A chromosome is a deoxyribonucleic acid (DNA) molecule with part or all of the genetic material (genome) of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle. The prokaryotes — bacteria and archaea — typically have a single circular chromosome, but many variations exist. This structure is, however, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome.
Typical structure of prokaryotic chromosome
Probably the most striking difference between prokaryotes and eukaryotes is how their genetic material is packaged, Eukaryotic cells have two or more chromosomes contained within a membrane-delimited organelle, the nucleus. In contrast, prokaryotes lack a membrane-delimited nucleus. The prokaryotic chromosome is located in an irregularly shaped region called the nucleoid (other names are also used: the nuclear body, chromatin body, nuclear region).
Usually, prokaryotes contain a single circle of double-stranded deoxyribonucleic acid (D-NA), but some have a linear DNA chromosome, Recently it has been discovered that some bacteria such as Vibrio cholera have more than one chromosome, Although nucleoid appearance varies with the method of fixation and staining, fibers often are seen in electron micrographs and are probably DNA. The nucleoid also is visible in the light microscope after staining with the Feulgen stain, which specifically reacts with DNA. A cell can have more than one nucleoid when cell division occurs after the genetic material has been duplicated. In actively growing bacteria, the nucleoid has projections that extend into the cytoplasmic. Presumably, these projections contain DNA that is being actively transcribed to produce mRNA. Nucleoids have been isolated intact and free from membranes. Chemical analysis reveals that they are composed of about 60% DNA, 30% RNA, and the DNA circle measures approximately 1,400 gm obviously it must be very efficiently packaged to fit within the nucleoid. The DNA is looped and coiled extensively, probably with the aid of RNA and nucleoid proteins (these proteins differ from the histone proteins present in eukaryotic nuclei).
Membrane-bound DNA-containing regions are present in two genera of planctomycetes. Pirellula has a single membrane that surrounds a region, the cellulosome, which contains a fibrillar nucleoid and ribosome-like panicles. The nuclear body of Gemmata obscuriglobus is bounded by two membranes.
Typical structure and Structural organization of eukaryotic chromosome
A chromosome is an organized structure of DNA and protein that is found in the nucleus of the cell. It is a single piece of coiled DNA containing many genes, regulatory elements, and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.
Chromosomes are nucleoprotein structures that carry genetic information. In eukaryotes, they are located in the cell nucleus. Chromosomes are rod-shaped dark stained bodies seen during the metaphase stage of mitosis when cells are stained with a suitable basic dye and viewed under a light microscope. Chromosome the nucleoprotein structure which is generally more or less rod-like during nuclear division. The genes are linearly arranged on the chromosome. Each species has a characteristic number of chromosomes.
The eukaryotic genome is made up of DNA/protein complexes called chromosomes. Despite the compaction of the DNA (deoxyribonucleic acid) with proteins, gene sequences embedded within chromosomes must still be available for transcription by RNA (ribonucleic acid) polymerases and all of the DNA must be capable of being copied by DNA polymerases.
Chromosomes have two main functions: to ensure that the DNA is segregated equally to daughter nuclei at cell division and to ensure that the integrity of the genome is maintained and accurately replicated in each cell cycle.
Chromosomes are composed of thin chromatin thread these were earlier called chromonemata but now known as chromatin fiber.
Each species has a definite and generally constant and somatic chromosome number. Somatic chromosome number is the number of chromosomes found in somatic cells and is represented by 2n. Gametic chromosome number is precisely one-half of the somatic number is represented by n and denotes the number of chromosomes found in the gametes of species.
Chromosome size and shape
The size of a chromosome is normally measured at mitotic metaphase and may be as short as 0.25 gm in fungi and birds, or as long as 30 gm in some plants such as Trillium. However; most metaphase chromosomes fall within a range of 3 gm in fruit fly (Drosophila), to 5µm in man, and 5µm to 12 µm in maize.
