Sobti, Jaspreet Kochar, N.
Chromosomes Fact Sheet
Capalash, S. Molecular Genetics of Gastrointestinal Tract Cancers. Sharma, A. Joshi et al. Molecular Pathogenesis of Lung Cancer. Sobti, S.
Some Aspects of Chromosome Structure and Functions | SpringerLink
Janmeja, S. Molecular Genetics of Prostate Cancer. Sharma, Harsh Mohan. Back Matter Pages About this book Introduction It was at the end of the 19th century that a Swiss biologist, Karl Nageli first proposed the existence of hereditary organelles that carried information from parent to offspring. Ensuing decades experienced vigorous studies that led to the development of discovery that chromosomes are indeed the carriers of genetic information.
Subsequent studies, especially by Morgan and Bridges, established unequivocally the chromosome theory of inheritance. Today, the structure of chromosome is well established.
At the physical level, eukaryotic chromosomes are composed of a single, linear, double helix of DNA. The elementary helical structure involves nucleosomes, comprised of histones around which the DNA is wrapped. A hierarchy of higher order of chromosomal architecture may possibly be responsible for the regulation of gene helical structures expression.
The localized condensations of DNA constitute chromomeres. Uncoiled structures sometimes extending from chromomeres, which form loops, is the result of discontinuities in the regular coiling of the DNA in the chromosome. Editors and affiliations. Sobit 1 G.
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Obe 2 R. The stability of this structure requires the presence of the last member of the histone gene family, histone H1. Because experiments that strip H1 from chromatin maintain the nucleosome, but not the 30 nm structure, it was concluded that H1 is important for the stabilization of the 30 nm structure. The final level of packaging is characterized by the nm structure seen in the metaphase chromosome. The condensed piece of chromatin has a characteristic scaffolding structure that can be detected in metaphase chromosomes. This appears to be the result of extensive looping of the DNA in the chromosome.
The last definitions that need to be presented are euchromatin and heterochromatin. When chromosomes are stained with dyes, they appear to have alternating lightly and darkly stained regions. The lightly-stained regions are euchromatin and contain single-copy, genetically-active DNA. The darkly-stained regions are heterochromatin and contain repetitive sequences that are genetically inactive. Centromeres and Telomeres Centromeres and telomeres are two essential features of all eukaryotic chromosomes.
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Each provide a unique function that is absolutely necessary for the stability of the chromosome. Centromeres are required for the segregation of the centromere during me iosis and mitosis, and teleomeres provide terminal stability to the chromosome and ensure its survival. Centromeres are those condensed regions within the chromosome that are responsible for the accurate segregation of the replicated chromosome during mitosis and meiosis.
- DNA Structure, Replication and Eukaryotic Chromatin Structure.
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When chromosomes are stained they typically show a dark-stained region that is the centromere. During mitosis, the centromere that is shared by the sister chromatids must divide so that the chromatids can migrate to opposite poles of the cell. On the other hand, during the first meiotic division the centromere of sister chromatids must remain intact, whereas during meiosis II they must act as they do during mitosis.
Therefore the centromere is an important component of chromosome structure and segregation. Within the centromere region, most species have several locations where spindle fibers attach, and these sites consist of DNA as well as protein. The actual location where the attachment occurs is called the kinetochore and is composed of both DNA and protein. Because CEN DNA can be moved from one chromosome to another and still provide the chromosome with the ability to segregate, these sequences must not provide any other function.
Principles of Biochemistry/Chromosome and its structure
Mutations in the first two sub-domains have no effect upon segregation, but a point mutation in the CDE-III sub-domain com pletely eliminates the ability of the centromere to function during chromosome segregation. The protein component of the kinetochore is only now being characterized. Fur thermore, mutants of the genes encoding the Cbf-III proteins also eliminates the ability for chromosomes to segregate during mitosis. Additional analyses of the DNA and protein components of the centromere are necessary to fully understand the mechanics of chromosome segregation.
Telomeres are the region of DNA at the end of the linear eukaryotic chromosome that are required for the replication and stability of the chromosome. McClintock recognized their special features when she noticed, that if two chromosomes were broken in a cell, the end of one could attach to the other and vice versa. What she never observed was the attachment of the broken end to the end of an unbroken chromosome. Thus the ends of broken chromosomes are sticky, whereas the normal end is not sticky, suggesting the ends of chromosomes have unique features.