A typical chromosome with its structures. Often, the centromere has a three layered covering known as Kinetochore.Diagram of a duplicated and condensed (metaphase) eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2) Centromere – the point where the two chromatids touch, and where the microtubules attach. (3) Short arm (p). (4) Long arm (q).
Several chromosome regions have been defined by convenience and convention in order to talk about gene loci. The largest regions on each chromosome are the short arm p and the long arm q, separated by a narrow region near the center called the centromere. Other specific regions have also been defined, some of which are similarly found on every chromosome, while others are only present in certain chromosomes.
The centromere is the attachment location that connects the kinetochore and spindle microtubules.[1] Most centromeres are constructed of heterochromatin, but centromeres lack a single, specific nucleotide sequence that is common to all of them.[1] Instead, a common unifying trait in eukaryotes is the presence of a variant histone protein referred to as CENP-A or CenH3.[1] Rather than relying on a specific sequence, the presence of this histone causes epigenetic changes that rearrange the chromatin, allowing the spindle microtubules and kinetochore to attach.[1]
This connection between kinetochores and microtubules makes it possible for chromosomes to move in mitosis and meiosis.[1] If chromosomes lack a centromere, the kinetochore and microtubules will not associate, failing to form new cells.[1]
The exact position of the centromere on a chromosome determines its shape and classifies it into four distinct categories: metacentric, submetacentric, acrocentric, and telocentric.[1]
The position of the centromere on a chromosome also divides it into two sections, commonly referred to as arms.[2] The short arm on a chromosome is called the "p arm," while the larger one is referred to as the "q arm."[1]
Kinetochore
The kinetochore is a set of proteins that connect the microtubules to the centromere, allowing the cell to undergo metaphase and the rest of mitosis.[3] The kinetochore is made of 100+ proteins, which are assembled and disassembled before and after cell division.[3] The kinetochore has the ability to bind to both microtubules and chromosomal DNA, which is necessary for proper segregation to daughter cells.[4]
Telomere
At either end of a chromosome is a telomere, a cap of DNA that protects the rest of the chromosome from damage. The telomere has repetitive junk DNA and, hence, any enzymatic damage will not affect the coded regions.[1]
Regions of a chromosome
The areas of the p and q regions close to the telomeres are the subtelomeres, or subtelomeric regions.[1] The areas closer to the centromere are the pericentronomic regions. Finally, the interstitial regions are the parts of the p and q regions that are close to neither the centromere nor the telomeres, but are roughly in the middle of p or q.[2]
The telomeres consist of a string of thymine or adenine nucleotides and then a string of guanines.[1] These numbers depend on the species. The human repeated telomere sequence is 5'TTAGGG3'.[1] The complementary strand ends with cytosine nucleotides. The guanine sequence extends further than the cytosine sequence, forming the G-rich 3' overhang.[1] This G-rich 3' overhang can fold itself into a t-loop, which further protects the ends of the DNA.[1]
The telomeres prevent DNA repair mechanisms from viewing the ends of chromosomes as broken DNA. This can cause the DNA to become attached and further degrade.[5]
The telomere gets shorter as the cell goes through cycles of division due to the inability of cells to replicate the end of a DNA strand. The cell dies when the telomere is not long enough to properly divide.[5]
Satellite Chromosome, Trabants, and NORs
Diagram of a satellite chromosome. (1) Chromatid. (2) Centromere (First Constriction). (3) Secondary constriction. (4) Satellite.
An SAT chromosome is a chromosome that has a separate section that is isolated from the main structure of the chromosome by a secondary constriction.[6] This section of extra material is known as a satellite chromosome or trabant.[6] There are four subclasses of satellites on chromosomes: microsatellites, macrosatellites, linear, and pseudosatellites.[6]
These secondary constrictions of chromosomes house the genes needed for the assembly of the nucleoli.[7] Specifically, they possess the genes needed to construct ribosomal RNA.[7] These regions are called the nucleolar organizer regions (NORs).[7]
References
1234567891011121314Pierce, Benjamin A. (2024). Genetics: a conceptual approach (Seventh edition digital updateed.). Austin: Macmillan Learning. ISBN978-1-319-33778-0.
123Rieger, Rigomar; Michaelis, Arnd; Green, Melvin M. (1976). Glossary of genetics and cytogenetics: classical and molecular (4th completely rev.ed.). Berlin, New York: Springer-Verlag. ISBN978-0-387-07668-3.
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