2. introduction
Meiosis is a special type of cell division
necessary for sexual reproduction in
eukaryotes. The cells produced by meiosis
are gametes or spores. In many organisms,
including all animals and land plants (but not
some other groups such as fungi), gametes
are called sperm and egg cells.
3. chromosomes are mateched in
homologous pairs
In humans, somatic cells (body cells) have:
23 pairs of homologous chromosomes and
one member of each pair from each parent.
The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic composition.
The other 22 pairs of chromosomes are autosomes with the same size and
genetic composition
4. 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.
6. GAMATES HAVE A SINGLE SET OF
CHROMOSOMES
Humans and most animals and plants have diploid body cells.
That means they have two sets of chromosomes
(homologous chromosome pair) one from each
parent.
Diploid is written 2n.
It refers to the total number of chromosomes a cell can have.
8. MEIOSIS
Meiosis is a type of cell division that
produces haploid gametes from
diploid cells.
Two haploid gametes combine in
fertilization to restore the diploid
state in the zygote
9. IMPORTANCE OF MEIOSIS
It produces haploid gametes which
prevents the chromosome number
from doubling in every generation.
Produce gametes for fertilization
11. 2 STAGES OF MEIOSIS: meiosis 1and 2
MEIOSIS I consisting of 5 phases: NAMELY
Interphase I, Prophase I, Metaphase I, Anaphase I and
Telophase I.
MEIOSIS II consisting of 4 phases:
Prophase II, Metaphase II, Anaphase II and Telophase II.
12. interphase
DNA Replication (to make duplicated chromosomes
Cell build up energy
Cell doesn’t change structurally
13. prophase
Chromosomes coil and become individual chromo-somes, nucleolus and
nuclear envelope disappear.
Homologous chromosomes come together as pairs by synapsis forming a
tetrad (Each pair, with four chromatids)
Non-sister chromatids exchange genetic material through the process of
crossing over to ensure genetic variation.
Centrioli move to opposite poles with spindle fibers between them.
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 (non-sister)
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 (no sister)
chromatids
15. metaphase
Centrioli 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 centromere.
.
17. tepephase
Duplicated chromosomes have reached 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
18. Meiosis 2
Each of the two haploid products enters
meiosis II.
Follows meiosis I without chromosome
duplication.
19. Prophase 2
Chromosomes coil and become compact (if uncoiled after telophase I).
Nuclear envelope and nucleolus, if re-formed, dissappears again.
Centrioli move to opposite poles, forming spindle fibers between them
20. Metaphase 2
Individual duplicated chromosomes align on the equator.
One chromosome per spindle fiber attached by means of kinetochore of
centromere.
Centrioli has reached the poles.
21. Anaphase 2
Spindle fibers contract.
Duplicated chromosomes split in half (centromere dividing in 2)
Daughter chromosomes move to opposite poles.
22. Telephase 2
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.
Centrioli for centrosome
23. Accidents during meiosis can alter
chromosome number
Nondisjunction is the failure of chromosomes or chromatids to separate
normally during meiosis. This can happen during:
meiosis I, if both members of a homologous pair go to one pole or
meiosis II if both sister chromatids go to one pole.
Fertilization after nondisjunction yields zygotes with altered numbers of
chromosomes
