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