Difference Between Centromere and Telomere

Main Difference – Centromere vs Telomere

Centromere and telomere are two structures found on a chromosome. Centromere contains a highly constricted region of DNA in the form of centric heterochromatin. The two sister chromatids of the replicated chromosome are held together by the centromere during cell division. Telomeres are the end regions of a chromosome, containing highly repetitive sequences of DNA. Genes in the end regions of the chromosomes are capped by the presence of telomeres from being truncated and fusion with other chromosomes. The main difference between centromere and telomere is that centromere is the region in the middle of a chromosome, holding the two sister chromatids together of a replicated chromosome whereas telomere is the end region of a chromosome, protecting the genes in the end regions from degradation.

This article explains, 

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

What is Centromere

The centromere holds the two sister chromatids together in a replicated chromosome. It is a region on a chromosome that consists of centric heterochromatin. The centric heterochromatin is flanked by pericentric heterochromatin. Between the two sister chromatids, cohesin protein complexes are present, linking the two copies of the replicated chromosome. The role of the centromere is to provide a site for the binding with microtubules via kinetochores. Kinetochore is the protein complex which is assembled on the centromere of the chromosome.

Structure of Centromere

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

Two types of centromeres can be identified within 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 can also be formed on the other DNA sequences.

The chromosome is divided into two arms by the presence of a single centromere. The 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 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. 

Figure 1: Centromere on a chromosome

What is Telomere

A telomere is the end region of each chromosome, containing repetitive sequences. It protects the chromosomal end from degradation and end fashioning with other chromosomal ends. During DNA replication, DNA polymerase is unable to carry out the replication of the whole chromosome until its end. Thus, during each replication round, the length of the chromosome is reduced from its telomere end. In humans, the length of the telomere is around 11 kb at the birth and it reduces to 4 kb with aging.

Structure of Telomere

A telomere in vertebrates contains around 2,500 times repeating of TTAGGG nucleotide sequence. Prokaryotes lack telomeres due to the presence of circular chromosomes. The presence of telomere at the end of a chromosome leads to a loss of genetic material during the semi-conservative DNA replication. But, genes at the end regions of a chromosome are protected from being truncated while the loss of genetic material occurs from telomeres. Therefore, the telomeres cap the genetic material at the end of the chromosome. The structure of a telomere is shown in figure 2.

Figure 2: Telomere Structure

The distal 300 bp region of the telomere is a single-stranded DNA stretch. This DNA stretch forms a T loop, which is an analogue to a knot, stabilizing the telomere end by preventing the recognition of telomere’s end as a break point. Several proteins like TRF1, TRF2, TIN1, TIN2, TRP1, RAP1 and POT1 are associated, stabilizing the T loop. These protein complexes are collectively known as the sheltering complex. Sheltering complex at the T loop is shown in figure 3.

Figure 3: Proteins in the Sheltering Complex

Difference Between Centromere and Telomere

Location

Centromere: Centromere is found in the approximate middle of a chromosome.

Telomere: Telomere is found at the end of a chromosome.

Number

Centromere: Monocentric organisms contain a single centromere per chromosome. Holocentric organisms contain more than one centromere per chromosome.

Telomere: A single sister chromatid contains two telomeres, each at the two ends.

Composition

Centromere: Centromere is composed of double-stranded, centric heterochromatin DNA.

Telomere: Telomere is composed of single-stranded, repetitive DNA sequences.

Associated Proteins

Centromere: Centromere is associated with cohesin and kinetochore protein complexes.

Telomere: The T loop of the telomere is associated with proteins like TRF1, TRF2, TIN1, TIN2, TRP1, RAP1 and POT1.

Role

Centromere: Centromere holds the two sister chromatids together.

Telomere: Telomere protects the genes at the end regions of a chromosome from degradation.

Conclusion

Centromere and telomere are two regions found on a chromosome. Centromere is approximately located in the middle of a chromosome and telomere is located at the two ends of each sister chromatid. Centromere consists of constricted centric heterochromatin, which is associated with cohesins and kinetochores. Both proteins play a role in holding the two sister chromatids together during the nuclear division. Kinetochore proteins also provide sites for the attachment of spindle microtubules. The T loop of the telomere is stabilized with TRF1, TRF2, TIN1, TIN2, TRP1, RAP1 and POT1 like proteins. The main role of the centromere is to hold the two sister chromatids together. Telomeres protect the end region genetic material from degradation and truncation. The main difference between centromere and telomere is their location on a chromosome and role during the cell’s life.

Reference:
1. “Centromere.” Wikipedia. Wikimedia Foundation, 06 Mar. 2017. Web. 13 Mar. 2017.
2.”Telomere.” Wikipedia. Wikimedia Foundation, 13 Mar. 2017. Web. 13 Mar. 2017.

Image Courtesy:
1.”Condensed Eukaryotic Chromosome” By Zephyris at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Telomere” (CC BY-SA 3.0) via Commons Wikimedia
3. “Telosome”By Unknown – Linkage, a journal by DCEG, a NCI division (Public Domain) via Commons Wikimedia

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