How is genetic information stored in chromosomes, genes and DNA?
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 genes control the characteristics of an organism.
A focused CCEA GCSE Biology answer on chromosomes, genes and DNA, covering how they relate to the nucleus, the double helix and complementary base pairing, the human chromosome number, and how genes control characteristics.
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What this dot point is asking
CCEA wants you to explain how the nucleus, chromosomes, genes and DNA relate, describe the structure of DNA as a double helix with complementary base pairs, state the human chromosome number, and explain how genes control characteristics.
From nucleus to DNA
The structure of DNA
Because the base pairing is fixed, each strand can act as a template to make an exact copy, so DNA can be replicated accurately before a cell divides.
The human chromosome number
Human body cells have 46 chromosomes, arranged in 23 pairs. One chromosome of each pair came from the mother and one from the father. Sex cells (gametes) have only 23 single chromosomes, so that fertilisation restores the full 46.
Examples in context
- Example 1. Why genes control characteristics
- A gene is a code for making a protein, and proteins do most of the work in the body. For example, a gene codes for the protein melanin that gives skin and hair their colour, and a different version of a gene can give a different amount of melanin, and so a different hair colour. This is why your genes, the sections of DNA you inherited, determine inherited characteristics such as eye colour and blood group.
- Example 2. Why accurate copying matters
- Before a cell divides, all of its DNA must be copied exactly so that both new cells have a complete set of instructions. Complementary base pairing makes this possible: the two strands separate, and each acts as a template so that A always gets a new T, and C always gets a new G. A mistake in copying (a mutation) can change a gene and so change a protein, which is why faithful copying is so important. This links DNA structure to cell division and to variation.
- Example 3. The scale of the genome
- The complete set of genetic material in an organism is called its genome. The human genome is enormous: 46 chromosomes carry around 20000 genes, made of about three billion base pairs of DNA. If you stretched the DNA from one cell out in a line it would be about two metres long, yet it is coiled and packed into a nucleus far too small to see without a microscope. Scientists have now read the order of all the bases in the human genome, which helps them understand genetic disorders and develop new medicines. CCEA expects you to know that a gene is one small part of this much larger store of information, and that the chromosomes are simply the way the long DNA molecule is organised and packaged.
Try this
Q1. How many chromosomes are in a human body cell? [1 mark]
- Cue. 46 (in 23 pairs).
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 20204 marksDescribe the relationship between a chromosome, a gene and DNA.Show worked answer →
Four marks for the nested relationship, from large to small.
Chromosomes are found in the nucleus of a cell and are made of a long molecule of DNA.
DNA is a double helix, two strands twisted together.
A gene is a short section of DNA on a chromosome.
Each gene codes for a particular protein, and so controls a characteristic of the organism.
Markers reward chromosomes in the nucleus, made of DNA, a gene as a section of DNA, and a gene coding for a protein or characteristic.
CCEA 20193 marksIn DNA, the bases pair up. State the complementary base pairs and explain why pairing is important.Show worked answer →
Three marks for the pairs and the reason.
Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).
This is called complementary base pairing, and the two strands are held together by these base pairs.
Because the pairing is fixed, each strand can act as a template to make an exact copy, which is how DNA is replicated accurately before cell division.
Markers reward A with T and C with G, the strands held together, and the link to accurate copying.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)