After reading this essay you will learn about: 1. Definition of Cell Division 2. Factors Controlling Cell Division 3. Significance 4. Role of Amitosis 5. Role of Mitosis 6. Role of Meiosis.
Contents:
- Essay on the Definition of Cell Division
- Essay on the Factors Controlling Cell Division
- Essay on the Significance of Cell Division
- Essay on the Role of Amitosis in Cell Division
- Essay on the Role of Mitosis in Cell Division
- Essay on the Role of Meiosis in Cell Division
Essay # 1. Definition of Cell Division:
Cell division, cell reproduction or cell multiplication is the process of formation of new or daughter cell from the pre-existing or parent cells. In other words, the formation of new cells from the pre-existing ones and their enlargement are important factors in the growth of the plants.
A plant, in fact, it starts its life as a single cell. In course of time the cells undergo repeated division to produce many new cells, as a result of which they grow and develop into mature plants.
Essay # 2. Factors Controlling Cell Division:
There are some factors which can control cell division are:
i. Cell Size:
Cells are capable of division grow for some time, attain a particular size and then undergo division.
ii. Kernplasma or Karyoplasmic Ratio:
rise in cell volume disturbs kernpklasma ratio. It stimulates the cell to divide.
iii. Mitogens:
Mitogens are agents, factors or substances that triger cell division. The common plant mitogen is hormone cytokinin. There are several mitogenic substances known in human beings, e.g., lymphokines, EGF (epidermal growth factor), PDGF (Platelet derived growth factor).
Essay # 3. Significance of Cell Division:
1. Cell Multiplication:
Cell division is a means of cell multiplication or formation of new cells from pre-existing cells.
ii. Continuity:
It maintains continuity of living matter generation after generation.
iii. Asexual Reproduction:
Cell division is a means of asexual reproduction in lower organisms.
iv. Multicellular Organisms:
The body of a multicellular organism is formed of innumerable cells. They are formed by repeated divisions of a single cell or zygote. As the number of cells increases, many of them begin to differentiate, form tissues and organs. In fully formed multicellular individuals, only some of the cells retain the power of division, e.g., bone marrow, germinal tissues, stratum germinativum, meristematic regions (in plants).
v. Growth:
Growth of an organism involves growth and division of its cells.
vi. Cell Size:
Cell division helps in maintenance of a particular cell size which is essential for efficiency and control of cell activities.
vii. Genetic Similarity:
The common type of cell division or mitosis maintains genetic similarity of all the cells in an individual despite their being different structurally and functionally. It is helpful in proper coordination.
viii. Repair:
Cell division is a means of repair and healing of injured regions of the body. Old or worn out cells are similarly replaced by new ones.
ix. Regeneration:
Cell division helps in regeneration of a part or whole of the organism
x. Sexual Reproduction:
Sexual reproduction requires a special type of cell division called meiosis.
xi. Reshuffling of Genetic Traits:
Meiosis is a means of reshuffling of genetic traits. It introduces variability
xii. Mutations:
During cell division, there is replication of genetic material. Any change during this activity results in mutations.
Essay # 4. Role of Amitosis in Cell Division:
It is a simple method of cell division which is also called direct cell division. Amitosis was discovered by Remak (1841, 1855) and described by Flemming (1882). In this division there is no differentiation of chromosomes and spindle. The nuclear envelope does not degenerate.
The nucleus elongates and constricts in the middle to form two daughter nuclei. This is followed by a centripetal constriction of the cytoplasm to form two daughter cells. Amitosis is not a regular method of division because it does not divide the nuclear matter equitably.
It occurs in metabolic nucleus (e.g., meganucleus of Paramecium) of some protozoa. The growth of embryonic membrane of some vertebrates is due to this type of cell division. Amitosis also occurs in diseased cells. In Chara, intermodal nuclei divide by amitosis.
It is not followed by cytokinesis. This produces a large number of nuclei of unequal size. Some authors include cell division of monerans (e.g., bacteria) under amitosis due to absence of spindle formation. As compared to amitosis, other types of divisions (mitosis and meiosis) are called indirect cell divisions.
