Cell Division & Cell Cycle by Rafique1956

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									Cell Division &
  Cell Cycle
    Dr. Muhammad Rafique
      Assistant Professor
•   Discuss the different type of Cell Division
•   Definition & Stages of Cell cycle
•   Define & discuss the Interphase
•   Define the terms related to cell division
•   Different Stages of Mitosis
•   Different stages of Meiosis I
•   Different stages of Meiosis II
             Types Of Cell Division
Two major types of cell
 Produce two cells that are
 genetically identical to the
 parental cell.
 Produce haploid gametes
 from a diploid parental cell.
 Gametes are genetically
 different from parent and
 each other.
                        Cell Cycle
All the cells in the body are
  derived from a single cell
  that from the fertilized egg
  or zygote by process of
  division. The process of
  cells division for cellular
  replication is called as
  Mitosis. When a single cell
  divides by the process of
  mitosis, will produce two
  identical cells. The life
  cycle of single cell is called
  as Cell cycle
               Before Cell Division
Before a cell can divide it has
 Grow in size,
 Duplicate its
 Separate the
   chromosomes for exact
   distribution between the
   two daughter cells.
These different processes are
   coordinated in the cell
                   Cell Cycle
Cell Cycle usually occurs
  in the somatic cells, the
  sequence of events are:
A.A Interphase
     1. G1
     2. S Phase
     3. G2
B.M Phase
G1: longer than the other
  phases occurs after M
  phase & Part of
Interphase refers to all stages
  of the cell cycle other than
During interphase:
Cellular organelles double in
DNA replicates
Protein synthesis occurs
The chromosomes are not
  visible and the DNA
  appears as uncoiled
                   A. Interphase
Ninety percent or more of the
  cell cycle is spent in
  interphase. During
  interphase, cellular
  organelles double in
  number, the DNA
  replicates, and protein
  synthesis occurs. The
  chromosomes are not
  visible and the DNA
  appears as uncoiled
              Stages of Interphase
Interphase is divided into the
    following stages:
1. G1 phase: During G1
    phase, the period that
    immediately follows cell
2. Cell grows
3. Differentiates
New organelles are made but
    the chromosomes have
    not yet replicated in
    preparation for cell
                   G0 Phase

During the G1 phase,
 cells respond to
 extracellular signals
 by advancing
 towards another
 division or
 withdrawing from
 the cycle into a
 resting state G0
 example is Nerve
                  S phase

DNA synthesis
 occurs during S
 phase. The
 replicate in
 preparation for cell
Replication of DNA by Complementary
             Base Pairing
  Duplicated Vs Unduplicated Chromosomes
Chromosomes either have one or two
  molecules of DNA plus associated
  proteins. A chromosome with one
  molecule of DNA is called an
  unduplicated chromosome because
  it only contains one molecule of
  DNA. A duplicated chromosome
  contains two identical daughter
  DNA molecules that have come
  from an original DNA molecule. In
  the case of a duplicated
  chromosome, each molecule of
  DNA and associated proteins is
  called a sister
       Chromatid vs. Chromosome
When two DNA molecules are
 joined together, each
 molecule is called a
 chromatid and the two of the
 molecules are called a
 duplicated chromosome.
 When a DNA molecule (and
 proteins) is not attached to
 another one then that single
 molecule of DNA is not a
 chromatid but an
 unduplicated chromosome.
• When a chromosome is
  examined during mitosis or
  meiosis there is a pinched in
  region somewhere along the
  length of the chromosome
  called the centromere. The
  centromere is a region to which
  the spindle fibers attach to the
  chromosome and it is constant
  for different types of
  chromosomes. The centromere
  also contains a small ring of
  protein called a kinetochore
  which is important in the
  movement of chromosomes
  during mitosis and meiosis.
The ends of the chromosome
  in eukaryotes are called
  telomeres. This region is
  important because during
  DNA replication, the
  telomere does not always
  get duplicated properly and
  the chromosome shortens               Centromere
  repetitive, noncoding base
  sequences found at the ends
  of the DNA strands in          Telomere
  eukaryotic cells.
During certain times of the
 cell's life cycle the
 chromosomes are not visible.
 This is because the
 chromosomes are stretched
 out very thin to allow
 surfaces for the various
 chemical reactions that
 involve chromosomes to take
 place. When the nucleus is
 stained and examined, it
 appears uniformly colored
 and the chromosomes
 collectively are termed
         Homologous chromosomes
Chromosomes come in
  identical looking pairs
  called homologous
  chromosomes. The bites of
  DNA along the
  chromosomes determine
  characteristics of the
  organism, say eye color,
  hair texture etc. These bits
  of DNA are called genes.
  Homologous chromosomes
  have the same general kind
  of gene along their length.
  Alternate forms of a gene
  are called alleles.
   Autosomes Vs Sex Chromosomes
• Some organisms such as birds and mammals have a
  chromosomal system of sex determination. For example in
  humans, individuals which have two X chromosomes (XX)
  typically develop into females and individuals which have
  an X chromosomes and a Y chromosome (XY) develop into
• Autosome refers to those chromosomes that are not
  involved in sex determination. Human diploid cells have 22
  pairs of autosomes and 1 pair of sex chromosomes.
• What is curious is the X and Y chromosomes are not strictly
  homologous. For example since the X chromosome is much
  bigger than the Y chromosome, most of the genes on the X
  chromosome do not have corresponding genes on the Y
             3. G2 phase

