Genes and How They Work Chapter 15.
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Transcript of Genes and How They Work Chapter 15.
Genes and How They Work Chapter 15 The Nature of Genes Early
ideas to explain how genes work came fromstudying human diseases
Archibald Garrod 1902 Recognized that alkaptonuria is inherited via
a recessiveallele Proposed that patients with the disease lacked a
particularenzyme These ideas connected genes to enzymes Beadle and
Tatum 1941 Deliberately set out to create mutationsin chromosomes
and verify that theybehaved in a Mendelian fashion in crosses
Studied Neurospora crassa Used X-rays to damage DNA Looked for
nutritional mutations Had to have minimal media supplemented to
grow Beadle and Tatum looked for fungal cells lacking specific
enzymes
The enzymes were required for the biochemical pathwayproducing the
amino acid arginine They identified mutants deficient in each
enzyme of thepathway One-gene/one-enzyme hypothesis has been
modifiedto one-gene/one-polypeptide hypothesis Central Dogma First
described by Francis Crick
Information only flows from DNA RNA protein Transcription = DNA RNA
Translation = RNA protein Retroviruses violate this order using
reversetranscriptase to convert their RNA genome intoDNA
Transcription Translation DNA-directed synthesis of RNA
Only template strand of DNA used U (uracil) in DNA replaced by T
(thymine) in RNA mRNA used to direct synthesis of polypeptides
Translation Synthesis of polypeptides Takes place at ribosome
Requires several kinds of RNA RNA All synthesized from DNA template
by transcription
Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA) Small
nuclear RNA (snRNA) Signal recognition particle RNA Micro-RNA
(miRNA) Genetic Code Francis Crick and Sydney Brenner determined
how the order ofnucleotides in DNA encoded amino acid order Codon
block of 3 DNA nucleotides corresponding to an amino acid
Introduced single nulcleotide insertions or deletions and looked
formutations Frameshift mutations Indicates importance of reading
frame Marshall Nirenberg identified the codons that specify each
amino acid
Stop codons 3 codons (UUA, UGA, UAG) used to terminate translation
Start codon Codon (AUG) used to signify the start of translation
Code is degenerate, meaning that some amino acidsare specified by
more than one codon Code practically universal
Strongest evidence that all living things sharecommon ancestry
Advances in genetic engineering Mitochondria and chloroplasts have
some differencesin stop signals Prokaryotic transcription
Single RNA polymerase Initiation of mRNA synthesis does not
requirea primer Requires Promoter Start siteTranscription unit
Termination site Promoter Forms a recognition and binding site for
the RNA polymerase
Found upstream of the start site Not transcribed Asymmetrical
indicate site of initiation anddirection of transcription 15 TATAAT
Promoter (10 sequence) Holoenzyme Core enzyme
Copyright The McGraw-Hill Companies, Inc. Permission required for
reproduction or display. TATAAT Promoter (10 sequence) Holoenzyme
Core enzyme Downstream 5 9 3 Start site (+1) Template strand
TTGACAPromoter (35 sequence) Coding strand Prokaryotic RNA
polymerase Upstream 5 3 a. b. binds to DNA RNApolymerase bound to
unwound DNA Transcription bubble 5 3 dissociates ATP Helix opens at
10 sequence Start site RNA synthesis begins 5 3 15 Elongation Grows
in the 5-to-3 direction as ribonucleotidesare added Transcription
bubble contains RNA polymerase,DNA template, and growing RNA
transcript After the transcription bubble passes, the now-
transcribed DNA is rewound as it leaves the bubble Termination
Marked by sequence that signals stop to polymerase
Causes the formation of phosphodiester bonds to cease RNADNA hybrid
within the transcription bubble dissociates RNA polymerase releases
the DNA DNA rewinds Hairpin Prokaryotic transcription is coupled to
