勁急20點!!! Meiosis and Mitosis

MEIOSIS
In biology, meiosis is the process by which one diploid eukaryotic cell divides to generate haploid cells often called gametes. Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes, including single-celled organisms that reproduce sexually.

MITOSIS
Mitosis is the process by which a cell duplicates its genetic information (DNA), in order to generate two, identical, daughter cells. It is generally followed immediately by cytokinesis which divides the cytoplasm and cell membrane. This results in two identical daughter cells with a roughly equal distribution of organelles and other cellular components. The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.

SEXUAL REPRODUCTION
Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring. It is characterized by two processes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve genetic recombination.

ASEXUAL REPRODUCTION
Asexual reproduction is a form of reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea, bacteria, and protists. Many plants and fungi reproduce mostly asexually as well. While all prokaryotes reproduce asexually (without the formation and fusion of gametes), mechanism for lateral gene transfer such as conjugation, transformation and transduction are sometimes likened to sexual reproduction.

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Mitosis係有絲分裂:
係一個prcess 去produce 2個cell,,2個都有一樣數目同埋kinds 既chromosomes

主要有幾個步驟..
1.resting stage [[interphase (分裂期間)]]
姐係個chromosome既複製(relication)

stage1:
1,,chromosomes become thicker & shorter ***chromosomes can not seen clearly***
2,,each chromosomes and its copy are seen as 2 chromatids joined together
3..nuclear membrane begins to disapper

stage2
1..chromatids(染色單體p lie up individually at middle plane of cell

stage3
1. chromosomes and theirｃｏｐｉｅｓseparate as indicidual daughter chromosomes and move apart towards the OPPSIRE ends of the cell.

stahe4
1. each daughter nuclieus has the same number and kinds of the chromosomes as the parent cell.

******daughte cell姐係分左出黎果2個cell, parent cell 姐係分佢地出黎果個cell


Meiosis係減數分裂
多數係sexual reproduction(有性生殖), 有involoves 2個gametes既融合嫁!
點解既個咁重要呢,,因為如果2個gametes融左一齊,以1個人有23 choromosomes為例
以mitosis分落去, 個cell 既chromosomes數目會變成46, 長此下去,chromosomes數目會倍增嫁!

所以meiosis就係一個nuclear division 用黎reduce chromosomes number,佢有係2個successice division嫁!
1. 2 pairs of homologous chromosomes become thicker and shorter, to form rod-like structure.
2. pairing of homologous(同源) pairs, lie together side by side.
3.replication of chromosomes before division
4. homologous chromosomes arrange in pairs at the middle plane of cell
5.nuclear membrane disapper
6.members of the homologous chromosomes move apart to the opposie end os cell.
7.cytoplasm divide to form two daughter cell.
-----------first meiotis division-----------
8. chromatids lie at the middlw plane of cell
9.chromatids sparate from each others as individual daughter chromosomes and moves opposite end of cell
--------second meiotic division-----------------
10. 4 gametes formed
**chromosomes are reduced by half in each gamete)

**2者重要分別係chromosome度
mitosis chromosome 係同parent cell 一樣
meiosis chromosome 係parentcell 一半


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asexual reproduction 係無性生殖
係由1個parent produce offspring, WITHOUT gametes 既融合
而且係遺傳上係相同既~~
例如有二分裂(binary fission) ,, ie. amoeba (divides by mitosis)
呢個多數都係bacteria度有

仲有vegetative propagation (營養繁殖), 係flowering plants.
佢地係vegetative organs 由parent plant body 不斷生長變成一個new plants既

sexual reproduction 係有性生殖
係有gametes 既融合, undergo meiosis
會有fertilization (受精作用), produce 一個合子, 係genetic 亦同parent唔同
咁係可以有variation (變異)同埋增加後代既適應力。






*************如果仲2唔明,,可以e-貓問我,,我都係今日會考生黎嫁

2007-04-04 23:27:53 補充：
來源黎自--自己 bio書 pastpaper marking 撈埋一齊既bio知識

mitosis vs meiosis
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
呢度有成個flash講解有乜唔同，有animation，好清楚。

希望幫到你!! ^_^