How is genetic information stored in DNA, and how does it control an organism?
The relationship between the nucleus, chromosomes, genes and DNA, the structure of DNA as a double helix with complementary base pairs, the human chromosome number, and how a gene codes for a protein.
A focused CCEA GCSE Double Award Science (Biology Unit B2) answer on genetic material, covering the relationship between the nucleus, chromosomes, genes and DNA, the double helix and complementary base pairs, the human chromosome number, and how a gene codes for a protein.
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What this dot point is asking
CCEA Double Award wants the nesting of nucleus, chromosomes, genes and DNA, the structure of DNA as a double helix with the base-pairing rules, the human chromosome number, and the idea that a gene codes for a protein. This is the foundation for inheritance and variation.
The nesting of the genetic material
It helps to picture this as a set of nested boxes: nucleus contains chromosomes, chromosomes are made of DNA, and short stretches of that DNA are genes.
The human chromosome number
Human body cells contain 46 chromosomes, arranged in 23 pairs. One of each pair came from the mother and one from the father. Twenty-two pairs are ordinary chromosomes, and the last pair are the sex chromosomes (XX in females, XY in males). Sex cells (gametes) have only 23 single chromosomes.
The structure of DNA
DNA is shaped like a twisted ladder called a double helix. The two strands are held together by pairs of bases. There are four bases, labelled A, T, C and G:
The order of the bases along a gene is the code that determines which protein is made.
How a gene codes for a protein
The order of bases in a gene determines the order of amino acids in a protein. Different proteins have different shapes and jobs - enzymes that speed up reactions, hormones such as insulin that carry messages, and structural proteins such as keratin in hair and nails - so by coding for proteins, genes control the characteristics of the organism. If the base sequence changes (a mutation), the protein may change too, which is how new alleles arise.
Most cells in the body contain the same full set of genes, but different cells only switch on the genes they need. A muscle cell and a nerve cell carry identical DNA, but they make different proteins because different genes are active, which is why the cells look and work differently. This idea links DNA to specialised cells.
Genes, alleles and characteristics
A single gene can exist in different versions called alleles. For example, a gene for eye colour might have a brown allele and a blue allele. The combination of alleles a person inherits, together with the environment, decides the characteristic shown. Because each person inherits a unique mix of alleles from two parents, no two people (apart from identical twins) have exactly the same DNA - the basis of DNA fingerprinting used in forensics.
Examples in context
Example 1. Why a mutation can change a person. A mutation changes the base sequence of a gene, which can change the amino acid order and so the protein. If the protein is an important enzyme, the change can affect a characteristic or cause a genetic disorder, linking DNA to inheritance.
Example 2. Why base pairing allows copying. Because A only pairs with T and C only with G, the two strands carry the same information in mirror form. When DNA is copied, each strand acts as a template, so the two new molecules are identical to the original. This accuracy is vital for cell division.
Try this
Q1. Where in the cell are chromosomes found? [1 mark]
- Cue. In the nucleus.
Q2. Which base pairs with cytosine? [1 mark]
- Cue. Guanine.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksDescribe the relationship between a chromosome, a gene and DNA.Show worked answer →
Three nested ideas for three marks.
A chromosome is a long, coiled molecule of DNA found in the nucleus.
A gene is a short section of DNA on a chromosome.
Each gene codes for a particular protein, which controls a characteristic.
Markers want chromosome made of DNA, gene as a section of it, and gene coding for a protein.
CCEA-style3 marksDNA has the bases A, T, C and G. Explain what is meant by complementary base pairing.Show worked answer →
State the pairing rule and why it matters for three marks.
The two strands of DNA are held together by pairs of bases.
Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).
Because the pairing is fixed, one strand acts as a template to make an exact copy, so DNA can be copied accurately. Markers reward A-T and C-G and the idea of a template.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)