Lecture 2 1 2012 Topic RNA and Protein Reading Chapter 3 pp 53 57 Web links http learn genetics utah edu This website has many resources http www youtube com watch v 41 Ne5mS2ls feature related This is a cool video showing the process of translation protein production 1 a Structure of RNA RNA is a single strand or chain of nucleotides Like DNA the nucleotide is composed of three parts The Base the Sugar and the Phosphate group There are also four Bases in RNA A G C and U Uracil In RNA U replaces T A is complementary to U G is complementary to C Making RNA in the cell this process is called Transcription Transcription begins in the nucleus of the cell Like DNA replication the first step of Transcription involves separation of the DNA double helix into two strands i e the hydrogen bonds between complementary DNA bases are broken Unlike DNA replication only one 1 of the two DNA parent strands acts as a template The single strand of RNA links of nucleotides grows using the same base pairing rules G pairs with C U pairs with A The RNA strand is then processed and exported out of the nucleus into the cytoplasm This RNA is now called messenger RNA or mRNA Making protein this process is called Translation See Figure 3 17 Translation occurs in the cytoplasm of the cell A protein is a string of amino acids that are joined by a peptide bond All proteins are made from 20 amino acids Translation is a complicated process It involves interaction between mRNA another type of RNA called transfer RNA or tRNA and big proteins called ribosomes Scientists developed the Genetic Code to interpret the information in DNA that leads to the production of proteins b c 2 a b c d 3 a b c d mRNA copied from one strand of DNA can be grouped into a series of non overlapping triplet nucleotides or codons An mRNA that is 30 nucleotides long contains a maximum of 10 codons 30 3 10 The Genetic Table see below lists codons and amino acids There are 64 codons but only 20 amino acids In other words sometimes more than one codon specifies the same amino acid This means the genetic code is degenerate Generally speaking there are 61 unique tRNA molecules in a cell Each tRNA is unique by it s anticodon sequence The anticodon is also a three nucleotide sequence that is complementary to each codon following the rules of base pairing G C and A U Each tRNA also carries one of the 20 amino acids a tRNA will bind to the mRNA via base pairing between the anticodon on the tRNA and the codon on the mRNA This occurs on the ribosome The tRNA will drop off the specific amino acid that it carries The first codon of any protein in any organism is AUG The tRNA with an anticodon UAC carries the amino acid Methionine Thus the first amino acid of any protein in any organism is Methionine Methionine starts the chain of amino acids i e the protein Translation continues as the ribosome moves along the mRNA molecule one codon at a time The process stops when the ribosome comes to a codon that specifies STOP there are three codons see Genetic Table Once this happens the ribosome and mRNA separate and translation stops e f g h i
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