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 Posted: Thu 11:37, 24 Mar 2011 Post subject: Active transcription units cheap nhl jerseys |
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Active transcription units are clustered in the nucleus,[link widoczny dla zalogowanych], in discrete sites called transcription factories or euchromatin.[link widoczny dla zalogowanych] Such sites can be visualized by allowing engaged polymerases to extend their transcripts in tagged precursors (Br-UTP or Br-U) and immuno-labeling the tagged nascent RNA. Transcription factories can also be localized using fluorescence in situ hybridization or marked by antibodies directed against polymerases. There are ~10,000 factories in the nucleoplasm of a HeLa cell, among which are ~8,000 polymerase II factories and ~2,000 polymerase III factories. Each polymerase II factory contains ~8 polymerases. As most active transcription units are associated with only one polymerase, each factory usually contains ~8 different transcription units. These units might be associated through promoters and/or enhancers, with loops forming a ‘cloud’ around the factor.
Some eukaryotic cells contain an enzyme with reverse transcription activity called? telomerase. Telomerase is a reverse transcriptase that lengthens the ends of linear chromosomes. Telomerase carries an RNA template from which it synthesizes DNA repeating sequence, or "junk" DNA. This repeated sequence of DNA is important because, every time a linear chromosome is duplicated,[link widoczny dla zalogowanych] it is shortened in length. With "junk" DNA at the ends of chromosomes, the shortening eliminates some of the non-essential, repeated sequence rather than the protein-encoding DNA sequence farther away from the chromosome end. Telomerase is often activated in cancer cells to enable cancer cells to duplicate their genomes indefinitely without losing important protein-coding DNA sequence. Activation of telomerase could be part of the process that allows cancer cells to becomeimmortal. However, the true? in vivo? significance of telomerase has still not been? empirically? proven.
The last 50 years have seen a yet more rapid increase in the rate of population growth[6] due to medical advancesand substantial increases in agricultural productivity, particularly beginning in the 1960s,[7] made by theGreen Revolution.[8] In 2007 the United Nations Population Division projected that the world's population will likely surpass 10 billion in 2055.[9] In the future, world population has been expected to reach a peak of growth, from there it will decline due to economic reasons, health concerns, land exhaustion and environmental hazards.[link widoczny dla zalogowanych] There is around an 85% chance that the world's population will stop growing before the end of the century.[citation needed] There is a 60% probability that the world's population will not exceed 10 billion people before 2100, and around a 15% probability that the world's population at the end of the century will be lower than it is today.
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