DNA Structure and Protein Synthesis

The Double Helix

DNA is a large molecule made of two strands twisted into a double helix shape (like a twisted ladder).

Nucleotides — The Building Blocks

Each strand of DNA is made up of repeating units called nucleotides. Each nucleotide consists of:

• A sugar molecule (deoxyribose)
• A phosphate group
• One of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

The sugar and phosphate groups form the backbone (sides of the ladder). The bases form the rungs.

Base Pairing Rules

The two strands are held together by complementary base pairs joined by hydrogen bonds:

$A \longleftrightarrow T \quad \text{(Adenine pairs with Thymine)}$ $C \longleftrightarrow G \quad \text{(Cytosine pairs with Guanine)}$

Memory tip: A-T (think "AT") and C-G (think "CG"). Or: A pairs with T because they both have a "T" sound. C and G are the other two.

Because of complementary base pairing, if you know one strand's sequence, you can determine the other:

• Strand 1: A T C G G T A
• Strand 2: T A G C C A T

• Chromosomes are long, coiled molecules of DNA found in the nucleus
• Humans have 46 chromosomes (23 pairs) in most body cells
• A gene is a section of DNA that codes for a specific protein
• Each gene contains a specific sequence of bases that provides the instructions for making one protein
• The human genome is the entire set of DNA in an organism (approximately 20,000-25,000 genes in humans)

The Human Genome Project

The Human Genome Project (completed in 2003) mapped all the genes in human DNA.

Benefits:

• Identifying genes linked to diseases (e.g., cystic fibrosis, breast cancer)
• Developing targeted medicines (pharmacogenomics)
• Understanding evolutionary relationships between species
• Forensic science and paternity testing

Proteins are made through two main stages: transcription (in the nucleus) and translation (at ribosomes).

Why Proteins Matter

• Proteins control virtually everything in cells:
  • Enzymes — catalyse chemical reactions
  • Structural proteins — collagen (connective tissue), keratin (hair, nails)
  • Hormones — insulin, growth hormone
  • Antibodies — immune defence
  • Carrier proteins — haemoglobin (carries oxygen)

Step 1: Transcription (Nucleus)

1. The DNA double helix unwinds and the two strands separate (hydrogen bonds between bases break)
2. One strand acts as a template
3. Free RNA nucleotides line up against the template strand following complementary base pairing rules:
   • A (DNA) → U (RNA) — RNA uses uracil instead of thymine
   • T (DNA) → A (RNA)
   • C (DNA) → G (RNA)
   • G (DNA) → C (RNA)
4. A molecule of messenger RNA (mRNA) is formed
5. The mRNA molecule leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm

Step 2: Translation (Ribosome)

1. The mRNA attaches to a ribosome
2. The ribosome reads the mRNA in groups of three bases called codons
3. Each codon codes for a specific amino acid
4. Transfer RNA (tRNA) molecules carry amino acids to the ribosome
5. Each tRNA has an anticodon that is complementary to the mRNA codon
6. Amino acids are joined together by peptide bonds in the correct order
7. The chain of amino acids folds into a specific 3D shape to form a protein

The Genetic Code

• There are 64 possible codons (4 bases in groups of 3: $4^3 = 64$)
• These code for 20 amino acids (some amino acids have multiple codons)
• The code is universal — the same codons code for the same amino acids in almost all organisms
• Three codons are stop codons — they signal the end of the protein

A mutation is a change in the DNA base sequence of a gene.

Types of Mutations

• Substitution: One base is replaced by a different base
• Insertion: An extra base is added into the sequence
• Deletion: A base is removed from the sequence

Effects of Mutations

Causes of Mutations

• Spontaneous — random errors during DNA replication
• Mutagens — agents that increase mutation rates:
  • UV radiation
  • X-rays
  • Chemicals (e.g., tar in cigarette smoke)
  • Certain viruses

The sequence of bases in DNA determines the sequence of amino acids in a protein. Different sequences create different proteins with different shapes and functions.

• A change in just one base can change one amino acid, which can change the entire protein's shape and function
• This explains why organisms with very similar DNA (e.g., humans and chimpanzees share ~98.7% of DNA) can still have significant differences

Question: A section of DNA has the base sequence: TAC GGA CTT. Write the mRNA sequence and explain how it would be translated into amino acids. (4 marks)

Solution:

Using complementary base pairing (remembering that RNA uses U instead of T):

mRNA sequence: AUG CCU GAA

Each group of three bases (codon) codes for one amino acid:

• AUG → Methionine (also the start codon)
• CCU → Proline
• GAA → Glutamic acid

The amino acids would be joined by peptide bonds at the ribosome to form part of a protein chain: Methionine – Proline – Glutamic acid.

• Remember: DNA bases = A, T, C, G. RNA bases = A, U, C, G (uracil replaces thymine).
• A codon = three bases on mRNA. An anticodon = three complementary bases on tRNA.
• Transcription = DNA → mRNA (in nucleus). Translation = mRNA → protein (at ribosome).
• When describing mutations, always link the base change → codon change → amino acid change → protein shape change → function change.

• DNA is a double helix made of nucleotides (sugar, phosphate, base) with complementary base pairs: A-T and C-G.
• A gene is a section of DNA coding for a specific protein.
• Transcription copies DNA to mRNA in the nucleus; translation reads mRNA at ribosomes to build a protein.
• mRNA uses codons (3 bases) each coding for one amino acid; tRNA carries amino acids to the ribosome.
• Mutations are changes in DNA base sequences that may alter the protein produced, with effects ranging from none to significant.