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Lecture 18

Lecture 18th - BIOA01H3.docx

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Biological Sciences
Mark Fitzpatrick

thBIOA01H3Lecture 18 stWeek of October 21 Central Dogma Chapter 13131b The Pathway from Gene to Polypeptide Involves Transcription and TranslationTranscription is the mechanism by which the information encoded in DNA is made into a complementary RNA copy Information in one nucleic acid type transferred to another nucleic acid type Translation is the use of the information encoded in the RNA to assemble amino acids into a polypeptide Information in a nucleic acid in the form of nucleotides converted into different kind of moleculeamino acidsTranscription o Enzyme RNA polymerase creates RNA sequence thats complementary to DNA sequence of a given gene o Process follows same basic rules of complementary base pairingnucleic acid chemistry in DNA replication o Template strand is red by the RNA polymeraseo RNA transcribed from a gene encoding a polypeptide is called messenger RNA mRNA Translation o mRNA associates with a ribosome a particle on which amino acids are linked into polypeptide chains o As ribosome moves along mRNA amino acids specified by mRNA joined one by one to form polypeptide encoded by the gene Processes of transcription and translation similar in prokaryotic and eukaryotic cells except for one difference prokaryotic cells can transcribetranslate given gene simultaneously eukaryotic cells transcribe and process mRNA in nucleus before exporting to cytoplasm for translation on ribosomes 131c The Genetic Code Is Written in ThreeLetter Words Using ForLetter AlphabetBreaking the Genetic Code Nucleotide information that specifies the amino acid sequence of a polypeptide is called the genetic codeGenetic information in DNA first transcribed into complementary threeletter RNA codons T is replaced by UFeatures of the Genetic Code Scientists write the codons in the 53 direction as they appear in mRNAsOf the 64 codons 61 specify amino acids AUG specifies the amino acid methionine First codon translated in any mRNA and is the start or initiator codonThree amino acidsUAA UAG UGAare stop codons also called nonsense or termination codons that act as periods indicating the end of a polypeptideencoding sentenceOnly two amino acids methionine and tryptophan are specified by a single codon3 Features of genetic code 1 Rest represented by at least 2 in other words many synonyms in nucleic acid code a feature known as degeneracy or redundancy2 Commaless the words of the nucleic acid code are sequential There is only one correct reading frame for each mRNA at the first base of the first threeletter codon at the beginning of the coded message 3 Universal with few exceptions same codons specify the same amino acids in all living organismseven in viruses132 Transcription DNADirected RNA SynthesisCertain aspects similar to DNA replication transcription differs1
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