Chemical Basis of Genetics

The DNA Story

Chromosomes

• What is the nature of this material?–Long threads of Genetic Material

• existence wasn’t postulated until 1856-1865

Gregor Mendel

• first proposed the existence of chromosomes

• called them factors–did not observe theses “factors”–consequently ideas not readily accepted by the scientific community

Year 1869• Friedrich Miescher (Swiss Botanist)

–nature of the cell– treated cell with Pepsin

•enzyme found in stomach that dissolves proteins to watery solution

–noticed - nucleus shrank a little, but essentially remained intact

–Conclusion made?

Fredrich Miescher (continued)

–Nucleus doesn’t behave like a protein

–Contains P along with C, H, O, and N

–Nucleic Acids - several types•many not found in the nucleus•typed studied by Meischer -> DNA

Deoxyribonucleic Acid

• De - less• Oxy - oxygen• Ribo - Ribose• Nucleic Acid

Year 1903-1907

• Chromosomes were first observed–Mendel’s “factors” rediscovered

• Seem to be affected by / controlling cell division

Year 1914

• Robert Feulgen (German Chemist)–devised method to stain DNA -> Bright Red

–applied to cells •DNA restricted to chromosomes

•others used staining technique for Quantitative Studies (How much of a substance is present)

•Somatic Cells = all have the same amount of DNA

•Gametes = all have 1/2 the amount of DNA as somatic cells

Year 1914 (continued)

–Results of Quantitative Studies suggested to scientists that DNA was the essential material in controlling the expression of inherited traits

•DNA not readily accepted by the scientific community - too simple of a molecule!!–Many believed key must be a protein because only a protein was complex enough to encode so much information•even though the amount of protein fluctuated with the cell’s activity.

DNA vs. Protein

• Which chemical is the controlling chemical in controlling the expression of inherited traits?

Year 1928

• Fred Griffith (English Medical Bacteriologist)–bacteria causing pneumonia– injected mice with 2 types of bacteria•Strain S = Lethal•Strain R = Non-lethal

Year 1928 (continued)

Results: Lives

Dies

Lives

DIES !

Strain R =

Strain S =

Dead S =

Strain R + Dead S

Year 1928 (continued)

• How could a mixture of non-lethal and dead lethal bacterial cause a mouse to die?

• Examine mouse - full of live lethal bacteria!

• Live Strain R transformed into live Strain S–hereditary information from the dead lethal bacteria entered non-lethal bacteria and changed them into a lethal variety.

• Still not convince most biologists to importance of DNA (Why would it?)

Year 1944• DNA isolated and shown to be the item

causing the transformation– isolated DNA injected into cell - cell change trait expression

• 1944-1954 at least 30 examples of bacterial transformations by purified DNA described

• Many still not convinced of importance of DNA

Year 1952

• Hershey/Chase (Carnegie Lab of Genetics)–Virus

•Head - DNA (Phosphorus, but NO Sulfur)

•Stalk•Tail - Protein (Sulfur, but NO Phosphorus)

1952 (continued)

• Virus Life Cycle

• (See Drawing on Board)

1952 (continued)

• labeled Phosphorus / Sulfur with radioactive isotopes–Head = labeled Phosphorus (DNA)

–Tail = labeled Sulfur (Protein)

1952 (continued)

• results– in the new generations of bacterial cells•All Radioactive Phosphorus - No Sulfur

• conclusion - DNA was inserted to produce new virus

• Conclusive evidence to support the importance of DNA in controlling the expression of inherited traits.

DNA Structure

• 1920’s Phoebus A. Levene (American Biochemist)–DNA composed of 4 bases

•Adenine •Guanine•Thymine•Cytosine

•all bases contain Nitrogen•also present - Phosphate,

Sugar (Deoxyribose)

DNA Structure (continued)

• *nucleotide = basic unit of DNA consisting of a Base, Phosphate, and Sugar

DNA Structure (continued)

• How are the elements arranged in the DNA?–Chemical Analysis showed the amounts of•Guanine = Cytosine•Adenine = Thymine•Sugars and Phosphates were variable and not equal to each other

DNA Structure (continued)

• 1953 Watson-Crick Model of DNA

•Double Helix in which 2 chains composed of alternating sugar and phosphate groups bonded together by bonds between A+T and G+C from opposite sides.

DNA Model

Mutations

• Gene Mutations (Point Mutations)–one nitrogenous base substituted for another

–most common• (See Board Diagram)

• Chromosomal Mutation– involves many genes

•1) Deletion - usually lethal•2) Inversion

–middle breaks, turns around, recombines

–may not harm if occurs in right place - single genes present

• Translocation–fragment of one Chromosomes attaches to non-homologous chromosome•may produce serious abnormalities in people

Somatic and Germ Mutations

• Somatic - affect individual (not offspring)–example: cancer

• Germ - not affect individual - (can affect offspring)

Effects and Frequencies of Mutations

• most mutations produce recessive genes–only expressed if combine with a similarly mutated gene

• sometimes dominant - a form of dwarfism

• nearly all chromosomal mutations are harmful–deformities/traits make it hard to survive

• some changes help organisms survive when the environment changes–mosquitoes resistant to insecticides

–bacteria resistant to antibiotics

How often do Mutations Occur?

• Believed to be a low rate–1 or 2 / 100,000 genes / generation

• Rates affected by mutagens–UV (bacteria, skin cancer)–X-Rays / other radiation–Tars in tobacco smoke (secondary person - cancer)

–Smog, viruses, various chemicals, drugs

–Mustard Gas (War)•removes Guanine from DNA

–Nitrous Acid•removes Nitrogen from bases in DNA

Recombinant DNA Technology

• transferring of DNA segments from one organism into the DNA of another species

• bacteria used to combat human diseases–human insulin–clotting protein

• future–agriculture (disease resistant crops)

–food processing–energy production

Q: How do experimenters know which bacteria contain recombinant DNA so they can allow only those bacteria to multiply?

A: Along with segments of DNA, the Experimenters attach marker genes to the plasmids. These genes provide resitance to streptomyocin, an antibiotic that kills certain bacteria.

If the recipient bacteria are grown on a medium containing streptomyocin, the colonies that survive must contain marker genes and therefore recombinant DNA.