Chromosome Structure: Definition, Structure and Key Points of Chromosomes in DNA

A strand of DNA wrapped in associated proteins gives the connected nucleic acid bases a structure.

During the cell cycle interface, the chromosome exists in a loose structure so that proteins can be translated from DNA and DNA can be replicated.

During mitosis and meiosis, the chromosome condenses, organizes, and separates. The substance consisting of all the chromosomes in a cell and their associated proteins is known as chromatin. There is usually only one chromosome in prokaryotes, which exists in a ring or linear shape.

The chromatin of most eukaryotic organisms consists of multiple chromosomes. Each chromosome carries part of the genetic code necessary to produce an organism.

Having the entire genetic code divided into different chromosomes allows the possibility of variation through the various combinations of chromosomes with the other alleles or genetic variations that they contain.

Recombination and mutation of chromosomes can occur during mitosis, meiosis, or interphase. The result is organisms that function and behave in different ways. This variation allows populations to evolve in response to changing environments.

Chromosome structure

Key points

  • In eukaryotic cells, chromosomes are made up of a single DNA molecule with many copies of five types of histones.
  • Histones are protein molecules and are rich in lysine and arginine residues; they have a positive charge. Therefore, they bind tightly to negatively charged phosphates in the DNA sequence.
  • Many proteins are not histones; these are mainly transcription factors. Transcription factors regulate which parts of DNA are transcribed into RNA.
  • During most of the cell’s life cycle, chromosomes are elongated and cannot be seen under a microscope.
  • During the S phase of the mitotic cell cycle, chromosomes duplicate.
  • The duplicated chromosomes are held together in the centromere region.
  • Centromeres in humans are approximately 1-10 million base pairs of DNA.
  • Centromere DNA is short, primarily repetitive DNA sequences; the sequences repeatedly repeat in tandem and bursts.
  • The attached duplicate chromosomes are commonly called sister chromatids.
  • Kinetochores are the point of attachment for the spindle fibers that help separate sister chromatids as the mitosis process moves to the anaphase stage. Kinetochores are a complex of approximately 80 different proteins.
  • The shorter arm of the two arms of the chromosome that extends from the centromere is called the pm, and the more extended arm is known as the q arm.

Chromosomes have a very complex structure. DNA or deoxyribonucleic acid forms the basis of the system. DNA is made up of two strands of nucleic acid-base pairs. The base pairs in DNA are cytosine, adenine, thymine, and guanine.


The spiral structure formed by the two strands of DNA is due to the complementary pairing between each base with its pair on the opposite strand. Adenine pairs with thymine, and guanine pairs with cytosine. The opposite side of the grounds forms a phosphate-deoxyribose backbone, which keeps the chains intact.

When DNA is duplicated, the strands separate, and a polymerase molecule creates a new strand that corresponds to each side. In this way, DNA is perfectly replicated.

This can be done artificially by a polymerase chain reaction in which special enzymes and heat are used to separate and replicate the strings multiple times, to produce many copies of the same DNA. This makes it much easier to study any strand of DNA, even whole chromosomes or genomes.

Cell division can occur after the cell has expressed and duplicated the DNA. This happens in both prokaryotes and eukaryotes, but only eukaryotes condense their DNA so that it can be separated.

Prokaryotic DNA is so simple that relatively few structural proteins are associated with the chromosome. In eukaryotes, many structural proteins are used.

The first of these proteins are the central histones. Many individual histone proteins join together to form a central histone.

DNA can wrap around one of these histones, giving it a wound structure. This structure, and the associated histone, are known as the nucleosome. These nucleosomes form “beads on a string.”

The string is turned back and forth by another histone, histone H1, and finally, fibers are produced. The next type of protein, scaffold proteins, begin to coil the yarn into a loose structure.

When the chromosome must condense during cell division, more scaffold proteins are activated, and the structure becomes much denser.

Individual chromosomes cannot be discerned even with a microscope until about halfway through the cell division cycles when the chromosome becomes very dense.