Download 15 Detecting environmental conditions

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21 Cell division
21.1 How do cells divide? (Book 3, p.110)

Cells are able to undergo a process called (1)
cell
division
to give
new cells for growth or reproduction.

Cell division involves the passing of (2)
genetic
material
from the parent
cells to the daughter cells.

(3)

There are two types of cell division: (4)
Replication
(有絲細胞分裂) and
(5)
(複製) of
DNA molecules takes place before cell division.
mitotic
meiotic
cell
cell
division
division
(減數細胞分裂).
Chromosome (Book 3, p.110)

The genetic information of an organism is carried by (6)

DNA is present in the (7)

A DNA molecule coils up around some special proteins to form a (8)

When a cell is going to divide, each chromosome is seen to consist of two threads called
(9)
chromatids
nucleus
(染色單體) joined at
DNA
(deoxyribonucleic acid).
of a cell.
chromosome
.
a point.
one chromosome
DNA molecule
nucleus
cell
chromatid chromatid
special protein
 Structure of a chromosome
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
Chromosomes always exist in (1)

Humans have (2)
23
pairs
)
Date:
in the body cells.
pairs of chromosomes.
 A full set of human chromosomes (top) male and (bottom) female

A cell with two sets of chromosomes is said to be a (3)

A cell with only one set of chromosomes is said to be a (4)
diploid
(2n) (二倍體) cell.
haploid
(n) (單倍體) cell.
For example:
nucleus
nucleus
 Sperm (精子)
 Ovum (卵子)
E Mitotic cell division (Book 3, p.111)
Mitotic cell division starts with (5)
(6)
42
cytoplasmic
nuclear
division
division
which is followed by
.
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Mitosis (Book 3, p.111)

(1)
Mitosis
(有絲分裂)
is the nuclear
cell membrane
division in mitotic cell division.
nuclear membrane

During mitosis, the duplicate set of chromosomes
invisible chromosomes
is separated from the original set.

Each new nucleus receives (2)
one
complete set of chromosomes so that two
 DNA molecules are replicated just
before mitosis
identical nuclei are formed.

Mitosis consists of four main stages which occur continuously:
Stage 1
chromosomes

Chromosomes become visible.

The nuclear membrane (3)
disintegrates
.
Stage 2

The chromosomes line up at the (4)
middle
of the cell.
Stage 3

The (5)
chromatids
separate and move to the
opposite ends of the cells.
Stage 4

New (6)
nuclear
membranes
form
around each set of chromosomes.

The chromosomes become invisible again.
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Cytoplasmic division (Book 3, p.112)
The cytoplasm is divided into (1)
two
equal halves. Two identical cells are formed.
a
cell membrane constricts at the middle
b
new cell walls and cell membranes are formed
between the two nuclei
 Cytoplasmic division in (a) animal cells and (b) plant cells
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Practical 21.1 Examination of mitotic cell division
(Student’s Book 3 p.113; Activity Book 3 p.57)
E Meiotic cell division (Book 3, p.114)

Meiotic cell division includes (2)
(3)

(減數分裂)
followed by division of
.
Meiosis is a type of nuclear division which reduces the number of chromosomes by
(4)

cytoplasm
meiosis
half
.
Meiotic cell division consists of first meiotic division and second meiotic division.
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 Stages of meiotic cell division (only two pairs of homologous chromosomes are shown)
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Practical 21.2 Examination of meiotic cell division
(Student’s Book 3 p.114; Activity Book 3 p.59)
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Significance of cell division (Book 3, p.116)
Mitotic cell division (Book 3, p.116)
Mitotic cell division is important to produce new cells for:
1
(1)
growth
2
(2)
repair
3
(3)
asexual
;
;
reproduction
(無性生殖)
in some organisms.
 Mitotic cell division is
important for growth
Meiotic cell division (Book 3, p.116)
1
Meiosis reduces the number of chromosomes by (4)
half
. It ensures that the
gametes produced are haploid so that the diploid number of chromosomes can be
(5)
2
restored
(還原) at
fertilization.
The random distribution and (6)
independent
assortment
(獨立分配) of
homologous chromosomes produce gametes with different genetic combinations. This causes
(7)
genetic
variations
among individuals of the same species.
 Independent assortment of chromosomes in meiotic cell division
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 Practice question
Mr and Mrs Chan are infertile. They undergo in vitro fertilization (IVF) to help them to have their
own baby. The diagram below shows the process.
sperm
2
1
a
3
1 Ova are collected from the
ovaries by suctioning through
a long needle.

2 The ova from Mrs Chan and
the sperm from Mr Chan are
put in a Petri dish. Any
zygotes are allowed to
develop for two more days.

3 Several of the embryos are
placed into the uterus of Mrs
Chan.
Suggest one cause for infertility in men and in women respectively.
(2 marks)
In men: no or few sperm produced / malformation of sperm / low sperm activity. (1m)
In women: ovulation problem / blocked oviducts. (1m)
b
Before undergoing IVF, Mrs Chan is given hormones to stimulate the maturation of a
number of ova in her ovaries. Explain why it is done.
(2 marks)
So that more mature ova can be collected (1m) for fertilization and to increase the
pregnancy rate (1m).
c
According to the diagram, state three ways in which the IVF process differs from the
natural female reproductive process.
(6 marks)
In IVF, more than one follicle undergo maturation. In the natural process, only one
follicle matures every month. / In IVF, the ova have to be retrieved from the ovaries and
fertilization is carried out in test tubes or Petri dishes. In the natural process, the ovum is
released into the oviduct where fertilization takes place. / In IVF, more than one embryos
are formed and transferred to the uterus. In the natural process, normally only one embryo
is formed from the fertilized ovum. (2m x 3)
Total: 10 marks
－ END －
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