A gene is a set of instructions written in DNA, and almost everything it does, it does by getting the cell to build a protein. So the path from gene to trait runs through a message and a molecule, not straight from DNA to the body. Having a gene is also different from using it: cells constantly decide which genes to read and how much, and the surroundings can change the outcome. The idea that information flows from DNA to RNA to protein, and never back from protein to DNA, was set out by Francis Crick in 1958.1National Human Genome Research Institute. Central Dogma
How a Gene Becomes a Protein
Inside the nucleus, an enzyme called RNA polymerase reads the gene and builds a matching copy called mRNA (messenger RNA). Starting is the hard part: only about 1 in 100 times the enzyme lands on a gene does it finish a full message.2PubMed Central. In vivo dynamics of RNA polymerase II transcription3PubMed Central. In vivo dynamics of RNA polymerase II transcription
Before leaving the nucleus, the messenger copy is cut and spliced. Some pieces are kept and others dropped. One gene can be spliced in different ways to give several versions of a protein.4PubMed Central. Mechanism of alternative splicing and its regulation Different tissues and life stages use different versions, which is part of how one set of genes builds very different organs.5PubMed Central. Alternative splicing and related RNA binding proteins in human health and disease
Outside the nucleus, the messenger copy is read by a ribosome, a machine made of RNA and proteins.6Wikipedia. Translation (biology) Its two halves clamp onto the message like a sandwich and pull in carrier molecules, each bringing one protein building block, called an amino acid.7Nature Education. How Genes Work: From DNA to Protein and Traits A chain of amino acids grows in the order the message spells out.8Nature Education. How Genes Work: From DNA to Protein and Traits
The reading follows a three-letter code. DNA uses only four letters, and the ribosome reads them three at a time. Each three-letter word names one amino acid or says stop. About 20 amino acids combine into all human proteins, which is why changing even a single letter can swap one building block for another. Once built, the chain folds into a precise 3D shape, and that shape is set by the order of its amino acids.9News-Medical. Protein Folding
How Proteins Turn Into Traits
Proteins do most of the work in the body, from building pigments to carrying oxygen to sending nerve signals. So a trait like eye color reflects how much of a pigment-making protein the eye’s cells produce. The pigment gene is read in cells of the iris, and its protein helps make the pigment melanin. More pigment gives brown eyes, less gives blue. Cells also break proteins down on purpose, and in nerve endings the speed of that removal helps set how strongly nerves signal.10PubMed. The ubiquitin proteasome system acutely regulates presynaptic protein turnover and synaptic efficacy
This is why “a gene for” a trait is misleading. Genes don’t contain traits, only instructions for proteins. The phrase is shorthand for a protein whose job affects the trait, and many traits involve dozens or thousands of proteins.
How Cells Switch Genes On and Off
Every cell carries the same DNA, but a skin cell and a nerve cell read different genes. Part of the reason is that some stretches of DNA act like dimmer switches, gathering control proteins that turn on nearby genes.11PubMed Central. Regulatory enhancer–core-promoter communication via transcription factors and cofactors Some switches work only when several control proteins land side by side, which allows very cell-specific control, as in nerve cells.12PubMed. Enhancer Activation by Transcription Factors and Underlying Mechanisms
Small chemical tags attached to DNA usually keep a gene quiet.13PubMed Central. The Role of DNA Methylation and Histone Modifications in Transcriptional Regulation in Humans They block activating proteins from landing or attract proteins that shut the gene down.14PubMed Central. The Role of DNA Methylation and Histone Modifications in Transcriptional Regulation in Humans DNA is also wrapped around spool proteins, and tags on those spools loosen or tighten the packing into open, readable regions or closed, silent ones.15PubMed Central. The Role of DNA Methylation and Histone Modifications in Transcriptional Regulation in Humans
These tag patterns are copied when cells divide, so a liver cell’s daughters stay liver cells.16PubMed. Epigenetic interplay between histone modifications and DNA methylation in gene silencing The patterns can go wrong, though, and faulty silencing of protective genes is seen in tumours.17PubMed. Epigenetic interplay between histone modifications and DNA methylation in gene silencing
Tags change which genes are read, not the letters themselves, which is what “epigenetic” means in news stories. Tags can shift over a lifetime while the DNA stays fixed, which is one reason identical twins can diverge.
