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; genome  halving of that genome during mitosis (M phase). mitosis  The period between M and S is called G 1 ; that between S and M is G 2.

 So, the cell cycle consists of: G 1 = growth and preparation of the chromosomes for replication;  S = synthesis of DNA [see DNA Replication] and duplication of the centrosome; DNA Replication centrosomeDNA Replication centrosome  G 2 = preparation for  M = mitosis. 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) kinases  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 in the cell cycle.

Stages of Cell Cycle  

 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  In a typical animal cell, mitosis can be divided into four principals stages 1. Prophase: 2. Metaphase: 3. Anaphase 4. Telophase: 5. 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.  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

Errors in mitosis  In 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.