biology (chapter 5 - notes)
TRANSCRIPT
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Chapter : Cell Division
5.1 Mitosis
Mitosis
A process of nuclear division which produces two genetically identical daughter
nuclei.
The significance of mitosis
1. Replaces dead cells. E.g. skin cells
2. Repaired and replaced damaged cells.
3. Basis of asexual reproduction in unicellular organism.binary fission
4. Increases the number of cells in all living organisms, thus, allowing growth and
development in multicellular organism.
5. Results in the formation of two daughter nuclei which are genetically identical to
each other and to parent nucleus. The nuclei contain the same number of
chromosomes, same genetic material as parent nucleus. This is to ensure that
the daughter cells perform the same function as the parent cell.
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Mitosis
Definition:
Process of nuclear division
Formation of two daughter cells
Two daughter cells are genetically identical to the parent cell
Maintains the chromosomal number of species
Ensures genetic material is passed on to the offspring
Cell cycle:
The period that extends from the time a new cell is produced until the time the cell
completes a cell division.
2 major phases:
a. Interphase
G1 (Growth phase 1)
S phase (DNA synthesis)
G2 (Growth phase 2)
b. Mitotic cell division (M Phase)
Mitosis
i. Prophase
ii. Metaphase
iii. Anaphase
iv. Telophase
Cytokinesis
Interphase:
- Interphase accounts for about 90% of the
cell cycle.
- Interphase is also the stage at which the cell grow larger and prepare for cell
division
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Interphase Description
G1(gap or growth phase) Begins to acquire and synthesis the materials
required for cell division.
Synthesis new organelles and proteins, cellsgrow larger
Metabolic rate of the cell is high.
Crucial phase: the cell will decide whether o not
to divide and complete the cycle to form new
cells
Chromosomes at this stage are known as
chromatin.
S phase DNA undergoes replication.
A duplicated chromosome consists of two
identical sister chromatids = 2 identical DNA
double helices
G2 Cell continues grow and remains metabolically
active.
Enzymes and proteins are synthesized for cell
division
Cell accumulates energy and completes its final
preparations for division.
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progressively until the cell separates
into two daughter cells
New cell walls and plasma
membranes are formed from
the contents of the cell plate
Cell plate divides the cell intotwo daughter cells
Cellulose fibres are produced
by the cells to strengthen the
new cell walls
Controlled mitosis
1. The cell cycle is controlled by genes of the chromosomes. Each type of cell
has its own timingand rateof cell division. The ability of the cell to divide at
its own rate and timing is called controlled mitosis.
2. Mitosis ensures that the genetic content and the number of chromosomes in
the parent cells are maintained in the daughter cells from one generation to
the next
3. The rate and timing of cell division are important for normal growth,
developmentand maintenanceof the organism.
4. For example:
- Regenerate a lizards lost tail
- Heals skin in a wound.
5. Cell with different division frequencies:
- Skin = throughout lifespan
- Liver = replace damaged & injured tissues
- nerve & muscle = do not divide once mature
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Uncontrolled mitosis
1. Uncontrolled mitosis is the situation where cells undergo uncontrolled
division when the genes that regulate the cell cycle are mutated or damaged.
2. When a cell divides by mitosis repeatedly, without control and regulation, it
can produce cancer cell.
3. Cancer (disease) caused by uncontrolled mitosis
4. Cancer cells compete with other normal cells to obtain sufficient nutrients
and energyfor their own growth.
5. A cancer cell will divide uncontrollably in forming a tumor(abnormal mass of
cells)
6. a) Benign tumor- remains at the original site, do not cause serious problem,
can be removed.
b) Malignant tumorinvasive, spread to other to neighboring tissue, impairing
the functions of one or more organs.
7. Cancer can caused by many factors such as:
- Damage of DNA
- Change in genes (mutation) that control cell division.
