A Brief History of Genetics 1 2 Early Concepts of Heredity Ancient Greeks – believed in “pangenesis”, in which miniature versions of body parts were.

A Brief History of Genetics

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Early Concepts of Heredity Ancient Greeks –

believed in “pangenesis”, in which miniature versions of body parts were transmitted during sexual reproduction;

Darwin - despite his evolutionary theories believed in a modified sort of pangenesis, via “gemmules”

1865 Gregor Mendel–studied inheritance of seven traits in pea plants and first used the terms dominant and recessive.

Laws of independent assortment and segregation are based on his work.

Proposed similar but separate inheritable characters, one from each parent, later to be called genes.

1869

Fredrich Miescher Successfully isolated nuclein

from pus cells obtained from discarded bandages.

He noticed that nuclein was acidic, contained a lot of phophorus and nitrogen and was found in the nucleus of cells.

Nuclein eventually became known as DNA

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Theodor Boveri – 1888-1890

Investigated chromosomes Found that:

Chromosomes are organized and individual structures throughout the process of cell division.

Sperm and egg contribute the same number of chromosomes.

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1900 Gregor Mendel

Mendel’s work is rediscovered by three scientists (one German, one Dutch and one Austrian) Hugo de Vries, Carl Correns, and Erich von Tsernak

16 years after his death, and 34 years after publication, Mendel’s work is finally recognized.

The first 20 years of the 20th century build upon the rediscovered experiments of Mendel

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1902 Walter Sutton

The Boveri-Sutton Chromosome Theory:

suggested that chromosomes are paired and may be the carriers of heredity

suggested that Mendel’s "factors" are located on chromosomes

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1909 Wilhelm Johannsen

First used the term gene not knowing quite what it was, just wanting a distinction from Darwin’s “gemmules”.

“The word gene…is completely free from any hypotheses, it expresses only the evident fact that in any case, many characteristics of the organism are specified in the gamete by means of special conditions and conditions and determine which are presented in unique separate, and thereby independent ways—in short precisely what we wish to call genes.” 7

1910

Thomas Hunt Morgan Morgan proved that

genes are carried on chromosomes.

He also demonstrated the existence of sex-linked genes.

His work eventually lead to the to a fundamental understanding of the mechanisms of heredity

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The Nobel Prize in Physiology of Medicine 1933

Morgan's experimental and theoretical work inaugurated research in genetics and promoted a revolution in biology.

Evidence he gathered from embryology and cell theory pointed the way toward a synthesis of genetics with evolutionary theory.

Morgan himself explored aspects of these developments in later work, including Evolution and Genetics published in 1925, and The Theory of the Gene in 1926.

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T. H. Morgan’s Fruit Flies 1907-1930s

1928

Fred Griffith Studied different strains of

Streptococcus pneumoniae, the bacteria that causes pneumonia.

This bacteria come in two strains: S and R

the S-form has a capsule and looks smooth under the microscope. It is virulent and kills infected mice because the immune system cannot break through the cell wall of the S bacterium.

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1928

Fred Griffith

The R-form of the bacterium doesn't have a capsule and appears rough.

The immune system is able to destroy the cell wall of the R bacterium. This makes the R form non-virulent. The mice infected by this form of the bacteria will survive.

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Griffith's Experiments

Griffith injected mice with the S form of bacteria and all the mice died from pneumonia.

He injected mice with the R form of bacteria and the mice survived the infection.

He then killed the S form by exposing them to high temperatures. Mice injected with these heat-killed bacteria survived with no ill effects.

He mixed his heat-killed, disease-causing bacteria with live, harmless ones and injected the mixture into mice.

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Griffith's Experiments He was expecting the mice to survive

because both strains were harmless. But, the mice died from pneumonia!!!

And he found living S cells in the mice! Somehow the heat-killed S strain passed their ability to cause disease to the live R strain.

Griffith called this Transformation one strain of bacteria had been changed

into another. Some factor was transferred from heat-killed cell into the live cells. He also found that the change was permanent.

