Chromosomes are made of DNA, proteins, and a small amount of RNA, all wound together into a compact structure that fits inside a cell’s nucleus. The dominant components by mass are proteins, which act as a structural scaffold, and DNA, which carries the genetic instructions. The exact proportions vary depending on the cell type and the chromosome’s state, but the interplay between these molecules is what gives chromosomes both their shape and their function.
DNA: The Informational Core
DNA is the molecule most people associate with chromosomes, and for good reason. It’s the long, double-stranded helix that encodes genes. In humans, individual chromosomes range enormously in size. Chromosome 1, the largest, contains roughly 249 million base pairs of DNA. Chromosome 21, the smallest, holds about 47 million. The Y chromosome falls in between at around 57 million base pairs. Stretched end to end, the DNA from a single human cell would measure about two meters, yet it all fits inside a nucleus only a few millionths of a meter across. That feat requires extreme compaction, and that’s where the proteins come in.
Histone Proteins and the Nucleosome
The primary packaging system uses proteins called histones. There are five histone families: four “core” histones (H2A, H2B, H3, and H4) and one “linker” histone (H1, sometimes called H5 in certain cell types). Two copies of each core histone form an eight-protein spool called a histone octamer. A stretch of 147 base pairs of DNA wraps around each octamer roughly 1.7 times, creating a bead-like unit called a nucleosome.
This first level of wrapping achieves a six- to sevenfold compaction of the DNA, depending on how much “linker” DNA sits between nucleosomes. The linker histone H1 sits on top of the nucleosome where the DNA enters and exits, helping lock the wrap in place and drawing neighboring nucleosomes closer together.
Higher Levels of Folding
Nucleosomes don’t just sit in a loose string. They coil into a thicker fiber roughly 30 nanometers in diameter, packing about 7 to 10 nucleosomes per turn. This step adds another roughly sixfold compaction, bringing the total to about 40-fold compared to bare DNA. During cell division, additional folding condenses the fiber into loops and coils that eventually produce the dense, X-shaped structures visible under a microscope. Some researchers have observed even thicker fibers, around 100 nanometers, consisting of looped 30-nanometer segments in dividing cells.
Between divisions, chromosomes relax into a less compact state so genes can be read. Even in this “open” configuration, the 30-nanometer fiber organization appears to persist as a baseline. Sections simply unfold temporarily when the cell’s machinery needs access to a particular gene.
Non-Histone Proteins
Histones get most of the attention, but chromosomes contain a large amount of non-histone protein as well. High-resolution imaging of mammalian chromosomes has found that non-histone proteins can make up the majority of a chromosome’s mass. In one detailed measurement, the content of non-histone proteins relative to DNA was more than four times greater than the DNA itself, while histones were only about 0.28 times the DNA content. These non-histone proteins include structural scaffold proteins that help organize chromosome loops, enzymes that copy and repair DNA, and regulatory proteins that control which genes are active. Their composition changes throughout the cell cycle, which helps explain why chromosomes behave differently during division compared to normal cell activity.
RNA as a Structural Component
Chromosomes also contain RNA, which was long considered a temporary byproduct of gene reading rather than a structural ingredient. That view has shifted. Chromosome-associated RNAs, many of them produced from repetitive “junk” DNA sequences, appear to be widespread components of chromosomes between cell divisions. These RNAs, together with a protein called SAF-A, form a mesh-like network that helps organize large-scale chromosome architecture and protects the genome from instability. Some of these repeat-rich RNAs promote a more open, gene-accessible state in their local neighborhood, while others help keep regions tightly packed and silent.
Specialized DNA Regions
Not all DNA in a chromosome codes for proteins. Two regions with distinctive compositions deserve mention because they’re essential to chromosome survival.
Telomeres cap each chromosome end with thousands of short, repeated DNA sequences. In humans and all other vertebrates, the repeat motif is the six-letter sequence TTAGGG, repeated over and over. These caps prevent the chromosome from fraying or fusing with neighboring chromosomes, much like the plastic tips on a shoelace. Telomeres shorten with each cell division, which is one reason cells have a limited lifespan.
Centromeres sit near the middle (or off-center) of each chromosome and consist of long stretches of repetitive DNA wrapped in a specialized set of proteins. During cell division, the centromere is where the molecular machinery attaches to pull copies of a chromosome apart into two daughter cells. Without a functional centromere, a chromosome is lost during division.
Chemical Modifications That Change Behavior
The composition of a chromosome isn’t fixed. Small chemical tags get added to both the DNA and the histone proteins, altering how tightly a region is packed and whether its genes are active. Three major types of modification shape chromosome behavior.
DNA methylation adds a methyl group (a carbon atom with three hydrogens) directly onto a cytosine base in the DNA. When methylation occurs in clusters near a gene’s starting point, it typically silences that gene. Histone acetylation attaches an acetyl group to positively charged spots on histone tails. This weakens the grip between the histone and the negatively charged DNA, loosening the chromatin and making genes easier to read. Histone methylation is more nuanced: one, two, or three methyl groups can be added to specific amino acids on histone tails, and the effect can either activate or silence a gene depending on the exact location.
These modifications are collectively called epigenetic changes. They don’t alter the DNA sequence itself, but they effectively change the chromosome’s working composition. A chromosome in a liver cell and a neuron carry the same DNA, yet their histone modifications differ dramatically, which is why those cells look and function so differently.
Bacterial Chromosomes Are Built Differently
Everything described so far applies to eukaryotic cells, the kind found in animals, plants, and fungi. Bacteria take a simpler approach. A typical bacterial chromosome is a single circular loop of DNA rather than a linear strand. It sits in an open region of the cell called the nucleoid rather than inside a membrane-bound nucleus. Bacteria do not use histone proteins. Instead, they rely on a different set of small, abundant proteins to help organize and compact their DNA. The result is a streamlined structure that fits the needs of a single-celled organism dividing as often as every 20 minutes.