Meiosis

overview
o Chromosomes:
- Occur in all living cells in the nucleus.
- consist of a DNA molecule coiled around histones (proteins).
- DNA segment(gene) in each chromosome strand controls a
particular inherited characteristic.
- Cell is not dividing- chromatin network.
- Cell is dividing- chromatin network condenses, threads shorten and
thickens as single chromosomes.
o Before a cell divides, DNA replication must occur (genetic material
duplicated)
- Necessary to ensure that both daughter cells formed during mitosis
have the identical genetic information to the mother cell.
o After replication, the single chromosome becomes chromatids which are
joined by a centromere.
o Somatic cell-body cell; gametes- sex cell

Chromosomes
- Each somatic cell (body cell) contains the same number of
chromosomes.
- For Organisms that reproduce sexually, the number of chromosomes is
even because they are diploid (two sets of chromosomes; 2n) (2n=46)
o Each chromosome of one set, has a partner
in the other set which is similar in size,
shape, and genetic composition.
o One pair comes from the mother and the
other from the father, these pairs are known
as homologous chromosome pairs. This
ensures that the organism has the
characteristics of both parents.
- Gametes (sex cells: egg and sperm) are haploid
chromosome number (one set of chromosomes) (n=23)
- During fertilization, the egg and the sperm each with one set of
chromosomes (n) fuse to form a zygote (2n) which contains two sets of
chromosomes (diploid chromosome number)
- The zygote divides by mitosis and forms an embryo consisting of many
cells each with a diploid chromosome number.

Karyotype- The diploid set of chromosomes arranged according to their size,

number in homologous pairs, and shape within a somatic cell
- The human karyotype has 22 pairs of autosomes and one pair of
gonosomes (sex chromosomes), known as the X and Y chromosomes.
- Human diploid chromosome is 46 (23 homologous chromosome pairs)
- A female has two X-gonosomes and a male has one X and Y lonesome.
o Chromosome composition in females is 44+XX
 o Chromosome composition in males is 44+YY.

Cell division
 Cell cycle is the period during which a cell grows, replicates genetic material,
and divides.
 It’s divided into:

Interphase
the period between two consecutive dividing phase
cell divisions.
-Chromatin network is visible as it -the cell nucleus and chromosome
only forms chromosomes in cell divide during karyokineses.
divisions. -this is followed by cytokinesis which
-Cell growth and replication of DNA includes the divisions of the
take place. (DNA replication ensures cytoplasm, organelles, and cell
that the chromosomes are double- membrane.
stranded in preparation for mitosis)
Preparation of cell division
Two types of cell divisions are mitosis and meiosis.
1. mitosis (a division of somatic cells)
- new cells (the same number of chromosomes) are formed during growth,
repair of damaged tissues, and asexual reproduction.
- two daughter cells formed from mitosis have the same number of
chromosomes as the mother cell and are identical to each other.

Process of mitosis
Prophase o chromatin network threads shorten and thicken to become visible as
chromosomes.
o the chromosomes are double-stranded as DNA replication occurred during
interphase.
o Chromosomes consist of 2 identical chromatids joined by a centromere.
o Nuclear membrane and nucleolus disappear.
o Centrosome duplicates and the centrioles move to opposite poles of the cell.
o Spindle fibers develop
Metaphase o chromosomes move to the equator and position themselves in a single row
on the equator of the cell.
o some spindle fibers attach to the centromeres

Anaphase o the centromere of each chromosome divides into two

o chromatids separate and move to opposite poles of the cell.
o chromatids are now known as daughter chromosomes.
o cytokinesis begins

Telophase o the daughter chromosomes position themselves at the poles.

o spindle fibers disappear.
o a nuclear membrane is formed around each group of daughter’s
chromosomes and a nucleolus forms in each nucleus
o the cytoplasm divides (cytokinesis)
 o in animals: invagination forms in the cytoplasm and plants: a cell plate
forms in the cytoplasm.
o cytokinesis is complete and two identical daughter cells are formed.
o the daughter chromosomes form a chromatin network after cell division.
The daughter chromosomes are single stranded, after cell division they from a
chromatin network again. DNA replication ensures that the chromosomes are
double-stranded in preparation for mitosis.

Meiosis
Meiosis is a reduction division where the
chromosome number in each cell is halved
from the diploid to the haploid. in animals and
plants, it results in the production of gametes
and spores.

 Meiosis Takes place during the

formation of gametes (sex cells) in
animals or spores in plants.
 Reduction revision occurs: Chromosome
number is halved in the nucleus, diploid
to haploid.
 Daughter cells formed have only one
set of chromosomes and aren’t
identical to each other and the
mother cell.

If the gametes had a full set of

chromosomes the zygote would have twice
the normal chromosome number. The
chromosome number of an Organism would
thus double with each generation. Meiosis
prevents this doubling by halving the
chromosome number. When gametes with
haploid chromosome numbers fuse the diploid number is restored.

