A chromosome is made of DNA and protein, with protein making up roughly twice the mass of the DNA itself. The DNA carries genetic instructions, while the proteins provide structure and help control which genes are active. Together, this DNA-protein complex is called chromatin, and it folds through several levels of compaction to create the dense, rod-shaped chromosomes visible under a microscope during cell division.
DNA: The Core Material
At its most basic level, a chromosome is a single, extraordinarily long molecule of DNA. This molecule is built from four chemical building blocks called nucleotides, each containing a sugar, a phosphate group, and one of four bases (adenine, thymine, guanine, or cytosine). Two strands of nucleotides twist around each other to form the famous double helix, which is only about 2 nanometers wide.
What makes this remarkable is the sheer length involved. Human chromosome 22, one of the smaller chromosomes, contains about 48 million base pairs. If you stretched its DNA out end to end, it would measure roughly 1.5 centimeters. Yet during cell division, that same chromosome compacts down to about 2 micrometers long, a compression ratio of nearly 10,000 to 1. Fitting that much material into such a tiny space requires an elaborate packaging system.
Histone Proteins: The Spools
The packaging system starts with histone proteins. Histones are small, positively charged proteins that attract the negatively charged DNA and act like spools for it to wrap around. Four types of core histones do this work: H2A, H2B, H3, and H4. Two copies of each come together to form an eight-protein cluster called a histone octamer.
About 146 base pairs of DNA wrap 1.65 times around each octamer, forming a structure called a nucleosome. This is the fundamental repeating unit of a chromosome. Short stretches of “linker” DNA connect one nucleosome to the next, creating a structure that looks like beads on a string when viewed under an electron microscope. A fifth histone, H1, sits on the linker DNA and helps pull nucleosomes closer together.
How Chromosomes Fold Into Shape
The beads-on-a-string arrangement is just the first level of packing. From there, the chain of nucleosomes coils into a thicker fiber about 30 nanometers across. This 30-nanometer fiber then organizes into large loops, each containing roughly 50,000 to 100,000 base pairs of DNA, attached to a protein scaffold that runs along the interior of the chromosome.
During cell division, the folding becomes even more extreme. The loops of 30-nanometer fiber fold upon themselves into progressively thicker fibers of about 100 nanometers, then 200 to 250 nanometers, and finally coil into the compact, visible chromosomes that can be distributed to daughter cells. This hierarchical folding model explains how a molecule centimeters long fits into a structure only micrometers across.
For most of a cell’s life, chromosomes are not in this fully condensed state. Between cell divisions, most of the chromatin stays relatively loose (called euchromatin), allowing the cell’s machinery to read genes and copy DNA. About 10% of the most active genes unpack even further into a very open configuration. Another 10% stays tightly wound at all times (called heterochromatin), particularly around specialized regions like the centromere.
Non-Histone Proteins
Chromosomes also contain a variety of proteins beyond histones. These non-histone proteins play roles in gene regulation, DNA repair, and structural support. Some stimulate or suppress the copying of genes into RNA. Others help mediate how tightly histones bind to DNA at different points in the cell cycle, effectively opening or closing access to specific stretches of genetic information. The protein scaffold that anchors the large DNA loops during cell division is also made of non-histone proteins.
Chemical Tags That Change Chromosome Behavior
The histone proteins sticking out from each nucleosome have flexible tails that can receive small chemical tags. Two of the most important are acetyl groups and methyl groups. These tags don’t change the DNA sequence itself, but they alter how tightly the chromosome is packed in that region, which controls whether nearby genes are switched on or off.
Acetylation (adding an acetyl group) loosens the grip between histones and DNA by changing the electrical charge on the histone tail. Regions rich in acetylation marks tend to be open and actively producing proteins. Methylation (adding a methyl group) can have different effects depending on where it’s placed. The same spot on histone H3, known as position 27, illustrates this perfectly: when acetylated, it marks active genes, but when methylated three times over, it silences them. These chemical modifications are a major reason why different cell types, all carrying identical DNA, can look and behave so differently.
DNA itself can also be methylated directly, typically on cytosine bases. This tends to suppress gene activity in that region and plays a role in long-term gene silencing during development.
Specialized Structures: Centromeres and Telomeres
Not all parts of a chromosome are alike. Two regions have distinct compositions that serve essential functions.
The centromere is the pinched-in section near the middle (or off-center) of each chromosome. It’s built from a repetitive DNA sequence called alpha satellite DNA, made up of roughly 171-base-pair units arranged in tandem. These units form larger repeating patterns called higher-order repeats that can span millions of base pairs. The centromere serves as the attachment point for the machinery that pulls chromosomes apart during cell division. Specialized proteins bind here instead of standard histones, creating a unique landing pad.
Telomeres sit at each tip of the chromosome and consist of a different repetitive DNA sequence (TTAGGG, repeated thousands of times in humans). They act as protective caps, preventing the ends of chromosomes from fraying or fusing with neighboring chromosomes. Telomeres shorten slightly each time a cell divides, which is one reason cells have a limited lifespan.
Putting It All Together
A chromosome, then, is not simply a strand of DNA. It is a highly organized structure made of one continuous DNA molecule, hundreds of thousands of histone protein spools, a supporting cast of non-histone proteins, and a layer of chemical modifications that regulate the whole system. The DNA stores the genetic code. The histones package it. The non-histone proteins maintain structure and control access. And the chemical tags act as switches that determine which parts of the code are read in any given cell, at any given time. All of these components work together to compress meters of genetic material into a space small enough to fit inside a cell nucleus just a few micrometers across.