Acrocentric Chromosome: What is it? Function, Centromere and Classification of Chromosomes

Deoxyribonucleic acid (DNA) is the genetic code with information that is transferred from one generation to another.

Function

Chromosomes are found within the nucleus of most living cells and consist of DNA that coils tightly into wire-like structures.

Deoxyribonucleic acid (DNA) is the way in which genetic information is stored within cells.

The additional protein structures called histones are compatible with the DNA molecule within the chromosome.

DNA molecules consist of two linear chains that are wrapped around each other forming a double helix structure.

The chromosomes are divided into two parts with a point of constriction in the center that is known as centromere.

An acrocentric chromosome is one in which the centromere is located near the end of the chromosome.

Humans usually have five pairs of acrocentric chromosomes.

Down syndrome is caused by an extra-monocrocentric chromosome.

Centromere

The chromosomes are divided into two parts with a point of constriction in the center that is known as centromere.

The centromere is the specialized DNA sequence of a chromosome that binds a pair of sister chromatids (a dyad).

During mitosis, spindle fibers are attached to the centromere through the kinetochore.

First it was thought that the centromeres were genetic loci that direct the behavior of the chromosomes.

The physical role of the centromere is to act as the assembly site of the kinetochores, a highly complex multiprotein structure that is responsible for the actual events of chromosome segregation.

That is, unite microtubules and signaling the machinery of the cell cycle when all the chromosomes have correctly adopted attachments to it, so that it is certain that the cell division is complete and that the cells enter anaphase.

With respect to the structure of the mitotic chromosome, the centromeres represent a contracted region of the chromosome (often called primary constriction) where two identical sister chromatids are in closer contact.

When cells enter mitosis, the sister chromatids (the two copies of each chromosomal DNA molecule resulting from the replication of DNA in the form of chromatin) are linked along their length by the action of the cohesin complex.

Now it is believed that this complex is released mainly from the arms of the chromosomes during prophase, so that when the chromosomes are aligned in the middle plane of the mitotic spindle (also known as the metaphase plate), the last place where one is linked with the other is in the chromatin in and around the centromere.

Classifications of chromosomes

There are four main types of chromosomes: metacentric, submetacentric, acrocentric, and telocentric.

Each chromosome has two arms, labeled p (the shortest of the two) and q (the longest).

The short arm ‘p’ is named after the French word ‘petit’ meaning ‘small’.

The four types of chromosomes in animal cells are classified by the position of the centromere.

Metacentric

These are chromosomes in the form of X, with the centromere in the middle, so that the two arms of the chromosomes are almost equal.

In a normal human karyotype, five chromosomes are considered metacentric: chromosomes 1, 3, 16, 19 and 20.

In some cases, a metacentric chromosome is formed by a balanced translocation: the fusion of two acrocentric chromosomes to form a metacentric chromosome.

The metacentric chromosomes have the centromere in the center, so that both sections have the same length.

Submetacentric

If the length of the arms is unequal, the chromosome is said to be submetacentric. Its shape is in the form of L.

The submetacentric chromosomes have the centromere slightly deviated from the center, which leads to a slight asymmetry in the length of the two sections.

Human chromosomes 4 to 12 are submetacentric.

Acrocentric

If the p (short) arm is so short that it is difficult to observe, but is still present, then the chromosome is acrocentric (the suffix “acro” refers to the Greek word for “peak”).

The human genome includes six acrocentric chromosomes: 13, 14, 15, 21, 22 and the Y chromosome.

In an acrocentric chromosome, the p arm contains genetic material that includes repeated sequences, such as the nucleolar organizing regions, and can be translocated without significant damage, as in a balanced Robertsonian translocation.

Acrocentric chromosomes have a centromere that is severely deviated from the center and leads to a very long and a very short section.

Human chromosomes 13, 15, 21 and 22 are acrocentric.

Telocentric

The centromere of a telocentric chromosome is located at the terminal end of the chromosome.

Telomeres can extend from both ends of the chromosome, their shape is similar to the letter “i” during anaphase.

The telocentric chromosomes have the centromere at the end of the chromosome.

Humans do not possess telocentric chromosomes, but they are found in other species, such as mice.

Subtelocéntrico

If the centromere of the chromosome is located closer to its end than its center, it can be described as subtelocentric.

Holocéntrico

With the holocentric chromosomes, the entire length of the chromosome acts like the centromere.

You can find examples of this type of centromere disseminated by the animal and plant kingdoms, the Caenorhabditis elegans nematode being the best known example.