2. Homologous Chromosomes
V In human being, somatic cells (autosomes) have 23 pairs of
Homologous chromosomes
V One from each parent.
v Humans have 1pair of sex chromosomes called Gonosomes,
(X and Y) which differ in Genetic information and size.
V The other 22 pairs have the same size and genetic information.
3. Homologous chromosomes are matched in:
length,
centromere position, and
gene locations (locus).
A locus (plural, loci) is the position of a gene.
Different versions (alleles) of a gene may be found at
the same locus on maternal and paternal
chromosomes.
4.
5. Gametes
Most animals and plants have a diploid number of body (somatic) cells.
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
haploid nuclei.
6. • 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 cell
• Fertilization is fusion of a sperm and egg cell.
• The zygote has a diploid chromosomes number,
one set from each parent
7.
8. MEIOSIS
Meiosis is a type of cell division that
produces haploid gametes from diploid
diploid cells.
Two haploid gametes combine in
fertilization to restore the diploid state
in the zygote.
10. 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
11. MEIOSIS 1:
ANAPHASE 1
DNA replication: Duplication
of Chromosomes
Cell builds up energy for the
process
12. 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.
• Centriole move to opposite poles with spindle fibers between them.
13.
14. CROSSING OVER
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 homologous
chromosomes.
Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment.
Genetic material are exchanged between maternal and paternal chromatids
16. MEIOSIS 1: METAPHASE 1
Centriole has reached the poles.
Homologous pairs align at the cell
equator.
The two chromosomes attach to
spindle fiber by means of the
kinetochore of the centromere.
18. MEIOSIS 1:TELOPHASE 1
Duplicated chromosomes reach the poles.
A nuclear envelope and nucleolus re-forms
around chromosomes.
Each nucleus now has the haploid number of
chromosomes.
Cell invaginates forming a cleavage furrow,
which extends to for 2 separate haploid cells.
19. MEIOSIS II
• This stage follows
Meiosis I however, no
chromosome duplication
will occur
• All the haploid products
enters meiosis II
20. MEIOSIS II : PROPHASE II
Chromosomes coil and become
compact (if uncoiled after
telophase I).
Nuclear envelope and nucleolus, if
re-formed, disappears again.
Centriole move to opposite poles,
forming spindle fibers between
them.
21. MEIOSIS II: METHAPHASE II
Individual duplicated chromosomes
align on the equator.
One chromosome per spindle fiber
attached by means of kinetochore
of centromere.
Centriole has reached the poles.
22. MEIOSIS II : ANAPHASE II
Spindle fibers contract.
Duplicated chromosomes
split in half (centromere
dividing in 2)
Daughter chromosomes
move to opposite poles
23. MEIOSIS II:TELOPHASE II
Daughter chromosomes has
reached the poles.
Two cells invaginate and form 4
daughter haploid cells (gametes)
They uncoil and form chromatin.
Nuclear envelope and nucleolus for
around chromatin again.
Centriole for centrosome.
24. Similarities and Differences Between Mitosis and
Meiosis
Mitosis and meiosis both
begin with diploid parent cells that
have chromosomes duplicated during the previous
interphase.
However the end products differ.
Mitosis produces two genetically identical diploid somatic
daughter cells.
Meiosis produces four genetically unique haploid gametes.
27. • www.cybercambridge.com 2010.
• Ms J Williamsons : Meiosis slides for Life Sciences 3A
FET 2018.
• Mabaso M.P. (2013) Slideshare Powerpoint
Presentation. Available from slideshare at
http://www.slideshare.net/mpmabaso-
140310070926-phpapp01-180824084918-
180827101123 [Accessed 27/08//2018]
