Cell cycle

Interphase

Interphase is termed as the portion of the cell cycle between divisions. It is considered as the preparatory period for cell division.

 Stages of interphase

There are three stages of interphase. The stages of interphase are:
The G1 phase or pre-DNA synthesis phase
It represent the time during the cell cycle in which cell prepare itself metabolically to go through DNA synthesis. This phase involves accumulation and synthesis of specific enzymes needed to control DNA synthesis and production of DNA base units so that the supply is at hand when synthesis begin.
The S- Phase or DNA-Synthesis Phase
During this portion of cell cycle the DNA molecules are actually duplicated. If preparatory events in G1are not completed, the S-Phase cannot occur.
The G2-Phase or Pre-Mitosis Phase
It is the time when cells are active metabolically and are preparing for mitosis. For example, energy storage for chromosome movement, the production of mitosis-specific proteins and RNA and formation of microtubule sub-units needed to make spindle fibres, take place.
Duration of interphase
The duration of time spent in interphase and in each stage of interphase is variable and depends on both the type of cell and the species of organism it belongs to. Most cells of adult mammals spend about 20 hours in interphase, this account for about 90% of the total time involved in cell division.





Diagram:

                                             MITOSIS

It is the type of cell division which ensures the same number of chromosomes in the daughter cells as that in the parent cells. In spite small differences major steps of mitosis in animals and plants are same.

 PREPROPHASE:

In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the centre of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division.

PROPHASE:

Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. Almost one third of mitosis is completed during this stage. In animals and lower plants the two pairs of centrioles migrate towads the opposite poles. But in higher plants centeriols are absent and the cytoplasmic microtubules organise themselves into a series of spindle fibers.

                  Chromatin material get condensed and chromes appear as thin thread(0.025-50 micrometers). Chromosomes become more and more thick ultimately each chromosome is visible. Towards the end of prophase nuclear envelope disappears and nuclear material is released in cytoplasm, nucleoli disappear. Mitotic apparatus is organized. Cytoplasm become more viscous.

                         

METAPHASE: 

Metaphase came from the Greek μετα meaning "after." The distinguishing event of metaphase stage is the region of each chromosome towards the equatorial plane. Equatorial plane is the midline region of the cell, a line which runs perpendicular to the polarity established by centiol and spindles. It is also referred as metaphase plate. Chromosomes align themselves on metaphase plate. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres), it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint.

    Anaphase:

It is the most criticle phase of the mitosis . It ensures equal distribution of chromatids in the daughter cells .The kinetochore fibers of spindle contract towards their respective poles. They exert force on the centromere and sister chromatids are seprated from centromere .As a result , half of the sister chromatids travel towards each pole .At the time polar microtubule elongates .

                   

Telophase:

The anaphase ends with the reaching of the chromosomes at opposite poles and telophase starts . The chromosomes unfold and decondense .They ultimately disappear as chromatin .Mitoitic apparatus disorganize . The nuclear membrane and nuclei reorganize. Thus two nuclei are formed at two poles of the cell.

                                                                                       

Significance of mitosis:

1) The hereditary material is equally distributed in daughter cells during mitosis.The crossing over does not takeplace during mitosis.Thus genetic information remains unchanged ensures the continuity of similar information from parent to daughter cells .                                                               

2) Asexual reproduction takes place by mitosis in some organisms.                                                            3) Regeneration , healing of wounds and replacement of older cells also takes place  by mitosis.                                                                                                                                                        4) The development and growth  of multicellular organisms depends on the orderly controlled mitosis.                                                                                                                                      5) Tissue culture and cloning also occur during mitosis.                              

                                         MEIOSIS I

Meiosis is the special type of cell division in which number of chromosomes in daughter cell are reduced to half,as compared to parent cell.It takes place in diploid cell only,in animals at the time of  gamete formation, while in plants when  spores are produced.Each diploid cell after meiosis produce 4 haploid cells, because it involves two consecutive divisions after single replication of DNA. The first meiotic division  is reduction divisions and second meiotic division is simple divisions.

