REPRODUCTION IN PLANTS AND ANIMALS

Biology

REPRODUCTION IN PLANTS AND ANIMALS

Introduction

  • The process by which mature individuals produce offspring is called reproduction.
  • Reproduction is a characteristic of all living organisms and prevents extinction of a species.
  • There are two types of reproduction: sexual and asexual reproduction.
  • Sexual reproduction involves the fusion of male and female gametes to form a zygote.
  • Asexual reproduction does not involve gametes.

Cell Division

  • Cell division starts with division of nucleus.
  • In the nucleus are a number of thread-like structures called chromosomes, which occur in pairs known as homologous chromosomes.
  • Each chromosome contains-genes that determine the characteristics of an organism.
  • The cells in each organism contain a specific number of chromosomes.

There are two types of cell division:

Mitosis

  • This takes place in all body cells of an organism to bring about increase in number of cells, resulting in growth and repair.
  • The number of chromosomes in daughter cells remains the same as that in the mother cell.

Meiosis

  • This type of cell division takes place in reproductive organs (gonads) to produce gametes.
  • The number of chromosomes in the gamete is half that in the mother cell. Mitosis
  • Mitosis is divided into four main stages.
  • Prophase, Metaphase, Anaphase and Telophase.
  • These stages of cell division occur in a smooth and continuous pattern.

Interphase

  • The term interphase is used to describe the state of the nucleus when the cell is just about to divide.
  • During this time the following take place:
  • Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell.
  • Division of cell organelles such as mitochondria, ribosomes and centrioles.
  • Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process.
  • interphase, the following observations can be made:
  • Chromosomes are seen as long, thin, coiled thread-like structures.
  • Nuclear membrane and nucleolus are intact.
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Prophase

  • The chromosomes shorten and thicken.
  • Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere.
  • Centrioles (in animal cells) separate and move to opposite poles of the cell.
  • The centre of the nucleus is referred to as the equator.
  • Spindle fibres begin to form, and connect the centriole pairs to the opposite poles.
  • The nucleolus and nuclear membrane disintegrate and disappear.

Metaphase

  • Spindle fibres lengthen.
  • In animal cells they attach to the centrioles at both poles.
  • Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres.
  • Chromatids begin to separate at the centromere.

Anaphase

  • Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres.
  • Chromatids becomes a chromosome.
  • In animal cell, the cell membrane starts to constrict.

Telophase

  • The cell divides into two.
  • In animal cells it occurs through cleavage of cell membrane.
  • In plants cells, it is due to deposition of cellulose along the equator of the cell.(Cell plate formation).
  • A nuclear membrane forms around each set of chromosome.
  • Chromosomes later become less distinct.
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Significance of Mitosis

  • It brings about the growth of an organism:
  • It brings about asexual reproduction.
  • Ensures that the chromosome number is retained.
  • Ensures that the chromosomal constitution of the offspring is the same as the parents.

Meiosis

  • Meiosis involves two divisions of the parental cell resulting into four daughter cells.
  • The mother cell has the diploid number of chromosomes.
  • The four cells (gametes) have half the number of chromosomes (haploid) that the mother cell had.
  • In the first meiotic division there is a reduction in the chromosome number because homologous chromosomes and not chromatids separate.
  • Each division has four stages Prophase, Metaphase, Anaphase and Telophase.

Interphase

  • As in mitosis the cell prepares for division.
  • This involves replication of chromosomes, organelles and buildup of energy to be used during the meiotic division.

First Meiotic division

Prophase I

  • Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents.
  • Chromosomes shorten and thicken hence become more visible.
  • Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma).
  • Chromatids cross-over at the chiasmata exchanging chromatid portions. Important genetic changes usually result.

Metaphase I

  • Spindle fibres are fully formed and attached to the centromeres.
  • The bivalents move to the equator of the spindles.

Anaphase I

  • Homologous chromosomes separate and migrate to opposite poles.
  • This is brought about by shortening of spindle fibres hence pulling the chromosomes.
  • The number of chromosomes at each pole is half the number in the mother cell.
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Telophase I

  • Cytoplasm divides to separate the two daughter cells.

Second Meiotic Division

  • Usually the two daughter cells go into a short resting stage (interphase)
  • But sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division.
  • The second meiotic division takes place just like mitosis.

Prophase II

  • Each chromosome is seen as a pair of chromatids.

Metaphase II

  • Spindle forms and are attached to the chromatids at the centromeres.
  • Chromatids move to the equator.

Anaphase II

  • Sister chromatids separate from each other
  • Then move to opposite poles, pulled by the shortening of the spindle fibres.

Telophase II

  • The spindle apparatus disappears.
  • The nucleolus reappears and nuclear membrane is formed around each set of chromatids.
  • The chromatids become chromosomes.
  • Cytoplasm divides and four daughter cells are formed.
  • Each has a haploid number of chromosomes.

Significance of Meiosis

  • Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells.
  • It helps to restore the diploid chromosomal constitution in a species at fertilization.
  • It brings about new gene combinations that lead to genetic variation in the offspring.

See also

BIOLOGY PRACTICAL ACTIVITIES

HUMAN DISEASES (PARASITIC DISEASES)

HUMAN DISEASES – BACTERIAL DISEASES

POLLUTION

HYDROPHYTES (WATER PLANTS)

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