Cell Division
&
Meiosis
Prof. Muhammad Rafique
Types Of Cell Division
Two major types of cell
divisions
Mitosis
Produce two cells that are
genetically identical to the
parental cell.
Meiosis
Produce haploid gametes
from a diploid parental cell.
Gametes are genetically
different from parent and
each other.
Cell Cycle
All the cells in the body are
derived from a single cell
that from the fertilized egg
or zygote by process of
division. The process of
cells division for cellular
replication is called as
Mitosis. When a single cell
divides by the process of
mitosis, will produce two
identical cells. The life cycle
of single cell is called as
Cell cycle
Before Cell Division
Before a cell can divide it has
to
 Grow in size,
 Duplicate its
chromosomes
 Separate the
chromosomes for exact
distribution between the
two daughter cells.
These different processes are
coordinated in the cell
cycle.
Replication of DNA by Complementary
Base Pairing
Duplicated Vs Unduplicated Chromosomes
Chromosomes either have one or two
molecules of DNA plus associated
proteins. A chromosome with one
molecule of DNA is called an
unduplicated chromosome because
it only contains one molecule of
DNA. A duplicated chromosome
contains two identical daughter
DNA molecules that have come
from an original DNA molecule. In
the case of a duplicated
chromosome, each molecule of
DNA and associated proteins is
called a sister
Chromatid vs. Chromosome
When two DNA molecules are
joined together, each
molecule is called a
chromatid and the two of
the molecules are called a
duplicated chromosome.
When a DNA molecule (and
proteins) is not attached to
another one then that single
molecule of DNA is not a
chromatid but an
unduplicated chromosome.
Centromere
• When a chromosome is
examined during mitosis or
meiosis there is a pinched in
region somewhere along the
length of the chromosome
called the centromere. The
centromere is a region to which
the spindle fibers attach to the
chromosome and it is constant
for different types of
chromosomes. The centromere
also contains a small ring of
protein called a kinetochore
which is important in the
movement of chromosomes
during mitosis and meiosis.
Chromatin
During certain times of the
cell's life cycle the
chromosomes are not visible.
This is because the
chromosomes are stretched
out very thin to allow
surfaces for the various
chemical reactions that
involve chromosomes to take
place. When the nucleus is
stained and examined, it
appears uniformly colored
and the chromosomes
collectively are termed
chromatin.
Meiosis
Meiosis, a type of nuclear division,
occurs only in reproductive cells
and results in a diploid cell
(having two sets of
chromosomes) giving rise to
four haploid cells (having a
single set of chromosomes).
Each haploid cell can
subsequently fuse with a
gamete of the opposite sex
during sexual reproduction.
Consists of two cell division
Meiosis I
Meiosis II
Meiosis I
• Meiosis I refers to the first
of the two divisions and is
often called the reduction
division. This is because it
is here that the
chromosome complement
is reduced from diploid
(two copies) to haploid
(one copy). Interphase in
meiosis is identical to
interphase in mitosis.
Meiotic division will only
occur in cells associated
with male or female sex
organs.
Stages of Meiosis
Stages of Meiosis I are
• Prophase I
• Metaphase I
• Telephase I
• Anaphase I
Stages of Meiosis II are
• Prophase II
• Metaphase II
• Telephase II
• Anaphase II
Sub-Stages of Prophase I
Leptonema
During this stage, the
chromosomes begin to
condense and become visible.
Zygonema
The chromosomes continue to
become denser. The
homologous pairs have also
found each other and begin to
initially align at the equatorial
plate
Sub-Stages of Prophase I
Pachynema
Coiling and shortening
continues as the
chromosomes become more
condense. A synapsis forms
between the pairs, forming a
tetrad.
Diplonema
The sister chromatids begin to
separate slightly, revealing
points of the chiasma. This is
where genetic exchange
occurs between two nonsister chromatids, a process
known as crossing over.
Sub-Stages of Prophase I
Diakinesis
The chromosomes continue
to pull apart, but non-sister
chromatids are still loosely
associated via the chiasma.
