Reproduction » DNA and the Genome

DNA: the genetic material

The genetic material in the nucleus of a cell is made of a chemical called DNA. Every instruction your cells need to build and run your body is stored in it.

DNA is a polymer. That means it is a very long molecule made of many smaller units joined together in a chain, a bit like a long string of beads. In DNA, there are two strands of these chains. The two strands twist around each other to form a double helix. You can picture it as a twisted ladder or a spiral staircase.

Key terms:

• DNA: the chemical that makes up the genetic material in the nucleus of a cell.
• Polymer: a long molecule made up of many smaller units joined together.
• Double helix: the shape of DNA, two strands twisted around each other.

From DNA to genes

A DNA molecule is far too long to float around loose. It is contained in structures called chromosomes.

A gene is a small section of DNA on a chromosome. Think of a chromosome as a long book of instructions and each gene as one single instruction in it.

What does each instruction say? Each gene codes for a particular sequence of amino acids. Amino acids are joined together in that exact order to make a specific protein. So one gene holds the instructions for making one protein.

Remember, you met chromosomes and genes when you studied how cells divide. Here the new idea is what a gene actually does: it tells the cell which amino acids to join, and in what order, to make a protein.

The genome

The genome of an organism is the entire genetic material of that organism. It is all of the DNA, with all of its genes, in one complete set.

Every living organism has its own genome. The whole human genome has now been studied. Scientists have worked out all of the genetic material that makes a human being. This is expected to have great importance for medicine in the future.

Key terms:

• Genome: the entire genetic material of an organism.

Why the human genome matters

You need to be able to discuss why understanding the human genome is important. There are three main reasons.

Let us look at each one.

• Searching for genes linked to disease. Scientists search the human genome for genes that are linked to different types of disease.
• Understanding and treating inherited disorders. An inherited disorder is passed on in the genes from parents. If we know which gene is involved, we understand more about what has gone wrong and can work towards treating it.
• Tracing human migration. Similarities and differences in the genomes of groups of people can be used to trace how humans moved around the world in the past.

Exam-style question

(a) Describe the structure of DNA. [2 marks]

(b) What is meant by the term genome? [1 mark]

(c) Give two reasons why it is important to understand the human genome. [2 marks]