In this article we will discuss about Centromere:- 1. Structure of Centromere 2. Types of Centromere.
Structure of Centromere:
The site of constriction in a chromosome under light microscope is generally taken as the position of centromere. It is generally believed that constitutive heterochromatin is present in the centromeric region. The component of centromere is mainly the kinetochore, and DNA associated proteins.
Spindle fibres or microtubules are attached at this point which helps in moving the chromosomes or chromatids to the poles during cell division. When microtubules of the spindle are attached at the centromere of metaphase chromosomes consisting of two chromatids, then sister chromatids separate and move to opposite poles of the spindle—and next step of division proceeds.
Thus centromere has two functions, one is the attachment of sister chromatids, and second is the site for attachment of spindle fibre.
It has been observed under the electron microscope that a single spindle fibre is attached to the centromere of yeast, Saccharomyces cerevisiae, while multiple spindle fibres are attached to the centromere of other organisms.
The chromatin segment of the centromere in yeast has been analysed and found to contain a Protein-DNA complex of 220 to 250 base pairs. Four regions have been identified in the centromere of yeast as CDE 1, CDE 2, CDE 3 and CDE 4.
The base sequences of first three regions are similar in all yeasts but the variation in base sequence is found in CDE 4. CDE 2 region is peculiar in having 90% of base pairs as AT rich. Inverted repeat segments are found in CDE 3 region.
The centromeric DNA is protected from the digestion of nuclease by forming a structure called centromeric core particle. This particle contains more DNA than normal nucleosome core particle and associated proteins. The spindle fibre is attached to this particle that helps to separate chromosomes during cell division.
When the centromeric DNA sequences an protein sequences of centromeric protein are compared, it has been found that protein sequences are more conserved than DNA sequences indicating thereby that DNA sequences may not be the important determinant factor in the function of the centromeric regions.
The centromeric regions of higher organisms contain large amounts of heterochromatin consisting of repetitive DNA.
The centromere of human chromosome contains tandemly repetitive DNA of 170 bp. These repeats are called a-satellite DNA. The number of copies may vary from 5,000 to 15,000. This a DNA is responsible, in most cases as a binding site, for centromeric protein. However the role of a satellite DNA on mammalian centromeres is yet to be established fully.
Mammalian centromeres bind about 30 to 40 spindle fibres or microtubules whereas only one microtubule is attached to the centromere of yeast. The two species of yeast, S. cerevisiae and Schizocharomyces pombe, show wide divergence in the size of centromeric DNA.
The centromenic DNA of S. pombe is 1,000 times larger than those of S. cerevisiae. The centromere of S. Pombe is more complex in having the central core of unique sequence DNA and the flanking sequence of 3 tandem repeats. The function of centromeric DNA has been assayed clearly in yeast showing segregation of plasmids in daughter cells in mitosis when centromere is present.
Types of Centromere:
(a) Structure of Telomeres:
All chromosomes have a special DNA-protein structure at the end called Telomeres. The telomeres have some important role in chromosome replication and stability. Microscopic observations show that chromosomes with broken ends become degraded leading sometimes to cell death.
In an experiment, telomeres from Tetrahymena were transferred to the ends of linear plasmid DNA of yeast and these were then allowed to replicate in yeast. It has been noted that the addition of telomeric DNA helps the plasmid DNA to replicate as linear molecules showing thereby that telomeres are needed for replication.
Telemore consists of repetitive DNA of large Kilo bases and are highly conserved containing clusters of G residues. The telomere sequences of mammals, including human, axe AGGGTT. In Tetrahymena, the sequences is GGGGTT.
Molecular studies show that the telomere sequences of a large number of eukaryotes are similar consisting of repeats of DNA sequences preferably clusters of G residues. The sequence of telomere repeats in human is AGGGTT, in Terahymean it is GGGGTT (Table 13.1). These telomeric sequences are repeated hundreds or telomeric replication thousands of times up to several kilo-bases.
(b) Telomeric Replication:
DNA polymerase has the capacity to synthesise a growing DNA chain in 5′ → 3′ direction but cannot synthesise up to telomeric ends. The replication process in telomere is unique and different.
The problem of replication of telomeres has been done by a special mechanism with the help of an enzyme telomerase having reverse transcriptase activity. This enzyme (telomerase) is able to add telomeric repeats at 3′ end of the DNA strand forming a single-stranded overhang at the 3′ end of both template and new strand.
Hence the 5′ end of each strand is shorter than 3′ end. Now the telomerase molecule incorporates an essential RNA molecule called Guide RNA at 5′ end which has specific sequences that are complementary to the telomere repeat.
It then serves as a primer for telomere at 5′ end of the strand. When the elongation of the strand at 5′ end is complete—i.e., two ends of the strand are equal—then the splicing of RNA primer takes place and the gap is filled up by the polymerase.
The control mechanism of the elongation of length of the telomere is not clearly known. But in case of yeast, it has been found that a special type of protein, called Rap 1p, has played an important role in regulating telomere length. It binds to the yeast telomere sequence and the elongation stops. The special mechanism of telomeric replication was first explained by C. Greider and E. Blackburn in 1986.