Download Note 7

South Tuen Mun Government Secondary School
Biology Revision Note 7
Chromosome are made up of DNA and protein. They occur as homologous pair in the nucleus.
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A homologous pair of chromosomes has the same length, has the same gene at the corresponding position
and the genes are arranged in the same order. However, the two chromosomes in a homologous pair are
not identical.
When a cell has chromosomes all in a pair, it has two sets of chromosomes, one set comes from the mother
and the other set from father. The cell is described as diploid (2n). If a cell has only one set of
chromosomes, it is described as haploid (n).
Mitotic cell division includes mitosis and cytoplasmic division
Mitosis – division of the nucleus in mitotic cell division
Before mitotic cell division, DNA replication takes place.
Interphase
In this stage, the cell is active, DNA replicates at interphase before the mitotic cell division. The chromosomes
are thin and are not visible.
Prophase – The nuclear membrane disappears. The chromosomes become fat and short (= condenses). They
become visible as threads under the microscope.
Metaphase – chromosomes are lined up at the equator of the nucleus.
Anaphase – chromosomes separate and go to the opposite poles.
Telophase – chromosomes reach the opposite poles and nuclear membrane is made. Cytoplasmic division
starts.
Significance / importance of mitotic cell division – to produce identical cells for growth, repair and asexual
reproduction
Meiotic cell division / reduction division
Before meiotic cell division I, DNA replication takes place.
(i) Meiosis I
Prophase I – The nuclear membrane disappears. The chromosomes become fat and short. They become visible
as threads under the microscope. Homologous chromosomes pair up.
Metaphase I – homologous pair of chromosomes are lined up at the equator.
Anaphase I – homologous pair of chromosomes separate and go to the opposite poles.
Telophase I – each member of the homologous pair of chromosomes reach the opposite poles and nuclear
membrane is made. Cytoplasmic division starts.
(Note : The two new cells have only one set of chromosomes, they are haploid cells.)
(ii) Meiosis II (one of the two daughter cells in meiotic division I is taken as an example)
Prophase II – the nuclear membrane disappears. The chromosomes become fat and short. They become visible
as threads under the microscope.
Metaphase II – the chromosomes are lined up at the equator.
Anaphase II – the chromosomes separate and go to the opposite poles.
Telophase II – the chromosomes reach the opposite poles and nuclear membrane is made. Cytoplasmic
division starts.
Significance / importance of meiotic cell division:
 There are two divisions in one meiotic cell division. One mother cell makes 4 new daughter cells.
 The chromosome number of the daughter cells is half the number of the mother cell. The daughter cells
have one set of chromosomes, they are haploid (n) cells.
 The haploid cells are sex cells, gametes. When the male gamete fuses with the female gamete in
fertilization, a diploid (2n) fertilized egg / zygote is formed. The original diploid number of chromosomes
is restored. Meiotic cell division is important in sexual reproduction.
 Meiotic cell division produces genetic variation in the daughter cells by independent assortment, as shown
below:
Differences between mitotic cell division and meiotic cell division:
Mitotic cell division
Meiotic cell division
One mother cell divides once to make daughter cells.
One mother cell divides twice to make 4 daughter cells
No homologous pairing in mitosis.
Homologous chromosome pairing in meiosis leading to
independent assortment  genetic variation of
daughter cells
Daughter cells are genetically identical to mother cell
and to each other.
Daughter cells are genetically different with the mother
cell and with each other.
Mitotic cell division is used for growth and repair,
asexual reproduction.
Meiotic cell division produces haploid sex cells which
when fertilized during sexual reproduction, restore the
diploid chromosome number.