Process of meiosis
Karyokineses- a division of the nucleus. Cytokinesis- a division of the
cytoplasm. Before meiosis takes place, the cell is in interphase during which
DNA replication takes place. After replication, the chromosomes are double-
stranded in preparation for meiosis.
 first meiotic divison(meiosis I)
the chromosome number is halved and
genetic material is exchanged
prophase
metaphase
meisois (continuos
process) anaphase
telophase
second meiotic division(meiosis II)
proceeds like mitosis where the
daaughter chromosomes seperate from
each other

First meiotic division (meiosis I)

1. Prophase I
a. The chromatin network condenses, and the threads shorten and
thicken to become visible as chromosomes.
b. homologous chromosomes arrange
themselves in pairs. The two
chromosomes of each homologous pair
lie together to form a bivalent.
c. Each chromosome of the bivalent splits
longitudinally into two identical
chromatids known as sister
chromatids. The centromeres don’t
divide.
d. Crossing over the chromatids of
homologous chromosomes takes
place. Chromatid segments
(groups genes) break off and are
exchanged.
e. this results in
reshuffling/recombination of
genetic material between maternal
and paternal chromosomes.
f. Chiasmata- the points at which
the chromatids cross over and the
breakages occur.
g. The nucleolus and nuclear membrane disappear.
h. The centrosome duplicates and centrioles move to opposite poles of
the cell.
i. Spindle fibers/protoplasmic threads develop between the centrioles.
2. Metaphase I
a. The bivalents position themselves on the equator.
b. The chromosomes lie in a double row on either side of
the equator.
c. Spindle fibers attach to the centromeres.
3. Anaphase I
a. The spindle fibers contract and shorten, pulling
the chromosomes toward the poles.
b. Homologous chromosomes are separated.
 c. One chromosome of the bivalent moves towards one pole and the
other at the opposite pole.
d. Cytokinesis begins.

NB: no division of centromeres occurs, it’s the chromosomes that move

to the poles. Separation of the homologous chromosome pairs takes
place, resulting in a halving of the chromosome number.

4. Telophase I
a. The chromosomes group themselves at the poles.
b. The spindle fibers disappear.
c. A nuclear membrane forms around each group of chromosomes
and a nucleolus forms in each nucleus
d. Cytokinesis is complete after the invagination of the cell membrane
and two daughter cells are formed.
e. Each daughter cell has one chromosome from each homologous
pair (half the chromosome number; haploid (n) of the mother cell
(2n))
f. The chromosomes in each daughter cell are double-stranded with
exchanged material (recombinant chromatids)
g. Interkinesis takes place in a short interphase where no DNA
replication takes place.

Second meiotic division (meiosis II)

Both daughter cells formed during meiosis I will undergo further division during
meiosis II

1. Prophase II
a. no pairing of homologous chromosome occurs.
b. the nucleolus and nuclear membrane disappear.
c. a spindle forms consisting of centrioles at opposite poles and
spindle fibers.
2. metaphase II
a. the chromosomes move to the equator and align a single row.
b. spindle fibers attach to the centromeres.
3. Anaphase II
a. The centromere of each chromosome splits into two
b. The two chromatids separate and move to opposite poles.
c. The chromatids at each pole are now daughter chromosomes.
d. Cytokinesis begins.
4. Telophase II
a. The daughter chromosomes
group at the poles
b. The chromosomes in each
daughter cell are single-stranded
with recombinant
chromatids/exchanged genetic
material
c. Spindle fibers disappear.
 d. A nuclear membrane forms around each group of chromosomes and
nucleolus forms in each nucleus
e. Cytoplasm divides
f. Animal cell: invagination of the cytoplasm develops on the equator,
dividing the daughter cell in two.
Plant cell: a cell plate develops in the cytoplasm on the equator.
g. Cytokinesis is complete. Four daughter cells have been formed,
with haploid chromosome numbers.
h. Due to crossing over (exchange of genetic material), the genetic
information of the four daughter cells isn’t identical.
i. In females, three of the daughter cells disintegrate and one
develops into an egg.

Purpose of meiosis/ reduction division

 Meiosis is known as reduction division because the diploid number of
chromosomes in the mother cell is reduced to the haploid number in the
daughter cells during the division process.
 Purpose of meiosis: the formation of gametes in the sex organs of animals
 Gametogenesis- the process of gamete formation
 Spermatogenesis- sperm is formed and occurs in the testes
 Oogenesis- eggs are formed and occur in the ovaries

Exceptions in plants
- In plants:
o meiosis produces haploid spores.
o In plants (ferns), the diploid sporophyte generation produces
haploid spores by meiosis, the spores germinate and give rise to
the haploid gametophyte generation.
o haploid gametes are then formed by mitosis in the male and female
sex organs of the adult gametophyte plant.
o Haploid male and female gametes fuse and form a diploid zygote
which develops into a diploid sporophyte plant.

Alternation of generations- when the haploid gametophyte generation and

the diploid sporophyte generation alternate with each other in that one
generation gives rise to the other

 Meiosis produces haploid gametes in animals but haploid spores in

plants.
 Mitosis is responsible for the production of gametes in plants.

