13.4 DNA and the Genome

A free GCSE Biology lesson for AQA (8461) from OBA School: dNA, chromosomes, genes and the genome; why the human genome matters. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe DNA as a polymer made of two strands forming a double helix
• Describe how DNA, genes and chromosomes are related
• Define the genome of an organism
• Explain why understanding the human genome is important for medicine and for tracing human history
• Use orders of magnitude and percentages with genome data

Key words

• DNA: Deoxyribonucleic acid: the chemical that makes up the genetic material in the nucleus.
• Polymer: A large molecule made of many similar small units joined in a chain.
• Double helix: The shape of DNA: two strands twisted round each other like a twisted ladder.
• Chromosome: A structure in the nucleus made of one very long DNA molecule (with proteins).
• Gene: A small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
• Genome: The entire genetic material of an organism.

DNA is the genetic material

The genetic material in the nucleus of a cell is made of DNA. DNA is a polymer made up of two strands forming a double helix.

The DNA is contained in structures called chromosomes. In a human body cell there are 46 chromosomes, and each one is a single, very long DNA molecule.

Key fact: Order of size, biggest first: nucleus → chromosome → gene → DNA base. The genome is all of the DNA together.

Genes code for proteins

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein. For example, one gene carries the instructions for an enzyme that digests lactose; another codes for a protein in your hair.

Proteins decide many of your characteristics, so your genes affect what you look like and how your body works.

Worked example: gene, chromosome or genome?

Name the term for each: (a) all the DNA in a bacterium (b) the section of DNA that codes for insulin (c) one long DNA molecule in the nucleus.
(a) all the genetic material = genome (b) one section coding for one protein = gene (c) chromosome

Answer: genome, gene, chromosome

The human genome

The Human Genome Project worked out the order of bases in human DNA. An almost complete sequence was published in 2003, and a fully complete one in 2022. Scientists found that humans have about 20 000 genes that code for proteins, spread across about 3 × 10⁹ base pairs.

• Searching for genes linked to different types of disease; Example finding alleles that increase the risk of some cancers or heart disease
• Understanding and treating inherited disorders; Example knowing the faulty gene in cystic fibrosis helps design tests and new treatments
• Tracing human migration patterns from the past; Example comparing DNA from people around the world shows where groups moved

Worked example: orders of magnitude (maths skill)

The human genome has about 3 × 10⁹ base pairs and about 2 × 10⁴ protein-coding genes. On average, how many base pairs of genome are there for each gene?
(3 × 10⁹) ÷ (2 × 10⁴) = 1.5 × 10⁵ base pairs per gene.
Much of this DNA does not code for proteins at all; only about 1–2% of the genome is coding DNA.

Answer: about 1.5 × 10⁵ (150 000) base pairs

Seeing DNA for yourself

You can extract DNA from fruit such as kiwi or strawberries in a school lab:

• 1. Mash the fruit with a solution of washing-up liquid and salt. The detergent breaks open the cell and nuclear membranes.
• 2. Filter the mixture to remove bits of cell wall and other debris.
• 3. Pour ice-cold ethanol gently down the side of the tube so it forms a layer on top.
• 4. White, stringy DNA comes out of solution where the layers meet. Wind it onto a glass rod.

Exam tip: Each strand you see contains millions of DNA molecules clumped together. A single DNA molecule is far too thin to see.

Common mistakes

• Saying a gene is bigger than a chromosome. A chromosome carries many genes.
• Writing that a gene "makes" a characteristic directly. A gene codes for a protein; the protein affects the characteristic.
• Confusing genome (all the genetic material) with gene (one section of DNA).
• Saying the genetic material is made of RNA or protein. In the nucleus it is DNA.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the genome of an organism? (Choose from: The entire genetic material of the organism, One section of DNA that codes for a protein, A single chromosome, All the proteins in a cell.)

Answer: The entire genetic material of the organism

Working: The genome is all of an organism's genetic material.

Question 2. True or false? The genetic material in the nucleus of a cell is made of protein.

Answer: False

Working: The genetic material is DNA, which is found in chromosomes in the nucleus.

Question 3. Which list is in order of size, smallest first? (Choose from: gene, chromosome, nucleus, cell, chromosome, gene, nucleus, cell, gene, nucleus, chromosome, cell, cell, nucleus, chromosome, gene.)

Answer: gene, chromosome, nucleus, cell

Working: A gene is part of a chromosome; chromosomes are in the nucleus; the nucleus is in the cell.

Related lessons

• Previous lesson: 13.3 The Best of Both Worlds
• Next lesson: 13.5 DNA Structure and Protein Synthesis
• In this chapter: 13.1 Types of Reproduction
• In this chapter: 13.2 Cell Division in Sexual Reproduction
• In this chapter: 13.6 Gene Expression and Mutation

Continue in the full course

This lesson is section 13.4 (AGCBH1304) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.