Structure of Chromosomes: Prokaryotic and Eukaryotic
Structure of Chromosomes: Prokaryotic and Eukaryotic
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.
Structure of prokaryotic chromosome
- 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).
- 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 is also 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.
- 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.
Structure 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.
- The chromosome is the nucleoprotein structure that 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 are now known as chromatin fiber.
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.