Essay # 5. Role of Mitosis in Cell Division:
Mitosis (Gk. mitos- thread or fibril) is that type of division in which chromosomes replicate and become equally distributed both quantitatively and qualitatively into two daughter nuclei so that the daughter cells come to have the same number and type of chromosomes as are present in the parent cell.
It is, therefore, also called equational division. Mitosis was first observed by Strasburger (1870) in plant cells, Boveri and Flemming (1879) in animal cells. The term of mitosis was coined by Flemming (1882).
It is the most common method of division which brings about growth in multicellular organisms and increase in population of unicellular organisms.
Mitosis occurs in the formation of somatic body cells and is hence often named as somatic cell division. The sites of mitotic cell division in a plant are meristematic regions like stem tip, root tip, intercalary meristem, lateral meristem, growth of embryo, leaves, flowers, fruits, seeds, etc.
In animals, mitosis is found in embryo development and some restricted regions in the mature form like skin and bone marrow. It can be easily studied in smears or sections of root and stem tips.
While the plant cell does not show much change, the animal cell becomes spheroid, more viscous and refractile at the time of mitosis. Depending upon the type of cell and the species, mitosis takes 30 minutes to 3 hours for completion.
The somatic cells or the body cells divide by a much complicated process where the nucleus plays the most important role. The nucleus divides first into two exactly equal daughter nuclei, and that process is followed by cytoplasmic division to make the cell division complete.
i. Karyokinesis:
Nuclear division is called mitosis or karyokinesis and cytoplasmic division is known as cytokinesis. Vegetative parts of the plant grow by this method of cell division.
Mitosis (Fig. 133) is a much complicated and continuous process in which the nucleus undergoes a series of changes to divide division, though purely man-made, is recognised in the whole biological world.
a. Prophase (the early phase):
It begins with the earliest recognisable changes in the ‘metabolic’ or resting nucleus. The nucleus slightly enlarges and the crooked chromonemata, which form the reticulum, separate and become quite distinct. They undergo progressive shortening and thickening, and form threadlike bodies, called chromosomes.
The number of chromosomes is always constant for a particular species of plant. Deeply stainable matter, called ‘matrix’, appears in which chromonemata remain embedded. Each chromosome undergoes longitudinal splitting into two equal and identical halves, called chromatids, which may often remain coiled or twisted round each other.
In the meantime the nuclear membrane and the nucleolus gradually disappear, and some fibrils make their appearance in the nuclear field.
b. Metaphase:
It is a short phase when the nuclear membrane and the nucleolus completely disappear, and by further extension of the fibrils a bipolar spindle is formed. The two ends of the spindle are the poles and the central portion is called the equator. The chromosomes arrange themselves at the equator, just preparatory to migrating towards the poles.
Each of the chromosomes has a spindle-fibre attachment region, called centromere, to which a fibre remains attached (traction fibre). Other fibres of the spindle (supporting fibres) extend from pole to pole.
c. Anaphase:
It is the moving phase when the chromatids are repelled from the equator and move towards the poles. As the attachment regions move first, the chromosomes appear U- or L-shaped, often with unequal arms. One set of chromosomes moves towards one pole and the other set towards the opposite pole. The cause of the movement is still uncertain and has aroused a good deal of controversy. It is likely that a tractive force is exercised by the spindle fibres.
d. Telophase or Reconstruction Phase:
This is the last phase when chromosomes have reached the poles and are crowded together. Their individuality is lost. Matrix disappears and the crooked chromonemata form the reticulum. Nuclear membrane and nucleolus re-appear, and thus two daughter nuclei, with the same number of chromosomes, are reconstructed.
ii. Cytokinesis:
During telophase the fibres of the spindle expand outwards almost touching the lateral walls. Protoplasmic materials accumulate at the equatorial region in form of small droplets ultimately coalesce to form a plate called cell plate. The cell plate are by chemical and physical changes is transformed into the intercellular substance, middle lamella, on which cellulose particles are deposited by the protoplasts to form the primary walls.
