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Individual chromosomes are composed of DNA tightly coiled around proteins called histones that support their structure. Each chromosome contains hundreds to thousands of genes that carry instructions for making proteins. Genes, made up of a tiny stretch of DNA, are the basic functional unit of heredity. Chromosomes are DNA and related proteins that form a complex arrangement and are found in the nucleus as chromatin fibres. During cell division, chromatin fibres twist and condense even more, forming chromosomes.
B-Chromosomes
The B-chromosome is a small chromosome found on the second sex chromosome of all sexually reproducing mammals. The B-chromosome is responsible for the production of a single protein called beta-globulin. It is a nonessential part of the karyotype and occurs ubiquitously in all mammalian species. The B-chromosome is not inherited and is neither expressed nor recombined.
Necessity of B-Chromosomes
B-chromosomes are also known as supernumerary, lineage-specific or accessory chromosomes. These chromosomes are not necessary for an individual’s life but act as an extra chromosome to the standard complement. In maize, B chromosomes lead to a reduction in fertility and increase the percentage of abortive pollen. Most B chromosomes are heterochromatic and are not responsible for coding anything. In humans, B chromosomes interact with standard chromosomes and play an important role in the evolution of genetic material. Thus, B chromosomes can be useful for studying various evolutionary processes related to molecular biology.
Features of B-Chromosomes
- B-chromosomes are a type of chromosome in humans. They occur in females and are the primary cause of Turner syndrome, which affects development in females.
- B-chromosomes are found in all vertebrates, from fish to humans, and have been found to have significant roles in several pathways of the cell, but little is known about their regulation.
- A B-chromosome is one that has a complete set of genes. This is the default state for all cells, as most human cells do not have a B-chromosome. However, in certain cases, a cell may have an extra B-chromosome.
A-Chromosome vs B-Chromosomes
The A-chromosome is a sex chromosome that has no homologous counterpart on the Z-chromosome. B-chromosomes are usually present in a large number. The B-chromosome is not inherited as it is not coded for by genes on the A-chromosome.
B chromosomes are extra chromosomes that are different from the 46 standard pairs of chromosomes. They can be added to one or both copies of the 23 pairs in a human cell. They do not contribute to the normal functions of an individual, nor are they present in all organisms.
An A-chromosome in humans is present in all individuals of the species, whereas a B-chromosome in humans is inherited only when both chromosomes are present from the same parent
The chromosome is a double-stranded molecule of chromatin, the DNA-protein complex that is packaged in the nucleus. It is roughly the size of a large grapefruit.
B-Chromosome Structure
In some species, B-chromosomes can be quite large, with as many as eight copies of the chromosome. They contain large numbers of genes, as much as 1/4 of a chromosome’s DNA, which can either be expressed or silenced in the cell. The B-chromosome has the same properties as a normal chromosome in that its sequences can be transcribed and translated to produce proteins.
B-Chromosomes in Humans
B-chromosomes are a part of the DNA of the human genome. They are often used to indicate past and present human migration patterns and admixture. B-chromosomes are extra chromosomes that occur in about 15% of eukaryotes. When autosomes and sex chromosomes are derived in intra and inter species crosses, there is a maximum chance of generation of B chromosomes. B chromosomes can be useful for studying various evolutionary processes related to molecular biology. The B-chromosome is associated with reduced female fertility and also contributes to evolution.
B-Chromosomes in Plants/Animals/Insects
A B-chromosome is a special chromosome found in some species of plants, animals, and insects. It is distinguished by a centromere that is typically located at the centromeric region of the chromosome and contains heterochromatic regions at both ends. B-chromosomes may or may not be associated with an A-chromosome, and chromosomes containing a B-chromosome may be either homologous or heterologous.
Conclusion
B-chromosomes are present in humans and many other mammals, including other primates. It is a sex chromosome, meaning that the presence of the chromosome determines whether an individual is male or female. The chromosome does not recombine, meaning that the presence of one B-chromosome provides the same advantage or disadvantage as an extra copy of a Y-chromosome, which is a larger and more variable structure.
