Continuity and Variation: Section C of The CSEC Biology Syllabus

Continuity
and Variation
Section C of the CSEC
Biology Syllabus
Objectives
TABLE OF
CONTENTS
Mitosis 01
02 Meiosis
Continuity and
Variation
03
IMPORTANT GENETIC TERMS
Chromosome- Haploid (n)-
DNA It contains DNA and protein (histones) and is It refers to an
(deoxyribonucleic found in the nucleus of cells organism having half
acid)
(23 chromosomes)
This nucleic acid
the number of
contains all the
chromosomes as the
genetic information of
Alleles- parent (46
the organism
A pair (or more) of genes chromosomes).
which code for the same
characteristics, and are
found on the same locus of
Gene- homologous chromosomes Diploid (2n)-
A portion or segment It refers to an organism
of DNA that carries having the full number of
genetic information to chromosomes as the parent
produce a specific (46 chromosomes).
protein
01 Mitosis
Mitosis is a type of cell
division in which two
identical daughter cells
are formed from a parent
cell.
Stages of Mitosis
Interphase Prophase Metaphase Anaphase Telophase

INTERPHASE
This is the longest part of the cell cycle during which a cell increases in size and
produces organelles which will be divided between the two new daughter cells.
The amount of DNA is doubled during this period.
PROPHASE
During prophase the following changes take place:
1. The chromosomes condense (shorten and become thicker) and become visible as long thin
threads. The chromosomes are now referred to as pairs of chromatids which are joined at a
centromere.
2. In animal cells (and cells of lower plants), the centrioles, which have divided, move to the
opposite poles of the cells.
3. Protein micro-tubules form from each centriole and the mitotic spindle develops, extending
from pole to pole.
4. Towards the end of prophase the nuclear membrane disintegrates and the nucleolus disappears.
5. Pairs of chromatids can clearly be seen lying free in the cytoplasm.

METAPHASE
During Metaphase the chromosomes arrange themselves at the centre or equator
of the spindle and become attached to certain spindle fibres at their centromere.
Contraction of the micro-tubule spindle fibres draws the individual chromatids
slightly apart.
ANAPHASE
Anaphase is a very rapid stage. The centromere splits and the spindle fibres
contract and pull the now separated chromatids to the poles, centromere first.
TELOPHASE
Telophase is the final stage of mitosis. The chromosomes have now reached the
poles of the cell and are referred to as chromosomes again. They uncoil and
lengthen. The spindle breaks down, the centrioles replicate, the nucleus reappears
and the nuclear membrane reforms.
CYTOKINESIS
In animal cells cytokinesis occurs by the constriction of the centre of the parent
cell from the outside inwards. In plant cells, a cell plate forms across the equator
of the parent cell from the centre outwards and a new cell wall is laid down.
Significance of Mitosis
Maintaining Species Chromosome Number
Mitosis produces two cells that have the same number of

chromosomes as the parent cell (haploid) and each
chromosome is genetically identical to it's original parent
cell chromosome. Therefore the division allows the
production of cells that are genetically identical to the parent
and so gives genetic stability.
Repair and Growth and Self Replacement
By producing new cells, mitosis leads to growth of an

organism and also allows for repair of tissues and the
replacement of dead cells. An example of mitosis in plants is
in the root tip. In human skin, dead surface cells are replaced
by identical cells from below.
Regeneration
Some organisms can regenerate lost limbs.

