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Lecture 2

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Simon Fraser University
Health Sciences
HSCI 100
Nienke Van Houten

CHAPTER 21: GENE EXPRESSION DNA (deoxyribonucleic acid) - Double helix - Hydrogen bonds strands = anti-parallel - A-T | G-C = complementary base pairing DNA replication (in nucleus) - Enzyme unwinds 2 strands (breaks H-bonds) - DNA Polymerase adds complementary nucleotides - Replication = SEMICONSERVATIVE o One parent strand conserved RNA (ribonucleic acid) - - GENE EXPRESSION - Base uracil instead of thymine - Single strand - Functions primarily in cytoplasm - Three types o Ribosomal (rRNA) – combines proteins to form ribosomes where synthesis occurs o Messenger (mRNA) – carries DNA instructions for synthesizing protein from nucleus to cytoplasm o Transfer (tRNA) – brings specific amino to where it can be added to a polypeptide that is under construction - Other rna o Ribozymes – enzymatic RNAs o snoRNA – components of mRNA splicing o `miRNA – microRNA that regulate gene expression - AU | GC Explain how genetic code encodes for 20 aa’s from only 4 bases - 4 bases in nucleic acids o 20 amino acids o 3 bases = one amino o = CODON DNA MESSAGE TO RNA 1. TRANSCRIPTION - RNA polymerase synthesizes mRNA using DNA template - mRNA processed before leaving nucleus o Exons are expressed o Introns (intervening sequences) snipped away from mRNA - Promoter starts transcription RNA TO PROTEIN: 2. TRANSLATION - Genetic code: language of the genes that translates sequene of bases in DNA into the specific sequence of amino aicds in a protein o Codons: sequences of 3bases on mRNA that specify 1/20 amino or begin/end of protein chain o 4 RNA bases can form 64 combo of three seq sequences but several sets of codons code for same amino - Ribosomes – workbenches which proteins are built from aminos - 1. Imitation: mRNA binds to ribosome o Players in protein synthesis come together (mRNA, tRNA, ribosomal subunits) o AUG= start codon o Small ribosomal unit attaches @ mRNA strand @ start codon (AUG) o tRNA with complementary anticodon pairs with start codon. Larger ribosomal joins - 2. Elongation: polypeptide chain grows o Start codon recognized – next codon aligned o Peptide bond forms by enzymes o Ribosome movement - 3. Termination: protein synthesis stops (poly A tail) o Stop codon moves into ribosome. No complementary pair just termination o Parts disassemble - mRNA moves into cytoplasm - tRNAs with anticodons carry amino to mRNA - anticodon-codon complement
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