The shape of the chromosomes is changeable from a phase in the continuous process of cell growth and cell division. In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic, and contractile, thread-like stainable structures, the chromatin threads. In the metaphase and the anaphase, the chromosomes become thick and filamentous. Each chromosome contains a clear zone known as the centromere. Along their length the centromere divided the chromosomes into two parts; each part is called a chromosome arms. The position of the centromere varies from chromosome to chromosome and it provides different shapes to later.
Telocentric- The rod-like chromosome which has the centromere on the proximal end is known as the telocentric chromosome.
Acrocentric –The acrocentric chromosomes are also rod-like in shape but these have the centromere at one end and thus giving a very short arm and an exceptionally long arm. The locusts (acridine) have the acrocentric chromosome.
Sub metacentric – The submetacentric chromosomes are J or L shaped. In these, the centromere occurs near the center or at a medium portion of the chromosome and thus forming two unequal arms.
Metacentric –The metacentric chromosomes are V-shaped and in these chromosomes, the centromere occurs in the center and forming two equal arms. The amphibians have metacentric chromosomes.
Structure of chromosome
While describing the structure of chromosome during the various phase of the cell cycle, cell biologist have introduced many terms for the various components as follows –
At mitotic metaphase, each chromosome consists of two symmetrical structures called Chromatid. Each Chromatid contains a single DNA molecule. Both Chromatid is attached only by the centromere and has become separated at the beginning of anaphase.
During mitotic prophase, the chromosomal material becomes visible as a very thin filament called chromonemata. A chromonema presents a chromatid in the early stage of condensation. Therefore Chromatid and chromonemata are two names for the same structure, a single linear DNA molecule, and associated protein. Chromonemata form the gene-bearing portion of the chromosome, Chromonemata are embedded in the achromatic and amorphous substance called matrix. The matrix is enclosed in a sheath or pellicle. Both matrix and pellicle are non-genetic materials.
The Chromomere is a bead-like accumulation of chromatin material that is sometimes visible along interphase chromosomes. The Chromomere bearing chromatin has the appearance of a necklace in which several beads occur on a string. Chromomeres are regions of tightly folded DNA and have a great interest for the cell biologist which corresponds to the unit of genetic function.
4) Centromere and kinetochore
It was considered that the centromere consists of small granules or spherules. The chromonema segment of the chromosome, the primary constriction. Centromeres are found to contain specific DNA sequences with special proteins bound to them, forming a disc-shaped structure called the kinetochore.
Each extremity of the chromosome has a polarity and therefore it prevents other chromosomal segments to be fused with it. The chromosomal ends are known as the telomere.
6) Secondary constriction
The chromosome besides having primary constriction or the centromere possesses secondary constriction at any point of the chromosome. Secondary constriction is useful in identifying a particular chromosome in a set. Secondary constriction can be distinguished from primary constriction because the chromosome bends only at the position of centromere during anaphase.
7) Nuclear organizers
These areas are certain secondary constrictions that contain the genes coding for 5.8S, 18 S, and 28 S ribosomal RNA, and that induce the formation of nucleoli. The secondary constriction may arise because their rRNA genes are transcribed very actively and thus interfering with chromosomal condensation.
Sometimes the chromosomes bear round elongated or knob-like appendages known as satellites. The satellite remains connected with the rest of the chromosome by a thin chromatin filament. The chromosome with the satellite is designated as a sat chromosome
. Chromosome satellites are morphological entities and should not be confused with satellite DNA which is highly repeated sequences.
Chromatin consists of DNA, RNA, and proteins. DNA, RNA is the most important component of chromatin, since it plays the central role of controlling heredity. Proteins are of two types; histones and non-histones proteins.
Histones are basic and non-histones are acidic in nature. Histone consists of 80% of total chromosomal proteins, they are present in ratio with DNA. Non-histones proteins are 20% which include many important enzymes such as DNA and RNA polymerases.
A DNA molecule is composed of three kinds of moieties.
l) Phosphoric acid
2) Deoxyribose sugar and
3) Nitrogen base.