DNA Structure and Protein Synthesis

This lesson covers the structure of DNA, the concept of a gene and genome, and the process of protein synthesis (transcription and translation). This is essential content for Edexcel GCSE Biology (1BI0) Topic 3: Genetics.

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DNA Structure

DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic information in all living organisms.

Key Features of DNA

• DNA is found in the nucleus of cells, organised into structures called chromosomes.
• DNA has the shape of a double helix — two strands twisted around each other, like a twisted ladder.
• Each strand is made up of a long chain of nucleotides.

Nucleotides

Each nucleotide consists of three parts:

1. A deoxyribose sugar (a five-carbon sugar).
2. A phosphate group.
3. A nitrogenous base (one of four types).

The sugar and phosphate groups form the backbone of each DNA strand (the sides of the "ladder"). The bases extend inwards and pair with bases on the opposite strand (the "rungs" of the ladder).

The Four Bases

There are four bases in DNA:

Base	Abbreviation	Pairs With
Adenine	A	Thymine (T)
Thymine	T	Adenine (A)
Cytosine	C	Guanine (G)
Guanine	G	Cytosine (C)

Complementary Base Pairing

The bases on opposite strands pair up in a specific way:

• Adenine (A) always pairs with Thymine (T) — held together by 2 hydrogen bonds.
• Cytosine (C) always pairs with Guanine (G) — held together by 3 hydrogen bonds.

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

Exam Tip: A useful mnemonic for base pairing is: Apple Tree and Car Garage (A-T and C-G). If given a DNA sequence and asked for the complementary strand, simply swap A↔T and C↔G.

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Genes and Genomes

What Is a Gene?

A gene is a section of DNA that codes for a specific protein (or polypeptide). Each gene contains a particular sequence of bases that provides the instructions for assembling amino acids in the correct order to make a specific protein.

• Humans have approximately 20,000–25,000 genes.
• Genes are located on chromosomes.
• Different genes code for different proteins, which determine the characteristics of an organism.

What Is the Genome?

The genome is the entire set of genetic material (all the DNA) in an organism. This includes all the genes plus non-coding DNA (DNA that does not code for proteins).

Why Is the Human Genome Important?

Understanding the human genome is useful for:

1. Identifying genes linked to diseases — enabling earlier diagnosis and targeted treatments.
2. Understanding evolutionary relationships — comparing genomes of different species to see how closely related they are.
3. Developing personalised medicine — tailoring drug treatments to a person's specific genetic makeup.
4. Forensic science — DNA profiling to identify individuals.

Exam Tip: The question "Why is it important to understand the human genome?" appears regularly in exams. Make sure you can give at least three reasons with clear explanations.

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Protein Synthesis

Protein synthesis is the process by which cells make proteins using the instructions encoded in DNA. It occurs in two stages: transcription and translation.

Proteins are essential for life. They include:

• Enzymes (biological catalysts that speed up chemical reactions).
• Structural proteins (e.g., collagen in skin, keratin in hair).
• Hormones (e.g., insulin, which regulates blood sugar).
• Antibodies (part of the immune system).
• Transport proteins (e.g., haemoglobin carries oxygen in the blood).

The Triplet Code

• The sequence of bases in DNA is read in groups of three — each group of three bases is called a codon (or triplet).
• Each codon codes for a specific amino acid.
• There are 20 different amino acids that can be combined in different sequences to produce thousands of different proteins.
• The order of amino acids determines the shape and function of the protein.

For example:

• The codon ATG might code for the amino acid methionine.
• The codon GCA might code for the amino acid alanine.

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Stage 1: Transcription

Transcription occurs in the nucleus of the cell.

Steps of Transcription

1. The DNA double helix unwinds and the two strands separate (unzip) at the gene to be copied.
2. One strand of the DNA acts as a template.
3. Free RNA nucleotides pair up with the exposed DNA bases on the template strand, following complementary base pairing rules (with one key difference: in RNA, uracil (U) replaces thymine (T), so A pairs with U).
4. The RNA nucleotides are joined together to form a single strand of messenger RNA (mRNA).
5. The mRNA strand detaches from the DNA template.
6. The DNA strands re-join (zip back up).
7. The mRNA molecule is small enough to leave the nucleus through a nuclear pore.

DNA vs RNA

Feature	DNA	mRNA
Number of strands	Double-stranded (double helix)	Single-stranded
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Location	Stays in the nucleus	Made in nucleus, moves to cytoplasm
Function	Stores genetic information permanently	Carries a copy of the gene's instructions to the ribosome

Exam Tip: The key difference between DNA and mRNA for base pairing is that mRNA uses uracil (U) instead of thymine (T). So if the DNA template strand reads TAC, the mRNA will read AUG (A pairs with U in RNA, not T).

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Stage 2: Translation

Translation occurs at a ribosome in the cytoplasm.

Steps of Translation

1. The mRNA molecule travels from the nucleus to a ribosome in the cytoplasm.
2. The ribosome reads the mRNA sequence three bases at a time (codon by codon).
3. Each codon on the mRNA specifies a particular amino acid.
4. Transfer RNA (tRNA) molecules bring the correct amino acids to the ribosome. Each tRNA has an anticodon that is complementary to the mRNA codon.
5. The amino acids are joined together by peptide bonds in the sequence determined by the mRNA.
6. The chain of amino acids folds into a specific 3D shape, forming a functional protein.

Summary of Protein Synthesis

graph TD
    A["DNA in nucleus"] -->|"Transcription"| B["mRNA strand formed"]
    B --> C["mRNA leaves nucleus through nuclear pore"]
    C --> D["mRNA arrives at ribosome in cytoplasm"]
    D -->|"Translation"| E["tRNA brings amino acids to ribosome"]
    E --> F["Amino acids joined by peptide bonds"]
    F --> G["Polypeptide chain folds into functional protein"]

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Worked Example