Parts of a Chromosome: Structure, DNA, Proteins and All Their Essential Functions

In the nucleus of each cell, the DNA molecule is packaged in thread-like structures.

Each chromosome is made up of DNA tightly wrapped many times around proteins called histones that support its structure.

Chromosomes are not visible in the nucleus of the cell, even under a microscope, when the cell is not dividing.

However, the DNA that makes up the chromosomes becomes more compact during cell division and is then visible under the microscope.

Structure of chromosomes

Chromosomes are made of many strands of chromatin, each containing DNA and proteins, in the following proportions: 50% protein (structure, protection, activation and deactivation of genes) and 50% DNA (deoxyribonucleic acid).

Somatic cells contain 46 chromosomes, gametic cells contain 23 chromosomes, of which 44 chromosomes are autosomes, while 2 chromosomes are sexual.

Each chromosome contains many genes that act as a blueprint or set of instructions for the cell.


DNA is the most stable molecule that carries out the transfer of genetic information from one generation to another.

The DNA on the chromosomes forms the basis of the genetic code.

It only contains the language of life that determines how a cell works and all its characteristics.

DNA is made of long chains of nucleotides, it has a double helix structure where two strands of nucleotides come together and twist to form a spiral staircase or a twisted staircase.

Each nucleotide has 3 parts:

  • A phosphate group, which helps form the backbone of the DNA molecule.
  • A sugar (deoxyribose) that acts like a glue, forms the backbone with phosphate.
  • Nitrogen bases, which are of four types: guanine, cytosine , adenine, and thymine .

The base of one nucleotide forms a hydrogen bond with the base of another nucleotide, so the bases form the rungs of the DNA ladder and the sequence of the bases makes up the genetic code .

Therefore, the double-stranded DNA has the complementary bases hidden and protected by the intertwined sugar-phosphate backbones.

The functions of DNA are mainly the following:

  1. Control of cellular activities: to produce characteristics of an individual and species.
  2. Replication: for materials to pass from one cell to another, from generation to generation, and for growth, through meiosis and mitosis.
  3. Mutations: permanent changes that are passed on to offspring can advance the species through evolution.

Most m-RNA is transported to the cytoplasm, where it is translated into protein terms.

Some of the mRNA remains in the coil associated with the protein.


They have two types of proteins, that is, acidic and basic proteins that are associated with DNA on the chromosome.


These are simple proteins with a molecular weight of less than 4000 Daltons.

Rich in the basic amino acid arginine, which is found in the sperm of some fish (salmon), snail, birds, etc.

Protamine polypeptides consist of 28 amino acid residues (19 arginines, 8-9 non-basic amino acids).


They are small proteins related to the DNA of eukaryotic cells. They are structural proteins of chromatin and also act as gene repressors.

In a wide variety of plants and animals, the DNA: Histone ratio is equal to 1: 1.

There are five types of histones: HI (H5), H2A, H2B, H3, and H4. Histones are rich in arginine and lysine (basic amino acids). They lack tryptophan.

Histones are highly modified as well as conserved proteins; with very little difference in amino acid sequences.

On the basis of the contents of arginine and lysine, they are divided into 3 groups: rich in high lysine (HI), rich in lysine (H2A, H2B) and rich in arginine (H3, H4).

Histones tend to depress genetic activity, make DNA, and are structurally important in the makeup of DNA molecules.

Non-histone proteins

They have a structural, enzymatic and regulatory function in chromatin.

In general they are acidic proteins. The ratio of acidic to basic residues in these proteins is 1.2 to 1.6.

The molecular weight ranges from 11,000 to 21,500 Daltons. They can show structural variation in different species and even tissues.

These proteins are synthesized throughout the cell cycle and stimulate genetic activity.

Chromosomal metabolism enzymes such as nucleic acid polymerase, nucleases, DNA pyrophosphorylases and nucleoside triphosphatases, among others.

They are proteins that are not histones. These proteins play an important role in the interaction of steroid hormones with the nuclei of target cells.

Chromosomes also contain metal ions such as Mg ++ and Ca ++.

Parts of a common chromosome

  • Chromatids: are the longitudinal units that make up the chromosome, and are arranged in pairs, called sister chromatids and joined by the centromere. Sister chromatids contain the same information and are identical in morphology, since they originate from a duplicated DNA molecule.
  • The centromere : Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled “p-arm.” The long arm of the chromosome is called the “q arm.”
  • The short arm: this is the result of the division of the chromatid, by the centromere. Its common nomenclature is arm p (petit: small).
  • The long arm: this also results from the division of the chromatid by the centromere. It is called the q-arm (queue: tail, tail).
  • The telomere: this is the terminal portion of the chromosomes, which is morphologically indistinguishable and is responsible for preventing the chromosomal ends from merging. The secondary constriction: this is the region of the chromosome, which is located at the ends of the arms, where the genes that are transcribed as RNA are arranged.
  • The satellite: this is the spherical segment of a chromosome, and it is separated from the rest of the chromosome by a secondary constriction.