Lesson notes about Meiosis

1. MEIOSIS

2. Meiosis
• It provides the reduction of chromosome
number in diploid cell.
• By meiosis, we get haploid cells and these
cells have half the number of chromosomes
that are present in the parent cell.

3. • The chromosomes that make up each pair are
called homologous chromosomes.
• They are similar in size and shape, and they
have genetic content.
• Those cells that have all the homologous
chromosomes are referred to as diploid, or
2n.

4. • Cells that have only one chromosome from
each pair are said to be haploid, or
monoploid.
• Body cells have diploid chromosome number.
• Gametes have haploid chromosome number.

5. SUMMARY OF MEIOSIS IN A CELL
WHICH HAS 2n=4 CHROMOSOMES

6. STAGES OF MEIOSIS
• Meiosis is also known as reduction division,
takes place only in gonads.
• Each cell divides twice (Meiosis I and Meiosis
II)
• The chromosomes replicate only once which
takes place before the first division.

7. • As a result of the two meiotic divisions, each
original cell produces 4 haploid cells which are
known as daughter cells.
• Each daughter cell contains the haploid
number of chromosomes.

8. Interphase
• It is same with mitosis.
• DNA replication is occurred in interphase.

9. 1. PROPHASE- I
• Each chromosome has already replicated in
interphase, producing two chromatids, as in
mitosis.
• The homologous chromosomes begin to pair
up in a process called synapsis.
• Each group of four chromatids is called a
tetrad.

11. • The chromatids of the tetrad sometimes twist
about each other, and at this point, they may
exchange corresponding segments.
• The exchange of segments between
chromatids during synapsis is called crossing-
over

13. • Nuclear membrane and nucleolus start to
disappear. At the end of prophase, centrioles
are at opposite poles.
• As prophase I ends, the homologous
chromosome pairs, each made up of four
chromatids, move toward the equator of the
cell.

14. 2. METAPHASE- I
• The tetrads line up on the
equator.
• The tetrads are fastened to
the spindle microtubules at
their centromeres.

15. 3. ANAPHASE- I
• The homologous
chromosomes of each
tetrad separate from
each other and move to
opposite ends of the
cell.
• This process of
separation is called
disjunction.

16. • The cluster of chromosomes around each pole
is haploid.
• Centromere division is not observed so sister
chromatids remain attach.

17. 4. TELOPHASE- I
• The cytoplasm divides forming two daughter
cells, each of them has half the number of the
parent cell's chromosomes.
• But each chromosome is already in replicated
form.

18. • Sometimes at the end of telophase I nuclear
membranes form and short interphase follows
without DNA replication.
• However, in most cases the cells immediately
begin the second division.

19. 5. PROPHASE- II
• Each of the daughter
cells forms a spindle,
and the
chromosomes (still
containing sister
chromosomes) move
toward the middle of
the spindle.

20. 6. METAPHASE- II
• The chromosomes become fastened to spindle
microtubules at their centromeres, and the
chromosomes line up on the equator.
• Each chromosome still contains two
chromatids.

21. 7. ANAPHASE- II

22. • The centromeres divide, and the sister
chromatids separate, each becoming a single
chromosome.
• The two chromosomes then move toward the
opposite poles.

23. 8. TELOPHASE- II
• Both daugter cells divide, forming four haploid
cells.
• In each cell, chromosomes return to their
interphase stage, and the nuclear membrane
forms again.

25. COMPARISON OF MITOSIS AND
MEIOSIS
• can be seen both in haploid
(n) and diploid (2n) cells
• can be seen both in diploid
(2n) cells
• provide reproduction in
unicellular organisms, growth
and repair in multicellular
organisms
• produces gametes (sex
cells=reproductive cells) in
sexually reproducing
organisms
• Two daughter cells are
formed after division
• Four daughter cells are
formed after division

26. • The genetic characters of
parental cell and daughter
cells are identical
• The genetic characters of
parental cell and daughter
cells are different, therefore
leads to genetic variations
• Chromosome number of the
parental cell and daughter
cells are identical
• Reduces chromosome
number to haploid =
monoploid
• No crossing-over • Crossing-over takes place.
Homologous pairs synapse
and genetic material is
exchanged between them.

27. • DIFFERENCES OF CELL DIVISION IN ANIMAL
AND PLANT CELLS
ANIMAL CELL PLANT CELL
Cytokinesis by cleavage
furrow
Cytokinesis by formation of
cell plate
Centrioles form spindle
fibers
No centrioles, spindle fibers
are formed by cytoplasmic
proteins