Genetic Material and the Avery Experiment

• Genetic material is a key topic in biology.

Genetic Material

• In 1902, Walter Sutton determined that genes are located in chromosomes.
• In the 1860s, Friedrich Miescher isolated nucleic acids in cell nuclei.
• Eukaryotic cells have both proteins and DNA in their nuclei.
• Protein is made of 20 different amino acids.
• DNA is made of 4 different nucleotides.
• In 1920, Griffith worked with Streptococcus pneumoniae.
• Proteins consist of carbon, hydrogen, oxygen, nitrogen, and sulfur.
• DNA consists of carbon, hydrogen, oxygen, nitrogen, and phosphorus.
• 32P is used to label DNA radioactively.
• 35S is used to label protein radioactively.

Avery, MacLeod, and McCarty Experiment (1944)

• This experiment involved extracting components from heat-killed SIII bacteria.
• The extract was treated to destroy different components.
• Type RII was added to each treated extract.
• The control with no components destroyed resulted in live type SIII bacteria.
• Destroying lipids and polysaccharides resulted in live type SIII bacteria.
• Destroying proteins resulted in live type SIII bacteria.
• Destroying RNA resulted in live type SIII bacteria.
• Destroying DNA resulted in no bacteria recovery.
• In 1952, Hershey and Chase performed their experiment.

The Hershey-Chase Experiment

• Phage DNA was labeled by growing phage in a 32P-containing medium.
• Phage protein was labeled by growing phage in a 35S-containing medium.
• New unlabeled bacteria were infected with 32P-labeled phage.
• New unlabeled bacteria were infected with 35S-labeled phage.
• After infection, agitation separated the empty phage from bacteria.
• Centrifugation separated the mixture; bacteria formed a pellet.
• Almost all the 32P label was in the pellet with the infected bacteria.
• Almost all the 35S label remained with the ghost particles in the supernatant.

Nucleotides

• DNA is made of nucleotides.
• A nucleotide consists of a base, sugar, and phosphate group.
• 3' and 5' refer to the carbon atoms on the deoxyribose sugar molecule.
• The 5' end has a phosphate group attached to the 5' carbon atom of the deoxyribose sugar.
• The 3' end has a hydroxyl group attached to the 3' carbon atom of the deoxyribose sugar.

Polynucleotide Chain Length

• A diploid human cell contains about 6 x 10^9 base pairs of DNA.
• These base pairs are distributed among 46 chromosomes.
• Each chromosome contains one DNA molecule.
• Therefore, each chromosome has approximately 130 x 10^6 bp.

DNA vs. RNA

• DNA contains deoxyribose sugar; RNA contains ribose sugar.
• DNA uses the pyrimidine base thymine (T); RNA uses uracil (U).

Chargaff's Rules

• G/C = 1, meaning the amount of guanine equals the amount of cytosine.
• A/T = 1, meaning the amount of adenine equals the amount of thymine.
• A+G/T+C = 1, meaning the total amount of purines equals the total amount of pyrimidines.

Watson and Crick DNA Structure

• DNA is a double helix of two polynucleotide strands.
• The helix is right-handed.
• The two polynucleotide chains run antiparallel.
• Sugars and phosphates face the outside of the helix.
• Nitrogenous bases face the helix's interior.
• Nitrogenous bases are almost perpendicular to the helix axis.
• The two polynucleotide chains interact via hydrogen bonds.
• H bonds can be broken.
• Adenine (A) pairs with Thymine (T) through 2 hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) through 3 hydrogen bonds.
• There are approximately 10 base pairs per turn of the helix.
• Each turn of the helix measures 3.4 nm or 34 Å.
• DNA is a “double helix”

Further Differences between DNA and RNA

• DNA is double-stranded; RNA is usually single-stranded.
• DNA and RNA can form secondary structures such as hairpin loops.
• DNA has a major and minor groove

Alternative Nucleic Acid Structures

• B structure (Watson and Crick): right-handed, 10 base pairs/turn, most common DNA.
• A structure: right-handed, 11 base pairs/turn, found in DNA-RNA hybrids.
• Z structure: left-handed, 12 base pairs/turn, found in certain DNA sequences.

DNA Replication

• Both DNA strands are copied.

Meselson and Stahl Experiment (1958)

• Experiment showed that DNA replication is semiconservative.
• Three possible modes of replication were considered: semiconservative, conservative, and dispersive.
• The experiment used old, heavy DNA made with 15N and new, light DNA made with 14N.
• DNA replication is semiconservative in bacteria and eukaryotic cells.
• DNA replication starts at specific sequences called origins of replication.
• Bacterial cells usually have one small, circular DNA molecule and one origin of replication.
• Eukaryotic cells have multiple linear DNA molecules and multiple origins of replication per chromosome.
• DNA replication proceeds bidirectionally from the origin of replication.

DNA Polymerases

• DNA polymerases are enzymes used to synthesize DNA.
• They require a template to copy.
• DNA polymerases need a primer with a 3'-OH group.
• They also require dNTPs (deoxyribonucleotide triphosphates).
• DNA polymerases can only synthesize in the 5' to 3' direction.
• DNA ligase makes the lagging DNA strand a continuous structure by catalyzing the formation of phosphodiester bonds
• Okazaki fragments are fragments of DNA made on the lagging strand
• Primers are RNA and are removed before the process is complete
• The DNA at the ends of chromosomes (telomeres) is synthesized by telomerase.
• Telomerase prevents the shortening of DNA molecules during replication.

Summary Point

• DNA polymerase synthesizes DNA in the 5' to 3' direction, and one strand is wrapped to allow for the same orientation of copying.

Explore the key experiments that identified DNA as the genetic material. Learn about Sutton's chromosome theory, Miescher's nucleic acid isolation, and Griffith's transformation experiment. Discover how Avery, MacLeod, and McCarty definitively proved DNA's role.