Chromosome: Structure and types

 Morgan worked on the eye color of Drosophila. He observed that the pattern of transmission of the white eye gene was identical to that of the X chromosome of Drosophila. This prompted Morgan postulate that the gene for eye color is located on the X chromosome. This was the first demonstration of an association between a specific gene and a specific chromosome. His work gave direct evidence in support of the chromosomal theory of inheritance. The white-eyed male fly was crossed with the red-eyed female, all the F1 offsprings were red-eyed. This gives an indication that the white eye is controlled by a recessive allele. The predictions match the F1 phenotypes, but this set of phenotypes could also be explained by a gene that is not on the X chromosome since all the flies were red-eyed (regardless of sex). So, the real test comes when the F1 is mated to make the F2 generation. Something strange happened in F2: all of the female F2 flies were red-eyed, while about half of the male F2 ...

 After the discovery of Mendel’s laws of inheritance, scientists turned naturally to the physical basis of heredity. Sutton demonstrated the similarities between the meiotic behavior of paired chromosomes and the behavior of pairs of Mendelian factors. He could explain Mendel’s principle of segregation on a cytological basis. The chromosomal theory of inheritance was proposed independently by Sutton and Boveri in 1902. This theory states that individual genes are found at specific locations on particular chromosomes and that the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel’s laws. Observations that support the chromosome theory of inheritance include Sutton and Boveri’s arguments for their chromosome theory of heredity were essential as follows: • Since the sperm and egg cells provide the only bridge from one generation to other, all hereditary characters must be carried in them. • The sperm cell lose practically all their cytoplasm...

 Recent work by a number of workers demonstrated that the network of threads consists of a single chromosome in the form of a ring. The exact three-dimensional arrangement by which 1100μ -1400μ long DNA chain, which forms 80% of the chromosome by mass (the remaining 20% being protein + RNA), is packed in an lμ long nucleoid, which could also be established now. At least two proteins, which bind DNA, resemble histones of eukaryotes and organize the DNA into structures comparable to nucleosomes of eukaryotes. It has been shown that the chromosome of E. coli is organized in about 45 loops, which radiate out from a dense proteinaceous scaffold, which is assumed to anchor the DNA. In each of the 45 loops, DNA is supercoiled and complexed with protein. The functional significance of looped domains is not clear but they do not represent units of transcription as in the case of lampbrush chromosomes.

 B-Chromosomes are a particular kind of supernumerary chromosomes, that may or may not be found in an organism as extra chromosomes over and above the standard diploid or polyploid chromosome complement. The standard complement consists of chromosomes described as A chromosomes. The B-chromosomes are found in the natural populations of many plants and animal species and are recognized on the basis of their following characteristics; (a) they are dispensable; are not found in all individuals of a species and may not be found in all cells of an individual organism; (b) they are not homologous with any of the basic A chromosomes; (c) their inheritance is non-Mendelian, sometimes due to non-disjunction during pollen mitosis (as in some plants); (d) they are usually smaller than A-chromosomes and have their own unique pattern of heterochromatin distribution; (e) in general, they are genetically inert, but may rarely organize nucleoli and carry functional genetic material; (f) when prese...

 Chromosomes of a special kind are found in a variety of primary oocyte nuclei in vertebrates (mainly amphibians) as well as in some invertebrates occur at the diplotene stage of meiotic prophase. These chromosomes, known as lampbrush chromosomes first observed by Walther Flemming (in 1882) in salamander (amphibian oocyte) and described in detail by R. Ruckert (1892) in shark oocyte. The lampbrush chromosomes are not condensed, instead, they are very long and stretched out. These chromosomes sometimes become even larger than giant salivary gland chromosomes. The largest chromosome having a length up to 1 mm has been observed in urodele amphibians. The lampbrush chromosome is present in the form of bivalent in which the maternal and paternal chromosomes are held together by chiasmata. Each bivalent has four chromatids, two in each homologue. The axis of each homologue consists of a row of chromomere from which lateral loops extend. Two loops are extended from each chromosome at a po...

