Chapter 11.4-11.6 From DNA to Proteins

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Chapter 11.4-11.6From DNA to Proteins

• College Prep Biology
• Mr. Martino

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Introduction

• Protein synthesis is the process of making
  proteins
• 2 stages
• 1. Transcription process of transferring the
  DNA gene onto an mRNA transcript
• 2. Translation process of tRNA and rRNA
  assembling the amino acids in the proper sequence
  according to mRNA
• Involves RNA

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11.4 Transcription

• 3 main classes of RNA
• mRNA messenger RNA carries the instructions
  from DNA
• tRNA transfer RNA delivers amino acids to
  ribosome to form protein
• rRNA ribosomal RNA a major component of
  ribosomes

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More Transcription

• RNA is similar yet different from DNA
• Single stranded
• 4 types of nucleotides (uracil substitutes for
  thymine pairing with adenine)
• Ribose substitutes for deoxyribose
• Different types of RNA
• Found in nucleus (like DNA) but also in cytoplasm

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Uracil is only found in RNA
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Transcription again

• Transcription is similar to replication but
  differs in 3 important ways
• 1. Only a small section of DNA (gene) serves as
  template
• Promoter a base sequence on DNA that signals
  the beginning of a gene
• 2. RNA polymerase attaches nucleotides instead
  of DNA polymerase
• 3. Results in a single strand of RNA not
  double helix

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• 3 Phases of transcription
• 1. Initiation when RNA polymerase attaches to
  the promotor DNA
• 2. Elongation growing of the RNA strand
• As it continues, RNA strand pulls away from DNA
  DNA rejoins
• 3. Termination RNA reaches a specific sequence
  that codes for stop
• Polymerase detaches from RNA and gene

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(No Transcript)
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• Not all of the new mRNA is used
• Introns base sequences that are removed before
  translation
• Exons parts of the mRNA that gets translated
  into a protein

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11.5 Genetic Code

• Gobind Khorana and Marshall Nirenberg realized
  ribsomes read nucleotides 3 at a time
• Codon set of 3 nucleotides amino acid
• 64 codons code for 20 amino acids
• AUG codes for methionine /start
• 3 codons for stop
• Several codons code for some amino acids
• Codons are universal for all living organisms

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Structure and Function of tRNA rRNA

• tRNA (transfer RNA) carry amino acids
• One end has anticodon while other carries a.a.
• Anticodon nucleotide triplet that base pairs
  with codon
• Specific for 1 amino acid
• rRNA component of ribosomes

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11.5 Translation

• Translation has 3 stages initiation,
  elongation, and termination
• 1. Initiation
• Step 1 mRNA binds to small subunit initiator
  tRNA locates binds to start codon (AUG)
• Step 2 large subunit binds to small one

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• 2. Elongation After initiation, A.A.s are
  added one by one
• Step 1 Codon recognition
• tRNA anticodon matches mRNA codon
• Step 2 Peptide Bond Formation
• A.A. separates from tRNA and attaches to
  polypeptide (P to A)
• Step 3 Translocation
• A site tRNA moves to P site

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• Termination stop codon on mRNA is reached
• Triggers a release factor to bind to ribosome
• Enzyme detaches mRNA and polypeptide
• Many polypeptides then float freely in cytoplasm
  others enter rough ER before being sent to
  their ultimate destinations

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11.6 Mutations

• Gene mutations change in nucleotide sequence of
  DNA genes
• Substitution one amino acid is substituted for
  another
• Ex. Sickle-cell anemia
• Frame shift mutations change down-stream
  codons
• Insertion an extra base is put into a gene
• Deletion loss of a base
• Transposable Elements (translocation) DNA
  segments that move spontaneously

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Causes of Mutations

• Many mutations occur spontaneously during
  replication
• Small number of mistakes do sneak by proofreaders
• Mutagen mutation causing agents in environment
• UV light, gamma rays, X-rays
• Carcinogen cancer causing agents disrupt base
  pairing
• Only spontaneous mutations of germ cells or
  gametes are passed on to offspring