Edexcel IGCSE Biology · Spec 3.14-3.15
DNA, Genes and Chromosomes
The relationship between DNA, genes and chromosomes, and the structure of a DNA molecule.
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DNA, Genes and Chromosomes
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Explained
DNA, genes and chromosomes: how the four terms fit together
These words describe the same material at four different scales, and questions almost always test whether you can place them in order. Chromosomes sit in the nucleus. Each chromosome is one long coiled molecule of DNA. A gene is a short section of that DNA. The genome is all of it, taken together.
From largest to smallest
- Nucleus: the part of the cell containing the chromosomes.
- Chromosome: one long, coiled molecule of DNA.
- Gene: a section of DNA that codes for a specific protein.
- Genome: the entire DNA of an organism.
Notice that a chromosome is not made of many DNA molecules. It is one molecule, coiled tightly. That is a common misconception and it is easy to avoid once you have read the definition carefully.
Defining a gene precisely
A gene is a section of DNA that codes for a specific protein. All three parts of that definition carry weight in a mark scheme.
Saying a gene "carries information" or "controls a characteristic" is too loose. It codes for a protein, and it is that protein which then affects the characteristic. Examiner reports across this part of the specification consistently reward the precise definition over the general description.
The structure of DNA
DNA is a double helix, which is the twisted ladder shape you will be asked to recognise and sometimes to draw. Two strands wind around each other, joined along their length.
The shape matters because it allows the molecule to be copied: the two strands can separate, and each can act as a pattern for a new partner strand.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology show questions in this area asking candidates to state what is meant by a term, with genome and gene both appearing. These are definition questions, and they are marked as definitions.
The lesson from the reports on similar items is consistent: a general statement about what something does is not the same as a definition, and only the definition scores. Learn the wording, not the gist.
How the scales connect in an answer
If asked to explain the relationship, work outward or inward but keep the order intact. A gene is a section of a chromosome, a chromosome is a molecule of DNA in the nucleus, and the genome is every chromosome an organism has.
Answers that jump between scales without stating the links tend to read as though the terms are interchangeable, which is precisely what the question is checking.
Spec 3.14-3.15
What you need to know
- Recall that chromosomes are found in the nucleus
- Link chromosomes, genes and DNA
- Define gene and genome accurately
Active recall
Quick check
Answer each question before opening the answer.
Define a gene and a chromosome.
A gene is a section of DNA that codes for a protein; a chromosome is a long DNA molecule carrying many genes.
Describe the structure of DNA.
A double helix of two strands held together by complementary base pairs: A with T, and C with G.
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