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Lecture 13

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Biology 1202B
Brenda Murphy

Biology 1202b Murphy 2012 Lecture 13: Chapter 13,14—Transcription DNA RNA Transcription in Eukaryotes vs. Prokaryotes Eukaryote—transcription and processing occurs in nucleus, before exporting mRNA into the cytoplasm for translation. Each process occurs separately because they are compartmentalized Prokaryote—transcription and translation of a gene can occur simultaneously because the genetic material is not confined to a nucleus. All the different enzymes that are need are free floating in the nucleus Transcription- DNA RNA  The information in DNA is transferred to RNA  DNA is double strand, but RNA is single stranded—transcription only one strand is read—the template strand  Template strand and mRNA are antiparallel, but when nucleotides are translated back it goes back to the template strand  RNA polymerase creates an RNA sequence complimentary to DNA template strand. Remember, RNA does not contain thymine, but instead, uracil: A-U instead of A-T. All others bind as in DNA replication  The single stranded RNA is called the messenger RNA (mRNA). Synthesis always occurs on a 5’ to 3’ direction because 3 prime end has the OH where nucleotides join  Codon—three letters of the nucleotide sequence (triplets) o RNA is read in triplets—3 RNA nucleotides can be read in the universal code chart. Codons are read in the 3’ to 5’ direction Application: If there is a mutation in the genetic code, what could change the protein? Certain genetic mutations alter how the body functions because of what proteins control Universal codon chart  Deciphers how codons correlate to amino acids an which proteins they make  Characteristics of the genetic code include commaless, universality and redundancy  Commaless—words of the nucleic acid code are sequential, with no indicators such as commas or spaces to mark the end of one codon and the beginning of the next—code can only be read correctly by starting at the right place  Universal—same codons specific the same amino acids in all living organisms but there are some expectations. Codon chart does not work for human mitochondria, plant chloroplasts or yeast  Redundancy—there are 64 different possible codons 4 different bases, A, U, G, T, times 3 sports (4x4x4)  There are only 20 amino acids. This is because only the first 2 nucleotides determine the amino acid (but there are some only specified by a single codon)  Using the chart to make sense of the mRNA to see the protein sequence  Codons we have to know!: o Start codon: AUG specified the amino acid methionine, and is the first codon translated into any mRNA in both prokaryotes and eukaryotes o Stop codons: UAA, UAG, UGA, do not code for any amino acid, which indicated the ending of a polypeptide sequence 3 major stages of transcription Initiation:  Assembly of the transcription machinery on the promoter  RNA polymerase binds to the promoter, unwinds the DNA in that region and begins synthesizing an RNA molecule  Another RNA polymerase may begin initiation as soon as there is room at the promoter Elongation  RNA polymerase moves along the DNA, unwinding it and adding new RNA nucleotides one by one in a 5’ to 3’ direction, to form a strand of pre-mRNA  Reading the string of nucleotides in a sequence of single stranded DNA template and changing this information into a string of nucleotides of single stranded pre-mRNA Termination  Releasing pre-mRNA RNA polymerase  Reassembly of dsDNA recoiling itself to be ready for another round of transcription Note: once polymerase has started transcription and moved out of the way of the promoter another molecule of RNA polymerase may start creating another pre-mRNA Eukaryotic processing of Pre-mRNA to mature mRNA 1. RNA polymerase II transcribes the gene. 5’ cap is added soon after transcription begins. There is no transcription terminator. Transcription continues past end of gene and eventually stops 2. Proteins bind to the polyadenylation signal and cleave the transcript just downstream of that point 3. Poly (A) polymerase adds a string of 50 to 250 adenine nucleotides, one nucleotide at a t
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