Structure, Functions and Types of Centromeres

The word centromere came from two Greek words “Centro” and “Mere”, which mean “central” and “part” respectively. Centromere is a unique and well-maintained chromatin structure that plays a crucial role in the replication of chromosomes throughout the process of cell division. Centromeres do a lot of important jobs, like keeping sister chromatids together, moving chromosomes, sticking to microtubules, forming heterochromatin, and controlling the mitotic checkpoint.

In simple terms, centromeres are like the glue that holds chromosomes together. It also ensures that chromosomes are divided correctly when a cell splits in two. Let’s discuss more about the centromere with its structure, functionalities, and types in detail.

What is Centromere?

A centromere is a constricted region of a chromosome that splits the whole chromosome during cell division.

• It keeps the two arms of the chromosome together and splits the whole chromosome into a short part (p) and a long part (q).
• The centromere has two important parts called kinetochore and DNA-associated proteins.
• The centromere also can be called a middle “hub” of a chromosome where spindle fibers are connected. This helps the chromosome to move properly when cells are dividing, especially in the anaphase stage.

Structure of the Centromere

Imagine a chromosome like a letter ‘X’, even if you’ve never seen one through a microscope. You’ll notice that the middle part where the two arms of the ‘X’ meet is thinner, right? That’s the centromere. Sometimes, it’s not right in the middle, though.

There are two major components of centromere:

Kinetochore

DNA-associated proteins

Kinetochore works like a traffic controller in cell division process and makes sure everything goes to the right place. It is an important component of cell division process as it holds onto spindle fibers. Kinetochore ensures that chromosomes are distributed correctly. It also ensures that chromosomes are safely attached to the spindle. It’s like an “all aboard” sign for cells to move to the next division stage, which is anaphase. Kinetochore makes sure that cell division happens safely and accurately.

The centromere contains a unique type of DNA called heterochromatin. This DNA is vital for the proper separation of sister chromatids during anaphase. Besides CENP-A nucleosomes, there are several other DNA-binding proteins that associate with centromeres. These DNAs have a significant impact on how the centromere functions.

Functions of Centromere

Centromeres are responsible to perform so many functions inside the chromosome. Here are some important functionalities mentioned below:

• Centrometers are responsible for the segregation and proper alignment process of the eukaryotic cell chromosomes during their division.
• Centromeres play a vital role in the new cell production.
• It is a chromosome binding spot, specifically for the sister chromatids.
• It is also an attachment point of the kinetochore.
• Centromeres are also responsible for microtubule attachment.
• It tracks the movements of chromosomes.
• Centromeres can establish heterochromatin and mitotic checkpoints in the process of coupling and separation of sister chromatids.

Classification of Centromere

Based on sequence specificity, centromeres can be divided into two groups:

Regional Centromeres

Point Centromeres

Regional centromeres are very common in eukaryotes. The DNA sequences (specifically, satellite DNA) are associated in the centromeres contribute to the function. They do not define the function of the centromere. These centromeres range several a 100 KBs in eukaryotic cells. Regional centromeres provide genetic flexibility and adaptability.

In point centromeres, the DNA sequence is vital to make them work. Point centromeres are often found in budding yeast, usually around 150 base pairs long. These centromeres are like tiny spots on the chromosome, and their job is super important because they help make sure chromosomes are split correctly during cell division. They link up with spindle fibers, which are like the “movers” for the chromosomes. It also makes sure the genetic material is distributed correctly to the new cells.

Depending on the position of centromere in a chromosome, they can be classified into several categories:

Metacentric	Metacentric chromosomes have their centromere positioned right in the middle. This centromere devides cromosomes in an even division into two equal arms when cells are dividing in the nucleus. Notably, certain human chromosomes, including numbers 1, 3, 16, 19, and 20, are classified as metacentric due to their centrally located centromeres. Metacentric centromeres play a key role in ensuring a balanced distribution of genetic information during cell division.
Sub-metacentric	The chromosome look like a letter “L.” This type of centromere is called sub-metacentric. Submetacentricity can be found in certain human chromosomes, including numbers 2, 4 through 12, 17, 18, and the X chromosome. These chromosomes have their centromeres positioned a bit off-center, and this special arrangement helps with proper cell division.
Acrocentric	The centromere is found at one end of the chromosomes. This arrangement makes one arm of the chromosome significantly longer than the other. Short arm contains very few genes. In humans, examples of acrocentric chromosomes include 13, 14, 15, 21, 22, and the Y chromosome.
Telocentric	Visualize it as though you have a flag at the very top of a flagpole when considering the organization of these chromosomes. The telocentric centromere predominantly observed in certain animal species, like mice, rather than in humans. The location of centromere is positioned right at one end of the chromosome, giving it an “i” shape.

There is a different type of chromosome, that is not usual;

Holocentric

• Certain organisms, like plants belonging to the Luzula genus, have holocentric chromosomes.
• The centromeres are spread out rather than concentrated in one spot.
• These chromosomes have centromeric heterochromatin, a special type of DNA.
• The spindle connects to this dispersed centromeric DNA.

FAQ’s on Centromere

1. What is Centromere?

The centromere is a compressed part of a chromosome and a specialized DNA sequence that holds the pair of sister chromatids together with cohesin protein molecules.

2. What are the Major Componets of a Centromere?

There are two major components:

1. Kinetochore: It is a megadalton protein structure that controls the attachments between chromosomes and microtubules with a proper feedback system for the cell cycle.
2. DNA-associated proteins: These are the heterochromatin formations that are the necessary components for both to perform the attachment and separation process of sister chromatids during anaphase.

3. What are the Major Classification of Centromeres?

According to the position of the centromere, there are four major types – Metacentric, Submetacentric, Acrocentric, and Telocentric. On the other hand, as per the sequence specificity, there are two particular groups – Regional and Point Centromeres.

4. What is the Major difference between Regional and Point Centromeres?

Regional centromeres are mostly found in the eukaryotic cells with a range of 100 KBs where point centromeres are common in budding yeast where 150 base pairs can be found.

5. Is there any Drawback of Centromeres?

There is a major problem with centromere. Dysfunction related to it can be a cause of cancer and miscarriage in some cases.