 Some of the chromosomes in some special tissues are found to be different from the normal chromosomes, they are called special type chromosomes. Polytene or giant chromosome Polytene chromosomes are giant chromosomes common to many dipteran (two-winged) flies. These chromosomes were first observed by E. G. Balbiani (1881) in salivary glands of dipteran species. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication), the number of chromonemata keeps on increasing leading to large, banded chromosomes. Along the linear axis, polytene chromosomes have variations in the concentration of the chromatin. Regions of high concentrations are known as chromomeres (bands), and regions with low concentrations are known as inter-chromomeres (inter-bands). For unknown reasons, the centromeric regions of the chromosomes do not endoreplication very well. As a result, the centromeres of all the chromosomes bundle together i...

Chromosome number in every species is generally constant. In higher organisms, the number of chromosomes in a somatic cell is called somatic number irrespective of the ploidy level (number of basic chromosome set) and is represented by 2n. In the gametes, the chromosome number is reduced to half, it is known as the gametic number or haploid number and is represented by “n”. Some organisms including many plants have three or more sets of chromosomes and are referred to as polyploid. The basic chromosome number is denoted by “x” so that the chromosome number of a diploid cell or individual is expressed as “2x”. Diploid: 2n = 2x, n = x Tetraploid: 2n = 4x, n = 2x Hexaploid: 2n = 6x, n = 3x

 Based on the position of the centromere Metacentric chromosome The metacentric chromosome has its centromere centrally located between the two arms. This gives the chromosome a typical 'V' shape that is seen during the anaphase. The arms of this chromosome are roughly equal in length. Submetacentric chromosome The arms of the submetacentric chromosome are said to be unequal in length. This is because the kinetochore is present in the submedian position. This gives rise to the 'L' shape of the submetacentric chromosome. Telocentric chromosome Also known as the monarchial type of chromosomes, they have a centromere that is located towards the end of the chromosome. Thus, telocentric chromosomes have a 'rod' shaped appearance. Acrocentric chromosome The location of the centromere in the acrocentric chromosome is subterminal (near to telomere). This causes the short arm of the chromosome to become really short making it very difficult to observe. Such chromosome as...

 Chromosome morphology can be well studied at mitotic metaphase. Under the light microscope, the following structural features can be seen: • Chromatid o Chromomere o Chromonema • Centromere • Secondary constriction • Satellite • Telomere • Pellicle and matrix Chromatid Chromatid Chromatid is the structural and functional unit of chromosomes. At metaphase, each chromosome consists of two longitudinal parts called chromatids. These chromatids are held together at a point called the centromere. The chromatids separate from each other during mitotic anaphase (and during anaphase II of meiosis) and move to the opposite poles during anaphase. During telophase and the G, phase of interphase each chromosome is composed of a single chromatid. Since the two chromatids making up a chromosome are produced through replication of a single chromatid during the synthesis (S) phase of interphase, they are referred to as sister chromatids. In contrast, the chromatids of homologous chromosomes are k...

 Strasburger discovered thread-like structures in 1875 which appeared during cell division. The term chromosome was given by Waldeyer in 1888 since these structures showed an affinity for basic dyes and could be stained deeply, while the cytoplasm took little or no stain. The term chromosome is derived from the Greek word 'chroma' which means 'color' and 'soma' which means 'body'. Chromosomes are the organized structures of DNA and proteins found in cells. They are thread-like structures located inside the nucleus of animal and plant cells. Chromosomes are composed of a single molecule of DNA with many copies of five types of histones. Histones are protein molecules and are rich in lysine and arginine residues, they are positively charged. Hence, they bind tightly to the negatively charged phosphates in the DNA sequence. A small number of non-histone proteins are also present, these are mostly transcription factors. Chromosomes play an important role tha...