A segment of DNA on a chromosome is called a gene. Genes are the basic physical and functional units of heredity, and each one occupies a specific location on a chromosome. The human genome contains roughly 19,300 protein-coding genes spread across 23 pairs of chromosomes, but genes make up only about 1 percent of your total DNA. The remaining 99 percent consists of non-coding sequences that play structural, regulatory, or still-unknown roles.
What a Gene Actually Does
A gene is essentially an instruction set. The information stored in its DNA sequence gets copied into an intermediate molecule called RNA, and that RNA is then read by cellular machinery to build a specific protein. Proteins do most of the work in your cells: they form structures, speed up chemical reactions, carry signals, and fight infections. The timing of when different genes are active determines what type of cell develops and how it behaves.
Not every gene codes for a protein, though. The human genome includes about 9,500 genes that produce functional RNA molecules instead. These RNA genes help regulate other genes, assist in protein assembly, or perform chemical tasks on their own.
Parts of a Gene
A gene isn’t one continuous stretch of useful code. It contains several distinct regions, each with a specific job.
- Promoter: A stretch of DNA at the beginning of the gene where the cell’s copying machinery latches on to start reading. Without it, the gene stays silent.
- Exons: The segments that contain the actual protein-building instructions. These are the parts that ultimately get “read” and translated.
- Introns: Stretches of DNA between exons that don’t code for any part of the protein. After the gene is copied into RNA, introns are snipped out before the message leaves the cell’s nucleus. Despite not coding for protein, their structure helps the RNA get processed correctly.
A single gene can have dozens of introns. One well-studied example, the gene for hemoglobin (the oxygen-carrying molecule in red blood cells), contains introns of about 130 base pairs tucked between its coding regions.
DNA Segments That Aren’t Genes
Since 99 percent of human DNA doesn’t code for proteins, chromosomes are loaded with other types of functional segments. Two of the most important are enhancers and silencers.
Enhancers are clusters of DNA that boost a gene’s activity. They can sit thousands of base pairs away from the gene they control, and they work regardless of their orientation or exact distance. They do this by providing landing pads for proteins called transcription factors, which help switch genes on in the right tissues at the right times. This is part of why a liver cell and a brain cell contain identical DNA yet behave completely differently.
Silencers do the opposite. They recruit repressor proteins that dial down or shut off a gene’s activity. Together, enhancers and silencers fine-tune gene expression with remarkable precision, ensuring genes activate only where and when they’re needed.
Structural Segments on Chromosomes
Beyond genes and regulatory regions, chromosomes contain specialized DNA segments that maintain their physical integrity.
Centromeres sit near the middle of each chromosome and are built from highly repetitive DNA sequences. Their job is critical during cell division: they serve as the attachment point for the fibers that pull duplicated chromosomes apart into two new cells. Without a functioning centromere, a chromosome can’t be inherited properly.
Telomeres cap the ends of every chromosome, much like the plastic tips on shoelaces. They consist of short DNA sequences repeated thousands of times, and they protect the chromosome from degrading or fusing with neighboring chromosomes. Telomeres shorten slightly each time a cell divides, which is one reason they’re closely studied in aging research.
How a Gene’s Location Is Identified
Every gene has a specific address on its chromosome, called a locus. The National Human Genome Research Institute describes a locus as “a physical site or location within a genome, somewhat like a street address.” Scientists use a standardized naming system that identifies the chromosome number, which arm of the chromosome the gene sits on (short arm or long arm), and its position along that arm.
Knowing a gene’s locus matters for diagnosing genetic conditions. When researchers link a disease to a particular locus, genetic testing can check that exact spot for mutations. The Telomere-to-Telomere (T2T) consortium recently completed the first truly gapless sequence of a human genome, revealing millions of base pairs that previous reference genomes had missed, including regions on chromosomes 9 and 10 that turned out to be relevant to rare genetic disorders.
How DNA Fits Inside a Chromosome
If you stretched out all the DNA in a single human cell, it would measure about six feet long. Fitting that into a cell nucleus roughly one-thousandth of an inch across requires extraordinary packaging.
The first level of organization involves spooling the DNA double helix around small proteins called histones, forming bead-like units called nucleosomes. Picture a thread wrapped around a series of tiny spools. These nucleosomes then coil and stack together into a thicker fiber called chromatin, which is the form DNA takes during most of a cell’s life. When a cell prepares to divide, chromatin condenses even further into the tightly packed, X-shaped structures we recognize as chromosomes under a microscope.
This packaging isn’t just about saving space. How tightly a region of DNA is wound affects whether genes in that region can be read. Loosely packed chromatin allows genes to be accessed and activated, while tightly packed chromatin keeps them silent. Your cells use this physical organization as yet another layer of gene control, on top of promoters, enhancers, and silencers.