Protein synthesis

Last time

Before we start: last time, in Monohybrid crosses and test crosses, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

From gene to protein

Your hair colour, your blood group, the enzymes in your gut: all of them come down to proteins. And every protein is built using instructions in your DNA. Today: how a gene, in the nucleus, is turned into a protein, at a ribosome.

Genes and chromosomes

A chromosome is a long thread of DNA, carrying genes. A gene is a length of DNA that codes for one protein. An allele is an alternative form of a gene.

The code

The sequence of bases in a gene decides the sequence of amino acids in the protein. The amino acid sequence decides how the protein folds, and so its shape. And the shape decides its function, for example the shape of an enzyme's active site.

The problem

DNA stays inside the nucleus. It is too large to leave. But proteins are made at ribosomes, in the cytoplasm. So a copy of the gene is carried out of the nucleus.

The steps

Here are the steps. First a copy of the gene is made as m R N A. It leaves the nucleus and goes to a ribosome. The ribosome reads the code and joins amino acids together in the right order, to make the protein.

mRNA

m R N A stands for messenger R N A. It is a short copy of one gene, small enough to leave the nucleus. It carries the code from the nucleus to the ribosome.

The ribosome

The ribosome reads the base sequence of the m R N A. It assembles amino acids in the order the code gives. The chain of amino acids then folds into the protein's shape.

Same genes, different proteins

Almost every body cell has the same genes. But each cell only uses some of them, making only the proteins it needs. That is how cells become specialised: a red blood cell makes haemoglobin, a stomach cell makes pepsin.