Main contents • Introduction • The importantance of cell division • The cell cycle • Meiosis • Nondisjunction and chromosomal abnormality
Introduction • Cell category （细胞类型）： non-nucleated prokaryoticcells nucleated eukaryotic cells • Cell division：The process in which a single cell divides into two separate cells，usually a small segment of a larger cell cycle • Cell division type （细胞分裂类型）： amitosis 无丝分裂 mitosis 有丝分裂 meiosis 减数分裂
Amitosis（无丝分裂） 图1 细菌裂殖
Mitosis（有丝分裂） Fig.2 the large yeast cell on the bottom is currently budding （芽殖）. The large cell in the top middle has already budded several times.
meiosis （减数分裂） 图4 减数分裂
The importantance of cell division • Grow and reproduce • Maintain orgamisms’bodies （replace） • Repair • Heredity and variation • Provides new cells
The cell cycle（细胞周期） • Interphase（间期）:G1,S,G2 and G0 • Cell division Some cells divide constantly: cells in the embryo, skin cells, gut lining cells, etc. Other cells divide rarely or never: only to replace themselves. Actively dividing cells go through a cycle of events that results in mitosis. Most of the cycle was called “interphase” by the microscopists who first studied cell division. During interphase the cell increases in size, but the chromosomes are invisible.
Meiosis（减数分裂） • Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. • Meiosis involves a reduction in the amount of genetic material. Meiosis comprises two successive nuclear divisions with only one round of DNA replication.
Meiosis • The first stage (meiosisI减数分裂I): • Separation of homologous chromosomes (同源染色体) • Result in the formation of 2 haploid (单倍体，having only half the original number of chromosomes of the parent) cells • The second stage (meiosisII减数分裂II): • Separation of chromatids（染色单体） • Result in 4 haploid daughter cells (gametes配子)
Interphase of Meiosis减数分裂间期 • Chromosomes replicated and become shorter and thicker • Before meiosis
7 7 前期I 中期I 后期I 末期I
ProphaseI of MeiosisI减数分裂I前期I • Synapsis ------ Each pair of homologous chromosomes lie side by side • 来自两个亲本的同源染色体双双配对，联结在一起，构成一对对较粗的染色体复合结构，称双价体或联会复合体，这种现象叫联会(synapse)。
ProphaseI of MeiosisI减数分裂I前期I • Each Chromosome is seen to be made up of two chromatids • Nuclear membrane soon disappears
MetaphaseI of MeiosisI减数分裂I中期I • The paired homologous chromosomes line up in the middle of the cell(equatorial plane) • Each chromosome is attached to spindle fibres
着丝粒 纺锤体极 着丝粒微管
AnaphaseI of MeiosisI减数分裂I后期I • Separation of pairs of homologous chromosomes • Spindle fibres contract • 2 members of each par of homologous chromosomes separate form each other (move to opposite ends) Independent assortment and Half the number
TelophaseI of MeiosisI减数分裂I末期I • Division of cytoplasm takes place • Formation of two haploid daughter nuclei
Attention to Meiosis 1 • crossing over（交换）in prophase, and the pairing of homologues in metaphase. • Crossing over. Homologues break at identical locations, then rejoin opposite partners creating new combinations（重组）of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.
Results of Meiosis 1 • Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each. • As a result of crossing over, each chromosome is the mixture of the original homologues.
ProphaseII of MeiosisII减数分裂II前期II • Start of second meiotic division • Each chromosome appears as a pair of chromatids
MetaphaseII of MeiosisII减数分裂II中期II • Chromosomes (consisting of 2 chromatids) line up in the middle of the cell
AnaphaseII of MeiosisII减数分裂II后期II • Spindle fibre contract • Each air of chromatids separate from each other (move to opposite ends)
TelophaseII of MeiosisII减数分裂II末期II • Each chromatid becomes a daughter chromosome • Nuclear membrane is then formed around each set of daughter chromosomes • 4 haploid cells
Summary of Meiosis • 2 cell divisions. • Start with 2 copies of each chromosome , each with 2 chromatids. • In meiosis 1, crossing over in prophase mixes alleles. • In metaphase I, homologues pair up, and in anaphase the homologues are separated into 2 cells. • Meiosis 2 is just like mitosis.
What would happen to the number of chromosomes in the zygote if gametes were formed by mitotic cell division? Roles of Meiosis减数分裂作用 • Heredity（遗传） • Produce haploid gametes with only half the number of chromosomes of body cells • Male and female gamete fuse together to form a zygote during fertilization. Normal diploid number chromosomes is restored • Evolution（进化） crossing over and recombination，mutation and diversity
Nondisjunction（不分离） and chromosomal abnormality（畸变）
Assigments • Types of cell divisions（细胞分裂类型） • Phases and their properties of meiosis1（减数分裂I的时期和特性）
Pair discussion • Why are different living individuals all called organisms ？ • How can living things grow and repair the damage？
Cancer cells • A cancer cell is a cell that grows out of control. Unlike normal cells, cancer cells ignore signals to stop dividing, to specialize, or to die and be shed. Growing in an uncontrollable manner and unable to recognize its own natural boundary, the cancer cells may spread to areas of the body where they do not belong.
In a cancer cell, several genes change (mutate) and the cell becomes defective. There are two general types of gene mutations. One type, dominant mutation, is caused by an abnormality in one gene in a pair. An example is a mutated gene that produces a defective protein that causes the growth-factor receptor on a cell's surface to be constantly "on" when, in fact, no growth factor is present. The second general type of mutation, recessive mutation, is characterized by both genes in the pair being damaged. For example, a normal gene called p53 produces a protein that turns "off" the cell cycle and thus helps to control cell growth.
Where would you expect mitotic cell division to occur in human and in plant? Mitosis • Cell divides to produce two identical daughter cells • The new cells contain the same number and kinds of chromosomes as their parent cells • Repeated mitotic cell division gives rise to a large number of cells containing identical genetic materials
Machinery of Mitosis • The chromosomes are pulled apart by the spindle, which is made of microtubules. The spindle fibers are attached to each centromere , and anchored on the other end to a centrosome . • There are 2 centrosomes, one at each end of the spindle. The chromosomes are lined up between the poles of the spindle. • When the spindle fibers contract, the chromosomes are pulled to the opposing poles. • The cell then divides to separate the two poles.
The process of Mitosis • Chromosome in the nucleus replicates itself • Chromosomes still exist as long, thin threads of chromatins • Not visible interphase
The Process of Mitosis • Chromosomes become shorter and thicker by coiling • Become visible under the microscope
The Process of Mitosis • Pair of chromatids • Held together at a point • Each chromatid carries an exact copy of genetic info. • Nuclear membrane disappear Prophase
The Process of Mitosis • Chromosomes line up in the middle of the cell • Each attached to thread-like structures called spindle fibres Metaphase
The Process of Mitosis • Spindle fibres contract • 2 chromatids in each chromosome separate from each other and move towards opposite ends of the cell (daughter chromosome) Anaphase
The Process of Mitosis • Daughter chromosomes extend into tiny threads (invisible) • Nuclear membrane is formed • 2 identical daughter cells Telophase
Summary of Mitosis • Prophase: • Chromosomes condense • Nuclear envelope disappears • centrosomes move to opposite sides of the cell • Spindle forms and attaches to centromeres on the chromosomes • Metaphase • Chromosomes lined up on equator of spindle • centrosomes at opposite ends of cell • Anaphase • Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes • Chromosomes pulled to opposite poles by the spindle • Telophase • Chromosomes de-condense • Nuclear envelope reappears • Cytokinesis: the cytoplasm is divided into 2 cells