Somatic cells contain identical genes genetic code for all proteins Inactive

Somatic cells contain identical genes genetic code

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Somatic cells contain identical genes, genetic code for all proteins! Inactive genes : DNA supercoiled not easily accessed Active “expressed” genes : DNA uncoiled, exposed for transcription Activation of gene →mRNA production Are all genes active in all cells to make all proteins?
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Transcription (DNA mRNA) Remember nucleotides? Read as 3 nucleotides ( base triplet ) DNA triplet → mRNA codon 1 amino acid DNA: 2 strands, in transcription- Template DNA strand Copied strand, contains promoter “start sequence” of gene to be transcribed Coding DNA strand Not transcribed, has same code as mRNA built, (RNA has U in place of T though)
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Transcription (DNA mRNA) Initiation : Transcription factors bind DNA promoter RNA polymerase binds promoter on template strand DNA helicase unwinds DNA Elongation : RNA pol moves along and assembles mRNA Complementary base-pairing: H bonds between template DNA and new mRNA ( U not T!! ) Covalent bonds between mRNA ribonucleotides DNA base triplet = 1 codon (mRNA -3 ribonucleotide sequence)
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When to stop? Termination : At end of gene: stop /termination signal Special nucleotide sequence RNA polymerase released review
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mRNA transcription summary
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RNA processing of pre-mRNA Result of transcription: pre- mRNA produced Many codons long But –immature and can’t leave nucleus yet! Pre-mRNA Contains: Exons : coding regions, codes for proteins: KEEP Introns : non-coding regions, does not code for proteins: REMOVE
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RNA processing before exits nucleus Splicing : cut out mRNA introns, connect exons Note: it isn’t always this easy Capping Poly A tail Spliced, capped and poly A tailed- mature mRNA leaves nucleus through nuclear pores Pre-mRNA Leaves nucleus!
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Protein synthesis: translation mRNA→protein Translation: 1 codon translated into 1 amino acid Purpose: make functional polypeptide (protein) Location: ribosomes Free: in cytoplasm Bound : on rough ER
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Reading the genetic code We started with a base triplet (3 nucleotides -DNA) We made a complementary codon (3 ribonucleotides -mRNA) 1 codon translates into 1 amino acid Each amino acid can be specified by >1 codon Polypeptides (proteins) made from combination of 20 amino acids
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mRNA→protein: who are the players? mRNA transcript with codons tRNAs: connection between amino acid and mRNA codon tRNA ends: Anti-codon at one end: binds codon Amino acid-other end Ribosome: “reading machines” Large subunit: forms peptide bond Small subunit: matches mRNA codon with tRNA
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Translation: Initiation Small ribosomal subunit binds sequence near mRNA cap Finds start codon : AUG Initiator tRNA (with anti-codon UAC and special aa “MET”) binds Small and large ribosomal subunit complex
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Translation: Elongation Additional amino acids added Another tRNA (with anti-codon and attached aa) pairs with next codon Ribosome links amino acids by peptide bond mRNA moved through ribosome After tRNA transfers aa, empty tRNA falls off and is “recharged”
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Translation: Termination “Stop” codon (UAA, UAG, UGA)-
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