Heredity in Living Organism

We need two things before we can start to build a house:
1. a plan of the house
2. a supply of building materials

The plan is needed because it tells us how and where to arrange the building materials. In the same way, when a new organism is built, a plan and building materials are also needed. 

The plan necessary to build a living organism is found inside the nuclei of its cells. The plan for building a house consists of lines drawn on a large sheet of paper. The plan for building a living organism consists of chemicals called genes. Genes instruct the body to make proteins. Proteins determine the shape of the body and how it behaves. Each gene controls the production of one particular protein, or occasionally a group of proteins. These genes are joined together to make long strands called chromosomes which are extremely small. 

Chromosomes and genes are made of deoxyribonucleic acid (DNA) which is a kind of plan showing how the body is to be built. It is often called the blue print for life - a blue is a kind of plan. Every cell in a living organism contains a copy of this plan. 

When cells divide, the DNA first copies itself. A copy of this plan is passed from one generation to the next. This is the reason we inherit characteristics  from our parents. 

Copying Genes

Every time a new cell is made in the body of an organism, the DNA is copied so that each new cell has its own identical copy of all the genes. The structure of the DNA molecule makes the genes easy to copy. A DNA strand separates down the middle, rather like the two halves of a zip fastener coming apart. Spare chemical bases then join up with the matching pairs on the separated strands. As a result, one strand of DNA makes two identical new strands.

Growth and Cell Division 

All living things grow from a fertilized egg-cell. The only way for the organism to grow is for this fertilized egg cell to divide into two, then four, eight, sixteen cells, and so on, until the new organism may contain millions of cells. 

This Is Natural Way To Divide The Cell Into Two

Stages of Cell Division As Shown In The Above Image:

1. Animal Cell About To Divide

2. The Nucleus Divides First

3. The Daughter Nuclei Separate And The Cytoplasm Pinches Off Between The Nuclei

4. Two Cells Are Formed

Each time a divides, the plans for making the organism are passed on to the new cell. Therefore, copies of the chromosomes and genes are made so that the plan is available in the new cells.

What Is Mitosis

In simple words we can define mitosis is a part of the cell cycle in which chromosomes in a cell nucleus are separated into two identical sets of chromosomes, and each set ends up in its own nucleus. When a cell is not dividing, we cannot see the chromosomes with an ordinary microscope. But when a nucleus starts to divide, the chromosomes can be seen as double threads. They show up quite clearly if the nucleus of the cell is stained with a special chemical dye. The chromosomes must make copies of themselves before the nucleus divides into two.