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BIOA01H3 (699)


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Biological Sciences
Mark Fitzpatrick

RNA DNA Two polynucleotide strands (double Single polynucleotide chain helix) Ribose Deoxyribose Uracil Thymine Found in nucleus, cytoplasm and ribosome Only found in nucleus 3 classes of RNA  mRNA (messenger)  tRNA (Transfer proper amino acids to build a protein *leaves afterpeptide bonds form b/w amino acids*)  rRNA ( structural component of a ribosome, along with proteins)  Genetic code There are 20 amino acids found in proteins, each amino acid is coded by 3 DNA nucleotides. Each triplet of nucleotides is called a CODON (64, 44 extra) More than one codon can code for a single amino acid  Transcriptionit occurs in the nucleus of eukaryotic cell  Initiation RNA polymerase binds to the promoter region (promoter is located upstream of the gene) Promoter is a TATA sequence; RNA polymerase opens the double helix  Elongation RNA polymerase starts to build the mRNA molecule in 5’ to 3’ direction using DNA as a template Primer is not needed, promoter sequence is NOT transcribed Template strand is used as a template, the other one is called coding strand mRNA sequence is complementary to the template strand and therefore identical to the coding strand (except mRNA contains uracil instead of thymine)  Termination RNA polymerase reaches termination sequence which signals it to stop mRNA detaches from DNA template strand, RNA synthesis stops and RNA polymerase is released  Post-transcriptional modifications Primary transcript must go through 3 modifications: 1. 5’ cap is added to the start of the primary transcript (7-methylguanosine) needed to initiate translation and to prevent mRNA from being digested 2. Poly-A-tail is added by poly-A-polymerase that consists of 200 adenine RNA nucleotides (Protect from degradation, possibly related with movement) 3. Splicing coding sections called EXONS and noncoding calledINTRONS Spliceosomes are responsible for cutting the introns out of the mRNA (If the introns are translated, the protein gene codes will not fold properly and therefore will not perform its function properly) Pre-mRNA combines with snRNPs and other proteins to form a spliceosome , within the spliceosome, snRNA base-pairs with nucleotides at the ends of the introns. The RNA transcript is cut to release the intron, and the exons are spliced together. Spliceosome comes apart and relases mRna.  Translation (the ribosome) consist of 2 subunits (large 60s, small40s) mRNA is clumpled by the subunits Ribosomes move toward de 3’ end, it read codons and adds aminoacids. The reading frame determines the sequence in which the codons are read tRNA delivers the amino acids to the ribosome. When AUG is in P site of ribosome and everything else is in A position tRNA joins the ribosome. The anticodons of some tRNAs recognize more than one codon (wobble hypothesis) the rules for base pairing between the third base of the codon and anticodon are relaxed. This is beneficial to avoid mutation 3erd base of the tRNA anticodon “wobbles”: it can H-bond with more than one type o
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