Cell Cycle: The cell undergoes a sequence of changes, which involves period of growth, replication of DNA, followed by cell division. This sequence of changes is called cell cycle.
It comprises two phases viz., interphase which is the period of non-apparent division and the period of division also known as mitotic phase. Each phase is further subdivided into different sub-phases.
The period of life cycle of cell (cell cycle) between two consecutive divisions is termed as the interphase or misleadingly called resting phase. Interphase also called as preparatory phase is the most important event in a cell cycle. It is the period of great biochemical activity and can further be divided into three phases named as G1-phase, S-phase and G2-phase.
G1 (Gap 1) is the first phase and is the period of extensive metabolic activity, in which cell normally grows in size, specific enzymes are synthesized and DNA base units are accumulated for the DNA synthesis or formation. Some of the cell leave the cell cycle and inter a new stage in which there is no cell process occurs, this phase is called G0, and remain for days, weeks, or in some cases (e.g., nerve cells and cells of the eye lens) even the life time of the organism without any growth. Following the G1 is the S-phase (synthesis phase) during which the DNA is synthesized and (chromosome are replicated) which initiates G2 phase (pre-mitotic phase), thus preparing the cell for division e.g., energy storage for chromosome movements, mitosis specific proteins,
RNA and microtubule subunits (for spindle fibres) synthesize. Cells then proceed to next phase which is the period of division). At each stage, there are specific checks points, which determine the fate of new phase according to cell’s internal, make up.
Length of each phase is variable. In the case of human cell, average cell cycle is about 24 hours, mitosis takes 30 minutes, G1 9 hours, the S-phase 10 hours, and G2 4.5 hours whereas full cycle in yeast cells is only 90 minutes.
It is the type of cell division, which ensures the same number of chromosomes in the daughter cells as that in the parent cells and divide a cell into two cells after completion. It means after completion of one mitotic cell two cells are formed and each daughter cell has the same number of chromosome like that of parents. For example human somatic cell has 46 chromosomes. After completion of one mitotic cycle two cells are formed each one has 46 chromosome. In spite of slight differences, major steps of mitosis are similar in plants as well as in animals.
Mitosis is a continuous process, but conventionally it may be divided into two phases, i.e., karyokinesis, which involves the division of nucleus and cytokinesis that refers to the division of the whole cell.
Note: The mitosis in plants and animal cells occurs in the same way with little differences.
Karyokinesis ; Karyon means nucleus and kinesis means division. It means it is the division of the nucleus.
At the very beginning of this process in an animal cell, the partition of the centriole occurs, which have been duplicated during interphase but present in the same centrosome.
Early in the mitosis the two pair of centrioles separate and migrate to opposite sides of the nucleus, establishing the bipolarity of the dividing cells.
Three sets of microtubules (fibres) originate from each pair of centrioles. One set the astral microtubules, radiate outward and form aster, other two sets of microtubules compose the spindles. The kinetochore microtubules attach to chromosomes at kinetochores part and polar microtubules do not interact the chromosomes but instead interdigitate with polar microtubules from the opposite pole. These microtubules are composed of a special protein called tubulin and traces of RNA (Ribonucleic Acid).
This specialized microtubule structure including aster and spindle is called mitotic apparatus. It means combination of aster and spindles is called mitotic apparatus. This is larger than the nucleus, and is designed to attach and capture chromosomes, aligning them and separating them so that equal distribution of chromosomes is ensured.
Karyokinesis is a continuous process but for our easiness we divide the whole process in to stages or phases. The four important phases are as under.
During interphase (non-dividing phase) of the cell cycle the chromosomes are not visible even with electron microscope, but using histologic stains for DNA, a network of very fine threads can be visualized. This network is called as chromatin.
The chromatin material gets condensed by folding and the chromosomes appear as thin threads (0.25mm – 50mm in length) at the beginning of prophase.
Chromosomes become more and more thick ultimately each chromosome is visible having two sister chromatids, attached at centromere. Nuclear envelope disappears or destroys and nuclear material is now in cytoplasm, at the same time nucleoli also disappear. Cytoplasm becomes more viscous.
Each metaphase chromosome is a duplicated structure which consists of two sister chromatids, attached at a point called centromere or primary constriction. The centromere has special area, the kinetochore, with specific base arrangement and special proteins where kinetochore fibers of mitotic apparatus attach.
The kinetochore spindle fibers link to the kinetochore region that is a specialized region in centromere of chromosome. These fibres align chromosomes at the equator of the spindle forming equatorial plate or metaphase plate. Each kinetochore gets two fibers one from each pole.
