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Lecture 13

Lecture 13- Transcription

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Department
Biology
Course Code
Biology 1202B
Professor
Brenda Murphy

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Lecture 13: Transcription – DNA to 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 by the nucleus. 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 needed are free floating in the nucleoid space. Transcription - DNA RNA - The information in DNA is transferred to RNA - DNA is double stranded, but RNA is single stranded. Therefore in transcription only one strand is read – the template strand. - Template strand and mRNA are antiparralel, but when nucleotides are trasnlated back it is goes back to the template strand. - RNA polymerase creates an RNA sequence complimentary to DNA template strand. Remember, that 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’ prime direction because 3 prime end has the OH where nucleotides join Codon: three letters of the nucleotide sequence (Triplets) - RNA is read in triplets – 3 RNA nucleotides can be read in the universal code chart. Codons are read in the 5’ to 3’ 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 and which proteins they make. - Characteristics of the genetic code include universality and redundancy. - Universal – same codons specify the same amino acids in all living organism but there are some exceptions. 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 spots (4x4x4) - However, there are only 20 amino acids. This is because only the first two 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: - Start codon: AUG specifies the amino acid methionine, and is the first codon translated into any mRNA in both prokaryotes and eukaryotes - Stop Codons: UAA, UAG, UGA, do not code for any amino acid, which indicate the ending of a polypeptide sequence. 3 Major Stages of Transcription Initiation - Assembly of the transcription components 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. Termination - Releasing pre-mRNA RNA polymerase - Reassembly of dsDNA by 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-RN- Polymerase and Transcription The promoter specifies where on the DNA transcription begins. This structure differs between the different types of cells: Prokaryotes: all genes have similar proteins so use same RNA polymerase for transcription - RNA polymerase recognizes the key DNA sequences on the promoter and binds to begin transcription Eukaryotes: have different promoters Protein coding genes: - Encode mRNA that is translated into a protein - RNA polymerase II - TATA box for Transcription factors Non-protein coding genes - Encode mRNA that itself is functional (rRNA, tRNA, snRNA) - RNA polymerase I and III Gene Organization Promoter Regions – Identify where the start point of transcription will begin for incoming RNA polymerase - RNA polymerase cannot bind to the promoter region itself, so proteins, called transcription factors recognize and bind to the TATA box to recruit the RNA
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