- Radioactive rays (X-rays, ultraviolet rays and gamma rays)
- Carcinogenic compound, e.g. carcinogenic smoke / benzene
- Genetic- Cancers that are inherited from the parents- Viruses- (Viruses like the EB and HIV-1) can cause cancer- Age- young people (leukemia) or older adults (colon cancers)
Normal cells Cancerous cells
Controlledgrowth Uncontrolledgrowth
A single-organized layer Multi-layered and disorganized
Nuclei and number of
chromosomes are normal
Nuclei and number of chromosomes
are abnormal
Cells are differentiated and carry
out specialized functions
Cells are undifferentiated and do not
carry out specialized functions.
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The application of knowledge of mitosis in cloning
The knowledge of mitosis is applied in cloning and the tissues culture technique.
1. Plants, animals and microbes can reproduce asexually through mitosis to
produce clones.
2. The clones share the same genetic content and chromosomal number with one
another as well as with the parent organism.
Cloning of animal
1. Somatic cells (from the mammary gland) are removed from a donor and grown in
a low culture medium.
2. The starved cells stop diving and enter a non-dividing phase.
3. An unfertilized egg cell is obtained. The nucleus is sucked out, leaving the
cytoplasm and organelles without the chromosomes.
4. An electric pulse stimulates the fusion between the somatic cell and the egg cell
without nucleus.
5. The cell divides to form embryo.
6. The embryo is then implanted into a surrogate mother. (the same breed of sheep
as the ovum donor sheep)
7. The sheep that is born is identical to somatic cell donor sheep.
Cloning
The process of producing clones or genetically identical organisms from a
single parent by mitotic division, or through asexual reproduction.
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4. The explants are incubated in a suitable temperature and at optimum pH level for
several weeks.
5. The explants divide by mitosis to produce callus ( an undifferentiated mass of
tissue)
6. The callus develop into an embryo
7. The embryo develops into a plantlet which can later be transferred to the soil for
growth into an adult plant.
Cloning
Advantages Disadvantages
Multiply copies of useful genes & clones The resistance of clones towards
diseases and pests is the same. If a clone
is affected with a disease / attacked by
pests, then all the clones will also be
affected ad die.
Clones can be produced in a shorter
time and in larger numbers
If the external environment changes, the
clones will be destroyed as cloning is
carried out under controlled environment.
Cloned plants can produce flowers and
fruits within a shorter period.
Clones may disrupt the natural equilibrium
of an ecosystem.
Cloning prevent endangered species
from extinction
The clones do not show genetic variation
Cloning involves vegetative
reproduction which does not need
pollinating agents
For reasons still unknown, cloned animals
have a shorter lifespan
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5.2 MEIOSIS
The Significance of meiosis
1. Meiosis (meio=reduce) is the process of nuclear division that reduces the number
of chromosomes in daughter cells to half that of the parent cell.
2. Meiosis produces haploid gametes. The gametes contain half the number of
chromosomes of the parent cell.
Each gamete receives only one chromosome from every pair of homologous
chromosome
3. Meiosis I begins with a single diploid (2n)parent cell and the end of meiosis II, four
haploid (n) daughter cells are produced, each genetically distinct from the other and
from the parent cell.
4. During sexual reproduction, the fusion of two gametes restores the complete
numbers of chromosomes and genetic material, forming a diploid zygote. The
offspring inherit trait from both parents to ensure continuation of life.
5.
Sexual reproduction
Sperm (n) Ovum (n)
Diploid zygote (2n)
2n
n
n
n
n
n
n
Parent cell 2 haploid cells Gamete cells
M
I
O
S
I
S
I
M
I
O
S
I
S
II
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METAPHASE I
1. The homologous chromosomes are lined up side
by sideas tetrad on the metaphase plate.
2. One chromosome of each homologous pair is
attached to the spindle fibres from one pole while
its homologue is attached to the spindle fibres
from opposite pole
ANAPHASE I
1. The spindle fibres pull the homologous
chromosomes awayfrom one another and move
them to the opposite poles.
2. Each chromosome still consists of two sister
chromatids
3. Although the cell started with four chromosomes,
only two chromosomes move towards each pole.
