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Ch. 17: From Gene to Protein

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Title: Ch. 17: From Gene to Protein


1
Ch. 17 From Gene to Protein
2
The Connection Between Genes and Proteins
  • The study of metabolic defects provided evidence
    that genes specify proteins
  • Garrod, suggested phenotypes had to do with
    expression of genes for enzymes
  • Transcription and translation are the two main
    processes linking gene to protein
  • Copy the information and interpret the
    information
  • In the genetic code, nucleotide triplets specify
    amino acids
  • Sequence of nucleotides primary protein
    structure
  • The genetic code must have evolved very early in
    the history of life
  • DNA code is universal..all cells use the same
    codons and amino acids to make their various
    proteins

3
Back to Mendel
  • One of Mendels factors for peas was stem
    length. We say height and the alleles are tall
    and short. Actually its length and the stems
    are long or not-long
  • Normal peas have a gene for the hormone called
    gibberellin which stimulates stem elongation.
  • Dwarf peas lack this gene and do not make
    gibberellin and are therefore not tall.
  • Dwarf peas will grow to normal height if
    gibberellins are added to their water
  • PROTEINS ARE THE LINKS BETWEEN GENOTYPE AND
    PHENOTYPE.

4
Scientific Evidence
  • 1909 inborn errors of metabolism
  • alkaptonuria
  • 1930 Beadle and Ephrussi, eye color in flies is
    due to an enzyme for pigment production
  • Beadle and Tatum minimal medium Neurospora
    crassa (bread mold), used x-rays to create
    mutations, complete media had 20 amino acids,
    looking for inability to metabolize amino acids
    from a limited source
  • Mutants had defects in metabolism
  • Must be enzymes related
  • Enzymes are proteins
  • One gene one enzyme hypothesis
  • now modified to one gene one (protein)
    polypeptide

5
Beadle and Tatums Neurospora crassa Experiment
6
Transcription and Translation
  • Genes have instructions for making proteins, but
    genes do not make proteins directly
  • Transcription is the synthesis of RNA under the
    direction of DNA. DNA provides the template. Get
    an accurate copy mRNA
  • Translation is the actual synthesis of a
    polypeptide, at the ribosome, under the direction
    of the mRNA
  • DNA ? RNA ? protein (polypeptide)

7
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9
Terminology
  • Triplet code three DNA nucleotides a word
  • mRNA carries message from DNA to ribsome
  • tRNA transports amino acids within
    cytoplasmrRNA ribosomes are composed of rRNA
    and proteins
  • Ribosome solid organelle found in cytoplasm of
    ALL cells used to manufacture protein
  • Template strand for each gene only one side of
    the DNA is transcribed
  • Codon mRNA triplets are called codons
  • Reading frame 5?3, starting at beginning,
    groups of three
  • The red dog ate the cat
  • xHer edd oga tat hec atx

10
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11
Cracking the Code
  • 1960 Marshall Nirenberg at NIH
  • National Institute of Health
  • www.nih.gov
  • Human genome projects library
  • Translated all the possible codons into amino
    acids
  • Found codons for start and for stop
  • Several amino acids can be coded for with more
    than one codon (redundancy) but no codons are for
    multiple amino acids (no ambiguity)
  • wobble effect

12
Code Evolved Early in the History of Life on
Earth (and any life anywhere else too)
  • Code is (near) universal to all know/studied
    organisms. Bacteria can translate human genetic
    information
  • All modern organisms have a common ancestor
  • Few exceptions are found in protista and in
    mitochondria DNA (. Endosymbiont hypothesis.)

13
  • Sketch a DNA molecule with chemically correct
    details
  • Show how it would replicate and
  • How it would transcribe and
  • List the amino acids in the short polypeptide it
    forms
  • It has the following template strand sequence of
    DNA triplets
  • TAC TTT GAG ATT

14
Genomic like information
  • Stthegeneticcodeisnearlyuniversalpsharedbyorganism
    sfromthesimplestbacteriatothemostcomplexplantsanda
    nimalspstthernacodonccgpforinstancepistranslatedas
    theaminoacidprolineinallorganismswhosegeneticcodeh
    asbeenexaminedpstinlaboratoryexperientspgenescanbe
    transcribedandtranslatedaftertheyaretransplantedfr
    omonespeciestoanotherpfponeimportantapplicationist
    hatbacteriacanbeprogrammedbytheinsertionofhumangen
    estosynthesizecertainhumanproteinsthathaveimportan
    tmedicalusesp

15
The Synthesis and Processing of RNA
  • Transcription is the DNA directed synthesis of
    RNA
  • Eukaryotic cells modify RNA after transcription

16
Transcription
  • RNA polymerase fits onto DNA (3) and moves in a
    5 ? 3 direction for the synthesis of the RNA
    strand.
  • C with G and this time, A with U (uracil)
  • DNA acts as a template
  • DNA is only opened at a small region (gene or
    genes of interest)
  • DNA helix reseals as RNA polymerase passes by.
    Completely intact and undiluted.

17
Bacterial transcription
  • Eukaryotic cells have 3 kinds of RNA polymerase
    (I, II used in RNA synthesis and III)
  • Bacteria have one kind it makes not only mRNA
    but also other types of RNA
  • Bacteria have one chromosome and many plasmids.
    Information is constantly being sent to ribosomes
    for translation into proteins needed by the
    bacterial cell

18
Steps of Transcription
  • Initiation
  • Promoter region where polymerase attaches and a
    dozen bases upstream start here and use this
    side of the helix.
  • Collection of transcription factors initiate the
    complex TATA box
  • Elongation
  • DNA exposed 20 bases at a time
  • 5 ? 3 synthesis of RNA strand
  • RNA peels away from DNA as completed
  • rate is 60 nucleotides per second
  • Termination
  • DNA contains a terminator sequence
  • polymerase continues to a AAUAAA sequence and
    10-35 nucleotides later the preRNA is cut free
  • other details are still murky

19
Modification of RNA
  • Initially RNA is called preRNA
  • The 5 end (transcribed 1st) is capped with
    special guanine provides protection and a start
    here signal for translation
  • Other end gets a ploy A tail (AAA-AAA) in
    addition to ribosomal attachment and protection,
    it seems to facilitate RNA as it leaves the
    nucleus
  • These regions are nontranslated

20
Further modification of RNA
  • Most of the pre RNA is actually removed. It
    didnt code for information about how to make a
    protein. We are uncertain of the function of this
    info, which does not make the info unimportant.
  • Initially the RNA can be 8000 bases, actual info
    for protein that goes to ribosomes is about 1200
    or 400 amino acids (1200 bases/ 3 bases per
    codon)

21
Cut and Paste
  • Called RNA splicing
  • Introns (intervening segments) are removed
  • they are noncoding, short, repetitive sequences,
    unique cause restriction enzymes to cut
    segments differently and create the DNA
    fingerprint
  • Probably have a role in gene expression and
    activity
  • May be place where new proteins evolve
  • Increase odds of crossing over during synapsis of
    tetrads (meiosis II)
  • Exons (expressed)
  • these are translated into amino acids for the
    polypeptide
  • 150 nucleotides
  • 5 cap exon exon exon . poly A tail
  • Process requires snRNPs - small nucleotide
    ribonucleoproteins. Sites to bind
  • Ribozymes RNA that functions as an enzyme.
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