Ch 5 and16 A Close Look at the Hereditary Molecules

• Protein sequence-->programmed by genes

• Genes are made of DNA, a nucleic acid

LE 5-25

NUCLEUS

DNA

CYTOPLASM

mRNA

mRNA

Ribosome

Aminoacids

Synthesis ofmRNA in the nucleus

Movement ofmRNA into cytoplasmvia nuclear pore

Synthesis of protein

Polypeptide

DNA

RNA

Protein

Flow of genetic information

The Roles of Nucleic Acids

• Two types:– Deoxyribonucleic acid (DNA)– Ribonucleic acid (RNA)

• DNA provides directions for its own replication.

• DNA directs synthesis of messenger RNA (mRNA)

• mRNA controls protein synthesis.

• Protein synthesis occurs on ribosomes.

LE 5-26a5 end

3 end

Nucleoside

Nitrogenousbase

Phosphategroup

Nucleotide

Polynucleotide, ornucleic acid

Pentosesugar

Nucleic acid building block

Nucleic Acid Structure

Monomers

nucleotide (3 parts)

1. nitrogenous base

2. 5 C sugar

3. Phosphate

Polymerpolynucleotide or nucleic acid

nucleoside

LE 5-26b

Nitrogenous bases

Pyrimidines

Purines

Pentose sugars

CytosineC

Thymine (in DNA)T

Uracil (in RNA)U

AdenineA

GuanineG

Deoxyribose (in DNA)

Nucleoside components

Ribose (in RNA)

Important Nucleic Acid Distinctions

Pyrimidines-one ring (T,U,C)

Purines- two rings (G,A)

DNA the sugar = deoxyribose

NO 2’ OH (hydroxyl)

Two kinds of bases

RNA the sugar= ribose

YES 2’ OH

Nucleotide Polymers

• Nucleotides (nt) connect through phosphodiester bond

5’ Phosphate--> 3’OH

• Creation of a sugar-phosphate backbone with bases as appendages.

• Sequence of bases along DNA or mRNA polymer unique for each gene.

LE 16-7

5 end

3 end

5 end

3 end

Space-filling modelPartial chemical structure

Hydrogen bond

Key features of DNA structure

0.34 nm

3.4 nm

1 nm

Two DNA strands bind togetherthrough complementary base-pairing.

FrancisCrick

JamesWatson

Structure of DNA double helix: published in 1953

Watson JD, Crick FHC. 1953. Molecular structure of nucleic acids: a structure for deoxyribonucleic acids. Nature 171:738.

LE 16-6

Franklin’s X-ray diffractionphotograph of DNA

Rosalind Franklin

Partly based on Franklin’s x-ray diffraction data

LE 16-8

Chargaff’s rules (1940s):

Amount of A=T

G=C

LE 16-UN298

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: widthconsistent with X-ray data

Watson & Crick: built model of DNA and tested possible combinations of bases

Did model support Chargaff’s observations and Franklin’s x-ray diffraction data?

LE 16-7

5 end

3 end

5 end

3 end

Space-filling modelPartial chemical structure

Hydrogen bond

Key features of DNA structure

0.34 nm

3.4 nm

1 nm

Antiparallel DNA strands

Two DNA strands bind togetherthrough complementary base-pairing.

The DNA Double Helix

• Two polynucleotides (strands) base-paired together GC, AT (complementary base-pairing)

• Double helix

• Two sugar-phosphate backbones run in opposite 5´ to 3´ directions - antiparallel

• One DNA molecule includes many genes

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

Sugar

Sugar

Sugar

Sugar

Complementary base pairs

G=C3 H-bonds

A=T

2 H-bonds

Behavior of DNA

Draw a 10 base pair double-stranded DNA (dsDNA) that is rich in AT.

Draw a 10 base pair double-stranded DNA (dsDNA) that is rich in GC.

If these were placed in a tube of boiling water what would happen?

DNA would become single stranded (ssDNA) (denatured or melted).

Which DNA would denature first. Why?

AT rich fragment less stable2 H-bonds/bp versus 3 H-bonds/bp

DNA Used as Evolutionary Ruler

• Linear sequences of DNA in chromosomes – passed from parents to offspring

• Two closely related species are more similar in DNA sequence than distantly related species

• Similarity of DNA sequence– Determines evolutionary relatedness

5’ GAACCTTCCAATTGATCT3’

5’ GAACCAACCAATTAAACT3’ 5’ GAACCTTCGAATTGATCT3’

1. Compare the human sequence to the frog and mouse. Which sequence is most similar to human?

human

mousefrog

2. Write in the complementary strand for each.

Earlier data suggested that DNA was hereditary material

Model system: Drosophila melanogasterInvestigator: Thomas Hunt Morgan (early 1900’s)Evidence: white eye phenotype associated with X-chromosome

Model system: bacteria and viruses

Investigators: Many

Evidence: various

Evidence That DNA Can Transform Bacteria

Evidence for genetic role of DNA (Frederick Griffith,1928)

Heat-killed pathogenic “S” Streptococcus pneumoniae+

“R”non-pathogenic bacterial strain

Some living bacteria became pathogenicTransformation of “R’ to ‘S”,

How could one determine pathogenicity experimentally?

LE 16-2

Living S cells(control)

Living R cells(control)

Heat-killedS cells (control)

Mixture of heat-killedS cells and livingR cells

Mouse dies

Living S cellsare found in blood sample

Mouse healthy Mouse healthy Mouse dies

RESULTS

• Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944)• Published results

– Showed DNA from bacteria NOT protein--> caused transformation of “R” to “S”

What molecule was responsible for conferring a new phenotypeinto an organism?

• Alfred Hershey and Martha Chase (1952)

– Used bacterial virus (bacteriophage) (T2) to ask whether DNA or protein was hereditary material

Independent confirmation

LE 16-3

Bacterialcell

Phagehead

Tail

Tail fiber

DNA

100

nm

LE 16-4

Bacterial cell

Phage

DNA

Radioactiveprotein

Emptyprotein shell

PhageDNA

Radioactivity(phage protein)in liquid

Batch 1:Sulfur (35S)

RadioactiveDNA

Centrifuge

Pellet (bacterialcells and contents)

PelletRadioactivity(phage DNA)in pellet

Centrifuge

Batch 2:Phosphorus (32P)

Hershey & Chase labeling experiment

Protein radiolabelled

DNA radiolabelled

Phage produced in and released from bacteriawith radioactive DNA.

Hershey & Chase results

-Suggest that DNA, not protein, is transferred to bacteria by phage.

-DNA programs the reproduction of more phage.

Contains important genetic instructions.

I’m a pretty cool molecule butI’ll still answer yourquestions.