In human being, somatic cells (autosomes) have 23 pairs
of Homologous chromosomes
One from each parent.
Humans have 1pair of sex chromosomes called
Gonosomes, (X andY) which differ in Genetic information
The other 22 pairs have the same size and genetic
Homologous chromosomes are paired according to:
Gene Location (Singular: Locus Plural: Loci)
Locus: Is the position in which a gene can be found on a chromosome.
Different versions (alleles) of a gene may be found at the same locus on the
maternal and paternal chromosomes.
Most animals and plants have a diploid number of body (somatic)
This means they have two sets of chromosomes (homologous pair),
one from each parent
A diploid number is written as 2n
It refers to the total number of chromosomes a cell can have.
• Meiosis, is a cell division process which converts the diploid nuclei into a
• Diploid cells have 2 sets of chromosomes
• Haploid cells have 1 set of chromosomes
• Meiosis only occurs in sex organs and results in gametes- sperm and an egg
• Fertilization is fusion of a sperm and egg cell.
• The zygote has a diploid chromosomes number, one set from each parent
o Meiosis is a special type of cell division necessary for sexual reproduction in
eukaryotes, such as animals, plants and fungi.
oMeiosis results in the formation of haploid gametes which combine during
fertilization to restore the diploid state in a zygote.
Meiosis is divided into 2 stages
• Meiosis I
• This process consist of 5 phrases:
• Interphase I,Prophase I, Metaphase I,Anaphase I,Telophase I.
• Meiosis II
• This process consist of 4 phrases:
• Prophase II, Metaphase II,Anaphase II;Telophase II
Meiosis I: Interphase I
• DNA replication: Duplication of Chromosomes
• Cell builds up energy for the process
•Events occurring in the nucleus:
• Chromosomes coil and become individual chromosomes, nucleolus and
nuclear envelope disappear.
• Homologous chromosomes come together as pairs by synapsis forming a
tetrad (Each pair, with four chromatids)
• Non-sister chromatids from homologous chromosomes exchange genetic
material through the process of crossing over to ensure genetic variation.
• Centrioli move to opposite poles with spindle fibers between them.
• Genetic recombination is the production of new combinations of genes due
to crossing over.
• Crossing over is an exchange of genes between separate chromatids on
• Non-sister chromatids join at a chiasma (plural, chiasmata), the site of
• Genetic material are exchanged between maternal and paternal
Meiosis I: Metaphase I
• Centriole has reached the poles.
• Homologous pairs align at the cell equator.
• The two chromosomes attach to one spindle
fiber by means of the kinetochore of the
Meiosis I: Anaphase I
• Spindle fibers contract.
• Duplicated chromosomes
move to opposite poles.
• Duplicated chromosomes reach the poles.
• A nuclear envelope and nucleolus re-forms around
• Each nucleus now has the haploid number of
• Cell invaginates forming a cleavage furrow, which
extends to for 2 separate haploid cells.
• This stage follows Meiosis I
however, no chromosome
duplication will occur
• All the haploid products enters
Meiosis II: Prophase II
• Chromosomes coil and become compact (if
uncoiled after telophase I).
• Nuclear envelope and nucleolus, if re-formed,
• Centrioli move to opposite poles, forming
spindle fibers between them
Meiosis II: Metaphase II
• Individual duplicated chromosomes align on
• One chromosome per spindle fibre attached by
means of kinetochore of centromere.
• Centrioli has reached the poles.
Meiosis II: Anaphase II
• Spindle fibers contract.
• Duplicated chromosomes split in half
(centromere dividing in 2)
• Daughter chromosomes move to opposite
Meiosis II:Telophase II
• Daughter chromosomes has reached the
• Two cells invaginate and form 4 daughter
haploid cells (gametes)
• They uncoil and form chromatin.
• Nuclear envelope and nucleolus for around
• Centrioli for centrosome.
Differences and Similarities between Mitosis and
Genetic variation in gametes
•Independent orientation at metaphase I
•Crossing over of genes during prophase I
Consequences of Abnormal Meiosis
• An extra copy of chromosome 21 causes Down Syndrome which is also
known as Trisomy 21
• This results in many debilitating characteristics eg. mental retardation
• Other diseases include: Klinefelter Syndrome andTurner Syndrome
Sources for my Presentation
• www.cybercambridge.com 2013
• Ms JWilliamsons : Meiosis slides for Life Sciences 3A FET 2014
• http://www.waukeshasouth.com/ 2011
• http://en.wikipedia.org/wiki/Meiosis 2010