Life on this planet is a miraculous thing, and how life developed is another enigma that has yet to be fully solved. All the forms of life found on earth are categorized widely into two classifications: eukaryotes and prokaryotes.
Chromosomes are the greatest units of DNA and protein arrangement. The prime chromosome function in cell include carrying the DNA and conveying the genetic information from the parents to their offspring in the next generation. During cell division, the most important part is played by the chromosomes.
The genome of prokaryotic cells is generally found in the shape of a circular chromosome situated within a chromosome. The genetic material in eukaryotic cells is found in the nucleus and is tightly packed into linear chromosomes.
The composition of the chromosomes consists of a complex or a mixture of DNA and proteins, which is known as chromatin. Chromatin is further organized into smaller subunits known as nucleososmes. Eukaryotes are tightly packed and their chromatin is arranged in such a way that a large quantity of DNA can be adjusted in a small space. This is also helpful as it can regulate gene expression.
In the cell, the cellular DNA is never found alone and is always accompanied by different proteins. Rather, the cellular DNA is always forming a complex or mixtures with other partners of protein that help them get fit in a small space. This complex of DNA-protein is known as chromatin, where the amount of nucleic acid and protein present is almost equivalent. In the cells, the chromatin gets folds into some characteristic formations which are known as chromosomes. A single chromosome consists of one piece of double stranded DNA in addition to the packaging proteins that are mentioned earlier.
The chromosomes of eukaryotic cells contain units of chromatin that repeats itself and are known as nucleosomes. Chemically degrading cellular nuclei and peeling away as much of the exterior protein packing from the DNA as practiceble led to the discovery of these. Nucleosomes are formed of DNA that is double stranded. They form complexes know as histones, which are tiny protein fragments.
Core particle of each of the nucleosome contains molecules of eight histones, as a pair of two of four various types of histones: H2A, H2B, H3, and H4. The structure and shape of histones has remained remarkably consistent throughout evolution, implying that the DNA’s packing role is critical to all cells that belong to eukaryotes. Histones are generally positive charged molecules and they bind together with the DNA that is negative in charge in a particular conformation.
A typical chromosome has the following parts and their functions:
(a) Centromere (Primary constriction):
In the stage of metaphase, the chromosome contains two twin sisters known as chromatids. These bind to one another at a point called primary constriction, or the centromere. At the stage of anaphase, the centromere divides the identical sisters to form two anaphasic chromosomes. Arms refer to the chromosomal segments on either side of the centromere.
So we can say that the chromosome of metaphase has four arms, whereas the chromosomes of anaphase have only two arms. Isobrachial chromosomes have equal arms, while heterobrachial chromosomes have asymmetrical arms. When the arms are uneven, the short arm is marked as “p” and the long arm is denoted as “q.”
At the location of the centromere, chromosomes are named:
- Telocentric (centromere terminal)
- Acrocentric (centromere subterminal and capped by telomere),
- Sub-metacentric (centromere is submedian),
- Metacentric (centromere median).
Generally, the chromosomes have one centromere and are called monocentric. The chromosomes may be dicentric, for example in maize and wheat; or sometimes polycentric. The centromere’s major purpose is to lay the groundwork for the kinetochore, a protein complex that is required for correct chromosomal segregation throughout mitosis.
The kinetochore, a specialised multi-protein complex on the surface of the centromere, is where spindle fibres (microtubules) attach. A metaphase chromosome’s centromere comprises two kinctochores that face in opposite directions. Lower plants have trilamilar kinetochore, while higher plants have ball and cup kinetochore.
The kinetochore is a multi-subunit proteinaceous construct in eukaryotes that generates load-bearing attachments of sister chromatids (replicated chromosomes kept together by the protein complex cohesin) to spindle microtubules throughout cell division.
(c) Secondary Constrictions:
In addition to the centromere, the achromosome may feature one or more secondary constrictions. Satellite or trabant refers to a chromosomal segment that is connected to the main region of the chromosome by a chromatin thread. The chromosome with satellites is referred to as they sat chromosome.
There are two types of secondary constrictions: NOR and Joint. They maintain a consistent location and are frequently used as markers. The nucleolar organiser region (NOR) is responsible for the production of the nucleolus and rRNA. Breaking and fusion of chromosomal segments can lead to the formation of joints. They serve as markers for where the nucleoli are assembled. It is involved in the rearrangement of the nucleolus at the end of cell division and is linked with the nucleolus throughout interphase of cell division.
The terminal ends of chromosomes are called telomeres. A telomere is a short repeated DNA sequence (GC rich) complex with proteins. They are synthesized separately and later added to the chromosomal tips. The main function of telomeres is to cap the chromosomal ends in order to prevent DNA loss during cell replication rounds.
The telomeres help in various ways:
(i) Provide stability by preventing end fusions of chromosomes,
(ii) Act as initiators of synapsis,
(iii) Shortening of telomeres causes senescence and aging.
Because of chromatin buildup, interphase chromosomes might appear beaded along their whole length. Chromo-meres are these bead-like structures. The chromo-meres are firmly coiled and no longer visible during the metaphase. Controlling gene expression can be aided by the layout of the chromosome structure. Chromosome maps can be created for use in genetic and evolutionary research.
