Chromosomes, genes and proteins

In the nucleus of your cells, your genetic information is organised into structures called chromosomes. Each chromosome is a very long, coiled-up molecule of DNA. Think of it like a thread (DNA) wound tightly around a spool to save space. DNA itself has a famous structure called a double helix, which looks like a twisted ladder. The 'sides' of the ladder are made of a sugar-phosphate backbone, and the 'rungs' are made of pairs of chemicals called bases. There are four bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). The rule is that A always pairs with T, and C always pairs with G. This specific pairing is crucial for copying DNA accurately.

A gene is a specific section of a DNA molecule. It's like a single recipe in a giant cookbook (the chromosome). The 'recipe' written in the gene is the instruction for making one specific protein. The language of this recipe is the genetic code, which is the sequence of the bases A, T, C, and G. A sequence of three bases (a triplet) codes for one specific amino acid. By reading the triplets in order, the cell knows exactly which amino acids to join together, and in what order, to build a particular protein.

Making a protein from a gene is a two-step process. First, inside the nucleus, the cell makes a 'photocopy' of the gene. This copy is not made of DNA, but a similar molecule called mRNA (messenger RNA). This process is called transcription. The mRNA molecule is small enough to leave the nucleus and travel into the cytoplasm. In the cytoplasm, the mRNA attaches to a tiny structure called a ribosome. The ribosome 'reads' the code on the mRNA, three bases at a time. For each triplet, it adds the correct amino acid to a growing chain. This process of building the protein from the mRNA code is called translation. The completed chain of amino acids then folds up to become a functional protein.

Everything you are, from your hair colour to how you digest food, is determined by proteins. Your observable characteristics are called your phenotype. These characteristics are the result of the proteins your cells make. For example, enzymes are proteins that act as biological catalysts, controlling all the chemical reactions in your body. Structural proteins, like collagen and keratin, provide support and form structures like skin and hair. Carrier proteins, like haemoglobin in red blood cells, transport substances around the body. Therefore, the specific set of genes an organism has determines the specific set of proteins it can make, which in turn determines its characteristics.

Most of your cells are diploid, meaning they contain two sets of chromosomes, arranged in homologous pairs. You inherit one chromosome from each pair from your mother, and the other from your father. A gene for a particular trait (e.g., eye colour) is found at the same position, or locus, on both chromosomes in a homologous pair. However, the version of the gene on each chromosome might be different. These different versions of the same gene are called alleles. For example, for the eye colour gene, you might have an allele for brown eyes and an allele for blue eyes. If the two alleles in a pair are the same (e.g., two alleles for blue eyes), you are homozygous for that trait. If the two alleles are different (e.g., one for blue, one for brown), you are heterozygous.

Your sex is determined by your 23rd pair of chromosomes, known as the sex chromosomes. Females have two X chromosomes (XX). Males have one X chromosome and one much smaller Y chromosome (XY). During meiosis to make gametes, all of a female's eggs will contain one X chromosome. In males, half of the sperm produced will contain an X chromosome, and the other half will contain a Y chromosome. Therefore, the sex of a child is determined by which type of sperm fertilises the egg. If an X sperm fertilises the egg, the resulting zygote is XX (female). If a Y sperm fertilises the egg, the zygote is XY (male). This means there is a 50% probability of having a boy and a 50% probability of having a girl at each conception.