translation
mRNA begins to be translated before transcriptionis finished Operon
Grouping of functionally related genes Multiple enzymes for a
pathway Can be regulated together Eukaryotic Transcription
3 different RNA polymerases RNA polymerase I transcribes rRNA RNA
polymerase II transcribes mRNA and somesnRNA RNA polymerase III
transcribes tRNA and someother small RNAs Each RNA polymerase
recognizes its ownpromoter Initiation of transcription
Requires a series of transcription factors Necessary to get the RNA
polymerase II enzyme to a promoter and to initiate gene expression
Interact with RNA polymerase to form initiation complex at promoter
Termination Termination sites not as well defined 24 Other
transcription factors RNA polymerase II Eukaryotic DNA
Initiation complex TATAbox 24 mRNA modifications In eukaryotes, the
primary transcript must bemodified to become mature mRNA Addition
of a 5 cap Protects from degradation; involved in translation
initiation Addition of a 3 poly-A tail Created by poly-A
polymerase; protection from degradation Removal of non-coding
sequences (introns) Pre-mRNA splicing done by spliceosome 5 cap HO
OH P P P CH2 + 3 poly-A tail 3 N+ A A A A A A A CH3
Methyl group A A U A A A mRNA P P P G 5 CH3 Eukaryotic pre-mRNA
splicing
Introns non-coding sequences Exons sequences that will be
translated Small ribonucleoprotein particles (snRNPs)recognize the
intronexon boundaries snRNPs cluster with other proteins to
formspliceosome Responsible for removing introns Eukaryotic
pre-mRNA splicing
Introns non-coding sequences Exons sequences that will be
translated Small ribonucleoprotein particles (snRNPs)recognize the
intronexon boundaries snRNPs cluster with other proteins to
formspliceosome Responsible for removing introns E1 I1 E2 I2 E3 I3
E4 I4 Transcription Introns are removed a. Exons Introns Mature
mRNA 33 poly-A tail 5 cap 3 poly-A tail Primary RNA transcript
DNAtemplate Alternative splicing Single primary transcript can be
spliced into differentmRNAs by the inclusion of different sets of
exons 15% of known human genetic disorders are due toaltered
splicing 35 to 59% of human genes exhibit some form ofalternative
splicing Explains how 25,000 genes of the human genomecan encode
the more than 80,000 different mRNAs tRNA and Ribosomes tRNA
molecules carry amino acids to theribosome for incorporation into a
polypeptide Aminoacyl-tRNA synthetases add amino acids tothe
acceptor stem of tRNA Anticodon loop contains 3
nucleotidescomplementary to mRNA codons 2D Cloverleaf Model
Acceptor end Anticodon loop 3 5 3D Ribbon-like Model Acceptor end
Anticodon loop Icon Anticodon end Acceptor end tRNA charging
reaction
Each aminoacyl-tRNA synthetase recognizes only 1amino acid but
several tRNAs Charged tRNA has an amino acid added using theenergy
from ATP Can undergo peptide bond formation without
additionalenergy Ribosomes do not verify amino acid attached to
tRNA The ribosome has multiple tRNA binding sites
P site binds the tRNA attached to thegrowing peptide chain A site
binds the tRNA carrying the nextamino acid E site binds the tRNA
that carried the lastamino acid mRNA 3 5 Large subunit Small The
ribosome has two primary functions
Decode the mRNA Form peptide bonds Peptidyl transferase Enzymatic
component of the ribosome Forms peptide bonds between amino acids
Translation In prokaryotes, initiation complex includes
Initiator tRNA charged with N-formylmethionine Small ribosomal
subunit mRNA strand Ribosome binding sequence (RBS) of
mRNApositions small subunit correctly Large subunit now added
Initiator tRNA bound to P site with A siteempty Initiations in
eukaryotes similar except
Initiating amino acid is methionine More complicated initiation
complex Lack of an RBS small subunit binds to 5 cap ofmRNA
Elongation adds amino acids
2nd charged tRNA can bind to empty A site Requires elongation
