A gene is the basic physical unitof heredity. Every living cell contains genetic information that determines an organism’s development, form, and function. This genetic information is encoded by two macromolecules: DNA and RNA.
DNA consists of two strands of phosphate and sugar molecules connected by pairs of nitrogenous bases to form a double helix structure. The four nitrogenous bases in DNA are adenine, thymine, cytosine, and guanine (abbreviated A, T, C, and G). Genes are sequences of nucleotides (composed of a sugar, a phosphate group, and a base) that provide the instructions that cells need to make molecules that give rise to an organism’s characteristics. Within the nucleus of each cell, DNA is tightly coiled around specialized proteins called histones, forming compact structures called chromosomes. Each gene occupies a particular position, or locus, on a chromosome.
Most genes contain instructions for creating proteins, amino acid-based macromolecules with a wide range of structures and functions. Among their numerous essential functions, proteins contribute to cell structure and repair, signal transmission between cells, and biochemical reactions within cells. Genes are used to create proteins through a two-step process. Double-stranded DNA is first transcribed into single-stranded messenger RNA (mRNA) that serves as a template for protein synthesis. mRNA exits the nucleus and interacts with cellular machinery called ribosomes. Ribosomes then read mRNA and translate its nucleotide sequence into long chains of amino acids, which then fold to form proteins.
An organism’s complete set of genetic material is called a genotype. The human genome is estimated to contain between 20,000 and 25,000 protein-coding genes, varying in size from thousands of nucleotides to over 2 million nucleotides. The complete set of observable traits that results from gene expression is called a phenotype. An organism’s phenotype includes all of its outward characteristics, including height and eye colour, as well as less apparent characteristics such as blood group and intelligence. For example, the genes that determine the amount of pigment in my skin are part of my genotype, but my skin colour is part of my phenotype. Whereas one’s genotype is determined solely by biological factors, one’s phenotype is determined by complex interactions between biological and environmental factors. This distinction between genotype and phenotype is evident in the case of identical twins – even though they have the same genotype, they often look and behave differently due to environmental and lifestyle factors.Continue reading “Snapshot: What is a Gene?”