dna: the molecule of inheritance mendel’s experiments proved that molecules from the parents were...
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DNA: The Molecule of InheritanceMendel’s experiments proved that
molecules from the parents were transferred to offspring
These molecules could store genetic info, be replicated, and undergo mutations
But what type of molecule? Could it be a lipid, protein, nucleic acid, carbohydrate?
Scientists narrowed it down to 2 possibilities: nucleic acids or proteins
What did they look like? What was their structure?
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Early DNA Experiments: Griffith Inject mice with live R bacteriamice live, no live R cells in
blood Inject mice with live S bacteriamice die, live S cells in blood Inject mice with dead S bacteriamice live, no live S cells in
blood Inject mice with live R bacteria + dead S bacteriamice die,
live S cells in bloodWhat happened in the last experiment?
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Early Experiments: GriffithInterpretation of resultsHeat killed S cells, but not hereditary materialThis material was transferred to R cells
transforming them into S cells Transformation
Permanent change, after 100s of generations transformed R cells still contained the instructions on causing infection
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Early Experiments: Avery (1944)
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Early Experiments: AveryInterpretation of resultsIf add protein digesting enzymes, protein
destroyed, but DNA intactR cells transformed into S cells, mice die
If add DNA digesting enzymes, DNA destroyed, protein intactR cells did not transform, mice live
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Early Experiments: Hershey and Chase (1950s)
• Used T4 bacteriophage-bacterial virus
• Made of a outer protein coat which protects its inner genetic material
• In order to infect its hosts its genetic material must be introduced into the host first
• Was it the protein coat or the inner genetic material that caused infection?
They knew that 1) T2 phages were 50:50 Protein:DNA, 2) Viral reproduction occurs inside bacterial cell
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Early Experiments: Hershey and Chase (1950s)
Radiolabel protein coat with 35S, after T4 infection of bacteria found label outside
Radiolabel inner material with 32P, after T4 infection of bacteria found label inside
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Early Experiments: Hershey and Chase
Interpretation of resultsInfection of bacteria cells is caused by the
transfer of it genetic material to the insideWhichever molecule was found on inside
must be molecule of inheritanceProved beyond any doubt that DNA was the
molecule of inheritanceBut what did it look like?
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Discovery of the Structure of DNADNA consisted of only 4 nucleotides: Adenine,
Thymine, Cytosine, Guanine deoxyribose sugar, phosphate group, base
T, C pyrimidines (one ring)A, G purines ( 2 rings)
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Discovery of the Structure of DNAChargaff’s Laws
(1949): A=T, G=C, DNA was species specific
Structure solved; it was a helix
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Discovery of the Structure of DNA: X-Ray Diffraction (Wilkins, Franklin)
X-ray diffraction of DNA fibers gave a regular repeating pattern of atoms in the DNA
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Discovery of the Structure of DNA: Watson and Crick Experiments (1953)• Using all previous
data from other scientists and experiments, Watson and Crick published a 1 page paper on solving the structure of DNA
• Watson, Crick, Wilkins received Nobel Prize in 1962
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Structure of DNA2 nm in diameterDistance between each base pair= 0.34nmDouble stranded helix, “twisted ladder”A base pairs with T, C base pairs with G, 2 H
bonds for A-T, 3 H bonds for C-GOne strand runs 5’3’ (right side up), the
other strand runs 3’5’ (upside down)
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DNA Replication• Duplication of DNA.
When does it occur?• Semiconservative-part
of the original DNA molecule is conserved during replication.
• 1 DNA molecule2 DNA molecules each consisting of a new and old strand
• New strand is made using old strand as template and base pair rules
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DNA ReplicationWhat would be the complementary strand of
the following DNA sequence? ACGTATACGTGCThe following piece of DNA is to be
replicated. Give the correct daughter strand.
TTACCGGTTC
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DNA Replication and Complementary StrandsACGTATACGTGC originalTGCATATGCACG complementary
TTACCGGTTC originalAATGGCCAAG replicated
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DNA Replication: Enzymes Involved• Topoisomerase and helicase-unwind and
uncoil DNA (break H bonds)• DNA polymerase adds the correct
nucleotide in the 5’-3’ direction only• DNA pol moves in the forward direction on
one strand, moves in the reverse direction on the other strand, however, always moving in the 5’-3’ direction
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DNA Replication
• Replication is continuous on leading strand; discontinuous on the lagging strand
• DNA pol only adds 5’3’
• Lagging strand is composed of Okazaki fragments that must be linked together by ligase
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Prokaryotic vs Eukaryotic Replication• Bacteria have a circular
chromosome replication occurs in 2 directions at the origin of replication
• Replication is fast minutes
• Eukaryotes have long, linear chromosomes replication begins at many locations replication bubbles, replication forks
• Replication is slower hours
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DNA Repair• What happens if the incorrect base is added
and a mismatch base pair occurs? Ex. A-G or C-T
• DNA Pol has a proofreading function• It will cut out the incorrect base and put back
the correct base• Ligase comes in and repairs the “cut” in the
DNA • Results in a very low error rate 1 out of
100 million base pairs
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From DNA to ProteinsDNA is the genetic instructions for
life, but DNA itself does not do work to sustain life.
Which type of molecule is responsible for all of a cell’s processes?
Flow of genetic information instructions translator worker DNA RNA
Proteins transcription translation nucleus cytoplasmWhere does transcription take place
in a prokaryotic cell?
