DNA structure, the genome and genes

What you'll learn

This guide covers everything you need to know about DNA structure, the genome and genes for AQA GCSE Biology. You'll understand how genetic information is stored in cells, the structure of DNA molecules, and how genes control characteristics. This topic is fundamental to understanding inheritance, variation and genetic engineering in later units.

Key terms and definitions

DNA (deoxyribonucleic acid) — the chemical that contains all the genetic information in an organism; it is a polymer made of two strands forming a double helix

Gene — a small section of DNA on a chromosome that codes for a specific sequence of amino acids to make a particular protein

Genome — the entire genetic material of an organism; all the DNA contained within its cells

Chromosome — a structure made of DNA molecules containing many genes; chromosomes are found in the nucleus of cells

Nucleotide — the repeating unit (monomer) that makes up DNA, consisting of a sugar, phosphate group and one of four bases

Base pair — complementary bases that join the two strands of DNA together (A pairs with T, C pairs with G)

Allele — a different version of the same gene; for example, the gene for eye colour has different alleles for blue, brown or green eyes

Protein synthesis — the process by which cells make proteins using the genetic code in DNA

Core concepts

DNA structure and organisation

DNA is a polymer made up of two long strands twisted together to form a double helix structure. Each strand is made of repeating units called nucleotides. Understanding this structure is essential for GCSE Biology.

Each nucleotide consists of three components:

• A sugar molecule (deoxyribose)
• A phosphate group
• One of four different bases: adenine (A), thymine (T), cytosine (C) or guanine (G)

The two strands of DNA are held together by base pairs. The bases follow specific pairing rules called complementary base pairing:

• Adenine (A) always pairs with thymine (T)
• Cytosine (C) always pairs with guanine (G)

This complementary pairing is crucial because it allows DNA to be copied accurately during cell division. The sugar and phosphate molecules form the "backbone" of each strand, with the bases pointing inward toward each other.

In eukaryotic cells (cells with a nucleus), DNA is packaged into chromosomes located in the nucleus. Human body cells normally contain 46 chromosomes arranged in 23 pairs. Each chromosome contains hundreds or thousands of genes along its length.

Genes and their function

A gene is a section of DNA that codes for a specific protein. The sequence of bases in a gene determines the sequence of amino acids in the protein. This is fundamental to how characteristics are controlled.

Genes work by controlling protein synthesis. The process follows this sequence:

1. The gene's base sequence acts as a code
2. This code determines the order of amino acids
3. Amino acids join together to form a specific protein
4. The protein carries out a particular function in the cell

Different proteins have different functions:

• Structural proteins like collagen provide strength to tissues
• Enzymes like amylase catalyse specific reactions
• Hormones like insulin regulate body processes
• Antibodies like immunoglobulin G fight infections

Each protein's function depends on its specific shape, which is determined by the sequence of amino acids, which in turn is determined by the sequence of bases in the gene. This demonstrates why genes are so important in controlling characteristics.

Not all parts of DNA code for proteins. Some sections of DNA have regulatory functions or may not code for anything at all. However, at GCSE level, focus on understanding that genes code for proteins.

The genome

The genome is the entire genetic material of an organism. In humans, this includes all the DNA contained in the 46 chromosomes found in most body cells, plus the small amount of DNA found in mitochondria.

Understanding the genome has important applications:

Medicine and health:

• Identifying genes linked to diseases such as cystic fibrosis or sickle cell anaemia
• Developing targeted treatments based on individual genetic makeup (personalised medicine)
• Predicting disease risk and implementing preventive measures
• Understanding how pathogens evolve and spread (relevant for tracking diseases in Caribbean and UK populations)

Evolutionary biology:

• Tracing human migration patterns by comparing genomes from different populations
• Understanding relationships between species
• Studying how organisms have adapted to different environments

Ancestry and migration:

• Scientists have used genome studies to trace human origins in Africa
• Understanding how populations migrated across continents, including to the Caribbean
• Identifying genetic diversity within and between populations

The Human Genome Project, completed in 2003, was an international effort to sequence all the DNA in human chromosomes. This project has enabled many of the applications listed above and continues to provide valuable data for research.

Relationship between DNA, genes and chromosomes

Understanding the hierarchy of genetic organisation is crucial:

DNA → Genes → Chromosomes → Nucleus → Cell

• DNA is the chemical molecule that carries genetic information
• Genes are sections of DNA that code for specific proteins
• Chromosomes are long molecules of DNA packaged with proteins
• Chromosomes are found in the nucleus of cells
• The complete set of chromosomes makes up the genome

A useful analogy: if the genome is like a complete encyclopedia, each chromosome is like one volume, each gene is like one article, and the DNA bases are like the individual letters that spell out the information.

In human cells:

• 46 chromosomes in body cells (23 pairs)
• Each chromosome contains hundreds to thousands of genes
• Each gene typically codes for one protein
• The entire genome contains approximately 20,000-25,000 genes

DNA and inheritance

Genes are inherited from parents through reproduction. This explains why offspring show characteristics similar to their parents.

