13.5 DNA Structure and Protein Synthesis

A free GCSE Biology lesson for AQA (8461) from OBA School: nucleotides, complementary base pairs, the triplet code and how proteins are made. 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 of nucleotides, each made of a sugar, a phosphate group and one of four bases
• Use the base-pairing rules A–T and C–G to work out a complementary strand (Higher tier)
• Explain how the order of bases controls the order of amino acids in a protein (Higher tier)
• Describe protein synthesis on ribosomes (Higher tier)
• Explain why a protein's shape matters for its job (Higher tier)

Key words

• Nucleotide: The repeating unit of DNA: one sugar, one phosphate group and one base.
• Base: One of four chemicals in DNA: adenine (A), thymine (T), cytosine (C) or guanine (G).
• Base pair: Two complementary (matching) bases joined across the two strands: A with T, or C with G.
• Triplet: A sequence of three bases. It codes for one particular amino acid.
• Ribosome: A tiny structure in the cytoplasm where proteins are made.
• Template: A copy of the gene's base sequence that the ribosome reads (it is called mRNA).
• Carrier molecule: A molecule that brings a specific amino acid to the ribosome (it is called tRNA).

Nucleotides: the units of DNA

DNA is a polymer made from four different nucleotides. Each nucleotide is a common sugar and a phosphate group with one of four bases attached to the sugar.

The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases: A, C, G or T.

Key fact: The sugar–phosphate backbone is the same all along the strand. Only the bases change, so the order of bases is what carries the genetic code.

Complementary base pairs (Higher tier)

The two strands are held together by weak bonds between the bases. The pairs are always the same: A with T, and C with G. The bases on one strand are complementary to the bases on the other.

Worked example: writing the other strand

One strand reads A T T G C A C G. Write the complementary strand.
Swap each base for its partner: A→T, T→A, G→C, C→G.

Answer: T A A C G T G C

Worked example: base percentages (maths skill)

In a sample of DNA, 22% of the bases are A. What percentage are C?
A pairs with T, so T = 22%. A + T = 44%.
C + G = 100 − 44 = 56%, and C = G, so C = 56 ÷ 2 = 28%.

Answer: 28%

The triplet code (Higher tier)

A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are joined to make a particular protein.

• TAC GGA TCA; Triplets 3; Amino acids in the chain 3
• 450 bases; Triplets 450 ÷ 3 = 150; Amino acids in the chain 150
• a protein with 312 amino acids; Triplets 312; Amino acids in the chain 312 × 3 = 936 bases

Exam tip: Divide the number of bases by 3 to get the number of amino acids; multiply amino acids by 3 to get bases. Real genes also have some extra non-coding bases.

Protein synthesis (Higher tier)

DNA cannot leave the nucleus, but proteins are made on ribosomes in the cytoplasm. So the cell makes a copy of the gene to carry the code out.

• 1. The two DNA strands of the gene separate. A template copy of the base sequence is made (a molecule called mRNA).
• 2. The template leaves the nucleus and moves to a ribosome in the cytoplasm.
• 3. The ribosome reads the bases three at a time (one triplet per amino acid).
• 4. Carrier molecules (tRNA) bring specific amino acids to add to the growing chain in the correct order.
• 5. When the chain is complete it folds up to form a unique shape. This shape lets the protein do its job.

• Enzymes; Why shape matters the active site must fit the substrate
• Hormones; Why shape matters must fit receptors on target cells (e.g. insulin)
• Structural proteins; Why shape matters shape gives strength, e.g. collagen in skin and tendons

Exam tip: Use the key words in order: template → ribosome → triplets → carrier molecules → amino acids joined → chain folds.

Common mistakes

• Pairing A with G or C with T. Learn it: A–T and C–G only.
• Saying a triplet codes for a protein. A triplet codes for one amino acid; a gene codes for a protein.
• Writing that proteins are made in the nucleus. They are made on ribosomes in the cytoplasm.
• Forgetting the last step: the chain must fold into a specific shape to work.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher) What does a sequence of three bases code for? (Choose from: One amino acid, One protein, One gene, One chromosome.)

Answer: One amino acid

Working: Each triplet of bases codes for one particular amino acid.

Question 2. (Higher) Write the complementary strand for this DNA sequence: A G C T T A

Answer: TCGAAT

Working: A→T, G→C, C→G, T→A, T→A, A→T gives TCGAAT.

Question 3. (Higher) A protein is made of 280 amino acids. What is the smallest number of bases needed to code for it?

Answer: 840

Working: 280 × 3 = 840 bases.

Related lessons

• Previous lesson: 13.4 DNA and the Genome
• Next lesson: 13.6 Gene Expression and Mutation
• In this chapter: 13.1 Types of Reproduction
• In this chapter: 13.2 Cell Division in Sexual Reproduction
• In this chapter: 13.3 The Best of Both Worlds

Continue in the full course

This lesson is section 13.5 (AGCBH1305) 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.