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Lecture 4

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Biological Sciences
Bebhinn Treanor

Lecture 4  Direct product of Transcription: called the primary transcript of precursor RNA  Type of RNA that encodes the info for amino acids: mRNA  Transcription of protein coding genes are 1 transcribed as: larger precursor molecules, that have both 5’ and 3’ untranslated regions that brackets exon/intron region TRANSLATION  Process: making proteins. Making polypeptides using info encoded in mRNA RNA PROCESSING  Every different protein consists: has own shape, size and biological function  After 5’ end of the transcript is synthesized: gets pushed away from the RNA polymerase and a  Size, shape and function of protein: determined by amino acids that makes it up CAP is put on it  Order of the bases mRNA: encodes the info for the order of amino acids of the protein  Component of CAP: GTP nucleotide  # of ribonucleotide that is sufficient enough for 20 amino acids: 3 rd 4  Method of addition of CAP: Cap is added on so that the GTP nucleotide’s free OH group (3 o 3 =64 combinations (3 pairs for 4 bases) carbon) is the face of the 5’ end and the phosphate groups of the GTP are adjacent to the 5’ free o Wrong: 2 is not sufficient since 2 =16 ( there are 20 amino acids!!!) phosphate group of the first ribonucleotide.  Poly A Tail: string of adenine containing nucleotides which I attached to the 3’ end of the GENETIC CODE precursor DNA  Genetic Code: order of the bases along the mature messenger RNA  Addition of CAP and Poly A tail: added on enzymatically on all eukaryotic precursor mRNAs and do not relate to the DNA sequence of the specific gene  Unit of code: triplet (sequence of 3 consecutive bases)  Codon: sequence of 3 consecutive bases in a mRNA molecule coding for a specific amino acid o No thymidine nucleotides in the gene that was used as a template for poly A tail  Degeneracy: more than 1 different codon is used to specify the same amino acid o When Poly A Tail is added: added transcription is terminated  Significance of Poly A tail: contributes to the stability of m|RNAs by protecting them from RNA-  Stop translation: 3 amino acid codes encode to stop  Start translation: one codon if it is near the 5’ cap of the mRNA and also is the code for adding degrading enzymes the amino acid methionine (double duty)  Exons: regions of the primary transcript that might survive and be part of the mature, processed RNA  # of codons that encode an amino acid: 61 (64-3=61stop codons)  Introns: regions of the precursor mRNA that must be removed as the mature mRNA is created. TRANSLATING THE CODE o Do not code for proteins  Last step in RNA processing: Post transcriptional which is removing of introns  Translate the code to amino acids: need tRNAs  Ribonucleoproteins: RNA containing proteins which remove introns  Function of tRNA: essential in translating the language of nucleic acids as expressed as an order of bases) into the language of proteins (an order of amino acids)  Spliceosomes: collection of ribonucleoproteins that acoomplish the removal of introns  ‘Splicing Process’: o Making tRNA: via ttranscription (precursor tRNA made to produce mature tRNA) 1. Recognize specific RNA sequence that brackets introns. There are specific ‘left boundary’  Required materials to translate the genetic code: set of tRNAs and set of aminoacyl tRNA synthetases and ‘right boundary’ sequences. The spliceosomes first recognize the 5’ boundary and then o Need a spricifc tRNA and specific aminoacyl tRNA synthetases for each different amino move along until it finds a 3’ boundary sequence 2. It then breaks phosphodiester bonds holding intron to exon and then creates new acid phosphodiester bonds to link the (now) adjacent exons  DNA template for tRNA: different gene for each different tRNA  Characteristic shape of tRNA: internal base pairings after tRNA is transcribed o Breakage of the phosphodiester bonds: RNA components  Difference in transcription from eukaryotes and prokaryotes: In EUKARYOTES, there is addition  Cloverleaf structure: single stranded transcript fold up on itself, using base pairing to form the of poly A tails and having introns spliced out mature transfer RNAs  What determines folding pattern: the rungs across the structure are places where there are o Once spliceosomes look for left boundary sequence, it will look for the R.boundary sequence. If the correct R. boundary sequence is unrecognizable by the spliceosomes, base pairings then the spliceosomes will continue to find R. boundary, and once it does that it will  Anticodon: important region of tRNA look all the introns between the Left and right boundary sequence o Bases of anticodon: complementary to the bases of a specific codon of a mRNA  Language of anticodon: Nucleic acids o Result: splicing mechanism found ways to block R. boundaries to make more mRNA  Reason for splicing introns in eukaryotes:  Attachment site for a specific amino acid: 3’ end of the tRNA and opposite end for anticodon 1. Sometimes one precursor mRNA can be spliced in more than 1ways to produce more than  3’ end for every tRNA: has 3’ ACC as 1 three ribonucleotides o Where the appropriate amino acid is attached to A one different mature mRNAalternative splicing o Alternative splicing: produce alternative mRNA. Thus, more than one protein can be  For a codon to be correctly translated: made from one gene o If a particular tRNA has an anticodon that is complementary to the codown that 2. Some introns when spliced out have regulatory roles in gene expression encodes the amino acid, methionine, the amino acid must be attached to the tRNA is  Location where RNA is made: nucleus methionine  Movement of RNA: RNA leaves nucleus and goes to cytoplasm to make protein 1 | P a g e o Example: if tRNA as an anticodon that base pair specifies lysine, then amino acid o Translation of the code will begin at the start codon: AUG attached to tRNA has to be lysine  Usually the start codon is not that the beginning of the mRNA in eukaryotes.  Aminoacyl tRNA synthetases: makes sure the correct amino acid is attached to the specific  5’ untranslated leader (UTR): RNA sequence before the first codon tRNA molecule
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