MitosisMitosis

In biology, In biology, mitosismitosis is the process by which a cell is the process by which a cell separates its duplicated genome into two separates its duplicated genome into two identical halves. It is generally followed identical halves. It is generally followed immediately by cytokinesis which divides the immediately by cytokinesis which divides the cytoplasm and cell membrane. This results in cytoplasm and cell membrane. This results in two identical daughter cells with a roughly equal two identical daughter cells with a roughly equal distribution of organelles and other cellular distribution of organelles and other cellular components. Mitosis and cytokinesis together is components. Mitosis and cytokinesis together is defined as the defined as the mitotic (M) phasemitotic (M) phase of the cell of the cell cycle, the division of the mother cell into two cycle, the division of the mother cell into two daughter cells, each the genetic equivalent of daughter cells, each the genetic equivalent of the parent cell.the parent cell.

Mitosis occurs exclusively in eukaryotic Mitosis occurs exclusively in eukaryotic cells. In multicellular organisms, the cells. In multicellular organisms, the somatic cells (all the body cells except the somatic cells (all the body cells except the reproductive (germ) cells) undergo mitosis, reproductive (germ) cells) undergo mitosis,

While germ cells - cells destined to While germ cells - cells destined to become sperm in males or ova in females become sperm in males or ova in females - divide by a related process called - divide by a related process called meiosis meiosis

In a diploid (having two sets of chromosomes) In a diploid (having two sets of chromosomes) eukaryotic cell, there are two versions of each eukaryotic cell, there are two versions of each chromosome, one from the mother and another chromosome, one from the mother and another from the father. The two corresponding from the father. The two corresponding chromosomes are called chromosomes are called homologous homologous chromosomeschromosomes. .

The pair of chromosomes in a diploid individual The pair of chromosomes in a diploid individual that have the same overall genetic content. One that have the same overall genetic content. One member of each homologous pair of member of each homologous pair of chromosomes is inherited from each parent.chromosomes is inherited from each parent.

Homologous chromosomes need not be Homologous chromosomes need not be genetically identical. For example, a gene genetically identical. For example, a gene for eye color at one locus (location) on the for eye color at one locus (location) on the father chromosome may code for green father chromosome may code for green eyes, while the same locus on the mother eyes, while the same locus on the mother chromosome may code for brown. chromosome may code for brown.

When DNA is replicated (duplicate copy of When DNA is replicated (duplicate copy of similar data), each chromosome will make similar data), each chromosome will make an identical copy of itself. The copies are an identical copy of itself. The copies are called called sister chromatidssister chromatids, and together they , and together they are considered one chromosome. After are considered one chromosome. After separation, however, each sister separation, however, each sister chromatid is considered a full-fledged chromatid is considered a full-fledged chromosome by itself. chromosome by itself.

The mitotic phase is a relatively short The mitotic phase is a relatively short action-packed period of the cell cycle. It action-packed period of the cell cycle. It alternates with the much longer alternates with the much longer interphaseinterphase, where the cell prepares itself , where the cell prepares itself for division. Interphase is divided into three for division. Interphase is divided into three phases, G1 (first gap), S (synthesis), and phases, G1 (first gap), S (synthesis), and G2 (second gap). During all three phases, G2 (second gap). During all three phases, the cell grows by producing proteins and the cell grows by producing proteins and cytoplasmic organelles. cytoplasmic organelles.

A eukaryotic cell cannot divide into two, the A eukaryotic cell cannot divide into two, the two into four, etc. unless two processes two into four, etc. unless two processes alternate: alternate: doublingdoubling of its of its genome ( (DNADNA) in ) in S phaseS phase (synthesis phase) of the cell cycle; (synthesis phase) of the cell cycle;

halvinghalving of that genome during of that genome during mitosis ( (M M phasephase). ).

The period between M and S is called The period between M and S is called GG11; ;

that between S and M is that between S and M is GG22. .

So, the cell cycle consists of: So, the cell cycle consists of: GG11 = growth = growth

and preparation of the chromosomes for and preparation of the chromosomes for replication; replication;

SS = synthesis of DNA [see = synthesis of DNA [see DNA Replication] and duplication of the ] and duplication of the centrosome; ;

GG22 = preparation for = preparation for

MM = = mitosis. .

When a cell is in any phase of the cell When a cell is in any phase of the cell cycle other than mitosis, it is often said to cycle other than mitosis, it is often said to be in be in interphaseinterphase. .

CyclinsCyclins Their levels in the cell rise and fall with the stages Their levels in the cell rise and fall with the stages

of the cell cycle. of the cell cycle. Cyclin-dependent Cyclin-dependent kinases (Cdks) (Cdks) Their levels in the cell remain fairly stable, but Their levels in the cell remain fairly stable, but

each must bind the appropriate cyclin (whose each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. levels fluctuate) in order to be activated.

They add phosphate groups to a variety of They add phosphate groups to a variety of protein substrates that control processes in the protein substrates that control processes in the cell cycle. cell cycle.

Stages of Cell CycleStages of Cell Cycle

G1- where the cell makes necessary G1- where the cell makes necessary preparations for initiation of DNA synthesis. preparations for initiation of DNA synthesis. Requires 30-50% of time taken by entire cell Requires 30-50% of time taken by entire cell cycle. In mammalian cells-4-9 hrs.cycle. In mammalian cells-4-9 hrs.

