Eukaryotic species consist of a specific number of chromosomes. For example, humans have a total of 46 chromosomes. An average human chromosome consists of less than 5 cm of DNA. The DNA gets highly organized and packed into the chromosomes. Hence, the chromosomes are the most efficient genome packets. The chromosomes have a different shape during every phase of the cell cycle.
However, the shape and the structure of the chromosomes get confusing in some species. The interphase and mitosis phase include the two phases of the cell cycle. The interphase consists of G1, S, and G2 phases. The chromosomes contain a thin and extended structure in the G1 phase. Hence, they do not appear clearly in this phase. The S phase is known as the phase of DNA replication. Hence, the chromosome gets duplicated by forming sister chromatids. The chromosome begins to condense in the G2 phase. The mitosis phase accompanies cell division. Hence, light microscopy helps to visualize the chromosomes in M phase. The metaphase chromosomes are thick and rod-shaped. However, the chromosomes in anaphase look like rods, J or V-shaped. Metaphase chromosomes form after DNA replication. The linking between the two chromatids involves a centromere. The chromosome arms are known as chromatids. A telomere is an extreme tip or the end of the chromosome. Metaphase chromosomes are present in a highly condensed form.
Image: Packaging of eukaryotic chromosomes
Nucleosomes:
DNA packaging becomes difficult to study without understanding the nucleosome. It is visible under an electron microscope. Each bead is known as a nucleosome. The string depicts the DNA. Each nucleosome consists of eight proteins known as histones. Five types of histones include H1, H2A, H2B, H3, and H4. Hence, each nucleosome consists of four main histones, each two in number. These proteins form a barrel-shaped core octamer with a DNA string wound twice around the barrel. The DNA linking the two nucleosomes is known as a linker DNA. The chromatin or a DNA histone complex gets stained with a suitable dye. The histones also contain large amounts of arginine and lysine. The charge of the amino acids is positive. Hence, it facilitates an easy binding with a negatively charged DNA. Like linker DNA, vertebrates consist of linker histones such as H1 a-e, H10, H1t, and H5. Each nucleosome gets attached to a linker histone acting like a clamp and preventing the coiled DNA from getting detached. Hence, the nucleosomes associate to form a 30nm fiber as revealed by the cell-breakage techniques. It involves interphase chromosomes in a highly condensed state or metaphase.
Non- histones are equally essential:
Non-histones prevail less abundantly as compared to the histones. However, they play an equally important role. DNA binding proteins are known as histone proteins. They play an important role in DNA replication, repair, transcription, and recombination. They bind with the histones due to their negative charge. The proteins remaining in the chromatin after the removal of the histones are known as non-histones. Hence, the construction of the scaffold structure involves non-histone proteins. A scaffold structure is also known as the central framework of the chromosome. To this, a DNA solenoid gets attached as a loop(s). It also includes a class of enzymes known as topoisomerases.
Two important models of a chromatin fiber:
1. Solenoid model: A solenoid structure is known as a supercoiled arrangement of the DNA in the nuclear chromosome. This arrangement gets produced due to nucleosome string coiling. The process of coiling takes place continuously. It consists of 7 nucleosomes per turn.
2. Helical ribbon model: It reveals the higher order structure of the chromatin.
Looped domains:
Selection micrographs show 30-90 kb loops of DNA. These loops get attached to a scaffold with a proteinaceous property. The X shape of the scaffold occurs due to the pairing of the sister chromatids. A human chromosome consists of 2000 looped domains. The location of the non-histone protein is at the base of the scaffold. Special regions get associated with the loops. They are known as scaffold-associated regions or SARs.
They involve a stretch of DNA binding to the non-histone proteins. The SARs determine the loops arranged spirally. The protrusion of the loops arises from the centromere. They resemble petals of a flower with 15 loops per turn. Thus, the complex scaffolding helps in understanding the chromosomes in a better way.
The study of human chromosome:
Study of human chromosome involves cytogenetic technique such as karyotyping. Different banding techniques help in identifying the chromosomes. A typical metaphase chromosome in a human consists of a satellite, a centromere, a secondary constriction, and chromatids. The study of the sex chromatin and Barr body involves a separate study.
CEN sequences (DNA-protein interactions) in the yeast:
The centromere of the yeast has a single sequence with an approximately 125 base pairs length. This sequence consists of short regions known as CDE I and CDE III. A region known as CDEII lies between CDEI and CDEIII. CDEII is a longer element and is variable and AT-rich. The sequence of CDE I and CDE III are highly conserved. A histone H3 analog is known as Cse 4 protein. It is present in the yeast. This protein combines with another protein MIf2 and forms a core around which the CDEII sequence gets wrapped. Two more proteins get involved in the process. Cbf1 recognizes and attaches to the CDEI sequence. Cbf3 protein attaches to CDEIII. They altogether form the kinetochore. Unlike humans, yeast lacks nucleosomes.
References:
[1] Cells: Molecules and Mechanisms, E.V. Wong
[2] Molecular Biology of the Gene, Watson
[3] Epigenetics, Lyle Armstrong
[4] Human Molecular Genetics, Tom Strachan
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