It is the most critical phase of the mitosis, which ensures equal distribution of chromatids in the daughter cells. The kinetochore fibers of spindle contract towards their respective poles, at the same time polar microtubules elongates exert force and sister chromatids are separated from centromere. As a result, half-sister chromatids travel towards each pole.
Reaching of the chromosomes at opposite poles terminates anaphase and start telophase. The chromosomes decondense due to unfolding, ultimately disappear as chromatin. Mitotic apparatus disorganizes nuclear membrane and nucleoli reorganize, resulting two nuclei at two poles of the cell.
During late telophase the astral microtubules send signals to the equatorial region of the cell, as a result actin and myosin (Special Proteins) are activated which form contractile ring, followed by cleavage furrow, which deepens towards the center of the cell, dividing the parent cell into two daughter cells.
Mitotic events in plant cells are generally similar to the events observed in animal cells but there are some major differences. Most higher plants lack visible centrioles,
instead they need its analogous region from that the spindle microtubules radiate. Moreover, shape of the plant cell does not change greatly compared with an animal cell- because it is surrounded by a rigid cell wall. At cytokinesis, in place of contractile ring a membrane structure, phragmoplast is formed from vesicle which originate from Golgi complex. These vesicles originate actually during metaphase, line up in the center of the dividing cell, where they fuse to form phragmoplast at the end of telophase.
The membrane of vesicles changes to the cell membrane of new cells. These vesicles also contain materials for future cell wall such as precursors of cellulose and
Importance of mitosis:
- Hereditary material equally distributed in daughter cells. A possible reason of this is the absence of crossing over or recombination.
- Hereditary materials transfers from generation to generation remain unchanged.
- Healing of wounds is possible
- Regeneration is another advantage
- Asexual reproduction in small organisms occurs through this process.
- Growth and development is another important function of mitosis.
Meiosis is the special type of cell division in which the number of chromosomes in daughter cells is reduced to half, as compared to the parent cell.
Two consecutive divisions occur in meiosis.
Two divisions, are meiosis I and meiosis II. The first meiotic division is the reduction division, whereas second meiotic division is just like the mitosis.
Both divisions can further be divided into substages like prophase 1, metaphase 1,
anaphase 1, telophase 1 and same names are used for meiosis II also.
This is very longest phase of meiosis, and differs from the prophase of mitosis as well, because in this chromosomes behave as homologous pairs. Each diploid cell has pair of chromosomes of each type, 1 member from each parent, It is due to the fusion of male and female gametes. Each chromosome has 2 chromatids, because chromosomes have been replicated during interphase. The interphase of meiosis lacks G2 stage. These similar but not necessarily identical chromosomes are called as homologous chromosomes.
Prophase 1 further consists of the followings stages.
The chromosomes become visible, shorten and thick. The size of the nucleus increases and homologous chromosomes start getting closer to each other.
Zygotene: First essential step of meiosis in which pairing of homologous chromosomes occur, called synapsis.
This pairing is highly specific and exactly pointed but with no definite starting point(s). Each paired complex structure of chromosome that is not completely fused is called bivalent or tetrad.
Pachytene: Synapsis occurs that is actually the pairing of homologous chromosomes. Crossing over occurs at this stage that is the exchange of the segments of the homologous chromosomes. The point at which the exchange of segments occurs is called chiasmata.
Diplotene: The paired chromosomes separate from each other but at the point of chiasmata they are still attached. Each bivalent has at least one chiasmata.
Diakinesis: During diakinesis the chromosome condensation is at the maximum stage. Separation of chromosomes completed and chromosomes are still attached at the end points. Nucleoli completely disappear.
Nuclear membrane disorganizes at the beginning of this phase. Spindle ibers originate and the kinetochore ibers attach to the kinetochore of homologous chromosome from each pole and arrange bivalents at the equator. The sister chromatids of individual chromosome in bivalent behave as a unit.
The kinetochore ibers contract and the spindle or pole fibers elongate, which pull the individual chromosome (each having two chromatids) towards their respective poles. It may be noted here that in contrast to anaphase of mitosis, sister chromatids are not separated. This is actually reduction phase because each pole receives half of the total number of chromosomes.
Nuclear membrane reorganizes around each set of chromosomes at two poles, nucleoli
reappear thus two nuclei each with half number of chromosomes are formed, later on
cytoplasm divides thus terminating the irst meiotic division. It is also to be noted that
chromosomes may decondense during this stage.
After telophase-1 two daughter cells experience small interphase, but in contrast to interphase of mitosis there is no replication of chromosomes.