factor called EF-Tu to bind totRNA and GTP Peptide bond can then
form Addition of successive amino acids occurs as acycle There are
fewer tRNAs than codons
Wobble pairing allows less stringent pairingbetween the 3 base of
the codon and the 5base of the anticodon This allows fewer tRNAs to
accommodate allcodons Termination Elongation continues until the
ribosome encountersa stop codon Stop codons are recognized by
release factorswhich release the polypeptide from the ribosome
Protein targeting In eukaryotes, translation may occur in the
cytoplasm orthe rough endoplasmic reticulum (RER) Signal sequences
at the beginning of the polypeptidesequence bind to the signal
recognition particle (SRP) The signal sequence and SRP are
recognized by RERreceptor proteins Docking holds ribosome to RER
Beginning of the protein-trafficking pathway Copyright The
McGraw-Hill Companies, Inc
Copyright The McGraw-Hill Companies, Inc. Permission required for
reproduction or display. RNA polymerase II RNA polymerase II 1. RNA
polymerase II in the nucleus copies one strand of the DNA to
produce the primary transcript. 3 3 Primary RNA transcript Primary
RNA transcript 2. The primary transcript is processed by addition
of a 5 methyl-G cap, cleavage and polyadenylation of the 3 end, and
removal of introns. The mature mRNA is then exported through
nuclear pores to the cytoplasm. Poly-A tail Poly-A tail 5 5 Primary
RNA transcript Primary RNA transcript Cut intron Cut intron Mature
mRNA Mature mRNA 5 cap 5 cap 3. The 5 cap of the mRNA associates
with the small subunit of the ribosome. The initiator tRNA and
large subunit are added to form an initiation complex. Large
subunit 5 cap mRNA Small subunit Cytoplasm Cytoplasm Empty tRNA
moves into E site and is ejected Lengthening polypeptide chain
Amino acids tRNA arrivesin A site 3 Emptyt RNA 3 3 mRNA A site P
site 5 E site 5 5 4. The ribosome cycle begins with the growing
peptide attached to the tRNA in the P site. The next charged tRNA
binds to the A site with its anticodon complementary to the codon
in the mRNA in this site. 5. Peptide bonds form between the amino
terminus of the next amino acid and the carboxyl terminus of the
growing peptide. This transfers the growing peptide to the tRNA in
the A site, leaving the tRNA in the P site empty. 6. Ribosome
translocation moves the ribosome relative to the mRNA and its bound
tRNAs. This moves the growing chain into the P site, leaving the
empty tRNA in the E site and the A site ready to bind the next
charged tRNA. 42 Mutation: Altered Genes
Point mutations alter a single base Base substitution substitute
one base foranother Silent mutation same amino acid inserted
Missense mutation changes amino acid inserted Transitions
Transversions Nonsense mutations changed to stop codon Nonpolar
(hydrophobic)
Copyright The McGraw-Hill Companies, Inc. Permission required for
reproduction or display. Normal HBB Sequence Normal Deoxygenated
Tetramer Abnormal Deoxygenated Tetramer Polar Leu Thr Pro Glu Glu
Lys Ser Amino acids 1 2 1 2 1 2 1 2 C T G A C T C C T G A G A A G A
A G T C T Nucleotides Hemoglobin tetramer "Sticky" non- polar sites
Abormal HBB Sequence Nonpolar (hydrophobic) Leu Thr Pro val Glu Lys
Ser Amino acids Tetramers form long chains when deoxygenated. This
distorts the normal red blood cell shape into a sickle shape. C T G
A C T C C T G T G A A G A A G T C T Nucleotides Chromosomal
mutations
Change the structure of a chromosome Deletions part of chromosome
is lost Duplication part of chromosome is copied Inversion part of
chromosome in reverse order Translocation part of chromosome is
moved to anew location Mutations are the starting point for
evolution
Too much change, however, is harmful to theindividual with a
greatly altered genome Balance must exist between amount of
newvariation and health of species