Accessed : 2014/03/06

Fred Phillips, Biology Teacher at Wando High School on Nov 30, 2008 . Meiosis Notes

http://www.slideshare.net/fred_phillips/meiosis-notes-presentation . Accessed : 2014/03/05

wja10255 on Feb 27, 2012 . Meiosis Notes

http://www.slideshare.net/wja10255/meiosis-notes-11773690 . Accessed : 2014/03/06

wraithxjmin on Apr 24, 2008. Lecture 6 Cell Division [Meiosis]

http://www.slideshare.net/wraithxjmin/lecture-6-cell-division-meiosis . Accessed :2014/03/06

docsawyer on Feb 05, 2010. Meiosis Notes

http://www.slideshare.net/docsawyer/meiosis-notes-3075315 . Accessed : 2014/03/06