Lecture 14 - DNA Structure and Analysis

What is a gene?

Fundamental units of inheritance in living organisms; together, they hold all the information necessary to reproduce a given organism and to pass on genetic traits to its offspring

Mendel’s Experiment

• Some traits such as height or flower color do not appear blended in their offspring

• traits are passed on as distinct, discrete entities

Thomas Hunt Morgan

genes are arranged linearly, and their ability to cross-over is proportional to the distance that separated them.

Frederick Griffith’s Experiments

• showed that a pneumonia strain could transform from one strain into a different strain

• first widely accepted demonstrations of bacterial transformation, whereby a bacterium distinctly changes its form and function

• critical experiment: Living IIR and heat-killed III S; inject mouse, mouse dies, tissue is analyzed. Living III s was recovered.

Barbara McClintock

Genetic linkage correspond to physical locations on chromosomes

Beadle and Tatum

Mutations in genes could cause defects in steps in metabolic pathways. This was states as “one gene, one enzyme” which is “one gene, one polypeptide” in modern genetics

Avery, McLeod, and McCarty

Test tube assay (not mice); experiment strongly implied that DNA is the transforming factor and not proteins or other materials

• experiment: III S cells in liquid culture medium, centrifuge, heat-kill and homogenize, after treated with deoxyribonuclease (kills DNA), no transformation occurred (compared to other tests, ribonuclease and protease treatments resulted in transformation)

Hershey-Chase Experiment

• helped confirm that DNA was the genetic material

• showed that when bacteriophages infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not

• Experiment: T2 phages are half DNA, half protein. marked DNA with phosphorus, protein with sulfur. Saw that when phages entered the nucleus, phosphorus was present not sulfur.

DNA Structure

Nucleotides are the building blocks of DNA

1. nitrogenous base

2. pentose sugar

3. phosphate group

Pyrimidines

cut the pie (PY)

CUT

• cytosine

• uracil

• thymine

Purines

1. Adenosine

2. Guanine

Erwin Chargraff

Showed that the amount of A is proportional to T, same for G and C

King’s College Group

Sir. John Randall, Raymond Gosling, Maurice Wilkins, Rosalind Franklin

• X-Ray Crystallography

• proved Helical structure

Caltech Group

Peter Pauling, Linus Pauling

Cambridge Group

Francis Crick, Jim Watson, Max Perutz

3 Groups looking for the DNA structure

Cambridge, King’s College, Caltech

Watson and Crick

Figured out all of the details

• DNA is a right-handed double-helix

• 2 strands run antiparallel

• bases are stacked on one another

• two strands connect A-T and G-C

• 10 bp per helix turn

3D Model Significance

• explained how DNA could function as the molecule of heredity

• base pairing explained how genetic information could be copied

• explained how occasional errors in replication could lead to a mutation in one of the daughter copies of the DNA molecule

Structure of polynucleotide chain

nucleotides are linked by a phosphodiester bond between the phosphate group at the C-5’ position and the OH group on the C-3’ position

Base pair function

• A-T and G-C base pairing provides complentarity of the two strands and chemical stability to the helix

• A-T base pairs form two hydrogen bonds, G-C base pairs from three hydrogen bonds

Fiers

First gene is sequenced and cloned

Human Genome Project

The human genome is sequenced

ENCODE Project

Identification of all functional elements in the human genome

What is a gene? Updated

• genes replaced with open reading frame (ORF) sequence patters

• defined by their predicted sequence rather than the genetic loc irresponsible for phenotypes

• identification of most genes in sequenced genomes is based on similarity to known genes, or statistically significant signature of a protein-coding sequence