Proteins are molecules made of amino acids. They are coded for by our genes and form the basis of living tissues. They also play a central role in biological processes. For example, proteins catalyse reactions in our bodies, transport molecules such as oxygen, keep us healthy as part of the immune system and transmit messages from cell to cell.
A gene is a segment of a DNA molecule that contains the instructions needed to make a unique protein. All of our cells contain the same DNA molecules, but each cell uses a different combination of genes to build the particular proteins it needs to perform its specialised functions.
Protein synthesis has 2 main stages. The 1st stage is known as transcription, where a messenger molecule (mRNA) is formed. This molecule is transcribed from the DNA molecule and carries a copy of the information needed to make a protein. In the 2nd stage, the mRNA molecule leaves the nucleus for the cytoplasm where the cell’s ribosomes read the information and start to assemble a protein in a process called translation
During translation, the ribosomes read the mRNA sequence of bases 3 at a time. These 3-letter combinations (called codons) each code a particular amino acid. For example, the base sequence TTT codes for the amino acid lysine.
There are 4 bases (adenine, thymine, guanine and cytosine) and therefore 64 (43) possible codons specified using some combination of 3 bases. However, only 20 amino acids are required to build all of the proteins in our bodies (some amino acids are specified by more than 1 codon). It is the particular sequence of amino acids that determines the shape and function of the protein.
Protein synthesis, like many other biological processes, can be affected by environmental factors. These include maternal nutrition, temperaturestress, oxygen levels and exposure to chemicals
Different types of proteins
There are many different types of proteins in our bodies. They all serve important roles in our growth, development and everyday functioning. Here are some examples:
- Enzymes are proteins that facilitate biochemical reactions, for example, pepsin is a digestive enzyme in your stomach that helps to break down proteins in food.
- Antibodies are proteins produced by the immune system to help remove foreign substances and fight infections.
- DNA-associated proteins regulate chromosome structure during cell division and/or play a role in regulating gene expression, for example, histones and cohesin proteins
- Contractile proteins are involved in muscle contraction and movement, for example, actin and myosin
- Structural proteins provide support in our bodies, for example, the proteins in our connective tissues, such as collagen and elastin.
- Hormone proteins co-ordinate bodily functions, for example, insulin controls our blood sugar concentration by regulating the uptake of glucose into cells.
- Transport proteins move molecules around our bodies, for example, haemoglobin transports oxygen through the blood.
Alternative roles for proteins
Each protein has a specific role in our body. However, scientists have discovered that some proteins perform more than 1 role.
For example, Dr Julia Horsfield leads the Chromosome Structure and Development Group at the University of Otago. Her lab investigates how cohesin proteins, which regulate chromosome structure during cell division, are also involved in making sure that genes are switched on or off at the correct times during development. Julia and her colleagues focus on the impact of a reduction in cohesin proteins on gene expression in zebrafish and use these results to better understand particular human diseases
Visit the Learn Genetics website to go on an animated tour covering DNA, genes, chromosomes, proteins, heredity and traits. The animation can be downloaded and run without an internet connection.