Mitosis in an Animal Cell

The word mitosis comes from a Greek word ‘mitos’ which means thread, and refers to the threadlike appearance of chromosomes during this period. Mitosis was first studied by Walter Flemming in animals and by Strassburger in plants.

Mitosis can be defined as “The division of the cell in such a manner that the chromosomes are duplicated and distributed equally to the daughter cells.” The process of mitosis reproduces cells and distributes equal DNA to each daughter cells. Thus, mitosis is a qualitative division of the cell. It takes place in the somatic (body) cells. The process of cell division can be divided into two main phases: karyokinesis and cytokinesis.

 

Karyokinesis: Division of Nucleus

Traditionally karyokinesis is studied in four stages, but actually it’s a continuous process.

1. Prophase (in Greek Pro means ‘before’)

2. Metaphase (in Greek Meta means ‘after’)

3. Anaphase (in Greek Ana means ‘again’)

4. Telophase (In Greek Telo means ‘end’)

 

Interphase (Preparing the scene)

Interphase (inter means ‘between’) is the phase between cell division. By G2 stage the cell has doubled much of its cell contents. The cytoplasm contains two microtubule organization centers (MTOCs) each with a pair of centriole.

Mitosis in an Animal Cell

The chromatin resembles interwoven fine threads. One or more nucleoli are present in the nucleus. Duplication of chromosomes take place, DNA replicates. Synthesis of RNA and proteins occur. The chromosomes are not visible under a light microscope, as they are still in the form of loosely packed chromatin.

 

Prophase: (Formation of Mitotic Apparatus)

Activities in the Nucleus: The chromosomes begin to shorten and thicken, coiling upon themselves a process called condensation. The condensation process continues throughout the prophase.

Nucleus disappears: The synthesis of rRNA ceases when that portion of the chromosome bearing rRNA genes, is condensed, as a result the nucleolus disappears.

Activities outside the nucleus: Early in the prophase, the two centriole pairs start to move apart, and continue to move until they reach the opposite poles of the cell establishing the bipolarity of the cell. Three sets of fibers i.e. microtubules originate from each pair of centriole. The microtubules are composed of protein tubulin and RNA. Two sets of microtubules (half spindle and spindle) compose the spindle. The microtubules do not interact with the chromosomes. They interdigtate with polar molecules from opposite poles.

Nuclear membrane disappears: During the formation of the spindle the nuclear membrane breaks down, and its components are reabsorbed into the endoplasmic reticulum.

Aster: When centrioles reach the poles of the cell, they radiate the third set of microtubules outward, thus bracing the centrioles against the cell membrane. This arrangement of microtubules is called aster. The function of the aster is probably mechanical (acting to stiffen the point of micro-tubular attachment during the later contraction of the spindle).

Mitotic Apparatus: The aster, spindle and centrioles are collective called mitotic apparatus.

Chromatids: At the beginning of the prophase the chromatin material is condensed and folded, as a result chromosomes appear as thin threads which range in length from 0.25µm to 50µm in length. By late prophase, each chromosome apparatus double. The two halves are called chromatids. Each pair of chromatids have a centromere. Specialized protein complexes called kinetochore having specific base arrangement develop on either side of each centromere. The second set of microtubules of spindle fibers are called kinetochore spindle fibers. The kinetochores of sister chromatids capture these fibers coming from opposite poles. Forces associated with spindle microtubules move the chromosomes toward center of the cell.

 

Metaphase: (Division of Centromeres)

The chromosomes move towards the equator of spindle, (halfway between the poles, the equatorial or metaphase plate). It is not a physical structure but an imaginary one. The kinetochore extends as spindle form fibers, the two poles of the spindle are attached to the kinetochore of centromere.

The centromeres of all the chromosomes are lined up on the metaphase plate. For each chromosome, the kinetochores of the two sister chromatids face opposite poles of the spindle. The microtubules attached to a particular chromatid come from one pole of the spindle, and those attached to its sister chromatid come from the opposite pole. At the end of the metaphase, the centromeres divide freeing the two sister chromatids from their attachment to one another. Centromere replication is simultaneous for all the chromosomes.

 

Anaphase: (separation of the sister chromatids)

The beginning of anaphase is marked by separation of the sister chromatids. Each sister chromatid now rapidly moves towards the poles, to which its microtubule is attached.

The poles move apart: The spindle fiber consists of microtubules which occur in the form of a pair. Each member of the pair is attached to opposite poles. The pair of microtubules of spindle fiber slide over one another, as a result the poles move apart and increase in length.

The centromeres move toward the poles: The kinetochore of the chromosomes are attached to the poles by half spindle fibers, so with the movement of pole apart, the chromosomes also move towards each pole. The spindle fiber do not get shorten by condensation. At the end of the poles the spindle fiber is broken down into its subunits by the action of enzymes. The tubulin from the spindle fiber is removed, as a result the spindle fiber becomes shorter and shorter, pulling the chromosomes closer to the pole of the cell. Anaphase is over when equivalent and complete collection of chromosomes have reached the two opposite poles of the cell.

 

Telophase: (reformation of nuclei)

At the beginning of the Telophase the two sets of chromosomes reach the opposite poles of the cell. The condensed and coiled chromosomes begin to recoil. They increase in their length and become thinner i.e. the chromosomes become like the chromosomes of interphase stage.

Disappearance of spindle: The spindle fiber break into tubulin subunits. Tubulins become part of the microtubules, which form the cytoskeleton.

Formation of the nuclear membrane: Each set of sister chromatids become surrounded by endoplasmic reticulum. This ER forms nuclear membrane around each set of chromosomes.

Appearance of nucleolus: The chromosomes in each set continue to uncoil. One of the early genes to regain expression are the genes for rRNA. The rRNA are synthesized. These newly formed rRNA form the nucleus. At the end of the Telophase the nuclear division is over.

 

Cytokinesis: The division of Cytoplasm

Nuclear division is over, but not the cell division. The new nuclei are still in the same cytoplasmic unit. Commonly nuclear division is followed by separation of the cytoplasm into two parts. This separation accomplished by pinching of the cell membrane when the astral microtubules send signals to equatorial region of the cell, where actin and myosin are activated which form the contractile ring. The pinching near the middle of an animal cell forma a cleavage furrow. The process of cytoplasmic division is called cytokinesis (Greek: cyto = cell, kinesis = movement). Cell organelles are distributed to the two daughter cells.