Chromosome

Jai Ganesh · 2023-10-25 17:03:31

Chromosome

Gist

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

Summary

A Chromosome is the microscopic threadlike part of the cell that carries hereditary information in the form of genes. A defining feature of any chromosome is its compactness. For instance, the 46 chromosomes found in human cells have a combined length of 200 nm (1 nm = {10}^{- 9} metre); if the chromosomes were to be unraveled, the genetic material they contain would measure roughly 2 metres (about 6.5 feet) in length. The compactness of chromosomes plays an important role in helping to organize genetic material during cell division and enabling it to fit inside structures such as the nucleus of a cell, the average diameter of which is about 5 to 10 μm (1 μm = 0.00l mm, or 0.000039 inch), or the polygonal head of a virus particle, which may be in the range of just 20 to 30 nm in diameter.

The structure and location of chromosomes are among the chief differences between viruses, prokaryotes, and eukaryotes. The nonliving viruses have chromosomes consisting of either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid); this material is very tightly packed into the viral head. Among organisms with prokaryotic cells (i.e., bacteria and blue-green algae), chromosomes consist entirely of DNA. The single chromosome of a prokaryotic cell is not enclosed within a nuclear membrane. Among eukaryotes, the chromosomes are contained in a membrane-bound cell nucleus. The chromosomes of a eukaryotic cell consist primarily of DNA attached to a protein core. They also contain RNA. The remainder of this article pertains to eukaryotic chromosomes.

Every eukaryotic species has a characteristic number of chromosomes (chromosome number). In species that reproduce asexually, the chromosome number is the same in all the cells of the organism. Among sexually reproducing organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the gender cells, or gametes. The haploid number is produced during meiosis. During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes.

Somatic cells reproduce by dividing, a process called mitosis. Between cell divisions the chromosomes exist in an uncoiled state, producing a diffuse mass of genetic material known as chromatin. The uncoiling of chromosomes enables DNA synthesis to begin. During this phase, DNA duplicates itself in preparation for cell division.

Following replication, the DNA condenses into chromosomes. At this point, each chromosome actually consists of a set of duplicate chromatids that are held together by the centromere. The centromere is the point of attachment of the kinetochore, a protein structure that is connected to the spindle fibres (part of a structure that pulls the chromatids to opposite ends of the cell). During the middle stage in cell division, the centromere duplicates, and the chromatid pair separates; each chromatid becomes a separate chromosome at this point. The cell divides, and both of the daughter cells have a complete (diploid) set of chromosomes. The chromosomes uncoil in the new cells, again forming the diffuse network of chromatin.

Among many organisms that have separate sexes, there are two basic types of chromosomes: gender chromosomes and autosomes. Autosomes control the inheritance of all the characteristics except the gender-linked ones, which are controlled by the gender chromosomes. Humans have 22 pairs of autosomes and one pair of gender chromosomes. All act in the same way during cell division.

Chromosome breakage is the physical breakage of subunits of a chromosome. It is usually followed by reunion (frequently at a foreign site, resulting in a chromosome unlike the original). Breakage and reunion of homologous chromosomes during meiosis are the basis for the classical model of crossing over, which results in unexpected types of offspring of a mating.

Details

A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity. These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.

Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form). Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.

Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.

Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

Etymology

The word chromosome comes from the Greek (chroma, "colour") and (soma, "body"), describing their strong staining by particular dyes. The term was coined by the German anatomist Heinrich Wilhelm Waldeyer, referring to the term chromatin, which was introduced by Walther Flemming.

Some of the early karyological terms have become outdated. For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state.

History of discovery

Otto Bütschli was the first scientist to recognize the structures now known as chromosomes.

In a series of experiments beginning in the mid-1880s, Theodor Boveri gave definitive contributions to elucidating that chromosomes are the vectors of heredity, with two notions that became known as 'chromosome continuity' and 'chromosome individuality'.

Wilhelm Roux suggested that each chromosome carries a different genetic configuration, and Boveri was able to test and confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson, Stevens, and Painter actually worked with him).

