F4 Biology teaching notes Ch 11: 2nd Term NE 22

11.3    Meiotic cell division (減數細胞分裂)(Book 2, p. 11-13)

Ø   occurs only in gamete-producing cells and results in the

formation of (2) gametes (配子).

Ø   reduces the chromosome number by (3) half. It gives rise to (4) four haploid (n) daughter cells which are genetically(5) different from the parent cell and among themselves.

A

How does meiotic cell division occur?(Book 2, p. 11-13)(CE-2006-I-B Q.8(b)

Ø   Meiotic cell division occurs after (6) interphase.

Ø   It involves two times of cell division. Each of them starts with nuclear division ((7) meiosis (減數分裂)), followed by cytoplasmic division.

n   First meiotic divisioninvolves the pairing and separation of (8) homologous chromosomes.

n   Second meiotic division involves the separation of the (9) sister chromatids of each chromosome.

Ø   Stages of first meiotic division:

First meiotic division 第一次減數分裂
Stage		Description
1.      Prophase I		l   Chromosomes shorten, thicken and become (10)visible. l   Members of each pair of (11) homologous chromosomes pair up. l   The (12) nuclear membrane breaks down.
2.      MetaphaseI		l   Homologous pairs line up along the (13) equator. l   (14) Spindle fibres attach to the chromosome.
3.      AnaphaseI		l   The spindle fibres contract. l   The two members of each (15) homologous pair separates and move to (16) the opposite poles of the cell. l   The (17) cytoplasm starts to divide.
4.      TelophaseI		l   New (18) nuclear membrane form around each group of chromosomes.

Second meiotic division 第二次減數分裂
Stage		Description
1.      ProphaseII		l   The (19) nuclear membrane break down again.
2.      MetaphaseII		l   (20)The chromosomes line up along the equator. l   New (21) spindle fibres attach to the chromosomes.
3.      Anaphase II		l   The spindle fibres(22) contract l   The (23) sister chromatids separate and move to opposite poles of the cell. l   The cytoplasm starts to divide
4.      TelophaseII		l   The new nuclear membranes form around each group of chromosomes. l   The chromosomes uncoil and become (24) chromatins again.

B

What is the importance of meiotic cell division?(Book 2, p. 11-17)

1      Producing haploid gametes

n   Meiotic cell division produces (25) haploid gametes for (26) sexual reproduction (有性生殖).

n  When the male gamete and the female gamete fuse at fertilization (受精) to form a

(27) zygote (合子), the (28) diploid number of chromosome (2n) can be restored.

2      Producing genetic variations in gametes

n   (29) Genetic variation (遺傳變異) within a species increase the chance for the species to survive when environmental conditions change.

n   Genetic variations in gametes are produced by:

l   (30) Independent assortment (獨立分配) of chromosomes

l   (31) Crossing over (互換).

i)      Independent assortment of chromosomes

²   In meiosis, pairs of homologous chromosomes line up (32) randomly along the equator of the cell. The members of each homologous pair then separate independently of the other pairs of chromosomes. This is known as independent assortment or (33) random segregation (隨機分離) of chromosomes.

²  Chromosomes are allocated to each gamete in different combinations. As a result,the genetic compositions among the gametes and the parents are different even the genetic materials all come from the parents.

Therefore, (34) variation (變異) exists in the offspring.

ii)     Crossing over (互換)

²  is an exchange of the corresponding segments of (36) non-sister chromatids (非姊妹染色單體) in a pair of homologous chromosomes.

²  Exchange of genetic materials between the members of the homologous chromosomes leads to  new genetic combination

²  The process:

During (37) metaphase I, homologous chromosomes pair up. è

Non-sister chromatids wrap around each other, forming a (29) chiasma (交叉). è

At the chiasma, the (30) chromatids break, exchange and rejoin.