Genetic information is passed on from parents to children via genes. But what exactly is a gene?
And what is the difference between a gene and a genome? And how do DNA, RNA and chromosomes fit into the picture?
What is DNA?
Deoxyribonucleic acid (DNA) is the familiar double helix shape, the chemical code that holds our genetic information inside our cells. If unravelled, the DNA from one cell would stretch out to 2 metres long. The DNA in each of our cells is the same copy. The double helix ‘backbone’ is made up of alternating sugar and phosphate molecules with nucleotides, or bases, attached.
The chemical bases are adenine (A), guanine (G), cytosine (C) and thymine (T), which can be combined in any order to form the DNA sequence. There is a specific way that they pair for the structure to be stable: A with T, and C with G. As they pair and unpair, the double helix forms or unwinds.
What are DNA variants?
Most people have the same letter (A,G,C,T) at each of the 3 billion points of the DNA code, but at about one million points, it is common to find differences in the letters between people.
These DNA variants are responsible for some of the differences between people and are used in research. The key question is which DNA variants are important for risk of disease.
What is a gene?
A gene is a segment of DNA that contains the instructions to build a protein.
Proteins do all the ‘jobs’ in our bodies – they include enzymes, hormones, antibodies, and structural proteins that support our cells and allow movement.
Gene expression occurs when genes are “read” by the cell and a protein is made.
Different genes are “turned on” in different cell types for the cells to perform their functions. For example, a liver cell has different functions to a heart cell, and therefore needs different proteins, which require different genes to be expressed.
What is a chromosome?
DNA is packaged into structures called chromosomes, ensuring that the DNA is tightly wrapped and fits in the nucleus of the cell. Each cell has 46 chromosomes, arranged in pairs – 23 inherited from each parent.
Cells are constantly dividing to create new cells to replace worn-out ones. When cells divide, all 46 chromosomes are replicated so a full set goes into each new cell, except when making egg or sperm cells, which only have 23 chromosomes. After fertilisation, when the egg and sperm fuse to make
a new human, the full complement of 46 is met again.
What is RNA?
Ribonucleic acid (RNA) is single-stranded rather than double-stranded like DNA, and can pass out of the nucleus to the areas of the cell where proteins are assembled. It has A, G and C bases like DNA, but uracil (U) instead of thymine (T). Every group of 3 bases code for an amino acid, and proteins are made up of chains of amino acids.
There are three types of RNA in our bodies – transfer RNA (tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA) – that enact the DNA instructions to make proteins. You may have heard of mRNA in the context of vaccines, in which mRNA is used to trigger an immune response.
What is a genome?
Your genome is the sum of all the genetic information in your body, which encompasses much more than just our genes. In fact, only 2 per cent of our DNA meets the traditional definition of a gene, i.e. it contains instructions for making proteins.
The remaining DNA (around 98 per cent) used to be known as ‘junk DNA’, because scientists thought it had no function. But this theory has been challenged by many researchers including IMB co-founder Professor John Mattick. It is currently thought nearly 75 per cent of DNA produces non-coding RNAs, which have numerous functions such as turning genes on or off, or fine-tuning their activity.
Genetics vs genomics: what’s the difference?Genetics refers to the study of how genes work, what effect they have and how they are inherited by our offspring. |
