DNA and Protein Synthesis

GCSE Biology · Inheritance

DNA Structure

DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for the development and functioning of all living organisms. It is found in the nucleus of cells, on structures called chromosomes.

Key Facts About DNA

DNA is a polymer — a long chain made of repeating units called nucleotides
Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a base
There are four bases: A (adenine), T (thymine), C (cytosine), G (guanine)
DNA has a double helix structure — two strands twisted around each other (discovered by Watson and Crick in 1953, building on X-ray data from Franklin and Wilkins)

Base Pairing Rules

The two strands are held together by complementary base pairs:

A always pairs with T (two hydrogen bonds)
C always pairs with G (three hydrogen bonds)

This is called complementary base pairing. It means that if you know the sequence on one strand, you can work out the other.

Example: If one strand reads A T C G G T, the complementary strand is T A G C C A

Genes and Chromosomes

A gene is a short section of DNA on a chromosome
Each gene codes for a specific sequence of amino acids, which makes a particular protein
Humans have approximately 20,000–25,000 genes
Humans have 23 pairs of chromosomes (46 total) — one set from each parent
The entire set of genetic material in an organism is called its genome

The Human Genome Project

The Human Genome Project (completed in 2003) mapped the entire human genome — all 3 billion base pairs.

Benefits:

Identify genes linked to diseases (e.g. cystic fibrosis, breast cancer)
Develop personalised medicine — drugs tailored to an individual's genes
Understand evolutionary relationships between species
Improve forensic science (DNA profiling)
Identify individuals at risk of genetic diseases for early screening

Protein Synthesis

Proteins are made in two stages: transcription and translation.

Step 1: Transcription (in the nucleus)

1. The two strands of DNA unzip (hydrogen bonds between base pairs break)

2. One strand is used as a template

3. Free RNA nucleotides line up along the template strand following complementary base pairing rules (but in RNA, uracil (U) replaces thymine — so A pairs with U)

4. A molecule of mRNA (messenger RNA) is produced

5. The mRNA detaches and moves out of the nucleus through a nuclear pore into the cytoplasm

Step 2: Translation (at a ribosome in the cytoplasm)

1. The mRNA attaches to a ribosome

2. The ribosome reads the mRNA three bases at a time — each group of three bases is called a codon

3. Each codon codes for a specific amino acid

4. tRNA (transfer RNA) molecules carry the correct amino acids to the ribosome

5. Amino acids are joined together in the correct order by peptide bonds

6. The chain of amino acids folds into a specific 3D shape → a protein

Types of Protein

Protein type	Example	Function
Structural	Collagen	Provides strength in tendons, bone, cartilage
Enzymes	Amylase, lipase	Catalyse metabolic reactions
Hormones	Insulin	Chemical messengers
Antibodies	Immunoglobulins	Defend against pathogens
Receptors	Hormone receptors on cell membranes	Detect chemical signals
Transport	Haemoglobin	Carries oxygen in red blood cells

Mutations

A mutation is a change in the DNA base sequence of a gene. Most mutations have no effect on the protein because:

The genetic code is degenerate (more than one codon can code for the same amino acid)
The mutation may be in a non-coding region of DNA

However, some mutations can:

Change the amino acid sequence → change the protein's shape → protein may not function
Be beneficial (rare) — produce a protein that gives an advantage (basis of evolution)
Be harmful — cause genetic disorders (e.g. sickle cell anaemia, cystic fibrosis)

Exam Tips

DNA bases pair A-T and C-G — in RNA, thymine is replaced by uracil (A-U)
Transcription happens in the nucleus; translation happens at a ribosome in the cytoplasm
A gene codes for a protein, not a characteristic directly — the protein determines the characteristic
Know what the Human Genome Project is and be able to discuss its benefits