The function of chromosomes in eukaryotic cells:
Eukaryotic cells have of genes that consists hereditary information and act like a hereditary vehicle. Cell division, growth, metabolism, and differentiation are all controlled by them.
The cell’s nucleus contains the eukaryotic chromosomes. This nucleus is known as the “control center” of cell that helps in the storage of the whole cell’s genetic material or the DNA. The nuclear envelope, also known as nuclear membrane consists of channels known as pores that help in regulating the movement of molecules through the nucleus.
The DNA present in the nucleus is arranged in the form of chromosomes. A chromosome is a DNA molecule which is coiled very tightly around the proteins to form histones. Eukaryotic cells contain several chromosomes that are linear in structure. The chromatin includes the whole DNA present in the nucleus and also the proteins associated with it. Chromatin consists of three basic layers of scaffolding that gives rise to a condensed molecule of DNA.
The presence of a membrane-bound nucleus is the most important feature that distinguishes a prokaryotic cell from a eukaryotic cell. This nucleus is referred to as the cell’s “control centre” since it aids in the storing of the complete cell’s genetic material or the DNA. Pores in the nuclear envelope, also known as the nuclear membrane, aid in the regulation of molecular transport across the nucleus.
The double stranded helix molecule of DNA together forms the chromosome but before that they are being coiled around the cluster of proteins known as histones. A nucleosome, which is the smallest unit of DNA-packing structure, is formed when a unit of roughly 200 DNA base pairs gets coiled around the eight histone proteins.
The linker DNA and the nucleosomes connect themselves, just like the beads on the string, to form 30-nm solenoid fibers which tightly packed. These fibers formed are further coiled and gets folded into the loops that are also packed together tightly. This scaffolding is the last stage where the chromosomes can be observed in the stage known as metaphase of meiosis and mitosis.
The process of supercoiling forms those 30-nm fibers. The application of tension is used by supercoiling so that it can twist a molecule of DNA, so it creates loops while wrapping around itself. With the cell division process, mitosis or meiosis, the individual chromosomes are clearly seen to be present in the nucleus with the help of a microscope.
The function of the chromosome in prokaryotic cells:
Similar to chromosome of eukaryotic cells, the chromosomes of prokaryotic cells helps in the storage and transmitting genetic information to other cell. To form RNA, DNA and protein, it transcribes, replicates and translates, respectively.
The chromosome present in prokaryotes form irregular kind of structure called nucleoid. Many prokaryotic cells obey the process of supercoiling in order to produce chromosome. The single chromosome of prokaryotes is circular shape, lacks histones. They follow the process of coiling and twisting and become compact so they can get fitted in nucleoid.
Prokaryotic chromosomes are single DNA molecules that are either linear or circular in shape and are found in the cytoplasm of a prokaryotic organism. Prokaryotes are the microscopic organisms that have single cell and primitive in nature. On the other hand, eukaryotes have multiple cells and belong to a high level organization. The meaning of prokaryotes stands for “before” nucleus. It evolved even before nucleus and thus lacks a recognisable nucleus-like pattern.
The entire genome or the prokaryotic genetic information is situated on a solitary chromosome which is linear in shape and is present in the cytoplasm. This prokaryotic chromosome is structurally different from the eukaryotic chromosome, though it also conveys the genetic information from one cell to the other. The prokaryotic genome not only contains the chromosomes but also plasmid which acts as an inheritance of traits.
The plasmid has a circular shape and is single-stranded DNA. They inherit many significant genes for the organisms. In contrast to eukaryotic cells, prokaryotes are deprived of the membrane-bound organelles in them. Some examples of prokaryotes may include algae, Archea, bacteria, and a few fungi.
Properties of the prokaryotic chromosome
- A chromosome of prokaryotes is either linear or circular in shape.
- It consists of an extrachromosomal DNA as regards a plasmid.
- It has a genome which is haploid in nature.
- The chromosome carries only one copy of the gene present.
- Prokaryotes include a single chromosome in them.
The genes present in the chromosomes of prokaryotic cells own a particular kind of mechanism which is known as the operon. Using this mechanism many genes can produce protein. Gene sequences that aren’t as vital aren’t found on the chromosome; rather, they’re found on plasmids. The chromosomes present in the prokaryotic cells has few number of repetitive and garbage DNA in them.
The genes present on the chromosome are situated very close to each. Out of whole, only 12% of the garbage DNA is found in the genome of the prokaryotic cells. They are usually tightly closed to each other and consist of less garbage genetic material in them. Both the process namely translation and transcription occurs in cytoplasm.
It is made possible by a number of proteins and enzymes, including topoisomerase I and II, gyrase, HU, H0NS, and IHF. All of them help in the regulation and preservation of supercoiling in prokaryotes.
Supercoiling can occur in the same order or direction as the double stranded DNA, and thus it is known as “negative supercoiling.” Whereas, if the supercoiling takes place in the reverse direction, the supercoiling is known as positive supercoiling. Prokaryotes, particularly bacteria, obey the process of negative supercoiling. Topoisimerase is the name of the class of enzymes that help in the regulation of the tension that is caused by the mechanism of supercoiling during replication.
The prokaryotic chromosomes are tightly supercoiled. Thus the value of the replication is very slow here as compared to the eukaryotes. Also, due to its location in the cytoplasm, the mechanisms of translation and transcription take place side by side at the same position.