Gene Activity: How Genes Work

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Gene Activity How Genes Work

• Chapter 14

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Function of Genes

• Genes Specify Enzymes
• Beadle and Tatum performed experiments on
  Neurospora crassa and proposed each gene
  specifies the synthesis of one enzyme
• One gene-one enzyme hypothesis

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Beadle and Tatum Experiment
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Function of Genes

• Genes Specify a Polypeptide
• The one gene-one enzyme hypothesis suggests a
  genetic mutation causes a change in the structure
  of a protein
• A gene is a segment of DNA that specifies the
  sequence of amino acids in a polypeptide of a
  protein
• Linus Pauling and Harvey Itano Hb

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Hemoglobin
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Hemoglobin

• Vernon Ingram determine the structural
  differences in hemoglobin
• Charge difference caused by 1 amino acid
  substitution

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From DNA to RNA to Protein

• Central Dogma of Molecular Biology
• Genes code for the sequence of nucleotides in RNA
  molecules
• RNA brings about formation of a protein coded for
  by DNA of a particular gene
• DNA ? RNA ? protein

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RNA

• RNA is made up of 4 nucleotides
• Uracil (U) replaces thymine (T) of DNA
• 2 Purines
• Adenine
• Guanine
• 2 Pyrimidines
• Uracil
• Cytosine

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Types of RNA

• Messenger (mRNA) - Takes message from DNA in
  nucleus to ribosomes in cytoplasm
• Transfer (tRNA) - Transfers amino acids to
  ribosome when instructed
• Ribosomal (rRNA) - Help make up ribosomes which
  reads message in mRNA

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Structure of RNA
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Steps of Gene Expression

• Transcription DNA serves as a template for RNA
  formation
• Gene unzips and exposes unpaired bases
• Serves as template for mRNA formation
• Loose RNA nucleotides bind to exposed DNA bases
  using the CG AU rule
• When entire gene is transcribed into mRNA, result
  is an mRNA transcript of the gene
• The base sequence in the mRNA is complementary to
  the base sequence in DNA

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Steps of Gene Expression

• Translation an mRNA transcript directs the
  sequence of amino acids in a polypeptide
• An mRNA transcript migrates to rough endoplasmic
  reticulum
• Associates with the rRNA of a ribosome
• The ribosome reads the information in the
  transcript
• Ribosome directs various species of tRNA to bring
  in their specific amino acid fares
• tRNA specified is determined by the code being
  translated in the mRNA transcript

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Gene Expression
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The Genetic Code

• There is a genetic code for each of the 20 amino
  acids found in proteins
• Genetic code is a triplet code, with each codon
  consisting of three nucleotide bases
• 64 (43) possible combinations of the 4
  nucleotides

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The Genetic Code
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The Genetic Code

• Code Properties
• Universal
• With few exceptions, all organisms use the code
  the same way
• Encode the same 20 amino acids with the same 64
  triplets
• Degenerate
• There are 64 codons available for 20 amino acids
• Most amino acids encoded by two or more codons
• Unambiguous
• None of the codons code for two or more amino
  acids
• Each codon specifies only one of the 20 amino
  acids
• Contains start and stop signals
• Punctuation codons
• Like the capital letter we use to signify the
  beginning of a sentence, and the period to
  signify the end

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Transcription

• A single chromosomes consists of one very long
  molecule encoding hundreds or thousands of genes
• The genetic information in a gene describes the
  amino acid sequence of a protein
• The information is in the base sequence of one
  side (the sense strand) of the DNA molecule
• The gene is the functional equivalent of a
  sentence

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Transcription

• The segment of DNA corresponding to a gene is
  unzipped to expose the bases of the sense strand
• The genetic information in the gene is
  transcribed (rewritten) into an mRNA molecule
• The exposed bases in the DNA determine the
  sequence in which the RNA bases will be connected
  together
• RNA polymerase connects the loose RNA nucleotides
  together

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Transcription

• During transcription, an mRNA molecule is formed
  with a sequence of bases complementary to a
  portion of one DNA strand
• RNA polymerase joins the nucleotides together in
  5 ? 3 direction