The chiasma begin to move
toward the ends of the tetrad
as separation continues. This
process is known as
terminalization. Also during
diakinesis, the nuclear
envelope breaks down and
the spindle fibers begin to
interact with the tetrad.
Different Stages of Meioses
crossing over
• A process that occurs during
prophase I of meiosis in which
genetic material from the
chromatid of one chromosome
exchanges places with the
material from the same area of
a chromatid on it's homolog.
This process increases the
variation in gametes produced
by an individual. The images
illustrate a homologous pair of
chromosomes before (on the
above) and after (on the
below) crossing over has
occurred.
Crossing over
Crossing over and
independent assortment of
the homologous
chromosomes helps
genetic variation. Crossing
over is when chromatids
(still in bivalent pairs) cross
over, forming a chiasma.
When the two formed
gametes fuse at
fertilization randomly, yet
more variation is produced
amongst the offspring
Prophase I
Prophase I is identical to
prophase in mitosis,
involving the
appearance of the
chromosomes, the
development of the
spindle apparatus, and
the breakdown of the
nuclear membrane.
Metaphase I
Metaphase I is where the critical
difference occurs between
meiosis and mitosis. In mitosis,
all of the chromosomes line up
on the metaphase plate in no
particular order. In Metaphase
I, the chromosome pairs are
aligned on either side of the
metaphase plate. It is during
this alignment that the
chromatid arms may overlap
and temporarily fuse, resulting
in what is called crossovers
Anaphase I
During Anaphase I,
the spindle fibers
contract, pulling
the homologous
pairs away from
each other and
toward each pole
of the cell.
Telophase I
• In Telophase I, a cleavage
furrow typically forms, followed
by cytokinesis, the changes that
occur in the cytoplasm of a cell
during nuclear division; but the
nuclear membrane is usually
not reformed, and the
chromosomes do not disappear.
At the end of Telophase I, each
daughter cell has a single set of
chromosomes, half the total
number in the original cell, that
is, while the original cell was
diploid; the daughter cells are
now haploid.
Meiosis II
Meiosis II is quite simply
a mitotic division of
each of the haploid
cells produced in
Meiosis I. There is no
Interphase between
Meiosis I and Meiosis II,
and the latter begins
with Prophase II.
Prophase II
Prophase II. At this
stage, a new set of
spindle fibers
forms and the
chromosomes
begin to move
toward the
equator of the cell.
Metaphase II
Metaphase II, all
of the
chromosomes in
the two cells
align with the
metaphase plate.
Anaphase II
In Anaphase II, the
centromeres
split, and the
spindle fibers
shorten, drawing
the
chromosomes
toward each pole
Telophase II
In Telophase II, a cleavage
furrow develops,
followed by cytokinesis
and the formation of
the nuclear membrane.
The chromosomes
begin to fade and are
replaced by the
granular chromatin, a
characteristic of
interphase.
Meiosis II
When Meiosis II is complete,
there will be a total of four
daughter cells, each with half
the total number of
chromosomes as the original
cell. In the case of male
structures, all four cells will
eventually develop into sperm
cells. In the case of the female
life cycles in higher organisms,
three of the cells will typically
abort, leaving a single cell to
develop into an egg cell, which
is much larger than a sperm
cell.
Differences between male and Female
Meiosis
Dr. Muhammad Rafique
Anatomy
Objectives
Defined the Gametogenesis
Defined the Gametes
Types of cells in Human
Describe the process of spermatogenesis
Discuss the process of oogenesis
Differences between spermatogenesis
and oogenesis
Gametogenesis
Gametogenesis is
the process in
which primordial
germ cells are
mature to become
mature germ cells
Now these germ
cells have ability to
fertilized to
reproduce
Gametes are the cells which
are developed after a
number of cells division.
The gametes are developed
from primordial germ cells
during the developed of
embryo. Initially they are
Diploid cells after
maturation they become
haploid cells.
