2. Why do cells divide?
Growth
Development
Repair.
3. Cell division
All complex
organisms originated
from a single
fertilised egg
Every cell in your
body started here,
through cell division
the numbers are
increased
Cells then specialise
and change into their
various roles.
4. Essential Features of Cell
Division
Transmit a complete copy of
genetic information (DNA)
Transmit materials necessary for
cell to survive and use genetic
information.
6. Prokaryotic Cell
No nucleus – genetic material (DNA)
in cytoplasm
No membrane-bound organelles
Cell division is called binary fission
Example: bacteria.
10. Eukaryotic Cell
Membrane-bound organelles,
including a nucleus
Genetic material (DNA) contained
within the nucleus
Cell division of somatic cells called
mitotic cell division
Examples: fungi, protists, plants,
animals.
12. What is Mitotic Cell Division?
Division of somatic cells (non
reproductive cells) in eukaryotic
organisms
A single cell divides into two
identical daughter cells
◦ Maintains chromosome ploidy of cell.
13. Ploidy Level
Ploidy – refers to the number of pairs of
chromosomes in cells
haploid – one copy of each chromosome
– designated as “n”
diploid – two copies (=pair) of each
chromosome
– designated as “2n”
triploid – three copies of each chromosome
– designated as “3n”
14. Number of Chromosomes
Each species has a characteristic number
of chromosomes:
◦
◦
◦
◦
◦
◦
Prokaryotes = one chromosome
Crayfish (2n) = 200 chromosomes
Fruit fly (2n) = 8 chromosomes
Human (2n) = 46 chromosomes
Wheat (6n) = 42 chromosomes
Potato (4n) = 48 chromosomes.
15. Number of Chromosomes
Diploid organisms receive
◦
◦
one set of chromosomes from female parent
(= maternal)
one set of chromosomes from male parent
(= paternal)
A “matched” pair of maternal and
paternal chromosomes are called
homologues
gamete (n)
zygote (2n)
fertilization
gamete (n)
16. Structure of a eukaryotic
chromosome
unreplicated chromosome
arm
arm
centromere
17. Structure of a eukaryotic
chromosome
Prior to cell division:
chromosomes (DNA) are replicated
(duplicated)
duplicated chromosome
◦
◦
attached at their centromeres
as long as attached, known as sister
chromatids
duplicated
chromosome
sister
chromatids
19. Structure of a eukaryotic
chromosome
metacentric
submetacentric
acrocentric
telocentric
20. Structure of a eukaryotic
chromosome
sister
chromatids
daughter
chromosomes
21. The Cell Lifecycle
The cell lifecycle is well defined and
can be divided into four stages:
◦ Gap 1 (G1) - The growth phase in which
◦
◦
◦
most cells are found most of the time
Synthesis (S) - During which new DNA
is synthesized
Gap 2 (G2) - The period during which
no transcription or translation occurs
and final preparations for division are
made
Mitosis - Cell division.
22. The Cell Lifecycle
Gap 1 - Doubling
of cell size.
Regular cellular
activities.
transcription and
translation etc.
Synthesis of DNA Regular cell
activities cease
and a copy of all
nuclear DNA is
made
S
G1
G2
M
Gap 2 - Final
preparation for
division
Mitosis - Cell
division
24. Mitosis
Mitosis is the process by which new
body cell are produced for:
◦ Growth
◦ Replacing damaged or old cells
This is a complex process requiring
different stages.
26. Stages of Mitosis
During mitosis an exact copy of the
genetic material in the “mother” cell
must be distributed to each
“daughter” cell
Each stage of mitosis is designed to
achieve equal and exact distribution
of the genetic material which has
been copied during the S phase of
the cell cycle.
27. Stages of Mitosis
Interphase - The ‘in between’ stage - this
is the stage most cells spend their time in
doing the things that cells do and, if they
are preparing to divide, growing and
replicating their DNA
S
Interphase
G2
G1
M
29. Prophase
The chromosomes
condense
The nuclear
envelope and
nucleolus disappear
The centrosomes
move to opposite
poles
The spindle starts to
form, growing out of
the centrosomes
towards the
chromosomes.
30. Metaphase
Metaphase is a short
resting period
the chromosomes are
lined up on the equator
of the cell
with the centrosomes
at opposite ends and
the spindle fibers
attached to the
centromeres
Everything is aligned
for the rest of the
division process to
occur.
31. Anaphase
In anaphase, the
centromeres divide at
this point, each individual
chromosome goes from:
◦ 1 chromosome with 2
chromatids
◦ to:
◦ 2 chromosomes
with one chromatid each
Then the spindle fibers contract, and the
chromosomes are pulled to opposite
poles, towards the centrosomes.
32. Telophase
In telophase the cell
actually divides
The chromosomes are at
the poles of the spindle
The spindle disintegrates
The nuclear envelope reforms around the two sets
of chromosomes
The cytoplasm is divided
into 2 separate cells, the
process of cytokinesis.
33. Cytokinesis
The organelles get divided up into
the 2 daughter cells passively: they go
with whichever cell they find themselves in
In plant cells, a new cell wall made of
cellulose forms between the 2 new nuclei,
about where the chromosomes lined up in
metaphase
◦
Cell membranes form along the surfaces of this
wall
In animal cells, a ring of actin fibers forms
around the cell equator and
contacts, pinching the cell in half.
35. Summary of Mitosis
Prophase:
◦
◦
◦
◦
Chromosomes condense
Nuclear envelope disappears
centrosomes move to opposite sides of the cell
Spindle forms and attaches to centromeres on the chromosomes
Metaphase
◦ Chromosomes lined up on equator of spindle
◦ centrosomes at opposite ends of cell
Anaphase
◦ Centromeres divide: each 2-chromatid chromosome
becomes two 1-chromatid chromosomes
◦ Chromosomes pulled to opposite poles by the spindle
Telophase
◦ Chromosomes de-condense
◦ Nuclear envelope reappears
Cytokinesis: the cytoplasm is divided into 2 cells.
