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Category Archives: Chapter 3 – Heredity and Variation

Cell Division: Meiosis and Its Importance

The process of cell division for the production of gametes is called meiosis. In human beings and animals meiosis takes place in the testes and the ovaries. In plants meiosis take place in the anthers and ovaries.

Meiosis  results in gametes which have half the number of chromosomes present in the parent cell. This is necessary so that when two gametes fuse in fertilization , the original number of chromosomes in the parent cell is obtained.

 

 

The original number of chromosomes in a parent cell before meiosis is  known as the diploid  number       ( 2n chromosomes).  The number of chromosomes in a gamete at the end of meiosis is called the haploid number ( ( n chromosomes ).

Each parent cell in meiosis  produces four gametes with different combinations of chromosomes because the chromosomes are reshuffled during meiosis.

Importance of Meiosis

1. Production of gametes .

Meiosis enable the production of gametes, fertilization cannot take place without the presence of male and female gametes and new organisms cannot be produced.

2. Reduction in chromosome number.

Each gamete has only half of the number of chromosomes of the parent cell. This is necessary because fertilized ovum contains  chromosomes from both the male and female gamete.

3. Maintainence of chromosome number in somatic cells.

Meiosis enable us to maintain the number of chromosomes in our somatic cells at 46 chromosomes.

4. Production of variation.

Meiosis causes genetic variation in the descendants. New individuals do not resemble their parents closely.

This is brought about in two ways:  

(a) Different  combinations of chromosomes due to reshufflement of parental choromosomes during meiosis process.    

(b) Crossing over which took place when chromosomes are arranged along the equatorial plane. In the process the chromatids exchange genes and so increase genetic variation. Genetic variation in a species enables the species to survive.

 

 

 

 

 

 

 

 

 

 

Cell Division : Mitosis and Its Importance

Mitosis is the process by which mature cell divides into two new cells. Each new cells possesses the same number of chromosomes as the parent cell.
All cells except reproductive cells, divide by means of mitosis.
In plants, mitosis is actively carried out at the ends of plant shoots and roots.

 

 

Importance of Mitosis:

1. Mitosis produces new cells for :

(a) growth

(b) replacing aging and damaged cells.

2. Mitosis enables organisms to reproduce themselves:

(a) by asexual reproduction e.g binary fission in amoeba and budding in yeast.

(b)  by vegetative reproduction e.g growing tapioca using its stem, growing lotus using its rhizome and growing bryophllum using its leaf.

 

 

 

 

Mitosis and Meiosis

 

Chromosomes and DNA structures

CHROMOSOMES
Chromosomes are fine thread-like structures present in the nuclei of plant and animal cells. Chromosomes only can be seen clearly under a microscope just when a nucleus is about to divide in cell division. Chromosomes and the genes on them are composed of deoxyribonucleic acid (DNA). Chromosomes exist in pairs except in the reproductive cells. Two identical chromosomes in a pair is said to be homologous.

GENES
Genes are hereditary particles that carry the inherited characteristics of a person. Genes occur in pairs, therefore each inherited characteristic is carried by two genes. One gene from each pair comes from the father and the other from the mother. There are thousands of genes on each chromosome, all the genes in the body cells of a person are similar, but different from those in other people.

Examples of inherited characteristics carried by genes are:
(a) colour of the eyes
(b) pattern of finger prints
(c) type of hair
(d) blood group
(e) intelligence

Deoxyribonucleic Acid (DNA)

DNA is found in every living organism. in fact, it is present in every cell of the body. The DNA has all the information for creating a fully grown person with all the necessary organs and the inherited characteristics such as black curly hair and dark skin etc. All the information in the DNA is transmitted in the form of a genetic code.

 

Varieties of cell structure diagrams

plant cell structure diagram picture

Cell Wall

  • Found only in plants, the cell wall is made of cellulose (a polysaccharide).
  • It’s found just outside the cell membrane and is permeable to most substances.
  • It forms a rigid outer boundary and confers plant cells their unique shapes.
  • Main function is to provide support and protection for the plant cell.

Centriole

  • Exists as a pair near the nucleus
  • Plays a part in the separation of chromosomes during cell division.

Chloroplast

  • Organelles that can capture energy from sunlight and combine carbon dioxide and water to form glucose in a process called photosynthesis.
  • Contains the green pigment chlorophyll.

Chromatin

  • Consists of deoxyribonucleic acid (DNA) bound to proteins (found in the nucleus).
  • During cell division, chromatin condenses to form distinct threadlike structures called chromosomes.

Cyptoplasm

  • Can be in a liquid (sol) state or semi-solid (gel) state.
  • The site for most of the chemical activities of the cell
 
 
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