Chromosomes, genes and proteins

Inside the nucleus of almost every one of your cells are tiny, thread-like structures called chromosomes. Think of them as tightly-packed instruction manuals. Each chromosome is made from a very long molecule of DNA (deoxyribonucleic acid), which is coiled up neatly. Humans have 46 chromosomes in most cells, arranged in 23 pairs. One chromosome from each pair comes from your mother, and the other from your father. These chromosomes carry all the genetic information that makes you unique, in the form of thousands of genes.

DNA is a remarkable molecule with a structure called a double helix, which looks like a twisted ladder. The 'uprights' of the ladder are made of a repeating sugar and phosphate group. The 'rungs' of the ladder are made of pairs of chemicals called bases. There are four different bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). The way these bases pair up is very specific and is a crucial part of DNA's function: Adenine always pairs with Thymine (A-T), and Cytosine always pairs with Guanine (C-G). This is known as the complementary base pairing rule. A unit consisting of a sugar, a phosphate, and a base is called a nucleotide, which is the basic building block of DNA.

A gene is a specific section of the DNA molecule. The sequence of bases (A, T, C, G) along a gene acts as a code. This genetic code holds the instructions for making a specific protein. Proteins are made from long chains of smaller molecules called amino acids. The sequence of bases in the gene determines the sequence of amino acids that are joined together. This amino acid sequence is critical because it causes the protein chain to fold into a unique 3D shape. The shape of a protein determines its function. For example, an enzyme's shape creates an active site, and an antibody's shape allows it to bind to a specific pathogen. Ultimately, these proteins determine an organism's characteristics, like eye colour or blood type.

The process of using a gene's instructions to build a protein is called protein synthesis. It happens in two main stages. Because the master copy of DNA cannot leave the nucleus, a temporary copy of the gene is made.

1. Transcription: This happens in the nucleus. The DNA double helix unwinds at the site of the gene. A 'messenger' molecule called mRNA (messenger RNA) is created, which is a copy of the gene's base sequence. This mRNA molecule is small enough to leave the nucleus.
2. Translation: The mRNA molecule travels out of the nucleus and into the cytoplasm, where it attaches to a tiny cellular machine called a ribosome. The ribosome 'reads' the mRNA code three bases at a time (a 'codon'). For each codon, a specific amino acid is brought to the ribosome. The ribosome joins these amino acids together in the correct order, forming a long chain that folds into a protein.

You inherit your genes on the chromosomes you receive from your parents. For each characteristic, like eye colour, you have a gene. However, there can be different versions of that gene in the population. These different versions are called alleles. For example, for the eye colour gene, there might be a 'brown eye' allele and a 'blue eye' allele. Since you have two of each chromosome (one from each parent), you have two alleles for each gene. If both alleles are the same (e.g., two 'blue eye' alleles), you are homozygous for that trait. If the two alleles are different (e.g., one 'blue eye' allele and one 'brown eye' allele), you are heterozygous for that trait.

A gene mutation is a random and permanent change in the sequence of bases in a DNA molecule. This can happen spontaneously when DNA is being copied during cell division, or it can be caused by exposure to certain chemicals or radiation. Since the base sequence of a gene codes for the amino acid sequence of a protein, a change in the bases can lead to a change in the amino acids. This may alter the final 3D shape of the protein, which can stop it from functioning correctly, or at all. A classic example is sickle-cell anaemia. A single base change in the gene for haemoglobin results in one different amino acid in the protein chain. This causes the haemoglobin molecules to stick together and the red blood cells to become a stiff, sickle shape. These cells can block blood vessels and carry less oxygen.