Chapter 4
Cell replication
Cell Theory
• All organisms are made of cells (and the
products of cells).
• All cells come from pre-existing cells.
• The cell is the smallest living organisational
unit.
All embryos (eg chicken) develop from
a fertilised egg (zygote).
New cells are produced by mitosis.
Cell Division
Two forms:
 cell replication (or mitotic cell division),
which involves mitosis and cytokinesis;
 Reduction division (or meiotic cell division),
which involves meiosis and cytokinesis.
egg cell
Purposes of cell replication
• Growth & Development
• Multicellular organisms grow in size by increasing the
number of cells.
• These cells become specialised – muscle cells, blood
cells and bone in animals
• Maintenance & Repair
• Regular death of the cells lining the gut
• Starfish can produce an entire new individual from a
single arm.
•Q1-3
Mitosis
The production of new cells genetically
identical with the original cell – an essential
process in asexual reproduction.


Eukaryote cells typically have a nucleus containing DNA.
DNA is found in thread-like structures - chromosomes.
Chromosomes

Found inside the nucleus of a cell.

Only become visible during mitosis or meiosis when they
condense, becoming shorter and thicker.


Each chromosome is usually single-stranded (1 DNA molecule).
Each body cell has two of each chromosome, forming matching
pairs.
 Humans have a total of 46 chromosomes
 Eucalyptus has 22 chormosomes
chromatid = 1 strand
Structure of a chromosome
centromere
In duplicated chromosomes, the two threads (DNA molecules)
connected by the centromere, are called chromatids.
Draw a chromosome
Cell Cycle – DRAW THIS!

Varies – short as 20 minutes to as long as 2 weeks!

Usually 10 – 30 hours in plants and 18 – 24 hours in animals.
Interphase

G1 – cell growth, Synthesis – DNA synthesis, G2 – cell growth.

The cell grows in size

Accumulation of materials required for DNA synthesis

DNA synthesis occurs (replication of chromosomes)
* Chromosomes are too slender to be visible in a cell.
Early Prophase




Chromatin strands condense and become clearly visible as thick,
thread-like chromosomes.
Each chromosome is composed of two identical strands called
chromatids, joined together by a centromere.
Centrioles move to opposite sides of the cell.
Spindle fibres (very fine microtubules of protein) begin to form
around and between the centrioles.
Metaphase
Chromosome
consisting of two
sister (identical)
chromatids

Nuclear membrane has totally broken down and the nucleolus
has disappeared.

Spindle fibres form across the centre of the cell.

Chromosomes align at the middle (equator), half-way between
the two spindle poles, attached by their centromeres.
Anaphase

Centromeres split, so that sister chromatids go to opposite ends
of the cell.

Spindle fibres contract, pulling chromosomes towards the poles
of the spindle.
Late Anaphase
Cleavage furrow

Identical set of chromosomes appear at opposite ends of the cell.

Cleavage furrow forms.

Cytokinesis (division of the cytoplasm) begins.
Telophase

Spindle fibres degenerate.

Nuclear membrane and nucleolus reforms in each daughter cell.

Cytokinesis nears completion.

Each chromosome unwinds as the chromatin that forms it again
becomes extended and thin.
Interphase

Two new daughter cells produced, each with an identical set of
genetic material.

In plant cells, a cell plate forms between the two groups of
chromosomes in a newly replicated plant cell and gives rise to
the new cell wall.
Stages in mitosis
Identify the stages in mitosis
A:
B:
C:
D:
E:
F:
G:
H:
I:
J:
K:
L:
Specialised cells
• Cells begin to produce and
release substances (growth
factors) that affect the
development of nearby cells.
• Cells become different from
one another, specialised for
particular functions.
• Specialisation
(differentiation) is under the
control of genes.
Stem Cells
Relatively undifferentiated cells important in tissue replacement.
Some organisms - eg flatworms (planarians) and starfish - retain a
population of stem cells throughout their life that can develop into
any cell type in the body, giving them remarkable regenerative
capacity.
Stem cell: two types
 Different levels of ability to turn into
various types of specialised cells.
 Great medical potential.
Embryonic stem cells:
 obtained from embryos in the earliest stages of development.
 can make replacement cells for any type of tissue
Adult stem cells:
 exist in most mature tissues - supply them with replacement cells
as required.
 more limited in their ability to develop into cells of different
types.
 usually able to make cells for a particular type of tissue only.
Apoptosis

Controlled cell death

Common mechanism to remove excess tissue.

Plays an important role in growth and development (eg in
the development of the hind limb of a chicken).
Cancer cells
Normal








Cancerous
Divide at a faster rate than normal cells of the same type.
Not affected by normal signals that control the cell cycle (eg contact
inhibition).
Look different and may become less specialised.
Release factors that stimulate the development of their own blood
supply.
Their DNA mutates, making them different and sometimes resistant to
earlier successful treatments.
Can ‘colonise’ new parts of the body and continue to grow unchecked.
Continue dividing endlessly, whereas normal cells undergo a limited
number of cell cycles.
Avoid proceeding to ‘natural’ death by apoptosis.
Binary Fission
•
The bacterial cell grows until it has almost doubled in size.
•
The DNA molecule then replicates and the cell divides by splitting
into two relatively equal halves = binary fission.
•
A new cell wall and membrane material is laid down between the
separating chromosomes to divide the cell in two.
•
Prokaryotes replicate by binary fission.
Question 1
The identical number of chromosomes in a nucleus is
maintained by:
A.
B.
C.
D.
mitosis
binary fission
meiosis
cytokinesis
Question 2
An example of an undifferentiated cell in a mammal
is a:
A.
B.
C.
D.
skin cell
stem cell
sperm cell
nerve cell
Question 3
In a mature adult mammal, mitosis
A.
B.
C.
D.
does not occur.
occurs in the lens of the eye.
occurs in the skin.
occurs in nerves
Question 4
A cell undergoing division is viewed under a light
microscope and a cell plate is visible. The cell
would be from
A.
B.
C.
D.
a plant.
an animal.
a virus.
a bacterium.
Question 5
Examine the following diagram of a chromosome.
a. Name structures A and B.
b. During what phase of mitosis does the chromosome appear in
this state? Give reasons for your answer.
c. Chromosomes do not always look like the diagram depicted.
Describe the changes in the appearance of chromosomes during
the different phases of the cell cycle.
d. Draw a typical interphase cell from an animal. Label the key
structures.
Question 7
Name some places where mitosis would be
occurring in a pregnant woman.
Question 8
What phase of mitosis is the following cell in?
Give reasons for your answer.
Question 9
a. Where are adult stem cells?
b. What is their role?
a. How do they differ from embryonic stem cells?
Question 11
Contrast cytokinesis in plant and animal cells.
Similarities:
Cytokinesis is division of the cytoplasm and occurs after or
towards the end of mitosis (nuclear division).
In animal cells the plasma membrane pinches in, forming
two daughter cells.
In plant cells the presence of the cell wall prevents this. Instead a
cell plate forms in the middle of the cell during telophase. It
grows out from the centre and divides the cell into two daughter
cells.
Question 12
Suggest how passage through the cell cycle might be different
in stem cells than in fully differentiated cells.
Stem cells are likely to move through the cell cycle more
rapidly than differentiated cells.
Interphase in particular is likely to be much shorter:
• no cell differentiation
• limited growth