He hypothesized that factor could be a gene that could change the properties of bacteria. 13

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1928Frederick Griffith

Transformation of Bacteria

1929 Phoebus Levene

He identified the deoxyribose sugar in “thymus” nucleic acid and the ribose sugar in “yeast”nucleic acid

Also identified the nitrogenous bases: adenine, guanine, cytosine and thymine in “thymus” nucleic acid and uracil in “yeast” nucleic acid.

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What Could the Genetic Material Be?

Griffith had shown that “something” was passing from one type of bacteria to another.

This “something” could be Parts of the capsule Proteins Nucleic acids

Whatever “it” was, it had to be Duplicated when the cell divided Stable and resistant to heat

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George Beadle and Edward Tatum1941 (Nobel Prize 1958)

One gene-one enzyme hypothesis

1944

Avery, MacLeod and McCarty

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Avery and his colleagues decided to expand on Griffith’s experiments to try to identify

the “transforming” material.

Separated the Components

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Ruptured heat killed S strain bacteria to release their contents. Separated and purified the RNA, DNA, proteins and the

polysaccharide capsules from the bacteria into separate factions. Mixed each faction with live R bacterial cells and injected them into mice.

R-bacteria + RNA from S-bacteria = live mouse R-bacteria + proteins from S-bacteria = live mouse R-bacteria + polysaccharides from S-bacteria = live mouse

Only R cells were found in their blood.

Separated the Components

Ruptured heat killed S strain bacteria to release their contents.

Separated and purified the RNA, DNA, proteins and the polysaccharide capsules from the bacteria into separate factions.

Mixed each faction with live R bacterial cells and injected them into mice.R-bacteria + RNA from S-bacteria = live mouseR-bacteria + proteins from S-bacteria = live mouseR-bacteria + polysaccharides from S-bacteria =

live mouseOnly R cells were found in their blood.R bacteria + DNA from S-bacteria = DEAD

MOUSE!!!They concluded that DNA was the hereditary

material

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Despite the fact that the Transforming substance had to be resistant to heat, and proteins are inactivated by heat…

Most scientists thought that proteins were the hereditary material because they were more complex and varied than nucleic acids.

Scientists were generally skeptical, believing DNA to be too simple a molecule to contain all the genetic information for an organism.

1950Erwin Chargaff

Discovered that in DNA, the amount of Adenine is equal to the amount of Thymine and the amount of Guanine is equal to the amount of Cytosine.

Chargaff’s Rule:

A=T G=C

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1952

Alfred Hershey and Martha Chase

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1952 Alfred Hershey and Martha

Chase

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DNA IS NOW ACCEPTED AS THE GENETIC MATERIAL!!!

And the race is on to determine the structure of DNA

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1951-1953

Rosalind Franklin Franklin was responsible for much

of the research and discovery work that led to the understanding of the structure of deoxyribonucleic acid, DNA.

Headed a X-Ray crystallography unit at King’s College in London

Did extensive work to elucidate structure of DNA

Franklin resisted model building unlike Pauling and Watson and Crick

1951-1953

Rosalind Franklin Franklin made marked

advances in x-ray diffraction techniques with DNA.

She extracted finer DNA fibers than ever before and arranged them in parallel bundles.

She studied the DNA fibers' reactions to humid conditions.

All of these allowed her to discover crucial keys to DNA's structure.

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1951-1953

Rosalind Franklin She recognized that two

states of the DNA molecule existed (A and B) and defined conditions for the transition.

From early on, she realized that any correct model must have the phosphate groups on the outside of the molecule.

After the formation of the Watson Crick model she demonstrated that a double helix was consistent with the X-ray patterns of both the A and B forms.

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Linus Pauling 1953

Prominent American biochemist who wrote ”The Nature of the Chemical Bond”;

Elucidated the structure of amino acids and proteins, and associated helical structures;

Proposed a 3-chain helical structure for DNA, but with sugar-phosphate groups on the inside;

Evelynn Fox Keller (2000) notes that had Pauling used all available literature, his model might have been correct; Watson and Crick are sure that he was on to it.

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1953

James Watson and Francis Crick

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Watson and Crick’s approach was to make physical models to narrow down the possibilities and eventually create an accurate picture of the molecule.

