Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

Chapter 20 Cell Division

VIEWS: 11 PAGES: 59

									Lession 5 Cell
   Division
   细胞分裂
  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 prokaryotic cells
   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.
图3 有丝分裂
meiosis (减数分裂)




   图4 减数分裂
The importantance of
     cell division

Provides new cells

Grow and reproduce
Maintain orgamisms’bodies (replace)
Repair
Heredity and variation
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.
5
图6 酵母细胞裂殖周期
      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
Before meiosis   Chromosomes
                 replicated and
                 become shorter and
                 thicker
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.
Meiosis 2
减数分裂II
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.
     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) What would happen to the
                 number of chromosomes in the
crossing over and recombination,mutation
                 zygote if gametes were formed by
and diversity    mitotic cell division?
 Nondisjunction(不分离) and
chromosomal abnormality(畸变)
唐氏综合症
Differences between two cell divisions
             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?
See you
         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.
                 Mitosis
Cell divides to produce two identical
daughter cells
The new cells contain the same number
and kinds of chromosomes as their
parent cells       Where would you expect mitotic
Repeated mitotic cell division gives rise
                cell division to occur in human and
                in plant?
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
            After Mitosis
Cytoplasm of parent cell divides into two
equal parts
Plant cell
   Cytoplasm separated into two halves, new cell
    membranes and cell walls formed between the
    two newly formed nuclei
Animal cell
   Cytoplasm separates into two halves by
    constricting inwards between the two daughter
    nuclei
Plant Cells
Animal Cell
      Roles of Mitosis
Provides new cells
 For growth
 For repair
                      Do you think that the muscle
 For replace   cells, nerve cell, skin cells, liver
                cells and white blood cells in our
 Asexual reproduction
                   body contain the same genetic
                   materials? Why do these cells
                   have different structures and
                   functions?
                  Meiosis 2
Meiosis 2 is just like mitosis
No replication of DNA
between meiosis 1 and
meiosis 2.
Chromosomes line up
individually on the equator
of the spindle at metaphase.
At anaphase the
centromeres divide, splitting
the 2 chromatids.
The one-chromatid
chromosomes are pulled to
opposite poles.
       减数分裂的特点
   遗传物质只复制一次,细胞连续分裂两次,导致染
      色体数目减半;
   S期持续时间较长,进行部分DNA的复制;
   同源染色体在减数分裂期Ⅰ(MeiosisI)配对联会、
     基因重组;
   减数分裂同源染色体配对排列在中期板上,第一次
     分列时,同源染色体分开。
     减数分裂的意义

   有丝分裂确保世代间遗传的稳定性;
   增加变异机会,确保生物的多样性,增强生物
    适应环境变化的能力;
   减数分裂是生物有性生殖的基础,是生物遗
    传、生物进化和生物多样性的重要基础保证。
 Sexual Reproduction
Fusion of gametes during fertilization is
also a random process
Thus zygotes will contain different
combination of genetic materials
Cause genetic variation
Very important for evolution of species

								
To top