GROWTH

Growth is defined as an irreversible or permanent increase in size, dry mass or weight due to the addition of living protoplasmic materials in organism. In other words, Growth is defined as an irreversible increase in size and complexity of an organism brought about by the synthesis of new protoplasm. For growth to occur, the rate of synthesis or building up of materials (anabolism) must exceed the rate of breaking down (catabolism).

 

Differences Between Plants and Animals Growth

 

Growth in plants

  1. Mainly restricted to place growing regions/ body/ apical meristem.
  2. Continuous throughout life (in growing regions).
  3. Indefinite number of parts.

 

Growth in animals

  1. Diffuse/takes in all parts of the intercalary.
  2. Limited to certain periods in life.
  3. Definite number of part.

 

Basis of Growth

For any organism to grow, it must pass through three phases usually referred to as the basis of growth. These are:

  1. Cell division: Cell division involves cell multiplication. In order to multiply, the cells undergo certain divisions. One cell divides into two, two into four, four into eight and so on. Two types of cell division exist, according to the behaviour of chromosomes. These are mitosis and meiosis.
  2. Cell enlargement: This is the process which follows cell division in which the daughter cells increase in mass and in size. That is, it enlarges in size.
  3. Cell differentiation: This takes place after cell enlargement in which each cell develops into a special type of cell by changing its shape and structure in order to carry out a specialised or a particular function. The kind of cell it becomes depends on its position in the body of the organism. For example, in human body, a cell may develop into a nerve cell if it is in the brain. Cell differentiation is important in the growth and development of a mature multicellular organisms.

 

Types of Cell Division

(a) Mitosis: Mitosis is a cell or nuclear division following the duplication of the chromosomes, whereby each daughter cell or nucleus has exactly the same chromosome content as the parent. In other words, mitosis is a cell division in which daughter cells have the same number of chromosomes as the parent cell.

Mitosis takes place in somatic cells, i.e ., body cells that are not involved in the production of gametes. Mitosis takes place during an organism’s growth, development and asexual reproduction. In plants, mitosis takes place in the terminal bud of the shoot at the tip of the roots and shoots. In animals, mitosis occurs at growth centres which are everywhere.

Mitosis produces diploid cells. In other words, the number of chromosomes in each somatic cell of an organism is called the diploid number (2n).

 

Stages of Mitosis

There are five stages or phases of mitosis.

These are:

  1. Interphase: This is the latent or resting stage of the cell. At this stage, the cell has normal appearance of non-dividing cell condition. Chromosomes are too thread-like for clear visibility.
  2. Prophase: The prophase is divided into two: Early and Late prophase. During early prophase, the chromosomes become visible as they contract and nucleolus shrinks. Centrioles are formed at opposite sides of the nucleus. Spindle fibres start to form and during the late prophase, the chromosomes become shorter and fatter. Each is seen to consist of a pair of chromatids joined at the centromere and nucleus disappears. Prophase ends with the breakdown of nuclear membrane.
  3. Metaphase: Metaphase also exists in early and late forms. During early metaphase, the chromosomes arranged themselves on the equator of the spindle and at late metaphase , the chromatids draw apart at the centromere region. (Note that the chromatids of each chromosome are orientated towards the opposite poles of the spindle).
  4. Anaphase: Anaphase also exists in early and late forms. During early anaphase, the chromatids part company and migrate to opposite poles of cell while during late anaphase, the chromosomes reach their destination, i.e ., toward the poles.
  5. Telophase: Telophase stages is equally divided into early and late forms. During the early telophase, the cell starts to constrict across the middle. During late telophase, the constriction continues. The nuclear membrane and nucleolus reform in each daughter cell. Spindle apparatus degenerates. The chromosomes eventually regain their thread-like form and the cell returns to resting condition (i.e. interphase). (Note that the daughter cells have precisely the same chromosome constitution as the original parent cell).

 

Importance or Role of Mitosis

  1. Mitosis promotes cell growth.
  2. It helps in the replacement or repair of damaged tissues.
  3. It serves as basis of asexual or vegetative reproduction.
  4. It produces genetically or identical offspring which are identical to the parents.
  5. Mitosis helps to maintain the diploid number of the chromosome of the cell.

 

Life Processes Involved in Mitosis

Life examples of mitotic process animals include:

  1. Formation of new cells in the malphighian layer of the skin.
  2. Production of red blood and white blood cells in the bone marrow.
  3. Cell division in liver.
  4. Cell division in meristem or tip of root or in cambium.
  5. Binary fission.
  6. Growth in spermatogenesis.
  7. Repair or healing of wound.