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Transcription

• Promoter defines start of a gene, the direction
  of transcription, and the strand to be
  transcribed
• Elongation continues until polymerase comes to a
  DNA terminator sequence

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Transcription
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Transcription
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Processing Messenger RNA

• Newly formed mRNA molecule called the primary
  mRNA transcript
• Primary mRNA transcript is modified before it
  leaves the eukaryotic nucleus
• Primary transcript consists of
• Some segments that will not be expressed
  (introns)
• Segments that will be expressed (exons)
• Performed by spliceosome complexes in nucleoplasm
• Introns are excised
• Remaining exons are spliced back together

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Processing Messenger RNA

• Modifications
• Cap on 5 end
• Poly-A tail on 3 end
• Introns are removed during RNA processing,
  leaving only exons

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Functions of Introns

• As organismal complexity increases
• Number of protein-coding genes does not keep pace
• But the proportion of the genome that is introns
  increases
• Humans
• Genome has only about 25,000 coding genes
• Up to 95 of this DNA is introns
• Possible functions of introns
• More bang for buck (alternative splicing)
• Exons might combine in various combinations
• Would allow different mRNAs to result from one
  segment of DNA
• Introns might regulate gene expression
• Exciting new picture of the genome is emerging

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Translation

• Transfer RNA (tRNA) molecules transfer amino
  acids to the ribosomes
• 64 different kinds
• tRNA molecules bear a specific triplet code
  (anticodon)
• tRNA molecules bind with one particular amino
  acid
• tRNA synthetases attach correct amino acid to the
  correct tRNA molecule

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Transfer RNA
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Role of Ribosomal RNA

• Ribosomal RNA (rRNA) is produced off a DNA
  template in the nucleolus
• Packaged with proteins into ribosomal subunits,
  one larger than the other
• Contain 3 binding sites to facilitate
  complementary base pairing between tRNA
  anticodons and mRNA codons
• The E (for exit) site
• The P (for peptide) site, and
• The A (for amino acid) site

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Ribosome Structure and Function
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Three Steps in Translation

• Initiation
• Elongation
• Termination

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Initiation

• Components necessary for initiation
• Small ribosomal subunit
• mRNA transcript
• Initiator tRNA, and
• Large ribosomal subunit
• Initiation factors (special proteins that bring
  the above together)
• Initiator tRNA
• Always has the UAC anticodon
• Always carries the amino acid methionine
• Capable of binding to the P site

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Initiation

• Small ribosomal subunit attaches to mRNA
  transcript
• Beginning of transcript always has the START
  codon (AUG)
• Initiator tRNA (UAC) attaches to P site
• Large ribosomal subunit joins the small subunit

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Elongation

• Elongation - refers to the growth in length of
  the polypeptide
• RNA molecules bring their amino acid to the
  ribosome
• Ribosome reads a codon in the mRNA
• Allows only one type of tRNA to bring its amino
  acid
• Must have the anticodon complementary to the mRNA
  codon being read
• Joins the ribosome at its A site
• Methionine of initiator is connected to amino
  acid of 2nd tRNA by peptide bond

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Elongation

• Second tRNA moves to P site (translocation)
• Spent initiator moves to E site and exits
• Ribosome reads the next codon in the mRNA
• Allows only one type of tRNA to bring its amino
  acid
• Must have the anticodon complementary to the mRNA
  codon being read
• Joins the ribosome at its A site
• Dipeptide on 2nd amino acid is connected to amino
  acid of 3nd tRNA by peptide bond

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Elongation
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Termination

• Previous tRNA moves to P site
• Spent tRNA moves to E site and exits
• Ribosome reads the STOP codon at the end of the
  mRNA
• UAA, UAG, or UGA
• Does not code for an amino acid
• Polypeptide is released from last tRNA by release
  factor
• Ribosome releases mRNA and dissociates into
  subunits
• mRNA read by another ribosome

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Termination
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Gene Expression in Eukaryotes
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ribosome/ribosome1.jpg