It is the presentation on the MEIOSIS phase of the Cell division. It includes all the details and definitions that are related to the topic of meiosis with the labelled diagrams. If you have any query or a question, you may ask in the comment box. thanks.

1. Meiosis..
Prepared By:
Shivang Patel..

2. Introduction..
• A process of reduction division in which the number of
chromosomes per cell is cut in half through the
separation of homologous chromosomes in a diploid cell.
• Diploid – 2 sets of chromosomes.
It is the term used to refer to a cell that
contains both sets of homologous chromosomes.
• Haploid – 1 set of chromosomes.
it is the term used to refer to a cell that
contains only a single set of chromosomes.

3. Homologous chromosome….
Homologous - term used to refer to chromosomes that
each have a corresponding chromosome from the
opposite-sex parent.
Homologous chromosomes are similar in shape, position
and information, but not identical to each other.

4. Homologous chromosome.

5. Important definitions..
• Meiosis is a process of reduction division in which the
number of chromosomes per cell is cut in half through
the separation of homologous chromosomes in a diploid
cell.
• Tetrad - structure containing 4 chromatids that forms
during meiosis.
• crossing-over - process in which homologous
chromosomes exchange portions of their chromatids
during meiosis

6. Crossing over..

7. Crossing over..
• Formation of a tetrad while going through the process of
crossing-over
• This occurs during prophase I of meiosis.

8. Introduction..
• Meiosis have mainly two division..
Meiosis-I.
Meiosis-II.

9. Prophase I
1.Occupies 90% of meiosis
2.Chromosomes begin to condense
3.Homologous chromosomes loosely pair along their length
precisely aligned gene by gene
• In crossing over, DNA molecules rejoin each other

10. Prophase-1.

11. Prophase-1
• Each chromosome pair is visible (microscope) as a tetrad: 4
chromatids
• Each tetrad has one or more chiasmata (regions where
crossing over occurred), hold the homologues together until
anaphase I
• Centrosomes movement, spindle microtubules formation,
nuclear envelope breakdown & nucleoli dispersal occur as
in mitosis.
• In late prophase I, kinetochores of each homologue attach to
microtubules from one pole or the other
• Then, homologous pairs move toward the metaphase plate.

13. Metaphase-1.
• Pair of homologous chromosomes (form of tetrads) are
arranged on metaphase plate, with one chromosome of each
pair facing each pole
• Both chromatids of a homologue are attached to kinetochore
microtubules from one pole; those of the other homologue are
attached to microtubules from the opposite pole..

14. Metaphase-1..

15. Anaphase-1.
1. Chromosomes move toward poles..
2. Sister chromatids remain attached at centromere & move as a
single unit toward same pole..
3. Homologous chromosomes, each composed of 2 sister
chromatids, move toward opposite poles..

16. Anaphase-1..

17. Telophase-1
1. At beginning of telophase I, each half of the cell has a
complete haploid set of chromosomes
2. But each chromosome is still composed of 2 sister
chromatids

18. Telophase-1..

19. Meiosis-II
• No interphase..
• Similar to Mitosis…

20. Prophase-II..
• Cells have one set of sister chromatids
• Nuclear membrane breaks down
• Centrioles separate.
• Each of the daughter cells forms a spindle,
and the double stranded
chromosomes moves towards
the equator..

21. Metaphase-II..
• Sister chromatids line up at the middle of the
cell..

22. Anaphase-II..
• The centromeres of sister chromatids finally
separate.
• The sister chromatids of each pair
move toward opposite poles.
Now its individual chromosomes.

23. Telophase-II..
• Cells start to split in two
• Each cell has one set of chromosomes—it is a haploid
• Nuclei form at opposite
poles of the cell and
cytokinesis occurs.
• After completion of
cytokinesis there are
four daughter cells.
• All are haploid (n)..

24. Overview of Meiosis-I..

25. Overview of Meiosis-II…

26. Non-Disjunction..
• Non-disjunction is the failure of homologous
chromosomes, or sister chromatids, to separate
during meiosis.
• Non-disjunction results with the production of
zygotes with abnormal chromosome numbers……
remember…. An abnormal chromosome number
(abnormal amount of DNA) is damaging to the
offspring.

27. Impacts of Non-Disjunction..
• The first is called Monosomy, the second is called
Trisomy. If an organism has Trisomy 18 it has three
chromosomes in the 18th set, Trisomy 21…. Three
chromosomes in the 21st set. If an organism has
Monosomy 23 it has only one chromosome in the 23rd
set.

28. Common non disjunction
disorder..
•
•
•
•
Down’s Syndrome – Trisomy 21
Turner’s Syndrome – Monosomy 23 (X)
Kleinfelter’s Syndrome – Trisomy 23 (XXY)
Edward’s Syndrome – Trisomy 18