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1

Gregor Mendel (1866)

Heredity is the transmission of characteristic from parent to offspring by means of genes

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Mendel's Laws of Inheritance

The law of segregation
-During the formation of gametes, the paired heredity of heredity determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair.

The law of independent assortment
-Segregation of the members of any pair of heredity determinants is independent of the segregation of the other pairs in the formation of gametes.

The law of dominance
-For each physical trait, one member of any pair of heredity determinants is dominant so that the physical trait that it specifies appears in a 3:1 ratio. The alternative form is recessive.

3

DeVries, Correns and Tshermak (independently)

-Repeated Mendel's work around 1900. They established a distinction between genotype and phenotype.
-The realised 'factors' now known as heredity determinants were transmitted from generation to generation and influence certain traits.

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What properties does genetic material have?

-Replication of information
-Storage of information
-Expression of information
-Variation by mutation

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Miescher (1869)

Experiment: Studied the chemical composition of the cell nucleus of white blood cells. He chose these as they could be isolated in abundance from pus-covered bandages from the hospital.

Findings:
-He found a substance he called "nuclein" (DNA).
-He found that it could not be a protein, because it did not have properties of proteins (it was not digested by pepsin protease, it does not contain any sulphur).
-He found that it did contain hydrogen, carbon, oxygen and phosphorus.
-He did not know anything about its function.

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Flemming (1879)

Organism: Salamander cells (they have large chromosomes).

Experiment: He developed novel microscopic techniques to observe fixed, stained cells (new staining techniques).

Findings:
-He discovered chromatin; fibrous structure in the nucleus, that's easily stained.
-He postulated that chromatin transforms into thread-like chromosomes at mitotic division.
-He correctly deduced the way that chromosomes move during mitosis, and that this movement allows their precise division.

7

Sutton and Boveri (1902)

Organism: Sutton; grasshoppers, Boveri; Ascaris worms.

Experiment: They observed chromosomes during these species because their chromosomes are large and few in number, and therefore easy to observe.

Findings:
-They observed grouping in pairs and subsequent separation, as well as reduction in chromosome number in gametes.
-They noticed that their observation were consistent with Mendel's laws of heredity.
-They suggested that different combinations of chromosomes could cause variation.

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Sutton-Boveri chromosome theory of inheritance

-Chromosomes are required for embryonic development
-Chromosomes carry Mendel's "factors".
-Chromosomes are linear structures with genes along them.

9

Thomas Hunt Morgan (1915)

Organism: Drosophila melanogaster

Experiment: Morgan was working on inheritance in fruit flies, looking for mutations.
He found a fly with white eyes, instead of the wild-type red.

Findings:
He linked this phenotype with an unusual chromosome composition; breakage/joined incorrectly.
-He concluded that genes were carried on chromosomes.

10

Garrod (1908)

Alkaptonuria is a rare disorder characterised by black urine. Later in life, pain in the joints.

Organism: Humans

Experiment:
Garrod was interested in the composition of urine and what it could tell him about metabolism. He noticed his patients with alkaptonuria ran in families and so he undertook a detailed analysis of its inheritance.
-The build up of intermediates from the breakdown of proteins caused the disease, and the urine discolouration.
-This was the first time human disease was linked to Medelian inheritance and he was able to link human disorders to metabolic defects.

He proposed:
1. That these diseases were "inborn errors of metabolism", missing steps in the body's chemical pathways.
2. That there are differences in metabolism between healthy individuals (slightly different enzymes).

11

Beadle and Tatum (1941) Experiment

Organism: Neurospora crassa, bread mould often used as a model organism in genetics. It has both sexual and asexual stages and most of these are haploid.

Experiment: They chose niacin production as a model pathway to study.
-Niacin = vitamin B3. It is essential for cell growth. It is a precursor of NAD, which is a cofactor of many enzymes.
-Niacin is synthesised in a series of biochemical steps from tryptophan, by a series of enzymes. The intermediates in the pathway are needed for normal synthesis of niacin but are not essential in their own right.

Beadle and Tatum generated mutations in the DNA of individual cells using X-rays. They cause breaks in the DNA double helix which are repaired imperfectly, forming stable mutations in the DNA. (Only a few per cell was generated otherwise the cell would become too damaged.
This generated a series of auxotropic mutants = mutants unable to synthesis essential compounds.

They were then grown in minimal media supplemented with intermediates n the pathway.

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Minimal Growth Media

Contains only he nutrients that an organism cannot synthesise for itself. A wild-type organism should be able to grow on this type of media.

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Complete Growth Media

Contains extra nutrients, for example, amino acids. This should make up for some defects in metabolic pathways and allow mutant cells to grow.

Wild-type cells are able to grow on minimal media or complete media.
Autotrophic mutants are not able to grow on minimal media.
-They ARE able to grow on complete media, or minimal media supplemented with the specific molecule they are unable to make.

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Beadle and Tatum (1941) Conclusion

They therefore arrived at the conclusion that one gene codes for one enzymes - the one gene - one enzyme hypothesis. We can now call this the one gene-one protein hypothesis.
-It shows a link between Mendelian genetic and the biochemistry of metabolism.

15

Streptococcus pneumoniae

Streptococcus pneumoniae can cause pneumonia in humans and in mice. Only some strains are capable of causing infection and illness - Streptococcus can live in the nasal passages without causing disease.
-S strain - Smooth bacteria - polysaccharide coats - pathogenic
-R strain - Rough bacteria - no polysaccharide coats - not pathogenic

The polysaccharide coat form a capsule that protects some strains from the host immune system.
Colonies look different under the microscope as well as causing different effect in vivo

16

Griffith

Organism: Streptococcus pneumoniae in mice.