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Comparison of DNA to RNADNADouble strandedDeoxyribose sugarBases A, T, C, GA-T, C-G base pairs1 type/function Very stable over time
RNASingle strandedRibose sugarBases A, Uracil, C, GA-U, C-G base pairs3 types/functionsUnstable, easily degrades
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Types of RNAsMessenger RNA (mRNA)-carries the protein
building instructions: 1 gene=1 mRNA=1 proteinRibosomal RNA (rRNA)-this RNA along with
other proteins make up ribosomes (site of protein synthesis)
Transfer RNA (tRNA)-brings correct amino acid to the ribosome; binds to mRNA sequence
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DNARNAAmino Acids (proteins)
How many different nucleotides are there?
How many different amino acids are there?
How would the nucleotides specify each of the amino acids?
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The Genetic Code Solved!There are only 4 nucleotides to specify 20
different amino acidsIf use 1 nucleotide per amino acid, only can
specify 4 Using 2 nucleotides, only can specify 16If use 3 nucleotides per amino acid, can specify
64 amino acids3 nucleotides = 1 codon, 1 codon per amino
acid
41
42
43
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The Genetic CodeThe code is universalThe code is
degenerative most amino acids have more than 1 codon. Why?
Each codon has only 1 meaning, CCA is different than CUA, and can specify a different amino acid
Has 1 start signal and 3 stop signals
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Transcription: DNARNA
• Initiation of Transcription• RNA pol binds to specific
DNA sequence called a promoter which is usually at the beginning of a gene
• DNA unwinds using enzymes
• RNA pol adds nucleotides in 5’3’ direction: A-U, C-G
• RNA pol falls off at end of gene, releasing mRNA transcript
• Many RNA pols work simultaneously to produce mRNAs
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RNA Transcript Processing and Modification
• Newly synthesized transcripts are not ready to make a protein
• 3 Modifications are made:
• Introns are cut out by “Splicing”
• Genes exons and introns
• Exons coding sequences that make a protein
• Introns noncoding (junk) sequences
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RNA Transcript Processing and Modification5’ cap is addedProvides an docking area for the ribosome to
bind during translationPrevents degradation 3’ Poly A (adenine) Tail is added100-200 As added to prevent degradation by
enzymes
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Similarities Between Replication and Transcription
Occur in the nucleusUse DNA as template to build new strandsAdd nucleotides in the 5’3’ directionAdd nucleotides according to base pair rules:
A-T,U or C-GBoth use helicase and topoisomerase to
unwind DNA
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Differences Between Replication and Transcription
ReplicationDNADNA copyCopy all of the DNACopy is double
strandedUse DNA polUse A-T, C-G
TranscriptionDNARNA copyCopy only part of
DNACopy is single
strandedUse RNA polUse A-U, C-G
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Transcription: DNARNAThe following sequence is to be transcribed:
AATCGGTCGATGG
What is the sequence of the transcript?
AATCGGTCGATGG DNA UUAGCCAGCUACC RNA
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Translation: RNAProteinsOccurs in the
cytoplasm, inside of ribosomes (attached to ER)
Requires a mRNA with a start/stop codon
Requires a tRNARequires amino
acids
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Translation: RNAProteinsRole of tRNA
• tRNA-2 Binding sites• mRNA binding site
called the anticodon
• UAA CGC AAC mRNA• AUU GCG UUG tRNA
• amino acid binding site, to bring the correct amino acid to growing polypeptide chain
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Translation: Role of Ribosomes
• Site of protein synthesis• Composed of large and
small subunits• During translation:• Small subunit binds mRNA,
then combines with large subunit to form intact ribosome (initiation)
• When small and large subunits combine, form E site (exit), P site (peptide), and an A site (amino acid)
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Translation
Initiation: initiator tRNA + small ribosome unit, mRNA + large ribosome unit
Elongation: initiator tRNA (start codon) P site
Where does the 2nd tRNA add?
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Translation: RNAProteins2nd tRNA A sitepeptide bond 1st
+ 2nd aaInitiator tRNA
released E site 2nd tRNA P site3rd tRNA A site
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Translation: ElongationPeptide bond 2nd +
3rd aaThis is repeated
many times until a stop codon is reached
Proteins can have as little as 30 aa or up to 1000 aa
12-17 aa are added every second!!!!
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Translation• Termination: stop codon
is reached, mRNA released, and ribosome subunits separate, polypeptide chain is released
• What happens to the newly synthesized protein?
• Golgi for processing and shipping by exocytosis
• Used in the cell it was made
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Reading the Genetic Code• There are 64 codons (61
specify amino acids, 3 are stops)
• Some amino acids have more than one codon, ex. arginine, leucine
• For codon GAC, first find the first base on code, second, and finally third; where all lines intersect that is the amino acid specified. GAC = aspartate
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Genetic Code ProblemWhat would be the amino acid sequence
specified by the following DNA sequence?
TAC GCT ATA CCC ATT
How many amino acids would be made?
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Genetic Code Problem
TAC GCT ATA CCC ATT DNAAUG CGA UAU GGG UAA RNAstart-arginine-tyrosine-glycine-stop amino acid
(methionine)
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Structure of Eukaryotic Chromosomes
Chromosomes (made of DNA) have proteins that help tightly package DNA in the nucleus Histones
They arrange the DNA around “beads” called nucleosomes
heterochromatin