Not all RNA becomes protein. Tiny RNAs called microRNAs stick to messages and get them destroyed or blocked.18PubMed Central. MicroRNAs and long non-coding RNAs during transcriptional regulation and latency of HIV and HTLV Longer non-coding RNAs can soak up microRNAs like a sponge, letting the blocked messages work again.19PubMed Central. Functional interactions among microRNAs and long noncoding RNAs These RNA tug-of-wars help steer cell growth, specialization and cell death.20PubMed Central. Functional interactions among microRNAs and long noncoding RNAs
What Mutations Do to a Protein
A mutation is a change in the DNA spelling. Its effect depends on the kind of change:
- Silent: the letters change but the same amino acid is made, so the protein is usually unaffected.21LibreTexts Biology. How Genes Work: From DNA to Protein and Traits
- Missense: one amino acid is swapped for another, with effects that are hard to predict.22pearson.com. General Biology Protein Synthesis Study Guide SEO | Notes
- Nonsense: a word turns into stop, so the protein ends early.23pearson.com. General Biology Protein Synthesis Study Guide SEO | Notes
- Frameshift: letters added or lost in a number not divisible by three shift the reading, so everything after is misread.24pearson.com. General Biology Protein Synthesis Study Guide SEO | Notes
Early stops and frameshifts usually leave a protein too short or garbled to work.25LibreTexts Biology. How Genes Work: From DNA to Protein and Traits If they happen near the very end of the gene, the protein may still function.26LibreTexts Biology. How Genes Work: From DNA to Protein and Traits A swap to a similar amino acid, or in an unimportant spot, also often leaves the protein working.27LibreTexts Biology. How Genes Work: From DNA to Protein and Traits
Diseases that need two faulty copies (recessive) are mostly caused by a protein that no longer works.28PubMed Central. Loss-of-function, gain-of-function and dominant-negative mutations have profoundly different effects on protein structure Diseases needing only one faulty copy (dominant) can come from a protein that does something new or harmful, or that jams the healthy copy’s protein.29PubMed Central. Loss-of-function, gain-of-function and dominant-negative mutations have profoundly different effects on protein structure Across thousands of inherited conditions in dominant genes, these harmful-new-action mechanisms explain about half (48%).30PubMed Central. Prevalence of loss-of-function, gain-of-function and dominant-negative mechanisms across genetic disease phenotypes
These terms turn up on genetic test reports, which may say “missense” or “loss of function.” A missense label alone doesn’t mean harmful. Labs grade variants as benign, uncertain or pathogenic, and if yours comes back “uncertain significance,” a genetic counselor can explain what that means for you.
Mutations You Weren’t Born With
A change that happens after conception creates two genetically different groups of cells in one person.31PubMed Central. Somatic Mosaicism in the Human Genome It may affect only part of the body and is not passed on to children.32PubMed Central. Somatic Mosaicism in the Human Genome Such changes range from one letter to a whole chromosome, and cancer is the best-known result.33PubMed Central. Somatic Mosaicism in the Human Genome Many go unnoticed, while some cause visible differences.34PubMed. Mechanisms and consequences of somatic mosaicism in humans
Why Most Traits Involve Many Genes
Large studies scanning whole genomes hoped to find single genes behind common traits.35PubMed Central. Polygenic inheritance, GWAS, polygenic risk scores, and the search for functional variants Most links they found have such small effects that pinning down each one is nearly hopeless.36PubMed Central. Polygenic inheritance, GWAS, polygenic risk scores, and the search for functional variants So researchers combine them into a polygenic risk score, an overall estimate of a person’s genetic tendency toward a trait.37PubMed Central. Polygenic inheritance, GWAS, polygenic risk scores, and the search for functional variants Newer methods that use knowledge of gene function predict up to a third better for people of non-European ancestry.38PubMed Central. Leveraging functional genomic annotations and genome coverage to improve polygenic prediction of complex traits within and between ancestries
If a consumer test gives you one of these scores, it is a tendency, not a diagnosis. Results are less reliable if your ancestry was underrepresented in the studies behind the score.