After Wilkins showed them pictures of Franklin’s x-ray diffraction work, Watson and Crick took a huge conceptual step. They suggested that the DNA molecule was made of two chains of nucleotides, each in a helix (as Franklin had found) but one going up and the other going down.

1953

James Watson and Francis Crick

Crick had just learned of Chargaff's findings about base pairs. He added that to the model, so that matching base pairs interlocked in the middle of the double helix to keep the distance between the chains constant. This was consistent with Franklin’s measurements.

The key to Watson and Crick's discovery was the realization that because of its size, shape and chemical makeup, each base on one side of the ladder could pair by hydrogen bonds with only one other base on the other complementary side of ladder going down. 33

1953

James Watson and Francis Crick

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Specifically, the large adenine molecule could pair with only the smaller thymine and the large guanine molecule could pair with only the smaller cytosine.

The Nobel Prize in Physiology or Medicine 1962

"for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material“

1957Arthur Kornberg

Discovers and isolates DNA polymerase, which becomes the first enzyme used to make DNA in a test tube.

Also proves that the strands are anti-parallel and that replication proceeds only in one direction (5’to3’)

a template.35

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DNA Replication The Watson-Crick model of DNA clearly provided a mechanism for

replication of DNA. Each strand could act as a template for the formation of new strands.

Three models were suggested for the mechanism of replication:

1958Meselson and Stahl

Their experiment determined the mechanism of DNA replication

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1957 Matthew Meselson and Edward Stahl

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First, they grew bacteria for many generations in a growth medium containing 15N. This is a heavy isotope of nitrogen (in contrast to the normal isotope 14 N).

Over many generations, 15N would be incorporated into all nitrogen-containing molecules of the cells, including DNA. DNA isolated from these cells could be distinguished from normal DNA because it would have a higher density.

The bacteria grown in heavy nitrogen were then transferred to growth medium containing 14 N for one round of replication. This lighter isotope would incorporate into any newly synthesized DNA.

1957 Matthew Meselson and Edward Stahl

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If semi-conservative replication occurred, then each DNA molecule after replication would contain heavy nitrogen and light nitrogen, and would therefore have a density intermediate between the two.

Conservative replication would produce one DNA molecule containing heavy nitrogen and one molecule containing light nitrogen, so there would be two different densities.

Dispersive replication would produce a single intermediate density, just like semi-conservative.

1957 Matthew Meselson and Edward Stahl

After one generation:

Replication was therefore either semi-conservative or dispersive. These possibilities could be distinguished after a second round of replication.

After two rounds, semi-conservative replication would produce two DNA molecules containing only light nitrogen, and two DNA molecules containing one light strand and one heavy strand. Therefore there would be two different densities: light and intermediate.

Two rounds of dispersive replication would produce four DNA molecules, each of which would contain mostly light nitrogen and some heavy nitrogen.

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When density of the DNA was measured after two rounds, two densities were observed: light and intermediate, indicating that DNA replication is semi-conservative, and not dispersive or conservative.

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1957Matthew Meselson and Edward Stahl

The 1970’s This decade is when human beings began to systematically control, manipulate

and exploit DNA technology. It marked the beginning of recombinant DNA technology, gene splicing and the

first biotechnology company, Genentech.

1970. Isolation of "reverse transcriptase," a restriction enzyme that cuts DNA molecules at specific sites. This allows scientists to create clones and observe their function.

1972 - creation of the first recombinant DNA molecule. - The first successful DNA cloning experiment was performed in California.

1973 - Scientists successfully transferred DNA from one life form to another, creating the first recombinant DNA organism.

1976 – Herberg Boyer and Robert Swanson found Genentech Inc., the first biotechnology company dedicated to developing and marketing products based on genetic engineering technology.

1977- a man-made gene was used to manufacture a human protein in bacteria for the first time.

1978- successful production of human insulin using recombinant DNA technology 44

The 1980’s

A combination of the computer revolution and more easily available enzymes led to the creation of several new technologies like the polymerase chain reaction (PCR) and automated gene sequencers. These in turn would make the job of mapping the entire human genome possible

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1990 to Now…

Human clones are created in Petri dishes, genes are dissected, the infant field of gene therapy begins and the first mammalian living clone is created

2001 – The complete map of the human genome is published.

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