In some cases, more than one pair, even upto 9 pairs of loops may emerge from a single chromomere. This is why these chromosomes are also popularly known as polytene chromosomes and the condition is described as polyteny. They inferred that in each DNA molecule there are two long and parallel strands or chains helically coiled around the same axis giving an appearance of spiral staircase. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Another syndrome caused by imbalance in the number of sex chromosomes is Turner syndrome. About this book.

The role of chromosomes in heredity was suggested independently by Sutton and Bover in 1902. The giant chromosomes have a distinct pattern of transverse banding which consists of alternate chromatic and achromatic regions. According to this model, the single DNA molecule of a chromatin fibre is coiled in a manner similar to the wire in a spring; the coils being held together by histone bridges pro­duced by binding histone molecules in the large groove of DNA molecules. Chromosome structure consists of a long arm region and a short arm region connected at a central region known as a centromere.The ends of a chromosome are called telomeres. Each chromosome contains hundreds and thousands of genes that can precisely code for several proteins in the cell. 4. Completing the CAPTCHA proves you are a human and gives you temporary access to the web property. It is the ground substance of chromosome which contains the chromonemata. For example, in human cells 22 pairs of chromosome are autosomes. Chromosome number of few common animals and plants is given below: In a diploid cell there is two of each kind of chromosome termed as homologous chromosomes, except for the sex chromosomes. It contains short repeated sequences of DNA, called satellite DNA. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria. The two chromatids are joined together at a point called centromere. Any model of chromatin fibre structure has to account for (i) packaging of a very long DNA molecule into a unit length of fibre; (ii) production of very thick (230-300A0) fibres from very thin (20A°) DNA molecules and (iii) the beads-on-a-string ultrastructure of chromatin fibres observed particularly during replication. During early and mid-prophase stages, the heterochromatic regions are constituted into three structures namely chromomeres, centromeres and knobs. About 600 plant species and more than 100 animal species are reported to possess B- chromosomes. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. It is very thin and is formed of achromatic substances. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome. Giant chromosomes are found in certain tissues, e.g., salivary glands of larvae, gut epithelium, Malphigian tubules and some Diptera, e.g., Drosophila, Chironomous, Sciara, Rhyncosciara etc. There is extensive RNA synthesis at the thin ends of loops, while there is little or no RNA synthesis at the thick end. Thus each loop represents one chromatid of a chromosome and is composed of one DNA double helix. Proteins associated with chromosomes may be classified into two broad groups: (/) basic proteins or histones and (ii) non-histone proteins. 1. �3ON����1��[�8 nM/P��(�:� %���(���T�������[����b��$��]i�8����(����V��L��cc�. ⇒ All the chromosomes other than sex chromosomes are always found in pairs of homologous chromosomes in diploid cells and are known as autosomes.

Telomeres also play a role in cancer. Each human somatic (or … This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. Such a coiled structure that would be stabilized as a single histone molecule would bind to several coils of DNA.


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