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B12.3 DNA and the Genome

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: dNA, chromosomes, genes and the genome; why knowing 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, and why some people have concerns
  • Use standard form, orders of magnitude and percentages with genome data

Key words

  • DNA: The chemical that makes up the genetic material in the nucleus of a cell.
  • 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.
  • Chromosome: A structure in the nucleus made of one very long, tightly coiled DNA molecule.
  • 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 twisted to form a double helix.

The DNA is coiled up very tightly in structures called chromosomes. A human body cell has 46 chromosomes, and each one is a single, very long DNA molecule. Laid end to end, the DNA in one cell would be about 2 metres long.

Key fact: Order of size, biggest first: cell → nucleus → chromosome → gene. The genome is all of the genetic material 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. Proteins include enzymes, some hormones and structural proteins such as those in hair and muscle.

Proteins decide many of your characteristics, so your genes affect what you look like and how your body works. Humans have about 20 000 genes.

Worked example: gene, chromosome or genome?

Name the term for each: (a) the section of DNA that codes for the enzyme amylase (b) all the DNA of a pea plant (c) one coiled DNA molecule in the nucleus.
(a) one section coding for one protein = gene (b) all the genetic material = genome (c) chromosome

Answer: gene, genome, chromosome

The Human Genome Project

The Human Genome Project was a huge international collaboration. Scientists in many countries worked together from 1990 to 2003 to find the order of all of the bases in human DNA: about 3 × 10⁹ base pairs.

  • Insurance; Why companies might charge more to people whose genes show a higher risk of disease
  • Employment; Why employers might treat people differently because of their genes
  • Ethics and privacy; Why who should be allowed to see a person's genetic information?

Exam tip: Having an allele linked to a disease usually means a higher risk, not that the person will definitely get it. Lifestyle and the environment also matter.

Comparing genomes (maths skill)

Worked example: how many times bigger?

The human genome is about 3.1 × 10⁹ base pairs; a fruit fly genome is about 1.4 × 10⁸ base pairs. How many times bigger is the human genome?
(3.1 × 10⁹) ÷ (1.4 × 10⁸) = 22.1…
So the human genome is about 22 times bigger (about one order of magnitude).

Answer: about 22 times bigger

Worked example: base pairs per gene

About 3 × 10⁹ base pairs and 2 × 10⁴ genes: (3 × 10⁹) ÷ (2 × 10⁴) = 1.5 × 10⁵ base pairs per gene on average.
Most of this DNA does not code for proteins: only about 1–2% of the genome is genes that code for proteins.

Answer: 1.5 × 10⁵ base pairs per gene

Common mistakes

  • Saying a gene is bigger than a chromosome. A chromosome carries many genes.
  • Writing that a gene directly "is" a characteristic. A gene codes for a protein; the protein affects the characteristic.
  • Confusing genome (all the genetic material) with gene (one section of DNA).
  • Describing DNA as one strand. It has two strands in a double helix.

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 gene that codes for one protein, The pair of sex chromosomes, The nucleus of a cell.)

Answer: The entire genetic material of the organism

Working: The genome is all of the genetic material of an organism. In humans it is about 20 000 genes on 23 pairs of chromosomes.

Question 2. True or false? DNA is a polymer made of two strands twisted to form a double helix.

Answer: True

Working: DNA is a polymer of two strands that twist around each other to form a double helix.

Question 3. What does a gene code for? (Choose from: A particular sequence of amino acids, to make a specific protein, A particular sugar, to make starch, A whole chromosome, A type of cell division.)

Answer: A particular sequence of amino acids, to make a specific protein

Working: Each gene codes for a particular sequence of amino acids, which makes a specific protein.

Related lessons

Continue in the full course

This lesson is section B12.3 (AGCSC1203) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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