DNA, genes and the genomeIB MYP Biology: Revision notes
Section 1
The structure of DNA
DNA (deoxyribonucleic acid) is the molecule that carries genetic information. It is made of two strands twisted into a double helix, like a twisted ladder. The strands are held together by pairs of chemical bases in the middle.
There are four bases: A (adenine), T (thymine), C (cytosine) and G (guanine). They always pair in the same way: A with T and C with G. This is called complementary base pairing.
Worked example: if one strand reads ATTGC, the strand opposite it reads TAACG.
Section 2
Genes and proteins
A gene is a section of DNA that codes for a particular protein. The order of the bases in the gene decides the order of amino acids, which are joined to make the protein. Different proteins do different jobs, for example enzymes and haemoglobin, so genes control the features of an organism.
Section 3
Chromosomes and the genome
DNA is coiled into structures called chromosomes in the nucleus. Each chromosome is one very long DNA molecule carrying many genes. A human body cell has 46 chromosomes, arranged as 23 pairs, with one of each pair from each parent. Gametes have 23.
The genome is all the genetic material of an organism. The human genome has about 20 000 genes, and scientists have read the complete sequence of its bases.
Order of size: genome, then chromosome, then gene, then base.
Section 4
Extracting DNA
DNA can be extracted from fruit such as strawberries in the school laboratory:
- Mash the fruit to break open the cells.
- Add salt and detergent. The detergent breaks down the cell membranes and nuclear membranes to release the DNA, and the salt helps the DNA clump together.
- Filter to remove solid pieces.
- Pour ice-cold ethanol down the side of the tube. DNA is not soluble in cold ethanol, so it appears as white, stringy strands at the boundary.
The ethanol does not dissolve the DNA. It makes the DNA come out of solution so you can see it.
Section 5
Mutations
A mutation is a change in the sequence of bases in DNA. It can change a gene, so a different protein, or a protein that does not work, may be made.
The possible effects are:
- No effect, which is the most common.
- Harmful, for example a faulty protein causes an inherited disorder.
- Beneficial (rare), for example a change that makes the organism better suited to its environment.
Mutations happen by chance when DNA is copied, and happen more often with radiation or some chemicals.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA, genes and the genome
- A science museum in Dubai has built a large model of DNA for visitors to explore. The model shows two long strands twisted around each other, with the strands joined together in the middle by pairs of bases.Distinguish between a gene and the genome.2 marks
- A student in Manila extracts DNA from strawberries in a school laboratory. She mashes the fruit, adds a mixture of salt and washing-up liquid, filters it, and then carefully pours ice-cold ethanol on top of the filtered liquid. White, stringy strands slowly appear where the two liquids meet.Explain why the student mashes the fruit and then filters the mixture.2 marks
- A genetic counsellor in Cape Town explains to a family that every human body cell has a nucleus containing chromosomes made of DNA, and that a change in a gene can sometimes cause an inherited disorder.State the number of chromosomes in a human body cell and describe how chromosomes, genes and DNA are related.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).