How Environment Changes What Genes Do
In fruit flies, about half the variation in sense-of-smell behavior came from how genes and rearing conditions interacted.39PubMed Central. Phenotypic Plasticity and Genotype by Environment Interaction for Olfactory Behavior in Drosophila melanogaster In tiny worms, some gene versions changed growth and fertility only at certain temperatures.40PubMed. Mapping phenotypic plasticity and genotype-environment interactions affecting life-history traits in Caenorhabditis elegans In crops, the effect of flowering genes depends on day length and warmth.41PubMed Central. Genomic and environmental determinants and their interplay underlying phenotypic plasticity In people, diet, smoking and stress interact with genes in similar ways.
Genes Outside the Nucleus
Mitochondria, the cell’s energy units, have their own small set of genes, inherited almost entirely from the mother. In mice and flies, a mismatch between nuclear and mitochondrial DNA can lead to heart and metabolic problems later in life.42PubMed Central. Inheritance of mitochondrial DNA in humans: implications for rare and common diseases Whether this happens in humans is not yet known.43PubMed Central. Inheritance of mitochondrial DNA in humans: implications for rare and common diseases
How Bacteria Do It Differently
In human cells a nuclear wall separates copying DNA from building protein.44PubMed Central. Transcription–translation coupling: Recent advances and future perspectives Bacteria have no nucleus, so a ribosome can start reading a message still being copied.45PubMed. Transcription-Translation Coupling in Bacteria This lets them make proteins faster, but they mostly skip the message splicing that human cells rely on.
How New Genes Evolve
Thousands of enzymes arose when a gene was accidentally copied and the copies drifted apart.46PubMed Central. Evolution of new enzymes by gene duplication and divergence In fruit flies, many young duplicate genes survive because one copy gained a new function.47PubMed Central. Neofunctionalization of young duplicate genes in Drosophila In humans and related monkeys, a duplicated immune gene gained the ability to punch holes in bacteria.48Trends in Ecology and Evolution. Evolution by gene duplication: an update That new antibacterial protein arose from many quick changes after the copy was made.49Trends in Ecology and Evolution. Evolution by gene duplication: an update
How Gene Editing Rewrites the Instructions
CRISPR-Cas9 comes from a bacterial defense against viruses, cutting invaders’ DNA at specific spots.50PubMed. CRISPR-Cas9 Structures and Mechanisms In the lab, a designed guide RNA leads the Cas9 protein to a matching DNA sequence.51PubMed Central. Mechanism and Applications of CRISPR/Cas-9-Mediated Genome Editing Cas9 only cuts next to a short marker sequence, which limits where it can work.52PubMed. CRISPR-Cas9 Structures and Mechanisms At that spot it cuts both DNA strands.53PubMed Central. Mechanism and Applications of CRISPR/Cas-9-Mediated Genome Editing
The cell’s quick repair often adds or loses a few letters, causing a frameshift or early stop that disables the gene.54PubMed Central. Mechanism and Applications of CRISPR/Cas-9-Mediated Genome Editing If a correct DNA template is supplied, a precise repair can paste in the fix, though it works only at certain points in cell division.55PubMed Central. Mechanism and Applications of CRISPR/Cas-9-Mediated Genome Editing
Switching a gene off relies on the sloppy repair and is easier. Correcting a gene needs the template and is less efficient, which is why early treatments mostly disable harmful genes.