In male the mature germ cell
is called as mature
Spermatozoon while
female the mature germ
cell is called Ovum
Gametes
Types of Cells
Two types of cells are present in
the body
Somatic Cells
Germ Cells
The somatic cells are present
throughout the body, and
they reproduced by the
Mitosis
Gametes: They are only present
in Testes in male and ovaries
in female
They reproduced by the process
of Mitosis and Meiosis
Location of Testis
Spermatogenesis
Process of Production of
sperm
Results in four haploid sperm
from each diploid cell that
undergoes mitosis &
meiosis
Male primordial germ cells
are developed from
epiblast during the 2nd
week of developed and
finally migrated to resides
in the testes. The testes
consist of numerous
convoluted tubules, the
seminiferous tubules.
Spermatogenesis
With in the seminiferous
tubules testes the
germs cell remain
Dormant up to
puberty. At he time of
puberty the hormone is
secreted from pituitary
glands which
stimulates primordial
germ cells (also called
as Spermatogonia) to
develop into mature
form.
At first the
Spermatogonia enter
mitotic division and
produced large
numbers of
spermatogonia, which
form thick population
of spermatogonia
which located on
basement membrane.
Undifferentiated germ
cells called
spermatogonia
(diploid) undergo
mitosis to produce
daughter cells called
primary
spermatocytes.
Spermatogenesis
Spermatogenesis
The primary spermatocytes
undergo I meiotic division to
produce haploid Secondary
Spermatocytes. Two
secondary spermatocytes will
be produced after first
meiotic division.
The two newly produced
secondary spermatocytes
undergo second meiotic
division to produced four
Haploid spermatids.
The spermatids will then
eventually mature into
functional sperm cells.
Spermatogenesis
Oogenesis begins soon
after fertilization, as
primordial germ cells
travel from the yolk sac
to the gonads, where
they begin to proliferate
mitotically. In female the
primordial germ cells are
reside with in the
ovaries and they will
form oogonia. Now the
oognia entered the first
meiotic division to form
primary oocytes. There
about 1-2 million
primary oocytes are
present with each ovary.
Oogenesis
Location of Ovaries
Oogenesis
They become oocytes once
they enter the stages of
meiosis several months
after birth. Now called
primordial follicles, they
are made up of oogenic
cells from the primordial
germ cells surrounded by
follicle cells from the
somatic line. The oocyte is
then arrested in the first
meiotic prophase until
puberty.
Oogenesis
A normal baby girl had
about 2 million primary
oocytes in her ovaries.
By 7 years old about
300,000 remain, her body
reabsorbed the rest.
Primary oocytes have
already entered meiosis I,
but the nuclear division is
arrested in a genetically
programmed way.
Meiosis will resume in one
oocyte at a time, starting
with the first menstrual
cycle.
Oogenesis
Only about 400 to 500
oocytes will be released
during her reproductive
years.
Follicle – primary oocyte and
nourishing cell layers
around it.
Stimulated by hormones the
follicle continues to grow
and the primary oocyte
completes meiosis I.
Resulting in the formation
of a secondary oocyte
(ends up with most of the
cytoplasm) and the first of
three polar bodies.
Oogenesis
Oogenesis
Ovulation then occurs
releasing the secondary
oocyte and the polar body.
Penetration of the sperm
induces the secondary
oocyte and the first polar
body to complete meiosis
II. There are now three
polar bodies and one
mature egg or ovum.
As the sperm and egg nuclei
fuse, their chromosomes
restore the diploid number
for a brand new zygote.
Differences Between Male & Female
Gametogenesis
Features
Location
Male
Testes
Female
Ovaries
Primordial Germ Cells
2nd Generation of
cells
Dormant Stage
First Meiotic Division
Spermatogonium
Occur at puberty
Spermatogonium
Spermatogomium
Primary
Spermatocytes
Secondary
Spermatocytes
Oogomium
Before Birth
Primary Oocytes
Primary oocyte
Oognium
2nd Meiotic division
Primary Oocyte
Differences Between Male & Female
Gametogenesis
Features
Male
Female
Resultant Product
after 2nd Meiotic
division
Resultant Products
Four Spermatids
which mature to
spermatozoa
Four Haploid
daughter cells
One ovum and
three polar bodies
Period for
production
Puberty to
throughout life
One ovum & three
polar bodies with
haploid numbers
menarche to
menopause
Comparing Spermatogenesis and Oogenesis
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