Difference Between Centromere and Kinetochore

Main Difference – Centromere vs Kinetochore

Centromere and kinetochore are two structures found on the approximate middle of chromosomes and are involved in the sister chromatid segregation at cell division. The centromere is a region of highly condensed DNA where the kinetochores are assembled. The kinetochore is a type of protein which connects the chromosomes with the spindle microtubules. Spindle microtubules are attached with kinetochore which is assembled on the centromere. The main difference between centromere and kinetochore is that centromere is the region where the two sister chromatids are held together after the replication of chromosome where kinetochore is the protein complex on the chromosome where spindle fibers are attached during cell division.  

This article explains, 

1. What is Centromere
      – Definition, Structure, Function, Characteristics
2. What is a Kinetochore
      – Definition, Structure, Function, Characteristics
3. What is the difference between Centromere and Kinetochore

What is Centromere

The centromere is the region of a replicated chromosome, holding the two sister chromatids together. Centric heterochromatin is the type of heterochromatin found in the centromere of a chromosome. The centric heterochromatin is flanked by pericentric heterochromatin. Cohesin protein complexes are present in between the two sister chromatids, linking the two copies of the replicated chromosome. The role of the centromere is to provide a site for the binding with microtubules via the kinetochore, which is a protein complex assembled on the centromere of the chromosome. Two types of centromeres can be identified on the chromosomes: point centromeres and regional centrosomes. Point centromeres bind with specific proteins to form centromeres. Though the formation of centromere prefers a unique DNA sequence to form the centromere, regional centromeres also can be formed on the other DNA sequences. The structure of a chromosome, bearing a centromere is shown in figure 1.

Figure 1: Centromere in a Chromosome

Positions of Centromeres

A chromosome consists of two arms: long arm is known as q arm and the short arm is known as the p arm. Depending on the centromere position chromosomes can be divided into four major types: metacentric, submetacentric, acrocentric and telocentric. Metacentric chromosomes consist of equal lengths in both p and q arms. In submetacentric chromosomes, p and q arms are fairly unequal in lengths. In acrocentric chromosomes, q arm is longer than the p arm. In telocentric chromosomes, the centromere is located on the terminal end of the chromosome. Positions of the centromere in different chromosome types are shown in figure 2.

Figure 2: Positions of the Centromere
A – Short/p arm, B – Centromere, C – Long/q arm, D – Sister chromatid
I – Telocentric, II – Acrocentric, III – Submetacentric, IV – Metacentric

Organisms with a single centromere per one chromosome are known as monocentric organisms. Examples of the monocentric organisms are fungi and vertebrates. On the other hand, holocentric organisms consist of more than one centromere per one chromosome. Nematodes are examples for the holocentric organisms.

What is Kinetochore

A kinetochore is a protein complex, which is assembled on the centromere of chromosomes, attaching spindle microtubules to pull chromosomes or sister chromatids apart during the anaphase of the cell division. In monocentric organisms, a single kinetochore assembling point can be identified; in holocentric organisms, the assembling of kinetochores can be observed along the entire chromosome. Other than promoting the binding of spindle microtubules to the chromosome, kinetochores play a role in holding two sister chromatids together along with cohesin protein complexes. Kinetochores at the metaphase of a human cell are shown in pink color of figure 3. Green color shows the spindle microtubules, and the blue color shows the chromosomes.

Figure 3: Kinetochores on a Spindle

Structure of Kinetochore

Two regions of a kinetochore can be identified: inner kinetochore and outer kinetochore. The inner kinetochore holds the two sister chromatids together by tightly associating with the centromere region. It contains nucleosomes which are made up of specialized histones called CENP-A, substituting the histone H3. The outer kinetochore interacts with the spindle microtubules. It is assembled on the surface of the chromosome after the nuclear envelope is broken down. The outer kinetochore consists of about 20 sites for the anchoring of kinetochore microtubules. The assembly of spindle microtubules with the centromere is shown in figure 4.

Figure 4: Assembling of kinetochore

Difference Between Centromere and Kinetochore

Composition

Centromere: A centromere is a constricted region of DNA on a chromosome.

Kinetochore: A kinetochore is a protein complex, assembled on centromeres of chromosomes.

Observation

Centromere: Centromeres can be observed under the light microscope.

Kinetochore: Kinetochores are only visible under the electron microscope.

Structure

Centromere: Centromeres consist of centric heterochromatin.

Kinetochore: Kinetochores are made up of specialized types of histones like CENP-A.

Position

Centromere: Four centeromere positions can be identified: metacentric, submetacentric, acrocentric and telocentric.

Kinetochore: Kinetochores consist of two layers: inner kinetochore and the outer kinetochore.

Number

Centromere: Centromeres can be either monocentric or holocentric.

Kinetochore: One centromere contains one kinetochore complex.

Binding with Microtubules

Centromere: Centromeres cannot bind with microtubules by themselves. 

Kinetochore: Outer kinetochore consists of around 20 sites for the anchoring of kinetochore microtubules.

Conclusion

Centromere and kinetochore are two types of elements contributing to the cell division. A centromere is a region of constricted DNA, present in the form of centric heterochromatin which is flanked by pericentric heterochromatin. Centromeres can be either monocentric or holocentric. Each centromere is assembled with a kinetochore complex. Kinetochore complex consists of inner and outer layers of kinetochores. The inner kinetochores hold the two sister chromatids together. On the other hand, outer kinetochores provide places for the attachment of spindle microtubules with centromeres. The main difference between centromere and kinetochore is that the centromere is a DNA region whereas kinetochore is an assembling protein complex in the centromere.

Reference:
1. “Centromere.” Wikipedia. Wikimedia Foundation, 06 Mar. 2017. Web. 12 Mar. 2017. 
2. “Kinetochore.” Wikipedia. Wikimedia Foundation, 20 Feb. 2017. Web. 12 Mar. 2017.

Image Courtesy:
1. “Mitoticchromosome” By HeavyQuark – Created on Microsoft paint (CC BY-SA 3.0) via Commons Wikimedia
2. “Centromere Placement” By Fockey003 – Own work (CC BY-SA 4.0)(CC BY-SA 4.0) via Commons Wikimedia
3. “Kinetochore” By Afunguy at English Wikipedia – Transferred from en.wikipedia to Commons by Lijealso using CommonsHelper. (Public Domain) via Commons Wikimedia
4. “Chromosome cohesion – en” By Dawn08 – Own work (CC BY-SA 3.0) via Commons Wikimedia