Prophase II, metaphase II, anaphase II and telophase II are just like the respective phases of mitosis during which the chromosomes, condense, mitotic apparatus forms, chromosomes arrange at the equator, individual/sister chromatids move apart, and ultimately four nuclei at the respective poles of two daughter cells (formed after meiosis I) are formed. Cytokinesis takes place and four haploid cells, with half of the number of chromosomes (chromatids) are formed.
Importance of Meiosis
Crossing over and random assortment of chromosomes are two signiicant happenings of meiosis. During crossing over, parental chromosomes exchange segments with each other which results in a large number of recombination. During anaphase due to the random separation of homologous chromosomes gives wide range of variety of gametes.
Both these processes results in variations and modifications in the genome. These variations lead to evolution and also make every individual specific, particular and unique in his characteristics. Even the progeny of very same parents, i.e., brothers and sisters are not identical to each other.
Meiosis usually takes place at the time of sexual cell (gamete) formation and spore formation in plants. It maintains the number of chromosomes constant generation after generation.
MEIOTIC ERRORS (NON-DISJUNCTION)
Meiosis is an orderly occurring phenomenon, which ensures every phase with appropriate finish, but sometimes, at any point the result may be unexpected, causing abnormalities. One of such abnormalities is chromosome non-disjunction, in which chromosomes fail to segregate during anaphase and telophase and do result in unequal distribution of chromosome among all the daughter nuclei. The result is decrease or increase in number of chromosomes causing various disorders like mental problems, physical or social disorders. The non-disjunction may occur in autosome or sex chromosomes. Examples of non-disjunction are as under.
Down’s Syndrome (Mongolism)
It is autosomal nondisjunction in which the 21st pair of chromosome fails to segregate result in a gamete with 24 chromosomes. When the gamete with 24 chromosomes fertilize another gamete with normal 23 chromosomes, the resulting organism will have down syndrome with 47 (2n+1) chromosomes.
Non-disjunction appears to occur in the ova and is related to the age of mother. The chances of teenage mother having Down’s syndrome child is one in many thousands, forty years old mother, one in hundred chances and by forty-five the risk-is three times greater. The affected individuals have flat, broad face, squint eyes with the skin fold in the inner corner, and protruding tongue, mental retardation, and defective development of central nervous system.
Autosomal non-disjunction may occur in other than 21st chromosome which usually results in abortion, or death in very early age.
The individuals with klinefelters’s syndrome have one more sex chromosome than normal. For example, 47 chromosomes (44 autosome + XXY) mean three sex chromosomes. They are phenotypically male but have frequently enlarged breasts. They are normally tall, obese having small testes with no sperms or little sperms at ejaculation and under developed secondary sex characters.
Males with 48 chromosomes (44 autosomes + XXXY), with 49 chromosomes (44 autosomes + XXXXY) and with 47 chromosomes (44 autosomes + XYY) are also observed.
These afected individuals have one missing X chromosome with only 45 •chromosomes (44 autosomes + X).
Individuals with this condition typically don’t survive maternity and are aborted.
Those who survive have feminine look with short stature, webbed neck, without ovaries and complete absence of germ cells.
Syndrome Sex Chromosomes Frequency
Down M or F Trisomy 21 1/40 1/700
Patau M or F Trisomy 13 1/33 1/15,000
Edward M or F Trisomy 18 1/200 1/6,000
Turner F XO 1/18 1/6,000
Metafemale F XXX or (XXXX) 0 1/1,500
Klinefelter M XXY or (XXXY) 0 1/1,500
Jacobs M XYY ? 1/1,000
Necrosis and Apoptosis
Cells in an organism depend upon various extracellular and intracellular signals for its regulated, controlled activities like cell division, pattern formation, differentiation, morphogenesis and motility. Each cell is predestined to its fate i.e., what responsibility it has to take and in which way. Even the death of the cell is programmed.
Programmed cell death helps in proper control of multicellular development, which may lead to deletion of entire structure (e.g., the tail of developing human embryos) or part of structure (e.g., tissue between developing digits). Cell death even controls the number of neurons, because most of the neurons in the human body die during development.
Cell death in multicellular organisms is controlled by two fundamentally different ways, i.e., either the cell commits suicide in the absence of survival signals (trophic factors) or cells are murdered by killing signals from other cells.
Due to morphological changes cell decide to suicide, the process is called apoptosis. Apoptosis (a Greek word) meaning falling off or dropping of.
During this process the dying cells shrink and condense ultimately split up, thus releasing small membrane bounded apoptotic bodies, which are generally phagocytized by other cells Intracellular constituents are not released freely in extracellular atmosphere which otherwise might have deleterious effects. In contrast to suicide, the cell death due to tissue damage is called necrosis, during which the typical cell swells and bursts, releasing the intracellular contents, which can damage neighbouring cells and cause inflammation.