(based on diagram / question)
TELOPHASE I
1. The chromosomes arriveat the poles.
2. Each pole has a haploid daughter nucleus (contain
only one set of chromosomes)
3. The spindle fibres disappear.
4. The nuclear membrane reappears to surround
each set of chromosomes. The nucleolus then
reappears in each nucleus.
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b) Meiosis II
The stages of meiosis II are identical to mitosis which results in the
separation of sister chromatids.
PROPHASE II
1. The nuclear membrane disintegrates
2. The spindle fibres reform in each daughter cell
.
METAPHASE II
1. The chromosomes (still made up of two sister
chromatids) line up randomly at the metaphase
plate.2. Each sister chromatid is attached to the spindle
fibres at the centromere.
ANAPHASE II
1. The centromeres of the sister chromatids separate
2. The sister chromatids of each chromosome are
now individual chromosomes.
3. Each individual chromosomes moves to the
opposite poles of the cell
Cytokinesis
Occurs simultaneously with telophase I, resulting in two haploid daughter cells.
For most organisms, there is no interphase between meiosis I and meiosis II
Another cell division is required because the chromosomes are still duplicated
Chromosomes remain in a condensed state.
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TELOPHASE II
1. The spindle fibres disappear
2. The nucleoli and nuclear membrane re-form.
3. Cytokinesis follows and four haploid daughter cellsare formed. (contain half the no. of chromosomes)
4. Each haploid cell is genetically different from
parent diploid cell. These haploid cells become
gametes.
THE IMPORTANCE OF MEIOSIS
1. Meiosis ensures that the diploid number of chromosomes is maintained
from one generation to the next.
2. Meiosis provides forgenetic variationwhich occurs from one generation to
the next.
Meiosis leads to genetic recombination
crossing over(prophase I)
Independent assortment(metaphase I)The homologous
chromosomes is arranged independently and randomly at the
metaphase plate of the cell. The paternal or maternal
chromosomes may be oriented to face either one of the poles
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COMPARISON BETWEEN MITOSIS AND MEIOSIS
Mitosis Similarities Meiosis
1. Nuclear division happen
2. DNA replicates only once
MitosisDifferences
Aspects/eventsMeiosis
All somatic cells Type of cell Cells in the reproductive
organs
Produces new cells for growth
and repair
Role Producesgametesfor
sexual reproduction
Synapsis does not occur Synapsis Homologous chromosomes
pair up to form bivalents.
Crossing over between non-
sister chromatids does not
occur during prophase
Crossing over Crossing over between non-
sister chromatids occur
during prophase I
The individual chromosomes
are arranged randomly at the
metaphase plate
Metaphase of
mitosis/ Metaphase
I of meiosis
Homologous chromosomes
line up side by side at the
metaphase plate
Sister chromatids separate to
move to the opposite poles
Anaphase of
mitosis/ Anaphase I
of meiosis
- Homologous
chromosomes separate
to move to the opposite
poles.
- The sister chromatids still
remain attached to each
other
One Number of cell
divisions
Two
Twodaughter cells Number of
daughter cells
produced at the
end of the division
Fourdaughter cells
(gametes)
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Diploid (2n)/ the same number
of chromosomes as the parent
cell
Chromosomal
number of the
daughter cells
Haploid (n)/ half the number
of chromosomes of the
parent cell
Genetically identical to the
parent cell and to one another
Genetic content Different from the parent cell
and form one another
No genetic variation in any
generation
Genetic variation There is genetic variation
from one generation to the
next.
5.3 APPRECIATING THE MOVEMENT OF CHROMOSOMES DURING MITOSIS
AND MEIOSIS
1. The ability of organisms to reproduce ensures the continuity of life.
2. The organisms reproduce through mitotic or meiotic cell division is to ensure
the survival of each speciesfrom one generation to the next.
3. Asexual reproduction through mitotic division produces offspring that are
identical to the single parent; sexual reproduction through meiotic division
produces genetic variation in offspring.
4. If meiosis does not occur properly, the gametes formed will have an abnormal
number of chromosomes. As a result, the zygote that is formed later would
become abnormal.
Exp: down syndrome (2n = 47 chromosomes)