Describe DNA as a polymer, the genome and a gene, how DNA is extracted from fruit, and how the order of bases controls protein synthesis through transcription and translation.

What this dot point is asking

Edexcel statements 3.4 to 3.10B want you to describe DNA as a polymer with a double-helix structure, define the genome and a gene, describe how DNA can be extracted from fruit, and (Biology only, 3.7B to 3.10B) explain how the base sequence controls protein synthesis through transcription and translation and how genetic variants affect the phenotype.

The structure of DNA

The bases always pair in the same way: A pairs with T and C pairs with G. This is called complementary base pairing, and it means that if you know the bases on one strand you can work out the other. DNA is stored in the nucleus, wound up into structures called chromosomes.

The genome and genes

The Human Genome Project worked out the order of all the bases in human DNA, which has many uses in medicine (see the variation and mutation page).

Extracting DNA from fruit

DNA can be extracted from fruit such as strawberries or kiwi in a school lab. You mash the fruit with a mixture of salt and detergent (the detergent breaks open the cell and nuclear membranes; the salt makes the DNA clump together), filter the mixture, then add cold ethanol. The DNA is insoluble in cold ethanol, so it comes out of solution as white stringy threads at the boundary between the layers.

Protein synthesis (Biology only)

The base sequence of a gene is a code for building a protein. The code is read in groups of three bases, and each triplet codes for one amino acid. The amino acids are joined together in the order set by the gene to make a particular protein, which then folds into a specific 3D shape, such as an enzyme with its precise active site.

Protein synthesis has two stages:

• Transcription: in the nucleus, the gene's base sequence is copied into a messenger molecule (mRNA), which can leave the nucleus.
• Translation: at a ribosome, the mRNA code is read three bases at a time, and the matching amino acids are joined in order to build the protein.

Because the folded shape determines how a protein works, changes in the base sequence (genetic variants) can change the protein. A variant in the coding DNA can alter the amino acid order and so change the protein's activity; a variant in the non-coding DNA can affect how much protein is made by changing how the gene is switched on.

Try this

Q1. State which base pairs with cytosine in DNA. [1 mark]

• Cue. Guanine (C pairs with G).

Q2. Define the term "genome". [1 mark]

• Cue. The genome is the entire DNA of an organism.