In sexual reproduction:

• Offspring receive half their chromosomes (and therefore half their genes) from each parent
• This produces genetic variation in offspring
• Each parent passes on one allele of each gene to their offspring

Different versions of the same gene are called alleles. For example:

• The gene for earlobe attachment has two alleles: attached or free
• The gene for blood group has three alleles: A, B and O
• The gene for hair colour has multiple alleles producing different shades

The combination of alleles an individual inherits determines their characteristics. Some alleles are dominant (only one copy needed to show the characteristic) while others are recessive (two copies needed). This will be explored in more detail in the inheritance and variation topic.

DNA and protein synthesis overview

While detailed mechanisms aren't required at GCSE, you should understand the basic principle of how genes control characteristics:

1. The sequence of bases in a gene provides a code
2. Each set of three bases codes for one specific amino acid
3. The order of bases determines the order of amino acids in the protein
4. This creates proteins with specific shapes and functions
5. These proteins determine characteristics of the organism

For example:

• The gene for insulin codes for the amino acid sequence of insulin protein
• If this gene is mutated (altered), the sequence of bases changes
• This could change the amino acid sequence
• This could alter the shape and function of the insulin protein
• This could result in diabetes

Worked examples

Example 1: Describing DNA structure (4 marks)

Question: Describe the structure of DNA. (4 marks)

Mark scheme answer:

• DNA is a polymer / large molecule (1 mark)
• Made of two strands (1 mark)
• Twisted to form a double helix (1 mark)
• Contains four different bases: A, T, C and G (1 mark)

Alternative acceptable points:

• Made of nucleotides / repeating units (1 mark)
• Bases pair in complementary pairs: A with T, C with G (1 mark)
• Contains sugar and phosphate groups (1 mark)

Examiner tip: For a 4-mark "describe" question, aim to make 4 distinct points. Use correct scientific terminology like "polymer," "nucleotide" and "complementary base pairs."

Example 2: Explaining genes and proteins (3 marks)

Question: Explain how a gene controls a characteristic in an organism. (3 marks)

Mark scheme answer:

• A gene is a section of DNA that codes for a protein (1 mark)
• The sequence of bases in the gene determines the sequence of amino acids in the protein (1 mark)
• The protein produced controls/determines the characteristic (1 mark)

Examiner tip: The command word "explain" requires you to link cause and effect. Show the logical sequence: gene → protein → characteristic.

Example 3: Understanding the genome (2 marks)

Question: Define the term 'genome' and give one reason why understanding the human genome is important. (2 marks)

Mark scheme answer:

• The genome is the entire genetic material of an organism / all the DNA (1 mark)
• Important for: identifying genes linked to diseases / developing treatments for genetic diseases / tracing migration patterns / understanding inheritance (any one reason) (1 mark)

Examiner tip: When a question asks for a definition plus an application, make sure you provide both parts. One mark is typically awarded for each component.

Common mistakes and how to avoid them

• Confusing genes and chromosomes: Remember that chromosomes contain many genes. A gene is a small section of a chromosome, not the whole structure. Think of a chromosome as a very long molecule containing hundreds of genes arranged in sequence.

• Incorrect base pairing: Always pair A with T and C with G, never A with C or G with T. Learn the mnemonic "Apple Tree" (AT) and "Car Garage" (CG) to remember complementary base pairs.

• Thinking all DNA codes for proteins: At GCSE, focus on understanding that genes (sections of DNA) code for proteins, but remember that not all DNA is made up of genes. Some DNA has regulatory functions.

• Vague descriptions of DNA structure: Don't just say "DNA is a double helix." Include specific details: it's a polymer made of nucleotides, has two strands, contains four bases, and the bases pair in complementary pairs. Use precise scientific terminology.

• Confusing genome with gene: The genome is ALL the genetic material in an organism (the complete set of DNA), while a gene is just one small section of DNA. Don't use these terms interchangeably.

• Forgetting the gene → protein → characteristic link: In exam questions about how genes work, always explain the full chain: the gene codes for a protein, and that protein controls the characteristic. Don't jump from gene directly to characteristic without mentioning protein.

Exam technique for "DNA structure, the genome and genes"

• Command word awareness: "Describe" means state features without explanation (list key points). "Explain" requires you to link cause and effect using connecting words like "because," "therefore," or "this leads to." For "describe the structure of DNA," list components. For "explain how a gene works," show the mechanism.

• Use diagrams strategically: If asked to describe DNA structure, a clearly labelled diagram of the double helix showing nucleotides, base pairs, and the sugar-phosphate backbone can earn marks. Always label diagrams fully and add a brief written explanation to ensure you score all available marks.

• Be specific about scale: Know the hierarchy: bases are parts of nucleotides, nucleotides join to form DNA strands, DNA forms genes, genes are sections of chromosomes, chromosomes are in the nucleus. Questions may test whether you understand these different levels of organisation.

• Link to real-world applications: Questions about the genome often require examples of its importance. Prepare examples from medicine (identifying disease genes, personalised treatments), evolution (tracing human migration), or agriculture (selective breeding). Context examples from UK or Caribbean health issues can demonstrate understanding.

Quick revision summary

DNA is a double helix polymer made of nucleotides, each containing a sugar, phosphate and one of four bases (A, T, C, G). Bases pair complementarily: A with T, C with G. Genes are sections of DNA that code for specific proteins by determining amino acid sequences. Chromosomes are long DNA molecules containing many genes, found in the cell nucleus. The genome is all genetic material in an organism. Understanding the genome enables disease identification, treatment development and ancestry tracing. Genes control characteristics through the proteins they code for.