In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory (the names are sometimes reversed). Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Morgan, all of a rather dogmatic turn of mind. Eventually, complete proof came from chromosome maps in Morgan's own lab.

The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted 24 pairs, which would mean 48 chromosomes. His error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio.

Prokaryotes

The prokaryotes – bacteria and archaea – typically have a single circular chromosome, but many variations exist. The chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola and Candidatus Tremblaya princeps, to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum. Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.

Structure in sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins. The genes in prokaryotes are often organized in operons, and do not usually contain introns, unlike eukaryotes.

DNA packaging

Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid. The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. 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. In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.

Certain bacteria also contain plasmids or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. In prokaryotes and viruses, the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like proteins.

Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and replication.

Eukaryotes

Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called chromatin. Chromatin contains the vast majority of the DNA of an organism, but a small amount inherited maternally, can be found in the mitochondria. It is present in most cells, with a few exceptions, for example, red blood cells.

Histones are responsible for the first and most basic unit of chromosome organization, the nucleosome.

Eukaryotes (cells with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes.

In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around histones (structural proteins), forming a composite material called chromatin.

Human chromosomes

Chromosomes in humans can be divided into two types: autosomes (body chromosome(s)) and allosome (gender chromosome(s)). Certain genetic traits are linked to a person's gender and are passed on through the gender chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of gender chromosomes), giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Number of genes is an estimate, as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.

Additional Information

Chromosomes are the basic building blocks of life where the entire genome of an organism is essentially organized and stored in the form of DNA (deoxyribonucleic acid) which is present inside every cell making up that organism. A chromosome is a single chain of DNA that is coiled and super coiled to form dense thread-like pieces. The term chromosome is derived from the Greek words "chroma" or color and "some" or body and is so named because chromosomes have the ability to be stained with dyes.

DNA is the genetic instruction book for enabling the production of proteins and cell processes that are essential to life and inherited from generation to generation. Every piece of DNA is composed of gene sequences containing instructions for each cell's development, reproduction and ultimately death. Each of the DNA chains within chromosomes may contain anywhere between 10,000 to 1,000,000,000 nucleotides.

The DNA is broken up into single stranded polynucleotide chains to expose gene sequences that can be copied into messengerRNA (mRNA, ribonucleic acid). This mRNA has four nucleotide bases arranged in different combinations of three and is similar to DNA. Ribosomes read these three-based nucleotide sequences and translates it to form the amino acid sequence of a protein Each three-based nucleotide codes for one of 20 amino acids, also known of as the building blocks of proteins. First, the amino acids form a long chain called the polypeptide chain. This chain then undergoes conformational and structural changes, folding and refolding over itself until the final complex structure of a protein is achieved. Chromosomes also contain DNA-bound proteins or histones that consolidate and stabilize the DNA and regulate its functions.

DNA and histone proteins are packaged into structures called chromosomes.

A chromosome may be circular or linear in shape, with nucleated or eukaryotic cells having linear chromosomes, and prokaryotic cells usually containing circular chromosomes. Chromosomes may also exist outside of the nucleus in eukaryotic cells, within the mitochondria or the "power houses" of cells.

Chromosomes may have condensed DNA organized around histone proteins to form chromatin. Chromatin allows long chains of DNA to be fitted within a cell nucleus. On cell division, the chromosomes form dense small thread-like structures that must be replicated before being equally divided between two daughter cells, to ensure each has an equal number of chromosomes. When body cells divide (mitosis), two copies of the 23 chromosomes are passed onto each daughter cell, giving them both a complete set of 46 chromosomes. When gametes (egg or sperm cells) divide (meiosis), only half the amount of chromosomes is passed onto daughter cells, as these daughter gametes will only form a cell with a complete set of chromosomes on fusion with another gamete during fertilization, after which the resulting zygote will have 23 pairs of chromosomes with half from each parent.

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#1 2023-10-25 17:03:31

Chromosome

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