What is DNA, how is it structured, and what is meant by the genome?
DNA as the genetic material, its structure as a double helix of four bases, the meaning of a gene, chromosome and genome, and how the genome and its interactions are important in medicine and biology.
A focused answer to AQA GCSE Biology 4.6.1.2 to 4.6.1.4, covering DNA as the genetic material, its double helix structure of four bases, the meaning of gene, chromosome and genome, and the importance of understanding the human genome.
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What this dot point is asking
AQA wants you to describe DNA as the genetic material, explain its structure, define gene, chromosome and genome, and explain why understanding the human genome is important.
DNA and its structure
DNA is made of four bases. The order (sequence) of these bases along a gene codes for the order of amino acids in a protein. Higher-tier candidates meet how the bases are read in threes and how the protein then folds into a particular shape that determines its function, such as whether it is an enzyme or a structural protein. Because a different sequence of bases makes a different protein, the DNA controls all the proteins a cell makes, and so the characteristics of the organism.
Genes, chromosomes and the genome
- A gene is a small section of DNA on a chromosome that codes for a particular sequence of amino acids, which makes a specific protein.
- A chromosome is a long molecule of coiled-up DNA carrying many genes; humans have 23 pairs in each body cell.
- The genome is the entire genetic material of an organism (all of its DNA).
These three terms are about scale: the genome is the whole set, a chromosome is one large package within it, and a gene is one small section of a chromosome. A helpful comparison is a library: the genome is the whole library, each chromosome is one book, and each gene is one set of instructions (a recipe) within a book.
It is also important to understand that genes do not work in isolation. Most characteristics are controlled by several genes interacting, and the proteins they code for then build the body or act as enzymes that control its reactions. A single gene codes for one protein by setting the order of amino acids, and the protein then folds into a particular shape that determines its job. This is why a change to the DNA sequence (a mutation) can change a protein and so a characteristic, and why understanding the genome lets scientists work out which genes are linked to which proteins and which diseases.
Try this
Q1. Define a gene. [2 marks]
- Cue. A small section of DNA that codes for a particular sequence of amino acids to make a specific protein.
Q2. State the structure of a DNA molecule. [1 mark]
- Cue. A polymer of two strands twisted into a double helix.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksDescribe the structure of DNA and explain the relationship between a gene, a chromosome and the genome.Show worked answer →
A 3-mark question rewards the structure plus the three terms in order of size.
DNA is a polymer made of two strands twisted together into a double helix, and it is made of four different bases. A gene is a small section of DNA that codes for a particular sequence of amino acids to make a specific protein. A chromosome is a long molecule of coiled-up DNA that carries many genes. The genome is the entire genetic material of an organism.
Markers reward the double helix of two strands, a gene as a section of DNA coding for a protein, a chromosome carrying many genes, and the genome as all the genetic material.
AQA 20214 marksExplain why understanding the whole human genome is important for medicine and for our understanding of human history.Show worked answer →
A 4-mark explain question rewards distinct uses each with a reason.
Understanding the human genome lets scientists search for genes linked to different diseases, so they can identify people at higher risk and develop treatments. It helps in understanding and treating inherited disorders, because the faulty alleles responsible can be found. It also allows scientists to trace human migration patterns from the past, by comparing the genomes of people from different parts of the world.
Markers reward at least three uses (genes linked to disease, treating inherited disorders, tracing migration), each linked to why understanding the genome makes it possible.
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Sources & how we know this
- AQA GCSE Biology (8461) specification — AQA (2016)