Lecture 07 2B03 2017.pptx - Ends of Chromosomes Telomeres...

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Ends of Chromosomes Ends of Chromosomes Telomeres : the structures at the ends of eukaryotic chromosomes Made of short (5 to 8 bp) tandemly repeated , G-rich nucleotide sequences that form protective caps 1-12 kb long on the chromosome ends Vertebrate telomere consensus sequence: TTAGGG Telomerase (an RNA-dependent DNA polymerase) maintains telomere length by restoring telomeres at the 3'-ends of chromosomes Somatic cells , which lack telomerase, inevitably lose bits of their telomeres The telomere theory of aging suggests that cells senesce and die when their telomeres are gone 1
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Replication of Ends of Chromosomes Replication of Ends of Chromosomes In lagging strand synthesis, RNA primers (pink) are extended by DNAP. When the primers are removed, gaps are created at 5’ terminal of the progeny strand Telomerase restores the 3’ end of chromosome using an RNA template that is part of the telomerase DNAP then uses the restored 3’ end to perform normal DNA replication Asterisks indicate telomere sequences that can not be copied by normal DNA synthesis The RNA template sequence is CUAACCC UAA C, to create TTAGGG repeats 2
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Replication of RNA Genomes Replication of RNA Genomes Many viruses have genomes composed of RNA DNA is an intermediate in the replication of RNA viruses The viral RNA is used as a template for DNA synthesis The RNA-directed DNA polymerase is called reverse transcriptase All RNA viruses contain such an enzyme within their viral particle RNA viruses that replicate their RNA via a DNA intermediate are termed retroviruses The primer for reverse transcriptase is a specific tRNA molecule captured from the host cell 4
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