Exam 3: #1 Molecular Structure of DNA and RNA

Genetic Material (DNA) 4 Key Requirements

1. Information: must contain the information needed to make an entire organism

2. Transmission: must be passed from parent to offspring

3. Replication: must be copied in order to be passed from parent to offspring

4. Variation: must be capable of changes (mutations) for phenotypic variation in each species

Frederick Griffith Experiments with Streptococcus Pneumoniae

Streptococcus Pneumoniae: a bacteria that causes pneumonia and discovered it exists in two different forms

1. Type S (Smooth):

• Produces polysaccharide capsule that surrounds the bacterial cell

  • Capsule: protective shield, hide bacteria from animal’s immune system

• Deadly and cause disease

• Form smooth, shiny colonies

2. Type R (Rough):

• Cannot make protective capsule

• Immune system can easily detect and destroy these bacteria = harmless

• Form rough, bumpy colonies

Mouse Injected with Living Type S bacteria

Griffith injected a mouse with Type S bacteria

• Mouse died from pneumonia

• Dead mouse blood had live Type S bacteria

• Result: Type S bacteria = deadly because protective capsule allows them to evade the immune system

Mouse Injected with Living Type R 

Griffith injected a mouse with living Type R bacteria

• Mouse survived and stayed healthy

• Mouse blood had no living bacteria

• Result: Type R bacteria are harmless because mouse immune system destroyed them 

Mouse Injected with Heat-Killed Type S Bacteria

Griffith heated Type S bacteria to kill them, then injected the dead bacteria into a mouse

• The mouse survived and remained healthy

• No living bacteria were found in the mouse's blood

• Result: Dead Type S bacteria can't cause disease, even though they have the capsule genes

Mouse Injected with Live Type R and Heat Killed Type S Bacteria 

Griffith mixed living Type R bacteria (harmless) with heat-killed Type S bacteria (dead), then injected this mixture into a mouse

• The mouse died from pneumonia

• When he examined the blood, he found living Type S bacteria

• Result: Something from the dead Type S bacteria transformed the harmless Type R bacteria into deadly Type S bacteria

Transforming Principle

Griffith Concluded: Something from the dead Type S bacteria transformed the harmless Type R bacteria into deadly Type S bacteria

• Called this process Transformation

Substance that allowed this to happen = transforming principle

• He did not know what type of substance

Avery, Macleod, and McCarty

They recognized that Griffith’s transformation experiments could be used to identify what the genetic material actually was

• Experiments in the 1940s: they knew that living cells contained major molecules; DNA, RNA, proteins, and carbohydrates

Avery, Macleod, and McCarty Experiments

Extracted material from Type S bacteria and separated it into purified fractions containing different macromolecules (DNA, RNA, proteins, and carbohydrates)

• Findings: Only the extract containing DNA was able to convert Type R bacteria into Type S bacteria = DNA is the transforming principle

Enzyme Tests (Confirm results by treating DNA extract with different enzymes):

• RNase (destroys RNA): Transformation still occurred

• Protease (destroys proteins): transformation still occurred

• DNase (destroy DNA): transformation was eliminated; therefore DNA is needed for transformation to occur

Hershey and Chase

Conducted experiments that showed the DNA, not protein, is the genetic material.

• Studied T2 bacteriophage, a virus that infects bacteria, consisting of only DNA and protein

• Labeling Strategy:

1. 32P (radioactive phosphorus): labels DNA because phosphorus is found in DNA, but not in proteins

2. 35S (radioactive sulfur): Labels proteins because sulfur is in proteins but not in DNA

Radiolabeled phages infected non-radioactive E. Coli bacteria

• Allowed them to track where the DNA and protein went during infection

Blender

Important step to remove virus particle (residual phage) from the bacterial cells

• Used a kitchen blender to physically shear off the empty phage coats that were stuck to the outside of the bacterial cells

• Centrifuge: used a centrifuge to separate heavier bacteria (pellet at bottom) from the lighter viral remains (liquid solution)

Results:

• Most 32P entered the bacterial cells (DNA)

• Most 35S remained outside the cells (proteins)

What did Miescher name his discovery and why

Discovered a substance high in phosphorus in bandages and named this substance “nuclein” because it came from the nucleus of cells

How was "nuclein" renamed to "nucleic acid"?

Later research showed DNA and RNA release H⁺ ions in water, making them acids. Combined "nuclein" + "acid" = nucleic acid

DNA and RNA Structure

1. Nucleotides (Building Blocks): Basic repeating units of nucleic acids

2. Linear Strand (Primary Structure): Individual nucleotides link together to form a linear strand

3. Double Helix (Secondary Structure): In DNA, two complementary strands interact

4. 3-D Folding (Tertiary Structure): 3-D structure of DNA results from folding and bending of the double helix.

5. Chromosomes (Quaternary Structure): interaction of DNA with various proteins creates even higher levels of organization

Nucleotide Structure

Nucleotide: repeating structural unit of DNA and RNA

Three components of nucleotides:

1. A phosphate group

2. Pentose Sugar

   • DNA: Deoxyribose (Only has OH on 1st and 3rd carbon)

   • RNA: Ribose (Has an extra OH on the 2nd carbon)

3. A nitrogenous (nitrogen containing) base