The eukayotic chromosome is composed of DNA, basic proteins called histones, non-histone proteins(involved in transcription, replication, repair and recombination of DNA) and RNA . The histone proteins are basic proteins rich in arginine and lysine. They are of 5 types-H1, H2A, H2B, H3 and H4.
Eight molecules two each of H2A, H2B, H3 and H4. From an ellipsoid core (11nm long and 6.5-7nm in diameter) around which about 147-166bp of DNA coils in 13/4 turns. This DNA histone complex is called nucleosome, the building block of chromosome, the building block of chromosome found as repeating units. The nucleosomes appear as a string of beads in the chromatin. About 14-100bp of DNA between these beads forms the LINKER region.
The hsitone H1 associates with the linker to aid folding of DNA into a more complex chromatin (in the form of 10nm zigzag or 30nm solenoild fibire). DNA enters and exits the nucleosome at sites close to each other and two turns of turns of DNA are stabilized and “sealed off” by H1, during cell division there is maximum folding of the chromatin and hence visible as chromosomes.
Polythene chromosomes:
In certain tissues of insects belonging to the order dipteral (flies, mosquitoes). The cell nuclei have reached a high degree of enlargement accompanied by many extra replications of each new chromosome within a single nucleus (endopolyploidy), however instead of each new chromosome separating as an individual unit all replicates of the same chromosome are lined up together in parallel fashion. This parallel duplication or polyteny, results in very think chromosomes that magnify any differences in density along their length (eg chromosomes). The numbers of bands varies between different species but are constant for the member of any particular sp.
In polyteny the two homologous chromosomes of each diploid pair are also often lined up side by side (somatic pairing) so that if the total diploid number of chromosomes id eight (4 pairs). Only four very thick and long chromosomes appear. The same chromomere in many paired chromatids may expand to form “puffs”. Polytene chromosomes were alilized in genetic research first by painter in 1933.
Lampbrush Chromosomes
The oocytes of some vertebrates with large yolky eggs expand greatly during their growth period, forming correspondingly large nuclei at these stages. In some aphibia the meiotic prophase chromosomes of such nuclei can reach about 1000 μm in length with long lateral loops giving a hairy “lampbrush”. Each pair of loops arise from single chromosomes located at short intervals along the very thin and double stranded chromosome. Towards the end of meiotic prophase the loops begin to disappear and the chromosomes contract, so that the metaphase bivalents are of the usual small size.
Posted in: Microbiology
0 comments:
Post a Comment