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Group II introns Retrohome to intronless copies same gene by: Insertion of an RNA intron into the gene Followed by reverse transcription Then second-strand synthesis Retrotranspose by: Insertion of an RNA intron into an unrelated gene Target-primed reverse transcription Lagging-strand DNA fragments as primers Group II retrotransposition...

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When 2 similar, non-identical DNA sequences interact, possibility exists for gene conversion Conversion of one DNA sequence into that of another Sequences participating in gene conversions can be: Alleles, as in meiosis Nonallelic genes, such as the MAT genes that determine mating type in yeast

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Mammalian telomeres form looped structures that protect ss telomeric DNA Proteins TRF1 and TRF2 appear to help telomeric DNA in mammalian cells form a t loop in which ss 3’-end of telomere invades the ds telomeric DNA upstream TRF1 may help bend the DNA into shape for strand invasion TRF2 binds at point of strand invasion, may stabilize the disp...

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Errors in correcting UV damage lead to a variant form of XP, XP-V DNA polymerase  is active on templates with thymidine dimers and AP sites The polymerase is not really error-free With a gapped template, it is one of least accurate template-dependent polymerases known

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Amino acid selectivity of at least some aminoacyl-tRNA synthetases is controlled by a double-sieve mechanism 1st sieve is coarse excluding amino acids too big Enzyme accomplishes this with an active site for activation of amino acids just big enough to accommodate the cognate amino acid, not larger amino acids 2nd sieve is fine, degrades too sma...

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Most newly-made polypeptides do not fold properly alone Polypeptides require folding help from molecular chaperones E. coli cells use a trigger factor Associates with the large ribosomal subunit Catches the nascent polypeptide emerging from ribosomal exit tunnel in a hydrophobic basket to protect from water Archaea and eukaryotes lack trigger ...

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The let-7 miRNA shifts the polysomal profile of target mRNAs in human cells toward smaller polysomes This miRNA blocks translation initiation in human cells Translation initiation that is cap-independent due to presence of an IRES, or a tethered initiation factor, is not affected by let-7 miRNA This miRNA blocks binding of eIF4E to the cap of ta...

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MicroRNAs are 18-25 nt RNAs produced naturally in plant and animal cells by cleavage from 75-nt stem-loop precursor RNA In last step of miRNA synthesis, Dicer RNase cleaves ds stem part of the precursor to yield miRNA in ds form Single-stranded form of miRNAs can team up with Argonaute protein in a RISC to control gene expression by base-pairing ...

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CoTC element downstream of the polyadenylation site in the human b-globin mRNA is a ribozyme that cleaves itself This generates a free RNA 5’-end This cleavage is required for normal transcription termination It provides an entry site for Xrn2, a 5’3’ exonuclease that loads onto the RNA and chases RNA polymerase by degrading the RNA

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RNAs containing group II introns self-splice by a pathway using an A-branched lariat intermediate, like spliceosome lariats Secondary structures of the splicing complexes involving spliceosomal systems and group II introns are very similar

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