During G2 phase,
 molecules that
 will be required
 for cell
 replication are
                   M phase
M phase includes the
 overlapping processes
 of mitosis and
 cytokinesis. At the end
 of cytokinesis, the
 parent cell has formed
 its two G1 phase
 daughter cells and the
 cell is ready to repeat
 the cycle
               Stages of Mitosis
Mitosis is divided into
 five stages: prophase,
 metaphase, anaphase,
 and telophase.
 Cytokinesis usually
 begins during anaphase
 and ends at a point after
 the completion of
               Mitosis - Prophase
During the first stage of
 mitosis, prophase, the
 chromatin condenses
 and the chromosomes
 become visible. Also
 the nucleolus
 disappears, the nuclear
 membrane fragments,
 and spindle fibers are
          Mitosis - Metaphase
During metaphase, the
 nuclear membrane
 fragmention is
 complete and the
 chromosomes line up
 along the cell's
             Mitosis - Anaphase
During anaphase,
 diploid sets of
 chromosomes move
 toward opposite
 poles of the cell and
 cleavage) begins.
           Mitosis - Telophase
During telophase, the
 nuclear membrane
 and nucleoli reform,
 cytokinesis is nearly
 complete, and the
 eventually uncoil to
Meiosis, a type of nuclear division,
  occurs only in reproductive
  cells and results in a diploid cell
  (having two sets of
  chromosomes) giving rise to
  four haploid cells (having a
  single set of chromosomes).
  Each haploid cell can
  subsequently fuse with a gamete
  of the opposite sex during
  sexual reproduction.
Consists of two cell division
Meiosis I
Meiosis II
                          Meiosis I
• Meiosis I refers to the first
  of the two divisions and is
  often called the reduction
  division. This is because it
  is here that the
  chromosome complement
  is reduced from diploid
  (two copies) to haploid
  (one copy). Interphase in
  meiosis is identical to
  interphase in mitosis.
  Meiotic division will only
  occur in cells associated
  with male or female sex
                   Stages of Meiosis
•   Stages of Meiosis I are
•   Prophase I
•   Metaphase I
•   Telephase I
•   Anaphase I
•   Stages of Meiosis I are
•   Prophase I
•   Metaphase I
•   Telephase I
•   Anaphase I
Different Stages of Meioses
                   crossing over
• A process that occurs during
  prophase I of meiosis in which
  genetic material from the
  chromatid of one chromosome
  exchanges places with the
  material from the same area of
  a chromatid on it's homolog.
  This process increases the
  variation in gametes produced
  by an individual. The images
  illustrate a homologous pair of
  chromosomes before (on the
  above) and after (on the below)
  crossing over has occurred.
                    Crossing over
Crossing over and
  independent assortment of
  the homologous
  chromosomes helps genetic
  variation. Crossing over is
  when chromatids (still in
  bivalent pairs) cross over,
  forming a chiasma. When
  the two formed gametes
  fuse at fertilization
  randomly, yet more
  variation is produced
  amongst the offspring
                  Prophase I
Prophase I is identical to
  prophase in mitosis,
  involving the
  appearance of the
  chromosomes, the
  development of the
  spindle apparatus, and
  the breakdown of the
  nuclear membrane.
                          Metaphase I
Metaphase I is where the critical
  difference occurs between
  meiosis and mitosis. In mitosis,
  all of the chromosomes line up
  on the metaphase plate in no
  particular order. In Metaphase I,
  the chromosome pairs are
  aligned on either side of the
  metaphase plate. It is during this
  alignment that the chromatid
  arms may overlap and
  temporarily fuse, resulting in
  what is called crossovers
              Anaphase I
During Anaphase I,
 the spindle fibers
 contract, pulling
 the homologous
 pairs away from
 each other and
 toward each pole
 of the cell.
                          Telophase I
• In Telophase I, a cleavage
  furrow typically forms,
  followed by cytokinesis, the
  changes that occur in the
  cytoplasm of a cell during
  nuclear division; but the nuclear
  membrane is usually not
  reformed, and the chromosomes
  do not disappear. At the end of
  Telophase I, each daughter cell
  has a single set of
  chromosomes, half the total
  number in the original cell, that
  is, while the original cell was
  diploid; the daughter cells are
  now haploid.
                    Meiosis II
Meiosis II is quite simply
 a mitotic division of
 each of the haploid cells
 produced in Meiosis I.
 There is no Interphase
 between Meiosis I and
 Meiosis II, and the
 latter begins with
 Prophase II.
              Prophase II
Prophase II. At this
 stage, a new set of
 spindle fibers
 forms and the
 begin to move
 toward the equator
 of the cell.
            Metaphase II
Metaphase II, all
 of the
 chromosomes in
 the two cells
 align with the
 metaphase plate.
            Anaphase II
In Anaphase II,
  the centromeres
  split, and the
  spindle fibers
  shorten, drawing
  the chromosomes
  toward each pole
  of the cell.
                  Telophase II
In Telophase II, a
  cleavage furrow
  develops, followed by
  cytokinesis and the
  formation of the nuclear
  membrane. The
  chromosomes begin to
  fade and are replaced
  by the granular
  chromatin, a
  characteristic of
                       Meiosis II
When Meiosis II is complete,
 there will be a total of four
 daughter cells, each with half
 the total number of
 chromosomes as the original
 cell. In the case of male
 structures, all four cells will
 eventually develop into sperm
 cells. In the case of the female
 life cycles in higher organisms,
 three of the cells will typically
 abort, leaving a single cell to
 develop into an egg cell, which
 is much larger than a sperm cell.
Differences between male and Female

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