Importance of meiosis
 Prevents the chromosome number of an organism from doubling with
each generation.
 For Genetic variation in a population.

Genetic variation
It is the differences that occur as a result of different gene combinations
between the organisms of a species.
Different gene combinations in genes are obtained by:

1. Crossing over of chromatids and the exchange of genetic material during

prophase I
2. Random assortment of chromosomes on the equator during
metaphase I. this independent assortment of chromosomes leads to a
greater variation in the gametes.
Gregor Mendel (father of genetics) calls it the law of independent assortment.

The different arrangements of the homologous chromosomes of bivalents

results in gametes with different gene combinations produced at the end of
meiosis.

Therefore, the gene combinations of sperms and eggs are not identical. Each
gamete formed will contain a unique combination of paternal and maternal
chromosomes.

3. random fertilization: the possibility of which sperm will fuse in which

egg in fertilization results in a wealth of gene combinations in the zygote.

Abnormal meiosis – changes in the chromosome

number/structure
o The abnormalities are known as chromosome mutations or aberrations.
chromosome mutations cause a change in the karyotype of a cell.
o A condition known as aneuploidy occurs when there are extra
chromosomes or missing chromosomes in a cell.
 Down syndrome
o It’s an example of aneuploidy.
When there are 47 chromosomes
instead of 46 in each somatic cell
o occurs in the production of eggs
during meiosis when the
chromosome pair number 21 does
not separate during Anaphase 1,
non-disjunction occurred.
o The zygote develops into an
individual with 47 chromosomes in every somatic cell, this
chromosome abnormality is known as Trisomy 21

Characteristics of down syndrome

 Small. Almond-shaped eyes

 Flat forehead
 Large tongue
 Mental retardation

The risk of having a Down syndrome baby increases with age. After the age of
35, the risk increases. this phenomenon is called the maternal age effect.
Down syndrome is confirmed using amniocentesis. A small amount of the
amniotic fluid is extracted from the uterus and the cells of the unborn fetus are
analyzed

Polyploidy is the condition where there are more than two sets of chromosomes
in a plant cell. It occurs due to abnormal meiosis when a full set of chromosomes
does not separate during anaphase I (total non-disjunction)

Comparison between mitosis and meiosis

Similarities between mitosis and meiosis
are cell division processes Both require DNA replication during
interphase before the division phase
begins
Both have prophase, metaphase, The chromatin network forms
anaphase, and telophase chromosomes in the early prophase

Differences between mitosis and meiosis in location

Meiosis mitosis
Takes place in the reproductive Takes place in plants and animals
organs of animals (testes and ovaries) when the zygote develops to an
to produce gametes embryo to an adult.
Takes place in the sporangia of the Takes place in the meristematic tissue
sporophyte generation in plants to in plants (the apical and lateral
produce spores meristem
Takes place in damaged tissues of
plants and animals
Differences between mitosis and meiosis in purpose
Meiosis Mitosis
This prevents the diploid chromosome Daughter cells have the same
number from doubling with each chromosome number as the original
generation mother cell. Multiplication of cells with
no change in chromosome number
Introduces variation in the daughter Production of identical daughter cells
cells by crossing over and random (no variation)
assortment
Growth: zygoteembryoadult
Repair of damaged tissue
Asexual reproduction

Differences between mitosis and meiosis the in process

Meiosis Mitosis
Two divisions take place: meiosis I One division takes place
and II
Chromosome number halves from Chromosome number remains the
diploid to haploid in the cell nucleus same in the cell nucleus
homologous chromosomes arrange no pairing of homologous
themselves in pairs and form chromosomes and bivalent formation
bivalents during prophase 1 occurs
four daughter cells are formed with a 2 daughter cells with the same
haploid chromosome number as the mother
cell are formed
Crossing over between homologous no crossing over and exchange of
chromosomes and exchange of genetic material occurs
genetic material occurs during
prophase 1

definitions
Meiosis A reduction division where the chromosome
number in each cell is halved from diploid to
haploid
chromatin network when a cell is in interphase the chromosomes
in the nucleus appear as a mass of threads
chromosome when the cell divides the threads shorten,
thicken, and become visible as single
chromosomes
chromatid after replication has occurred the chromosome
consists of two identical units known as
chromatids, they are connected by a
centromere
diploid chromosome number the double set of chromosomes that appear in
pairs in each body cell of an Organism
haploid chromosome number the single set of chromosomes found in sex
cells after meiosis
homologous chromosome a set of two chromosomes that are similar in
pair size and shape and carry corresponding genes
that control the same characteristics. Thus it
combines the characteristics of the mother and
the father in the new Organism
Autosomes the normal chromosomes that are not involved
in sex determination
Gonosomes the sex chromosomes that determine the sex
of an individual
Karyotype the complete set of diploid chromosomes in a
somatic cell arranged in pairs according to size
shape and number
genome the complete set of genes in a particular
Organism