Transcript Mitosis

Mitosis

In biology, mitosis is the process by which a cell
separates its duplicated genome into two
identical halves. 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. Mitosis and cytokinesis together is
defined as the mitotic (M) phase of the cell
cycle, the division of the mother cell into two
daughter cells, each the genetic equivalent of
the parent cell.
 Mitosis
occurs exclusively in eukaryotic
cells. In multicellular organisms, the
somatic cells (all the body cells except the
reproductive (germ) cells) undergo mitosis,
 While germ cells - cells destined to
become sperm in males or ova in females
- divide by a related process called
meiosis

In a diploid (having two sets of chromosomes)
eukaryotic cell, there are two versions of each
chromosome, one from the mother and another
from the father. The two corresponding
chromosomes are called homologous
chromosomes.
 The pair of chromosomes in a diploid individual
that have the same overall genetic content. One
member of each homologous pair of
chromosomes is inherited from each parent.
 Homologous
chromosomes need not be
genetically identical. For example, a gene
for eye color at one locus (location) on the
father chromosome may code for green
eyes, while the same locus on the mother
chromosome may code for brown.
 When
DNA is replicated (duplicate copy of
similar data), each chromosome will make
an identical copy of itself. The copies are
called sister chromatids, and together they
are considered one chromosome. After
separation, however, each sister
chromatid is considered a full-fledged
chromosome by itself.
 The
mitotic phase is a relatively short
action-packed period of the cell cycle. It
alternates with the much longer
interphase, where the cell prepares itself
for division. Interphase is divided into three
phases, G1 (first gap), S (synthesis), and
G2 (second gap). During all three phases,
the cell grows by producing proteins and
cytoplasmic organelles.
A
eukaryotic cell cannot divide into two,
the two into four, etc. unless two
processes alternate: doubling of its
genome (DNA) in S phase (synthesis
phase) of the cell cycle;
 halving of that genome during mitosis (M
phase).
 The period between M and S is called G1;
that between S and M is G2.
 So,
the cell cycle consists of: G1 = growth
and preparation of the chromosomes for
replication;
 S = synthesis of DNA [see DNA
Replication] and duplication of the
centrosome;
 G2 = preparation for
 M = mitosis.
 When
a cell is in any phase of the cell
cycle other than mitosis, it is often said to
be in interphase.
 Cyclins
 Their
levels in the cell rise and fall with the
stages of the cell cycle.
 Cyclin-dependent kinases (Cdks)
 Their levels in the cell remain fairly stable,
but each must bind the appropriate cyclin
(whose levels fluctuate) in order to be
activated.
 They add phosphate groups to a variety of
protein substrates that control processes
Stages of Cell Cycle
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G1- where the cell makes necessary
preparations for initiation of DNA synthesis.
Requires 30-50% of time taken by entire cell
cycle. In mammalian cells-4-9 hrs.
S- DNA synthesis-30-40%, 5-7 hrs.
G2- cell makes preparations for initiation of
mitosis. Has high energy requirements to carry
mitosis, 10-20%.
Mitotic- shortest lasting less than 1 hr.
Recent evidences are Cdk along with cyclins are
control switches for cell cycle.
Interphase
 The
cell is engaged in metabolic activity
and performing its prepare for mitosis.
 Chromosomes
are not clearly discerned in
the nucleus, although a dark spot called
the nucleolus may be visible.
 The
cell may contain a pair of centrioles .
Interphase

Interphase
stages
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1.
2.
3.
4.
5.
In a typical animal cell, mitosis can be
divided into four principals stages
Prophase:
Metaphase:
Anaphase
Telophase:
Cytokinesis
Prophase:
 The
chromatin, diffuse in interphase,
condenses into chromosomes.
 Each
chromosome has duplicated and
now consists of two sister chromatids.

At the end of prophase, the nuclear
envelope breaks down into vesicles.
Prophase

Prophase
Metaphase:
 The
chromosomes align at the equitorial
plate and are held in place by
microtubules attached to the mitotic
spindle and to part of the centromere.
 Chromosomes interact with hollow tubular
filaments known as microtubules, which
become organized into a spindle and then
pull the chromosomes.
Metaphase

Metaphase
Anaphase:
 The

centromeres divide.
Sister chromatids separate and move
toward the corresponding poles.
Anaphase

Anaphase
Telophase:
 Daughter
chromosomes arrive at the poles
and the microtubules disappear.
 The
condensed chromatin expands and
the nuclear envelope reappears.
Telophase

Telophase
Cytokinesis
 The
cytoplasm divides, the cell membrane
pinches inward ultimately producing two
daughter cells
Cytokinesis

Cytokinesis
 In
Errors in mitosis
non-disjunction, a chromosome may fail to
separate during anaphase. One daughter cell
will receive both sister chromosomes and the
other will receive none. This results in the
former cell having three chromosomes coding
for the same thing (two sisters and a
homologous), a condition known as trisomy,
and the latter cell having only one chromosome
(the homologous chromosome), a condition
known as monosomy. These cells are
considered aneuploidic cells.
 Aneuploidy can cause cancer.
 An
arm of the chromosome may be broken and
the fragment lost, causing deletion.
 The fragment may incorrectly reattach to
another, non-homologous chromosome,
causing translocation.
 It may reattach back to the original
chromosome, but in reverse orientation,
causing inversion.
 An
arm of the chromosome may be broken and
the fragment lost, causing deletion.
 Duplication
occurs when a gene sequence is
repeated in excess of the normal amount.
A
translocation occurs when a part of one
chromosome is transferred to another
nonhomologous chromosome.
 Most translocations are reciprocal.
 An
inversion alters the position and sequence
of the genes so that gene order is reversed
within the chromosome.