Genetics in ~1920: 1. Cells have chromosomes Sketch of Drosophila chromosomes (Bridges, C. 1913)

Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes Genetics in ~1920: 1. Cells have chromosomes 2. Specific locations on chromosomes control different phenotypes Sketch of Drosophila chromosomes (Bridges, C. 1913)

Protein DNA Chromosomes are made of: Genetics in ~1920: Which one is the genetic material?

DNA: consists of nucleotides The structure of a nucleotide 1’ 2’ 3’ 4’ 5’

DNA: consists of nucleotides (4 types) The structure of a nucleotide Purines Pyrimidines 1’ 2’ 3’ 4’ 5’

Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Cytosine (C) Guanine (G) DNA nucleotide Sugar (deoxyribose) 3 end Phosphate DNA is a polymer -Nucleotides are joined together by linking 5’ and 3’ carbons of sugar groups to phosphate groups - A chain of nucleotides has a 5’ end and a 3’ end

Amino group Carboxyl group Protein: Consists of amino acids

Nonpolar Glycine (Gly or G) Alanine (Ala or A) Valine (Val or V) Leucine (Leu or L) Isoleucine (Ile or I ) Methionine (Met or M) Phenylalanine (Phe or F) Trypotphan (Trp or W) Proline (Pro or P) Polar Serine (Ser or S) Threonine (Thr or T) Cysteine (Cys or C) Tyrosine (Tyr or Y) Asparagine (Asn or N) Glutamine (Gln or Q) Electrically charged AcidicBasic Aspartic acid (Asp or D) Glutamic acid (Glu or E) Lysine (Lys or K) Arginine (Arg or R) Histidine (His or H) Amino group Carboxyl group Protein: Consists of amino acids (20 different types)

Peptide bond Fig Amino end (N-terminus) Peptide bond Side chains Backbone Carboxyl end (C-terminus) (a) (b) A polypeptide

Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Living S cells RESULTS EXPERIMENT Griffith (1928) Transformation of bacteria

Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Living S cells RESULTS EXPERIMENT Griffith (1928) Transformation of bacteria

Avery (1944) DNA is the transforming material How could he have shown this?

Bacterial cell Phage head Tail sheath Tail fiber DNA 100 nm Hershey + Chase (1952) T4 infection of E. coli

EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Hershey + Chase (1952) T4 infection of E. coli

EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Hershey + Chase (1952) T4 infection of E. coli

EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet Hershey + Chase (1952) T4 infection of E. coli

EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet Hershey + Chase (1952) T4 infection of E. coli

02_UnTable01.jpg Chargaff (1949)

Rosalind FranklinWatson and Crick 1953: The double helix

Watson and Crick- 1953

Key aspects of the Watson-Crick model - The 2 strands are in shape of a double helix base pairs per turn of the helix Hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3 end 5 end (b) Partial chemical structure (a) Key features of DNA structure 1 nm

Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Cytosine (C) Guanine (G) DNA nucleotide Sugar (deoxyribose) 3 end Phosphate Data used to deduce double helix: 1)Chemical structure of DNA polymer 2)Chargaff’s rules 3)Franklin’s X-ray diffraction data

(b) Franklin’s X-ray diffraction photograph of DNA This told them: -2 anti-parallel DNA strands -Helical shape -Width, period of helix

Key aspects of the Watson-Crick model -2 anti-parallel strands of DNA -Sugar-phosphate backbone on outside, bases on inside -Bases form pairs through hydrogen bonding Hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3 end 5 end (b) Partial chemical structure (a) Key features of DNA structure 1 nm

Cytosine (C) Adenine (A)Thymine (T) Guanine (G) Base pairing PurinePyrimidine

Why A and C can’t base pair:

Fig. 16-UN1 Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data

Watson and Crick “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

Fig A T G C TA TA G C A T G C T A T A G C (a) Parent molecule (b) Separation of strands

Fig A T G C TA TA G C (a) Parent molecule AT GC T A T A GC (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands A T G C TA TA G C A T G C T A T A G C

Fig Parent cell First replication Second replication (a) Conservative model (b) Semiconserva- tive model (c) Dispersive model

Fig a EXPERIMENT RESULTS Bacteria cultured in medium containing 15 N Bacteria transferred to medium containing 14 N DNA sample centrifuged after 20 min (after first application) DNA sample centrifuged after 20 min (after second replication) Less dense More dense

Fig b CONCLUSION First replicationSecond replication Conservative model Semiconservative model Dispersive model