Saturday, August 15, 2009


Stucture of DNA and Chromosome.


DNA molecules are found in chromosomes.

Chromosomes are thread-like structures in the nucleus of a cell.

Each chromosome contains a long DNA molecule enveloped by protein.

Cell growth:

2 types of cell division:

  1. Mitosis-is a cell division that occur in somatic cells.
  2. Meiosis-is a cell division that occur in reproductive organ.


Definition : Mitosis is a process of cell division which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell.

In a typical animal cell, mitosis can be divided into four principals stages:

  • Prophase: The chromatin, diffuse in interphase, condenses into chromosomes. Each chromosome has duplicated and now consists of two sister chromatids. At the end of prophase, the nuclear envelope breaks down into vesicles.
  • Metaphase: The chromosomes align at the equitorial plate and are held in place by microtubules attached to the mitotic spindle and to part of the centromere.
  • Anaphase: The centromeres divide. Sister chromatids separate and move toward the corresponding poles.
  • Telophase: Daughter chromosomes arrive at the poles and the microtubules disappear. The condensed chromatin expands and the nuclear envelope reappears.
  • Cytokinesis: The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells .

Phases of mitosis:






Importance of mitosis:

  1. To maintain the number of chromosomes.
  2. To maintain genetic materials.
  3. To ensure the body cells are diploid (2n).
  4. Growth, replace damage or death cells

Application of mitosis

  1. Tissue culture

a. isolate tissue from the shoots or roots.

b. transfer the tissue into culture medium.

c. stimulate growth by controlling pH, hormones, temperature.

d. cell replicate and form roots.

2. Cloning

a. nucleus from organism A combine with cell of an organism B.

b. stimulate the cell division.

c. implant embryo into the surrogate mother.

d. an offspring is born.

Advantages of cloning:

1. Produce a large quantity.

2. Maintain the genetic material.

3. Control the quality of crops or livestock.


Illustration of the process by which a single parent diploid cell (Both homologous chromosomes) divides to produce four daughter haploids cells (One homologous chromosome of the pair).

Meiosis is the type of cell division by which gametes cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material.

Meiosis comprises two successive nuclear divisions with only one round of DNA replication.
Four stages can be described for each nuclear division.

  • Interphase: Before meiosis begins, genetic material is duplicated.
  • First division of meiosis
    • Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.
    • Metaphase 1: Homologous chromosomes align at the equatorial plate.
    • Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
    • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
  • Second division of meiosis: Gamete formation
    • Prophase 2: DNA does not replicate.
    • Metaphase 2: Chromosomes align at the equatorial plate.
    • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
    • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.


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