Transcript 细胞生物学减数分裂-双语课件

Meiosis
减数分裂
Learning Objectives
• Definition of Meiosis
• Processes of Meiosis
• Significance of Meiosis
• 减数分裂(reduction division)是有性生
殖个体性成熟后，形成生殖细胞过程中
发生的一种特殊的细胞分裂方式。
• 减数分裂的主要特点是DNA复制1次，
细胞连续分裂两次，结果形成了染色体
数目减半的生殖细胞。
Phases of Meiosis
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First Division
Prophase I
Metaphase I
Anaphase I
Telophase I
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Second Division
Prophase II
Metaphase II
Anaphase II
Telophase II
减数分裂的过程
• 减数第一次分裂
前期Ⅰ
中期Ⅰ
后期Ⅰ
末期Ⅰ
• 减数第二次分裂
前期Ⅱ
中期Ⅱ
后期Ⅱ
末期Ⅱ
细线期
偶线期
粗线期
双线期
终变期
• First division :
• The result of this cell division is a
reduction of the number of
chromosomes per cell.
Interphase
• The DNA is replicated into two
identical copies, just as in mitosis.
During interphase, the DNA is not
visible. The only thing you could see
in a microscope is the nucleolus, an
area of intense RNA synthesis.
Prophase
is quite different to the mitotic prophase;
the chromosomes become visible.
They look very long, as they are not yet totally condensed.
Their ends are linked to one pole of the nucleus.
The chromosomes that belong to the same pair come next to
each other.
The chromosomes become thicker and shorter, as they
condense more and more. They are linked to each other in
pairs.
At this stage, the 2 chromatids of each chromosome become
visible.
The chromosomes then start to move away from each other,
but remain linked at points called chiasmata.
At this stage, some genetic material can be exchanged
between chromosomes : it is now that crossing-over may take
place.
• Metaphase I : the pairs of
chromosomes (tetrads) are situated
at the equator of the cell. At this
stage, the nuclear membrane has
disappeared. The centromeres are
orientated toward the poles of the
cell, and the chromosomes are fully
condensed.
• Anaphase I : the chromosomes (two
chromatids each) migrate toward
opposite poles. There is a separation
of chromosomes, and not of
chromatids as in mitosis. Each pole
will receive one set of chromosomes
of 2 chromatids.
• The telophase is very short, and
often mistaken with prophase II.
There is no time for the formation of
a new cell membrane, nor for
duplication of DNA. The next step
starts straight away.
Cell division II
• The second division conserves the
number of chromosomes but divides
the chromatids.
• Prophase II: it is very short.
Everything is ready. The two
centrioles migrate away from each
other, and a network of microtubules
forms in each daughter cell. The two
networks are parallel to each other,
perpendicular to the previous one.
• Metaphase II : the chromosomes are
situated on the equator. We
therefore see two equators
• Anaphase II : the chromatids,
although present at the first cell
division, only separate now. They
migrate toward the two opposite
poles of the cell.
• Telophase II: the reconstitution of
four daughter cells starts and
finishes.
减数分裂的意义
1. 产生单倍体,单倍体结合成二倍体,保持人类染色体
数目的相对稳定和人类各种形状在世代遗传中保持
相对稳定。
2. 同源染色体的分离是分离律的细胞学基础。
3. 非同源染色体之间可以随机组合进入同一生殖细胞,
是自由组合律的细胞学基础。
4. 位于同一条染色体上的许多基因都相互连锁,经过
减数分裂一起进入一个生殖细胞,同时同源染色体
间的部分交换产生新的连锁关系.这是连锁与互换
律的细胞学基础。
5. 为人类各种变异提供了细胞学 基础。
6. 减数分裂异常是导致染色体病的重要原因之一。
• Conclusion : in most cases, the division results in
spores or gametes. The cell has given rise to four
daughter cells, each with a different set of
chromosomes. Each has a set of chromosomes, and
will meet another set of chromosomes in another
cell in order to constitute a fertilised egg which
will give a fully working new individual (plant or
animal). Maybe you could think of all the different
processes that led to variation in the meiosis...
• how many chromosomes has the cell?
• how many pairs?
减数分裂与有丝分裂比较