Prophase 1:

This is very prolonged phase, and differ from prophase of mitosis because in which chromosomes behave as homologous pairs. These similar but not necessarily identical chromosomes are called as homologous chromosomes. Prophase 1 consists of following stages:

Leptotene:

The chromosomes become visible, shorten and thick. The size of the nucleus increases and homologous chromosomes start getting closer to each other.

Zygotene:

In this stage, the synapsis starts, which is the pairing of homologous chromosomes. This pairing is highly specific homologous chromosomes are paired but not fused , this complex structure is called as bivalent or tetrad.

.

Pachytene:

The pairing of homologous chromosomes is completed . Chromosomes become more and more thick . Each bivalent has four chromatids , which wrap around each other. Non-sister  chromatids of homologus chromosomes exchange their segments due to chiasmata formation, during the process called crossing over.In this way reshuffling of genetic material occurs which produces recombinations. Pachytene may lasts for days,weeks or even years , whereas leptotene and zygotene can last only for few hours.

Diplotene:

The paired chromosomes repel each other and begin to separate. Separation however, is not complete, because homologous chromosomes remain united by their point of interchange (chiasmata). Each bivalent has at least one such point, the chromatids otherwise are separated .

Diakinesis:

During this phase the condensation of chromosomes reaches to its maximum. At the same time separation of the homologus chromosomes (started during diplotene) is completed, but still they are united at one point , more oftenly at ends . Nucleoli disappear.

Metaphase I:

Nuclear membrane disorganize at the begning of this phase. Spindle fibres originate and the kinetochore fibres attach to the kinetochor of homologous chromosome from each pole and arrange bivalents at the equator. The sister chromatids of individual chromosome in bivalent behave as a unit.

 Anaphase I:

During anaphase 1, one half of each tetrad is pulled towards each pole of the dividing cell. The Centro mere do not divide. The products of separation of these homologous chromosomes are two dyads. in mitosis two sister chromatids pass to two different poles due to division of Centro mere but in meiosis sister chromatids do not separate and go to same pole.

Telophase I:

Nuclear membrane reorganises around each chromosome set at two poles nuclei reappear thus two nuclei each with half number of chromosome are formed.Later on cytoplasm  divides thus terminating the first meiotic division.It is also to be noted that chromosome may not decondense  during this state.

Meiosis II:

After telophase I, two daughter cells experience small interphase, but in contrast to interphase of mitosis there is no replication of chromosomes.

Prophase II:

Each dyad  is composed of one pair of chromatids attached by a common centromere, therefore, these chromosomes soon arrange themselves at metaphase plate.

Metaphase II:

The centromeres are directed to the equatorial plate. Then the centromere divide.

Anaphase II:

The sister chromatids of each dyad are pulled to opposite poles. Since the number of dyad is equal to the haploid number.

Telophase II:

It reveals one member of each homologous chromosome pair present at each pole. Each chromosome is referred to as a Monad.

                         During second meiotic division, not only the haploid state is acheived , but if crossing over has occurred, each monad is combination of maternal and paternal genetic information. As a result, the offspring produced by any gamete will receive from it a mixture of genetic information originally present in his or her grandparents.

 Following cytokinesis in telophase II, four haploid gametes are formed from a single meiotic event.

                      

Significance  of Meiosis:

Meiosis is a very significant process.The  process of  meiosis is necessaryfor the successful reproduction of all diploid organisms as during this process the diploid amount of the genetic information is reduced to the haploid amount. In, animals meiosis leads to the formation of gametes while in plants haploid spores are formed which produced haploid gametes.

Thus the significant points about meiosis are:

1. It maintains the same chromosome number in the sexually reproducing organisms. From a diploid cell, haploid gametes are produced which in turn fuse to form a diploid cell.
2. It restricts the multiplication of chromosome number and maintains the stability of the species.
3. Maternal and paternal genes get exchanged during crossing over. It results in variations among the offspring.
4. All the four chromatids of a homologous pair of chromosomes segregate and go over separately to four different daughter cells. This leads to variation in the daughter cells genetically.