Biol321 Molecular Genetics

Lecture 1

Transcription generates two units

PlacI

LacI

Plac O

DNALacZ LacY LacA

mRNAUnit 1 Unit 2

Repressor -Galactosidase Permease Trans-acetylaseMolecular

Genetics&Genomics

CT CTNT NT recA lexA

CT CTNT NT SOS genes

CT CTNT NT

CTNT

OFFlow expression

OFFlow expression

OFFlow expression

Dr. Fahd M. Nasr

Sixth EditionSeptember 2006

Department of Biology Faculty of SciencesLebanese University Hadath-Beirut Lebanon

References• Peter J. Russell. Igenetics: A Molecular Approach. 2013,

Pearson Education Limited, ISBN-13:9781292026336.

• James D. Watson. Molecular Biology of the Gene. 2013, Benjamin-Cummings Publishing Company, ISBN-13:9780321762436.

• Terry Brown. Introduction to Genetics: A Molecular Approach. 2011, Garland Science, ISBN:9780815365099.

• Benjamin A. Pierce. Genetics: a conceptual approach. 2012, W. H. Freeman & Company, ISBN-13: 978-1-4292-3250-0.

http://yeastwonderfulworld.wordpress.com/

What is the nature of the genetic material?

• Postulated by scientists to have three main properties:

– Stability

– Accuracy of replication

– Capacity for variation

What are the genes at the molecular level: Beadle and Tatum Experiment?

Wild type

Mutant

Minimal medium Minimal medium + A

+ +No +

Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis

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

"Transforming principle"

Griffith’s transformation Experiment

AveryAvery-- McCartyMcCarty-- Macleod experiment 1944Macleod experiment 1944

Attachment

2

1

5

43

Entry of phageDNA anddegradation of host DNA

Release

Synthesis ofviral genomesand proteins

Assembly

Phage assembly

Head Tail Tailfibers

Hershey-Chase experiment

• Using radioactive labels these researchers tagged either the DNA or protein population

– P32Why does thistag only DNA?

– S35Why does thistag only proteins?

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)

Activity: Hersey Chase Experiment

The helical structure of tobacco mosaic virus

Necrosis Necrosis

Chlorosis StuntingStunting

Mosaic disease: significant losses in yield & $ value

RNA is the genetic material TMV

The chemical composition of DNA and RNA

Pentose sugar Deoxyribose in DNA Ribose in RNA

The chemical composition of DNA and RNA

The major ribonucleotides

The major deoxyribonucleotides

Chargaff’s rules: Base Composition studies

X-Ray Crystallography Revealed the Basic Helical Structure of the DNA Molecule

Molecular Structure of DNA

1962: Nobel Prize in Physiology and Medicine

James D.Watson

Francis H.Crick

Maurice H. F.Wilkins

What about?Rosalind Franklin

The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The first step in replication is separation of the two DNA strands.

Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

The nucleotides are connected to form the sugar-phosphate back-bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

New strand5 end

Phosphate BaseSugar

Template strand3 end 5 end 3 end

5 end

3 end

5 end

3 end

Nucleosidetriphosphate

DNA polymerase

Pyrophosphate

Features of different conformations of the DNA double helix