Asexual Reproduction
Asexual reproduction results in complete offspring that are

identical to the parent. Asexual reproduction involving
mitosis takes place in certain flowering plants where organs
such as bulbs, tubers and runners produce large numbers of
identical offspring in a relatively short period of time. There
is no genetic variation between each individual. However,
most of these plants also reproduce sexually.
Artificial Propagation
Some horticulturists and agriculturists have extended asexual

reproduction to include cuttings, budding, layering and
grafting. These techniques are commonly used in gardening
and the commercial growing of plants.
Artificial Propagation- Cuttings
A stem is quite near a node and pushed into the soil. New
roots grow old from the submerged part of the stem,
particularly if treated with a plant growth substance. For
example sugar-cane and chrysanthemum.
Artificial Propagation - Grafting
A cutting, called the scion which is to be propagated is

inserted into a slit in the stem of another plant (the stock) and
the joint is bound up to seal it. The stock already has a root
system so this scion is able to grow into a new plant.
Tissue Culture
A form of vegetative propagation used to make large

numbers of identical plants. Like binary fission it also results
from mitosis. This technique can be used to clone whole
plants from very small pieces cut from the parent plant.
Advantages of Tissue Culture
1. Large Numbers of identical plants can be produced

reasonably quickly from superior individuals. This can
make them cheaper.
2. Tissue culture can be used to propagate plant species
which do not develop naturally through sexual
reproduction easily such as orchids. Growing orchids for
commercial sale is big business.
Disadvantages of Tissue Culture
1. Variety within plant species is being replaced with

similarity because it is cheaper. This is risky because if
that one kind becomes susceptible to a particular disease or
a pest, the whole crop may be lost.
CLONING OF
ANIMALS
A clone is an exact copy of an organism.
It is now easier for scientist to manipulate the
separation of the first four cells of a zygote and use
these to create clones of the organism. This is
practised mainly in the livestock and dairy
industries.
Why Clone Animals?
Its is financially advantageous to make clones of

superior animals, such as those which may produce
larger amounts of a high quality milk or high protein
meat.
It is also used to copy individuals which have special
genes added through genetic engineering, for example,
for producing human hormones in the animals milk.
Animal Cloning
A second way to create a clone is to take the nucleus of a

body cell from the superior individual and use it to replace
the nucleus of an unfertilised ovum. The cell can be made
to divide as it would have done if it was fertilised ovum
and implanted into the womb of a surrogate mother but all
the cells it makes now have the chromosomes from the
superior animal. The first example of this kind of cloning
is Dolly the sheep.
DIAGRAM SHOWING THE CLONING
PROCESS FOR DOLLY THE SHEEP (Cloned
Advantages and Disadvantages of Animal Cloning
Advantages Disadvantages
● Superior traits can be passed ● The effects of using a body cell as in

onto offspring without the risk creating Dolly are still being studied.
It is possible that what is in effect an
of losing them through a older nucleus may cause problems
genetic exchange during with the clones individual
meiosis ● The technique used to create Dolly
● The use of surrogate mothers could be used to clone humans. Many
means more superior offspring countries now have legislation to
prevent this because it is considered
can be created than could be unethical, for example, it might only
carried by just the genetic be done for purely selfish reasons.
maker.
02 Meiosis
Meiosis
This is a type of cell division

in which four unidentical
daughter cells are produced
from a parent cell.
Meiosis is important in Sexual Reproduction as it is
used in the formations of gametes (sex cells).
VENN DIAGRAM COMPARING MITOSIS AND MEIOSIS
That’s it for
now.

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Continuity

Interphase Prophase Metaphase Anaphase Telophase

5. Pairs of chromatids can clearly be seen lying free in the cytoplasm.

Maintaining Species Chromosome Number

Mitosis produces two cells that have the same number of

Repair and Growth and Self Replacement

By producing new cells, mitosis leads to growth of an

Some organisms can regenerate lost limbs.

Asexual reproduction results in complete offspring that are

Some horticulturists and agriculturists have extended asexual

Artificial Propagation- Cuttings

Artificial Propagation - Grafting

A cutting, called the scion which is to be propagated is

A form of vegetative propagation used to make large

1. Large Numbers of identical plants can be produced

1. Variety within plant species is being replaced with

Its is financially advantageous to make clones of

A second way to create a clone is to take the nucleus of a

● Superior traits can be passed ● The effects of using a body cell as in

This is a type of cell division

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