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Chapter 7
Chromosomes and Cell division
Chromosomes, DNA and genes
Chromosomes are present in the nucleus.
When the cell is at rest, the chromosomes
are invisible( 不 能 見 到 ) . When the cell
begins to divide, the chromosomes can
absorb a special stain (Aceto-carmine or
Aceto-orcein) and can be seen under
microscope.
Inside the chromosome are DNA (Deoxyribonucleic acid)
and histones molecules. Each DNA molecule contains a
number of genes which carry hereditory(遺傳) information
Homologous Chromosomes
It consists of
-Paternal chromosome: inherited from the male
parent.
- Maternal chromosome: inherited from the
female parent.
Which of the following statements is/ are true?
If the statement is false, try to amend the statement.
Homologous chromosomes exit in a diploid cell.
True
Homologous pair is separated during mitosis.
False! They are separated during meiosis.
Homologous chromosomes have identical genotype.
False
X and Y chromosomes are a pair of homologous chromosomes.
True
In human body, sex chromosomes are found in gametes only.
False! All cells in the human body contain sex
chromosomes.
The gene for insulin production can be found in the cells of
pancreas only.
False! The gene can be found in all body cells. But the gene
is expressed in the cells of pancreas, but suppressed in the
other body cells.
1
4
3
2
5
Arrange the above stages in correct sequence.
1
4
3
2
5
Chromosome
When the cell is not dividing, the chromosomes are
not visible.
At this stage, chromosome is called chromatin.
Appearance of Chromosome
when a cell is dividing
Chromatid
centromere
splitting
Sister chromatids
centromere
A pair of
homologous
chromosomes
Maternal
chromosome
A splitted
chromosome
Paternal
chromosome
A pair of
homologous
chromosomes
Cell division = Nuclear division + cytoplasmic cleavage
Two types of nuclear division: Mitosis and meiosis
Mitosis
Mitosis consists of four phases:
Prophase
Metaphase
Anaphase
Telophase
Between two cell divisions, there is a phase called
interphase.
Interphase
The chromatins are unidentified. During
interphase, the following events occur:
[
DNA duplication takes place just
before mitosis.
[Respiratory rate is very high for ATP
synthesis. ATP is essential for the various stages
of mitosis especially anaphase at which
separation of sister chromatids occur.
Prophase
The chromosomes become demonstrable under staining
technique. The chromatids become shorter, thicker and
disentangled.
The nuclear membrane dissolves and the nucleoli
disappear.
The two pairs of centrioles move to the opposite poles of
the cell.
1
2
Each chromosomes are splitted up longitudinally into two
thread like sister chromatids joining at the centromere.
The sister chromatids have identical genotype.
Spindle of fibres are formed from the centrioles.
3 4
Metaphase
The chromosomes move to and line up along
the equatorial region of the cell.
Anaphase
[1]The spindle fibres contract.
[2]The centromeres are splitted up.
[3]The sister chromatids are pulled by the
shortening spindle fibres to the opposite poles of
the cell.
Telophase
[1]The chromosomes become longer, thinner,
entangled, fainter and finally lose their
identity.
[2]The nuclear membranes are formed. Each
encloses the separated chromatids. Thus, two
new nuclei are formed.
Cytoplasmic cleavage
It is followed by cytoplasmic cleavage in
animal cell.
Special features of cell division in plant cell
After telophase, a series of flattened vacuoles made by Golgi bodies,
appear in the middle of the cell instead of cytoplasmic cleavage.
The membranes of these vacuoles fuse to form a cell plate, a double
membrane unit.
This grows outward to join the plasma membrane.
Cell wall materials are laid down between the two membranes of the cell plate.
Significance of mitosis
[1]It enables replication of genetic materials so that the daughter cells have the same
genotype as the parent cell. This results in genetic stability within population of cells.
[2]It takes place in somatic cells for production of new cells leading to growth, replacement
of worn-out tissues and regeneration.
[3]It is responsible for asexual reproduction of some species. e.g. Binary fission of Amoeba,
vegetative propagation of angiosperms.
The mammalian and angiospermic parts where mitosis takes place are as
follows:
[1]In mammalian body, with the exception of nerve cells, most somatic cells have
the capacity of mitosis.
[2]In angiosperm, mitosis takes place at
phloem.
(a) cambium: mainly for formation of secondary xylem and secondary
(b) cork cambium: for formation of secondary cortex.
(c) apical meristems in both shoot and root apexes: for growth
length of both parts.
in
Meiosis
Meiosis consists of two consecutive divisions:
[1] Meiosis I (or 1st meiotic division)
[2] Meiosis II (or 2nd meiotic division)
Interphase exist with identical diagnostic
features to those of mitosis.
The chromosomes become
condensed, shorter and thicker and come into
identity.
Pairing of homologous chromosomes
forming bivalents.
Each chromosome is splitted
longitudinally to form 2 sister chromatids
joining at the centromeres.
Chromatids from homologous
chromosomes may adhere at some points
called chiasmata. This process is called
crossing over, resulting in formation of
non-parental chromosomes.
The bivalents migrate to and line up along the
equatorial plane of the cell.
Members of homologous chromosomes (i.e. non-sister chromatids) move to opposite poles of
the cell leaded by spindle fibres contraction. The manner of migration of paternal and
maternal chromosomes are in random. This is known as independent assortment of
maternal and paternal chromosomes. This results in formation of different variety of
gametes which will be formed at the end of meiosis II. In anaphase I, sister chromatids of the
same chromosomes do not separate and they go towards the same pole.
Anaphase II
It involves separation of sister
chromatids.
Significance of meiosis
Meiosis results in formation of haploid gametes, each of which contain
half the chromosome number of the parent cells which are diploid. After fertilization with
another haploid gamete, it gives rise to a diploid zygote, therefore restoring the
chromosome number of the parent cell.
[1]
Therefore, meiosis ensures that the chromosome number remains constant throughout
generations.
[2]
Variation occurs during meiosis due to
(a)
Crossing over in prophase I results in formation of non-parental
chromosomes.
Independent assortment in parental and maternal chromosomes results in
formation of a great variety of gametes.
(b)
Subjected to natural selection, disadvantageous variations are eliminated. The
advantageous variations are adaptive to change in environment and is therefore
able to survive and reproduce. Through a number of generations, a species which
is completely different from the ancestor may form.
This is the basis of evolution.