Chromosome Structure/Centromere/Telomere

Chromosome Structure

Chromosomes contain centromeres and telomeres. This is an important region on chromosomes that is indispensable for chromosomal DNA to be transmitted from parent to child and from cell to cell without error.

The area where the two chromosomes are attached at the center is called the centromere.

The arms appear to extend from the centromere. The shorter arm is called the short arm (p) and the longer arm is called the long arm (q). The bridge of each arm is called the telomere.

Chromosomes in division are made of the same DNA.

One of them is called the chromosome segregant, and the two combined are called the sister chromatid segregants.

And although these sister chromatid fragments are made of the exact same DNA. Cells originally contain two chromosomal DNA strands in pairs. When these become chromosomes, they are called homologous chromosomes.

One is derived from the mother and the other from the father; chromosomes 1 through 22 are called autosomes, and X and Y are called sex chromosomes.

Role of the Centromere

Centromeres and kinetochores were thought to be one and the same, but centromeres often refer to chromosomal (DNA) regions associated with kinetochores, while kinetochores are more often referred to microtubules, the structures themselves to which they attach.

Centromeres have two roles in meiosis.

The first is that it is the region where chromosomal DNA is distributed exactly equally between the two daughter cells.

The second is controlled to separate sister chromatid segments in a regular manner.

Telomeres are a complex of specialized structures and proteins at the ends of the single double-stranded DNA that make up chromosomes. The older the telomere, the shorter the telomere DNA length tends to be.

Inactivation of X chromosome

Females have two X chromosomes and males have one.

The X chromosome has something called inactivation. Males have an X chromosome and a smaller Y chromosome, while females have two X chromosomes, resulting in a higher amount of genes on the X chromosome.

Therefore, one of the two X chromosomes is inactivated to suppress gene activity as a mechanism to equalize gene expression on the X chromosome in males and females. This inactivation of the X chromosome is also called lionization, and it occurs randomly regardless of maternal or paternal origin. Once inactivated, the X chromosome is never activated again. It is passed on to the daughter cells that arise from mitosis.

Inactivation when there are two or more X chromosomes

So what would happen to inactivation if there were three or four X chromosomes? This can be determined by a test called the FISH method.

If there are three X chromosomes, two of the three are inactivated; if there are four X chromosomes, three of the four are inactivated. One chromosome is always inactivated. What would happen if this inactivation did not occur? Inactivation is done to regulate the gene dosage, so an excess of genes can lead to severe developmental delay. Therefore, most cases are considered to result in miscarriage.

Also, if there were three X chromosomes and two were supposed to be inactivated but only a very small fragment was not, the child would be born with severe developmental delay. There are also rare cases where cells with maternal X inactivated and cells with paternal X inactivated are mixed together. This is called mosaicism.

ES cells and iPS cells, which are often heard of these days, are not inactivated by the cells themselves, but one of them is inactivated as the cells differentiate.