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The structure of DNA, how genes code for proteins, what the genome is, and how mutations can change a protein.
Specification references this topic covers: AQA 4.1.2, 4.6.1; Edexcel B3.4, B3.5, B3.21.
DNA is the instruction book for building and running a living thing. Every cell in your body carries a copy. This lesson shows what DNA looks like, how it is copied, and how its instructions are used to make proteins.
It matters because proteins do most of the work in your body. Enzymes, hormones and the parts that give structure are all proteins. A change in DNA can change a protein, and that can change how you work.
By the end you will be able to describe DNA and its base pairs. You will explain how a gene codes for a protein. You will also say what a mutation does.
DNA is a polymer, which is a long molecule made of repeating units. The units are called nucleotides. DNA has two strands that coil together in a double helix. It is found in the chromosomes in the nucleus.
Each nucleotide has a sugar, a phosphate and one of four bases: A, C, G and T. The two strands are held together by pairs of bases. A always pairs with T, and C always pairs with G.
So if one strand reads A C G T, the other reads T G C A.
Think of DNA as a huge recipe book. A gene is one recipe. It is a short section of DNA that codes for one particular protein.
The bases are read in groups of three, called triplets. Each triplet codes for one amino acid. Amino acids are the small units that proteins are built from.
The order of the bases sets the order of the amino acids. This decides the shape and job of the protein. Proteins include enzymes, hormones and collagen.
The genome is the entire genetic material of an organism. The Human Genome Project mapped the human genome. Knowing it helps scientists find genes linked to diseases. It also helps them develop medicines and trace how human populations moved.
Not all DNA codes for proteins. Non-coding regions can switch genes on and off. They control when, and in which cells, a protein is made.
Genetic testing raises ethical issues, such as who should have access to the data.
A mutation is a change in the sequence of bases in DNA. Mutations happen at random. Radiation and some chemicals can also cause them.
Most mutations change the protein very little, or not at all. This is because several triplets can code for the same amino acid. Some change one amino acid, which can change the shape of the protein. An enzyme may then no longer fit its substrate.
A mutation in a non-coding region can change whether a protein is made. Rarely, a mutation helps an organism, and it can be passed on.
DNA is a double helix of nucleotides. A pairs with T, and C pairs with G.
A gene codes for a protein. Three bases code for one amino acid, and the order of amino acids decides the protein’s shape.
A mutation changes the bases and can change the protein.
Name the shape of a DNA molecule.
Answer: a double helix
One strand of DNA has the base sequence T A C G G T. Write the sequence of the complementary strand.
Answer: A T G C C A
Explain how a mutation in a gene could change the function of an enzyme.
Answer: A change in bases changes the amino acids, so the shape of the active site changes
DNA, the genome and mutations practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
Name the four bases found in DNA. Give the one that pairs with guanine.
Answer: cytosine
What is the shape of DNA?
Answer: double helix
What is the name for the entire genetic material of an organism?
Answer: genome
What is a section of DNA that codes for one protein called?
Answer: gene
Which base pairs with thymine in DNA?
Answer: adenine
What are the small units that proteins are made from called?
Answer: amino acids
A DNA strand has the bases A A G C. Write the complementary strand.
Answer: T T C G
How many bases are needed to code for one amino acid?
Answer: 3
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) A section of DNA has the base sequence A T G C. Write the base sequence of the complementary strand. [1 mark] (b) Explain how the order of bases in a gene determines the structure of a protein. [3 marks]
Answer: (a) T A C G (b) the bases are read in groups of three, each triplet codes for one amino acid, so the sequence of bases sets the order of the amino acids, and this determines how the chain folds
Explain how a mutation in a gene can change the protein that the gene codes for. [4 marks]
Answer: A mutation changes the sequence of bases in the gene. This can change an amino acid in the protein. The change can alter the shape of the protein. An enzyme may then no longer fit its substrate.
Pairing A with C in DNA
A pairs with T, and C pairs with G.
Using uracil in DNA
DNA has A, C, G and T. Uracil is in RNA.
Saying all mutations are harmful
Most have no effect. Some are harmful and a few are helpful.
DNA, the genome and mutations is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 7.
Pairing A with C in DNA. A pairs with T, and C pairs with G.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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