The material of inheritance Inside nearly all cells there is a nucleus. The nucleus contains thread-like chromosomes. These chromosomes carry the genes that control all your characteristics. Look at the picture of human chromosomes: If you were to count them, you would find 46. This full number of chromosomes is called the diploid number. They are different shape and sizes. You can separate them into identical pairs. Chromosomes occur in pairs. But there are different numbers of chromosomes for different species of animal and plant. Humans have 23 pairs, cats have 19 pairs. Fruit flies have only 4 pairs. What’s a gene? Each chromosome is made up of thousands of genes arranged like beads on a necklace. It is the genes that carry the genetic information that affects how we grow and what we look like. For instance there is a gene for eye colour, and others for hair and for height. If we could unravel a chromosome, it would form an extremely long thread. That thread is made up of a chemical called DNA. DNA stands for deoxyribonucleic acid. A single gene is made of short length of DNA. So the long thread that makes up a chromosome contains hundreds of genes. A DNA molecule is made up of three molecules: A phosphate A sugar A base The sugar and phosphate molecule join up and form the backbone of the DNA strand. The bases are attached to the sugar molecules. In fact if you look at the diagram, you will see that DNA is made up of two strands of nucleotides. It is rather like a ladder. The sugars and phosphate make up the uprights of the ladder and the bases make up the rungs. And then the whole molecule is twisted into a double helix – a bit like a spiral staircase.

The material of inheritance and genes

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The material of inheritance

Inside nearly all cells there is a nucleus. The nucleus contains thread-like chromosomes. These chromosomes carry the genes that control all your characteristics.

Look at the picture of human chromosomes:

If you were to count them, you would find 46. This full number of chromosomes is called the diploid number. They are different shape and sizes. You can separate them into identical pairs.

Chromosomes occur in pairs. But there are different numbers of chromosomes for different species of animal and plant. Humans have 23 pairs, cats have 19 pairs. Fruit flies have only 4 pairs.

What’s a gene?

Each chromosome is made up of thousands of genes arranged like beads on a necklace. It is the genes that carry the genetic information that affects how we grow and what we look like. For instance there is a gene for eye colour, and others for hair and for height.

If we could unravel a chromosome, it would form an extremely long thread. That thread is made up of a chemical called DNA. DNA stands for deoxyribonucleic acid.

A single gene is made of short length of DNA. So the long thread that makes up a chromosome contains hundreds of genes.

A DNA molecule is made up of three molecules:

A phosphate A sugar A base

The sugar and phosphate molecule join up and form the backbone of the DNA strand. The bases are attached to the sugar molecules.

In fact if you look at the diagram, you will see that DNA is made up of two strands of nucleotides. It is rather like a ladder. The sugars and phosphate make up the uprights of the ladder and the bases make up the rungs. And then the whole molecule is twisted into a double helix – a bit like a spiral staircase.