Experiment:
1. Kill S bacteria by heating them to high temperatures. Is an infection established? (No, the mouse lives)
2. Inject dead S bacteria into mice together with live R bacteria. Is an infection established? (Yes, the mouse dies)
3. If an infection is established, of the bacteria have a polysaccharide coat?

Results:
-Inoculation with dead S bacteria and live R bacteria established infection. Bacteria from infected mice do have a polysaccharide coat.
-Therefore the R cells have undergone a "transformation".
-Some sort of hereditary material has passed from the S bacteria to the R bacteria, changing the phenotype.
-Griffiths called this material the "transforming principle".

17

Avery, MacLeod and McCarty (1944)

Experiment: These scientists systematically destroyed each component of the S strand extract using enzymes that specifically digest each type of molecule, before combining with live R bacteria to test for transformation.
-Do the modified extracts retain the transforming principle?

In order for the bacteria to be virulent, they require the DNA encoding for the polysaccharide coat - but not the polysaccharides themselves.

We not know that small pieces of DNA from the dead smooth cells were being taken up and integrated into the genome of the rough cells.
-Lots of different pieces of DNA from the S cell extract are being taken up by different individual R cells.
-Only he few cells that take up the gene coding for the polysaccharide capsule will become virulent. Other bacteria that have integrated different genes will not be virulent.

18

Bacteriophages

-A category of viruses
-All viruses require a host cell to reproduce, like parasites.
-The host cell for bacteriophages is a bacterium.
-Bacteriophage T2 - the host is specifically E. coli.

Life Cycle:
-DNA genome; destruction of the host cell's chromosomes.
-Transcription and translation of viral genes and replication of the viral genome.
-Assembly of new virus particles from DNA and protein subunits.

19

Hershey and Chase (1952) - Experiment

1. Label bacteriophage DNA or protein with a radioactive isotope (Different number of neutrons - unstable and decaying).
-They knew that DNA contained phosphate groups and that protein contained sulphur.
-32P and 35S are unstable isotopes of these elements. They can be detected by a Geiger counter.

2. Infect unlabelled bacteria with radioactive phage.

3. Separate phage ghosts from infected bacteria using a blender. This made the phage ghosts fall off the bacteria but the bacteria themselves remain intact.
-Then then centrifuged this mixture to separate out by weight, the bacteria are heavier so a pellet formed containing the bacteria (including the phage genetic material).
-The supernatant contains phage ghosts.

4. Test bacteria and phage ghosts for radioactivity using a Geiger counter

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Hershey and Chase (1952) - Findings

If protein were the genetic material: Phage ghost DNA and the bacteria's protein should be labelled

If DNA were the genetic material: Phage ghost protein and the bacteria's DNA should be labelled.

They found that DNA was the genetic material.

They also showed how radioactive DNA is carried over from generation to generation.

21

Kossel (1878)

Albrecht Kossel identified the different nucleobases in nucleic acids.
-Felix Hoppe-Seyer's lab - the same group as Friedrich Miescher and continued his research.
-Kossel identified A, G, C and U and discovered thymine.

-Purines: 2 rings, A and G
-Pyrimidines: 3 rings, C, T and U.

22

Levene (1929)

Levene's contribution was to show how nucleotides make up DNA: deoxyribonucleic acid.
-Nucleotides, made of deoxyribose, phosphate and a base, are assembled into strings and the components are joined by covalent bonds.

Chain grows in a 5' to 3' direction.

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The tetranucleotide hypothesis

Levene believed that the four nucleotides were repeated in the same order in each DNA molecule, and therefore than DNA was far too simple to make up the genetic material of cells.

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Chargaff (1951)

Chargaff was inspired by reading Avery et al's paper showing that DNA transforms bacteria.
He used paper chromatography to separate and isolate the nucleobase components of DNA from a number of species.

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Chargaff's Rules

%A = %T and %G = %C
-Or % purines = % pyrimidines

%AT ≠ %GC.
Composition varies from one species to another; some species are more AT-rich than others.

26

Wilkins and Franklin (1952)

They used X-ray crystallography to study the structure of DNA,
X-ray crystallography

-Make compound crystalline (evenly spaced atoms)
-Pass X-ray through
-Detect use photographic plate.

-X pattern indicates a helix.
-Regular pattern indicates repeating, even pattern
-Distance between the spots, indicated the distance of one turn (3.5nm). This suggests a helical structure.

27

Watson and Crick (1953)

Watson and Crick didn't actually experiment, they just compiled information.

Information they had:
-Structures of the nucleotides (Levene)
-Ratios of he different nucleotides in the DNA (Chargaff)
-Crystal structure (Wilkins and Franklin)

28

Main features of Watson and Crick's model of DNA

A-T and G-C hydrogen-bonded base pairs
-Hydrogen bonding is formed between a purine and a pyrimidine.
-Fairly weak attraction
-A-T = 2 bonds (weaker)
-C-G = 3 bond
-Restriction enzymes are likely to be AT rich as these are easier to break.
-These can be broken by heat (e.g. in PCR)

Antiparallel strands
-Sugar phosphate backbone
-Stacking of bases in the middle
-Anti-parallel so running in opposite directions
-Left = 5' to 3', Right = 3' to 5'

Right handed double helix

One helical turn every 10.5bp

Major and minor grooves