A human gene is the fundamental unit of heredity, acting as the instruction manual for building and operating a living person. Genes are responsible for passing on physical and biological characteristics from one generation to the next, determining traits like eye color and organ function. Human cells contain an estimated 20,000 protein-coding genes, each playing a specific role in orchestrating the complex biological machinery of the body.
The Physical Structure of a Gene
A gene exists as a specific segment of deoxyribonucleic acid, or DNA, which is a long, coiled molecule found within the nucleus of nearly every cell. DNA is constructed from a sequence of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The precise order of these bases provides the unique genetic code.
These long DNA molecules are tightly packed and organized into structures called chromosomes. Humans typically have 46 chromosomes, arranged in 23 pairs, with one set inherited from each parent. Each chromosome can contain hundreds to thousands of individual genes, each located at a specific spot. Genes themselves vary significantly in size, ranging from a few hundred base pairs to over two million base pairs in length.
From Gene to Function: Making Proteins
The function of most genes is to provide instructions for creating proteins, which are the workhorses of the cell. Proteins perform the vast majority of tasks required for life, such as catalyzing metabolic reactions and providing structural support. The process of making a protein from a gene’s code is known as gene expression, following a two-step sequence.
The first step, called transcription, involves copying the gene’s DNA sequence into messenger RNA (mRNA). This mRNA molecule acts like a portable working copy of the instructions, traveling from the nucleus to the cell’s cytoplasm. Next, translation occurs on cellular structures called ribosomes, which act as protein-assembly factories.
The ribosome “reads” the mRNA instructions in three-base segments, known as codons, where each codon specifies a particular amino acid. The ribosome links these amino acids together in a specific order dictated by the gene’s sequence. This long chain then folds into a complex three-dimensional shape, forming the functional protein.
How Genes Determine Traits
Genes influence observable characteristics, or traits, by determining the production of specific proteins that regulate various functions in the body. Traits like height, hair color, and even susceptibility to certain conditions are all influenced by the genetic instructions inherited from one’s parents. Because humans inherit one set of chromosomes from each parent, they receive two copies of every gene.
These copies are known as alleles, which are different versions of the same gene. For example, a gene for eye color might have an allele for blue eyes and an allele for brown eyes. The interaction between these two inherited alleles determines the resulting physical trait, or phenotype.
Some alleles exhibit a pattern called dominant inheritance, meaning that the trait they code for will be expressed even if only one copy is present. Other alleles are recessive, and the trait will only appear if an individual inherits two copies of that specific version. This basic principle explains why a trait might skip a generation or appear when neither parent outwardly displays it.
Genetic Variation and Mutation
Genetic variation refers to the small differences in DNA sequences between individuals. While the human genome is over 99% identical across all people, the minor differences in the remaining fraction give rise to diverse traits and unique characteristics. The ultimate source of this variation is a change in the DNA sequence known as a mutation.
Mutations can occur randomly during DNA replication or be caused by environmental factors like radiation. Most mutations are considered neutral, having no noticeable effect because they occur in non-coding regions or do not alter the resulting protein’s function. Harmful mutations, such as the one causing cystic fibrosis, produce a defective protein that leads to a disorder.
A small fraction of mutations can be beneficial, providing an advantage that may help an organism survive or reproduce, such as resistance to malaria. For a mutation to be passed down to offspring, it must occur in the germline cells (sperm or egg). Once present in the germline, the mutation is incorporated into the DNA of the new individual and can be passed to subsequent generations.