Chromosomes: what they are, types and parts

Definition of chromosome: Chromosomes are biological structures that compact DNA into genes . In addition to being the molecular units responsible for keeping DNA grouped and coded, they also store it and transmit genetic information from parents to descendants in each new generation.

Chromosomes can be in the form of threads, double helices or small rings , although their shape changes depending on the phase of the cell cycle in which they are. These chromosomes are made up of :

• Deoxyribonucleic acid (DNA).
• Various proteins such as histones.

Definition of chromosome: All living things have a specific number of chromosomes for their species inside their cells. This is known as chromosome endowment and regulates the characteristics and functioning of individuals of the same species. For example:

• In the case of prokaryotic cells : the chromosomes are scattered in the cytoplasm.
• In the case of eukaryotic cells : chromosomes are found inside the cell nucleus in the form of chromatin; which is an association of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) that has the appearance of thin, tangled threads. Discover the difference between eukaryotic and prokaryotic cells in this article that we recommend.

For the cell division process to develop successfully, it is necessary that the they remain intact . When the genetic material is divided or grouped irregularly or poorly, mutations occur that can have serious consequences such as problems in embryonic development or diseases such as leukemia.

Definition of chromosome: Human beings have 46 chromosomes distributed in 23 pairs. 22 pairs constitute the non-sex chromosomes or autosomes; and the remaining pair are the sex chromosomes or heterochromosomes: chromosomes “X” and “X” if it is a woman and chromosomes “X” and “Y” if it is a man.

Definition of chromosome: The gametes or sexual cells do not have a complete chromosomal load , in fact, they are the only human cells like this. These only have half the number of chromosomes, and when two of them unite (egg and sperm respectively) the resulting cells have a complete chromosome load inherited equally by both parents.

In this opportunity we will classify them according to three criteria: according to the position of the centromere, according to their shape and according to their function.

Types of chromosomes according to the position of the centromere

The centromere is the narrowest and most condensed region of a chromosome. It joins its sister chromatids and separates it into two sections with arms of specific lengths. Depending on the position of its centromere, a chromosome can be:

• Telocentric chromosome : it is the one whose centromere is located at one of its ends and, therefore, the chromosome has only one arm.
• Acrocentric chromosome : is one whose centromere is much closer to one telomere than to the other and, therefore, the chromosome has a very short arm and a very long arm.
• Submetacentric chromosome : is one whose centromere is located very close to the center of the chromosome, but slightly closer to one end than the other.
• Metacentric chromosome : is one whose centromere is located exactly in the center of the chromosome. Form two arms of equal lengths.

Types of chromosomes according to their shape

Depending on its physical shape, a chromosome can be linear or circular.

• Linear chromosomes : are those that have linear DNA chains that are arranged in pairs within cells. Most chromosomes in eukaryotic cells are of this type.
• Circular chromosomes : Consist of a single circular DNA molecule and some associated proteins. They are smaller than linear chromosomes and are found in prokaryotic organisms and in the mitochondria and chloroplasts of eukaryotic organisms.

Types of chromosomes according to their function

There are two types of chromosomes in sexually reproducing organisms: non-sex and sex.

• Somatic chromosomes : also called autosomes are all those that are not involved in determining the sex of individuals. In the case of the human being, chromosome pairs from 1 to 22 are autosomal chromosomes.
• Sex chromosomes : also called allosomes are those that are responsible for determining the sex of individuals. In the case of the human being, the pair of chromosomes 23 is the one that dictates the sex of the people.

They have four main parts: the chromatids, the centromere, the arms, and the telomeres.

Chromatids

Chromosome structure and function: Chromatids are identical coiled strands of DNA . They are made up of microfilaments called chromonemes and small granules or knots called chromomeres. Two longitudinal chromatids with the same type and number of genes form a linear chromosome.

Centromere

Chromosome structure and function: The centromere is the narrow, condensed region that joins the two chromatids of a linear chromosome . In addition, it helps keep chromosomes properly aligned during cell division and allows them to interact with spindle fibers during mitotic and meiotic anaphases. It also correctly performs the respective chromosomal movements of these phases.

Above them are protein structures called kinetochores. These are anchored to the microtubules of the mitotic spindle during mitosis and allow both halves of the chromosome to be brought to opposite poles of the cell.

Brazos

Chromosome structure and function: The arms are the two segments resulting from the horizontal division of the chromatids by the centromere. In acrocentric and submetacentric chromosomes, the arms have different lengths. To facilitate their study, the shorter arms are designated with the letter “P” and the longer arms are designated with the letter “Q”.

Telomeres

Chromosome structure and function: Telomeres correspond to the ends of linear chromosomes . They are stretches of DNA that do not encode any protein or provide genetic information. Its function is to protect the ends of the chromosomes and prevent them from fraying and disorganizing.