 

Life examples of mitotic process in plants include:

  1. Mitosis occurs in root tip or apex.
  2. It also occurs in stem tip or apex.
  3. It also occurs in cambium.
  4. It is found in meristems.

 

(b) Meiosis: Meiosis is a two successive cell division with only one duplication of chromosomes. Four daughter cells are produced in meiosis. Meiosis is a reduction in cell division and the resulting four daughter cells are haploid.

Meiosis takes place in reproductive cells, i.e ., ovules and pollen grains in plants, ovaries and testes in animals. In animals, meiosis occurs in the formation of gametes (sex cells such as eggs and spermatozoa).

The process of gamete formation is called gametogenesis. The process involved in the production of spermatozoa by the testes is called spermatogenesis while that of eggs or ova production by the ovaries is called oogenesis.

 

Stages of Meiosis

Meiosis consists of two successive divisions:

  • 1st meiotic division: when the parent cell splits into two;
  • 2nd meiotic division – when the products then divide again giving a total of four daughter cells.

 

1st Meiotic Division or Reduction Division

This 1st meiotic division is divided into five stages of:

1) Interphase: This is the resting stage of the cell. At this stage, the chromosomes are not seen.

2) Prophase I: Prophase I is divided into three:

  • Early prophase I: At this stage, the chromosomes contract, becoming more clearly visible and the nucleus shrinks.
  • Mid prophase I: The homologous chromosomes come together (synapsis) forming a bivalent.
  • Late Prophase I: During this stage, each chromosome is seen to consist of a pair of chromatid. At prophase I, crossing over takes place.Thepointsonthe chromosomes where crossing over takes place are known as chiasmata. (singular form is called chiasma).

3) Metaphase I: During metaphase I, the nuclear membrane disappears and the spindle is formed. The bivalents (chromosomes) assemble at the equatorial region of the spindle.

4) Anaphase I: At this anaphase, the bivalents separate completely and each member chromosome moves to the opposite pole of the spindle.

5) Telophase I: This stage produces two daughter cells with chromosome number, halved as in the parent cells.

 

 

2nd Meiotic Division

The second meiotic division is made up of four stages. The division is similar to mitosis. At the end of the second division, four groups of daughter cells are formed.

  1. Prophase II: The two daughter cells prepare for the second meiotic division. Here, the centrioles have replicated and a new spindle is formed.
  2. Metaphase II: The chromosomes arrange themselves on the spindle in the usual way. The chromosomes appear doubled and the centromere starts to divide in the position of the equatorial plane.
  3. Anaphase II: Each chromosome finally splits into two. The actual chromosomal division takes place at anaphase II and each one moves to opposite poles of the cells.
  4. Telophase II: The two cells are divided into four which is tetrad.

The four haploid cells are produced from the original diploid mother cell.

 

Importance or Roles of Meiosis

The importance of meiotic process are:

  1. It aids the formation of sperms or male gametes in animals.
  2. It aids the formation of ova (eggs) or female gametes in animals.
  3. It aids the formation of pollen grains in anthers of flowering plants.
  4. It also aids the formation of ovules in ovary of flowering plants.

 

Life examples or areas where meiosis occur in

(a) Plants

  • Meiosis is found in ovaries
  • It is also found in anthers.

 

(b) Animals

  • Meiosis is found in ovaries.
  • It is also found in testes.

 

Differences Between Mitosis and Meiosis

 

Mitosis

  1. Mitosis takes place during growth of body or somatic cells.
  2. The number of chromosome parent and new cell are the same.
  3. Two offspring cells are formed.
  4. Chromosomes are arranged in pairs in both parents and new cells.
  5. There is no exchange of material between the chromatids.
  6. There is no formation of bivalent.
  7. There is no crossing over.
  8. Mitosis involves only one stage of division.

 

Meiosis

  1. Meiosis takes place only in the production of gametes.
  2. The number of chromosome of new cells is half the number in the parent cell (haploid no. of cells ).
  3. Four offspring cells are formed.
  4. Chromosomes are arranged in pairs in the parent cell but new cells have only one of each homologous pairs.
  5. There is exchange of material that results in variation.
  6. Two whole chromosomes form bivalent.
  7. Crossing over occurs.
  8. Meiosis involves two stages of division.