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?

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?

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

Early Experiments: Avery (1944)

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

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

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

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?

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)

Discovery of the Structure of DNAChargaff’s Laws

(1949): A=T, G=C, DNA was species specific

Structure solved; it was a helix

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

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

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)

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

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

DNA Replication and Complementary StrandsACGTATACGTGC originalTGCATATGCACG complementary

TTACCGGTTC originalAATGGCCAAG replicated

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

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

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

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

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?

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

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

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?

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

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

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

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

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

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

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

Transcription: DNARNAThe following sequence is to be transcribed:

AATCGGTCGATGG

What is the sequence of the transcript?

AATCGGTCGATGG DNA UUAGCCAGCUACC RNA

Translation: RNAProteinsOccurs in the

cytoplasm, inside of ribosomes (attached to ER)

Requires a mRNA with a start/stop codon

Requires a tRNARequires amino

acids

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

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)

Translation

Initiation: initiator tRNA + small ribosome unit, mRNA + large ribosome unit

Elongation: initiator tRNA (start codon) P site

Where does the 2nd tRNA add?

Translation: RNAProteins2nd tRNA A sitepeptide bond 1st

+ 2nd aaInitiator tRNA

released E site 2nd tRNA P site3rd tRNA A site

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!!!!

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

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

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?

Genetic Code Problem

TAC GCT ATA CCC ATT DNAAUG CGA UAU GGG UAA RNAstart-arginine-tyrosine-glycine-stop amino acid

(methionine)

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