Somatic mitosis is the most important method of cell division which takes place in all the vegetative parts of the plants. It is usually confined to some regions called meristems, like root-tips and stem-tips. This is equational division, because the distribution of chromosomes in the two daughter nuclei are exactly equal, both qualitatively and quantitatively.
The daughter nuclei are exactly alike the mother nucleus, as they have the same number of chromosomes. The number of chromosomes is constant for a species. A cell may divide a million times, but the method is such that the number will remain same. The number of chromosome in pea is 14, in onion 16, in tobacco 48, in maize 20.
Some significance of mitosis are:
i. Growth:
Somatic cells are formed by mitosis. Therefore, mitosis is essential for growth and development of a multicellular organism. Human baby has about 6 x 1012 cells. All of them develop from a single celled zygote through repeated mitosis. Plants are able to grow throughout their life due to mitotic divisions in their apical and lateral meristems.
ii. Maintenance of Surface or Volume Ratio:
An overgrown somatic cell is induced to divide so that mitosis helps in maintaining a proper surface/volume ratio.
iii. Nucleocytoplasmic Ratio:
An efficient cell has a high nucleocytoplasmic ratio. Increase in size lowers the ratio. It is brought back to efficient level through division.
iv. Maintenance of Chromosome Number:
Mitosis involves replication and equitable distribution of all the chromosomes so that all the cells of a multicellular organism have the same number and type of chromosomes. This helps in proper co-ordination among different cells.
v. Regeneration:
Mitosis keeps all the somatic cells of an organism genetically similar, resembling the fertilized egg. They, therefore, are able to regenerate part or whole of the organism.
vi. Reproduction:
Mitosis is the method of multiplication of unicellular organisms.
vii. Repair:
It is a mechanism for replacing old or worn out cells. In human body roughly 5 x 109 cells are daily lost from surface of skin, lining of alimentary canal, RBCs, WBCs, etc. The same are replaced by new cells formed through mitosis.
viii. Healing:
An injury or wound is healed by repeated mitotic divisions of the surrounding healthy cells.
ix. Opportunity for Differentiation:
Mitosis produces multicellular condition. It provides opportunity for differentiation.
x. Cancer:
Uncontrolled mitotic division leads to cancer.
xi. Evidence of Basic Relationship:
The details of mitosis are similar in the majority of organisms, showing their basic similarity and relationship.
Essay # 6. Role of Meiosis in Cell Division:
Meiosis or reduction division (Fig. 134) is a very complicated method of cell division which is restricted only to the reproductive cells. This method involves two divisions, of which the first division is reductional and the second one is mitotic or equational.
During prophase slender thread-like bodies, the chromosomes, are formed as usual. Two chromosomes approach each other and become very intimately associated. They may remain coiled or twisted around each other, but do not actually fuse.
This selective pairing is called synapsis, which is a fundamental feature of meiosis. Taking arbitrarily, if there are six chromosomes, they are arranged in three pairs. The chromosomes forming the pair are called homologous ones, and the pairs are known as bivalents.
Soon, while in contact, each chromosome undergoes longitudinal splitting and become double. In metaphase the split chromosomes move to the equator of the spindle. Since each member of the homologous pair has split longitudinally into two halves the chromosomes at the equator appear as tetrads, consisting of four chromatids.
During the next stage, anaphase, the original chromosomes forming a pair, each consisting of two chromatids—dyads, are separated and move towards the poles. This is separation or disjunction of the two members which formed the homologous pair, since the two chromatids of each dyad constitute an original chromosome.
In the telophase two daughter nuclei with reduced or halved number of chromosomes are reconstructed. This number is designated as haploid or ‘n’ number; so the original number is diploid or ‘2n’. This division is immediately followed by a second division, which is mitotic or equational.
Thus four nuclei are formed at the close of meiosis, each nucleus receiving one of the four chromatids of each tetrad. Now cytokinesis occurs, resulting in the formation of four cells.
Meiosis or reduction division takes place somewhere in the life cycle of all plants having sexual method of reproduction. The gametes, male and female, have reduced or n chromosomes. ‘2n’ number is restored in the zygote, the product of fusion of the two gametes. Thus the number of chromosomes remains constant in a species.