S- DNA synthesis-30-40%, 5-7 hrs.S- DNA synthesis-30-40%, 5-7 hrs. G2- cell makes preparations for initiation of G2- cell makes preparations for initiation of

mitosis. Has high energy requirements to carry mitosis. Has high energy requirements to carry mitosis, 10-20%.mitosis, 10-20%.

Mitotic- shortest lasting less than 1 hr.Mitotic- shortest lasting less than 1 hr. Recent evidences are Cdk along with cyclins are Recent evidences are Cdk along with cyclins are

control switches for cell cycle.control switches for cell cycle.

InterphaseInterphase The cell is engaged in metabolic activity The cell is engaged in metabolic activity

and performing its prepare for mitosis. and performing its prepare for mitosis.

Chromosomes are not clearly discerned in Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleus, although a dark spot called the nucleolus may be visible. the nucleolus may be visible.

The cell may contain a pair of centrioles .The cell may contain a pair of centrioles .

InterphaseInterphase

Interphase

               

stagesstages

In a typical animal cell, mitosis can be In a typical animal cell, mitosis can be divided into four principals stages divided into four principals stages

1.1. ProphaseProphase: :

2.2. MetaphaseMetaphase: :

3.3. AnaphaseAnaphase

4.4. TelophaseTelophase: :

5.5. CytokinesisCytokinesis

ProphaseProphase: :

The chromatin, diffuse in interphase, The chromatin, diffuse in interphase, condenses into chromosomes. condenses into chromosomes.

Each chromosome has duplicated and Each chromosome has duplicated and now consists of two sister chromatids.now consists of two sister chromatids.

At the end of prophase, the nuclear At the end of prophase, the nuclear envelope breaks down into vesicles.envelope breaks down into vesicles.

ProphaseProphase

Prophase

               

MetaphaseMetaphase::

The chromosomes align at the equitorial The chromosomes align at the equitorial plate and are held in place by plate and are held in place by microtubules attached to the mitotic microtubules attached to the mitotic spindle and to part of the centromere. spindle and to part of the centromere.

Chromosomes interact with hollow tubular Chromosomes interact with hollow tubular filaments known as filaments known as microtubulesmicrotubules, which , which become organized into a spindle and then become organized into a spindle and then pull the chromosomes.pull the chromosomes.

MetaphaseMetaphase

Metaphase

               

AnaphaseAnaphase::

The centromeres divide.The centromeres divide.

Sister chromatids separate and move Sister chromatids separate and move toward the corresponding poles. toward the corresponding poles.    

AnaphaseAnaphase

Anaphase

               

TelophaseTelophase::

Daughter chromosomes arrive at the poles Daughter chromosomes arrive at the poles and the microtubules disappear. and the microtubules disappear.

The condensed chromatin expands and The condensed chromatin expands and the nuclear envelope reappears. the nuclear envelope reappears.

TelophaseTelophase

Telophase

               

CytokinesisCytokinesis

The cytoplasm divides, the cell membrane The cytoplasm divides, the cell membrane pinches inward ultimately producing two pinches inward ultimately producing two daughter cells daughter cells

CytokinesisCytokinesis

Cytokinesis

               

Errors in mitosis Errors in mitosis In In non-disjunctionnon-disjunction, a chromosome may fail to , a chromosome may fail to

separate during anaphase. One daughter cell separate during anaphase. One daughter cell will receive both sister chromosomes and the will receive both sister chromosomes and the other will receive none. This results in the other will receive none. This results in the former cell having three chromosomes coding former cell having three chromosomes coding for the same thing (two sisters and a for the same thing (two sisters and a homologous), a condition known as homologous), a condition known as trisomytrisomy, , and the latter cell having only one chromosome and the latter cell having only one chromosome (the homologous chromosome), a condition (the homologous chromosome), a condition known as known as monosomymonosomy. These cells are . These cells are considered considered aneuploidicaneuploidic cells. cells.

Aneuploidy can cause cancer.Aneuploidy can cause cancer.

An arm of the chromosome may be broken and An arm of the chromosome may be broken and the fragment lost, causing the fragment lost, causing deletiondeletion. .

The fragment may incorrectly reattach to The fragment may incorrectly reattach to another, non-homologous chromosome, another, non-homologous chromosome, causing causing translocationtranslocation. .

It may reattach back to the original It may reattach back to the original chromosome, but in reverse orientation, chromosome, but in reverse orientation, causing causing inversioninversion. .

An arm of the chromosome may be broken and An arm of the chromosome may be broken and the fragment lost, causing the fragment lost, causing deletiondeletion. .

Duplication occurs when a gene sequence is Duplication occurs when a gene sequence is repeated in excess of the normal amount. repeated in excess of the normal amount.

A translocation occurs when a part of one A translocation occurs when a part of one chromosome is transferred to another chromosome is transferred to another nonhomologous chromosome. nonhomologous chromosome.

Most translocations are reciprocal.Most translocations are reciprocal.

An inversion alters the position and sequence An inversion alters the position and sequence of the genes so that gene order is reversed of the genes so that gene order is reversed within the chromosome.within the chromosome.