© SSER Ltd.
The Process of Mitosis
maternal in
origin
paternal in
origin
centromere
This diagram represents a parent cell containing
a single pair of chromosomes – the pair of chromosomes are
the same size and shape and are therefore described as being
homologous chromosomes. One member of the pair was
donated by the male at reproduction and the other member
by the female. Each pair of homologous chromosomes thus has
one member that is paternal in origin and another member that
is maternal in origin. Humans possess 23 pairs of chromosomes
in each body cell. Cells in which the chromosomes occur in
homologous pairs are termed DIPLOID (two sets of chromosomes)
Each chromosome replicates itself to form a pair of identical
chromatids called sister chromatids that remain attached to
one another at a region called the centromere
The cell divides into two and each daughter cell receives
one of each of the sister chromatids
The daughter cells are genetically identical
to the parent cell
The function of mitosis is to construct an exact
copy of each chromosome and then to distribute,
through division of the parent cell, an identical
set of chromosomes to each of the
two daughter cells
The Process of Mitosis
Mitosis is a continuous process but for convenience of description is divided
into FOUR MAIN STAGES:
Prophase
Please
Metaphase
Make
Anaphase
Another
Telophase
Two
The following slides describe the process of mitosis in an animal cell containing
two pairs of homologous chromosomes
Summary of Mitosis
Interphase – cell prepares Early Prophase – chromosomes
Late Prophase –
spiralise and condense and
for division; DNA replicates,
chromosomes spiralise and
become visible as threads;
new organelles are
condense further and
manufactured and cell grows; centrioles move to opposite
can now be seen to consist
poles of the cell and spindle
chromosomes present as
of two sister chromatids
fibres begin to form
granular material (chromatin)
joined at the centromere
Metaphase –
Early Anaphase –
nuclear membrane
as the cell
has disintegrated and
enters anaphase,
spindle fibres have
the centromeres
grown across the cell;
divide into
chromosomes line up
two separating
independently along
the sister
the equator of the
chromatids of
spindle attaching to
each chromosome
the fibres via their
centromeres
Summary of Mitosis
Anaphase – spindle activity pulls the
chromatids apart and the separated
chromatids move to opposite
poles of the cell
Cytokinesis – as the membrane
continues to constrict, the
cytoplasm becomes divided
forming two genetically
identical daughter cells. Each
cell now possesses an exact copy
of each chromosome that was
present in the nucleus of
the original cell
Telophase – the chromatids are now
described as chromosomes and they begin
to uncoil. The spindle fibres disintegrate
and the cell begins to constrict along its
central axis. A nuclear membrane begins
to form around each set of chromosomes
Daughter
cells
A
D
B
These photographs are taken
from prepared slides of onion
root tip cells that were
undergoing mitosis:
Identify the photograph
showing Interphase and the
photographs showing the four
stages of mitosis – Prophase,
Metaphase, Anaphase and
Telophase
C
E
The Cell Cycle
G1
- First Growth Phase
The cell grows and
new organelles and
proteins are
manufactured
G2
- Second
Growth Phase
The cell grows
and prepares
for mitosis
E
PROPHAS
HA
S
TAP
ME
AN
AP
HA
SE
S IS E
E
IN AS
K
TO OPH
Y
C
L
TE
E
M
S
DNA replication
takes place
During interphase, preparations for
mitosis take place
An Introduction to Meiosis
Meiosis is another form of cell division that is associated
with reproduction in many organisms
In humans, meiosis is responsible for the formation
of the reproductive cells or gametes
In humans, these are the egg and sperm cells
Whereas most body cells have a complement of
23 pairs of chromosomes, human gametes possess
only 23 single chromosomes. A gamete’s complement of
23 single chromosomes is constituted by one chromosome
taken from each of the 23 pairs of chromosomes
Within the human ovaries and testes, gametes are
produced by meiosis and this process halves the
chromosome number
Human body cells are DIPLOID as they possess two sets of
chromosomes (23 pairs)
Human gametes are described as being HAPLOID as they
possess only one set of chromosomes (23 chromosomes)
SPERM
If the gametes were diploid then the number of
CELL chromosomes would double at every generation after
fertilisation
EGG
CELL
MITOSIS
MEIOSIS
Diploid body
cell
The nucleus
divides twice
Two diploid daughter cells
Meiosis is important as it ensures
that, when the gametes fuse at
fertilisation, the normal diploid number
of chromosomes is maintained; meiosis
is also an important source of
genetic variation
Four haploid, genetically different
gametes are produced