meiosis for 11th standers

1. By
JITESH DAUNDE

2. MEIOSIS
Complex type of cell division.
Responsible for evolution and diversity
Associated with spore formation (Plants and
Fungi) Gamets (Animals)
Gametes are formed from specialized diploid cells
The production of offspring by sexual reproduction
includes the fusion of two gametes
This specialised kind of cell division that reduces
the chromosome number by half results in the
production of haploid daughter cells.
This kind of division is called meiosis.
We come across meiosis during gametogenesis in
plants and animals

3. Two sequential cycles of
nuclear and cell division
called
Only a single cycle of
DNA replication during
meiosis I.
Pairing of homologous
chromosomes and
recombination between
them.
Four haploid cells are
formed at the end of
meiosis II.
Key features
of
MEIOSIS
Meiotic events can be grouped under the
following phases:

4. longer and more complex. subdivided based on chromosomal behaviour, i.e.,
i) Leptotene ii) Zygotene iii) Pachytene iv) Diplotene v) Diakinesis.
Leptotene (Leptos- thin, nema- thread)
• The chromatids condenses and form long and thin thread like
chromosome.
• Each chromosome consist two chromatids as single threads.
Zygotene/ Zygonema (Zygo – Pair, nema- thread)
• chromosomes start pairing together called synapsis.
• Such paired chromosomes are called homologous chromosomes
• The complex formed by a pair of synapsed homologous
chromosomes is called a bivalent or a tetrad.
Pachytene (Pachus- thick)
• Condensation of chromosomes progresses and they become short
and thick
• bivalent chromosomes ( four chromatids) now clearly appears
• This stage is characterized by the appearance of recombination
nodules, the sites at which crossing over occurs between non-sister
chromatids of the homologous chromosomes.
Crossing over is the exchange of genetic material between two homologous
chromosomes. Is an enzyme-mediated process and the enzyme involved is called
recombinase. Crossing over leads to recombination of genetic material on the
two chromosomes.
Meiosis I - Karyokinesis Prophase I

5. Diplotene/ Deplonema (Diplos- Two)
• the synaptonemal complex and homologous chromosomes of the
bivalents to separate from each other except at the sites of crossovers.
• These X-shaped structures, are called chiasmata.
Diakinesis
• This is marked by terminalisation of chiasmata.
• During this phase the chromosomes are fully condensed and the meiotic
spindle is assembled to prepare the homologous chromosomes for
separation
• By the end of diakinesis, the nucleolus disappears and the nuclear
envelope also breaks down.

11. •The spindle formation is completed
•Chromosomes move and arrange on the equatorial plane
•Chromosomal fibres extend from pole towards centromere
Metaphase I

12. •Chromosomal fibres contract and pull the homologous
chromosome.
•Interchromosomal fibres extend and chromosomes start moving to
opposite poles.
•Homologus chromosomes still attaches to chiasmata, finely get
separated called disjunction.
• At end of this phase half number of chromosomes gather at
respective pole.
Anaphase I

13. •Chromosomes uncoil to form chromatids
•Spindle disappear
•Nucleus and nucleolus reappear two daughter nuclei are formed
Telophase I

14. Cytokinesis I
•Division of cytoplasm
•Plasma membrane constrict in the middle in animal cells
•Formation of cell plate in plants
•Formation of two daughter cells with single nucleus consisting
single set of chromosomes.
Interkinesis- it is a short resting period between meiosis I
and meiosis II it may or may not be present

15. •It is divided in to sub stages Karyokinesis and cytokinesis
•Four haploid daughter cells are formed at the end of Meiosis II
Meiosis II - Karyokinesis

16. •Chromosomes become distinct with chromatids.
•Nucleolus and nuclear membrane disappear.
Prophase II

17. •Spindle formation takes place.
•Chromosomes moves towards the equatorial plane.
•Chromosomes get connected to respective poles by chromosomal
•fibres.
Metaphase II

18. •Centromeres divides and chromatids are separated.
•Each chromatids now called daughter chromosome.
•They moves towards respective poles by contraction of chromosomal
fibres and elongation of interchromosomal fibres.
Anaphase II

19. •Daughter chromosomes are gather at respective pole and
starts uncoiling
•Nucleolus reappears and nuclear membrane is formed forming
two daughter nuclei.
Telophase II

20. •Division of cytoplasm forming four haploid daughter cells
Cytokinesis II
Meiosis 1st is
Reductional
cell division
Meiosis 2nd is
Equational
cell division
Difference

21. Significance of meiosis
•Produces haploid gamets for sexual reproduction.
•Help in maintain chromosomal number constant
for given species
•Introduce genetic recombination leading to variation
and evolution
Spermatogenesis Oogenesis