Exam 1: Lecture 4

Question 1

Bacteria

Answer 1

• single celled prokaryotic organisms
• no nucleus
• transcription and translation not spatially separated as they are in eukaryotic cells

Question 2

Bacteriophage

Answer 2

• ancestors of viruses that only infect bacterial cells
• particles contain DNA genome that is packaged into the head of the phage
• upon attachment to a bacterial cell wall by the legs of the phage the baseplate comes in contact with the cell wall and a hole is burrowed into the membrane
• genome passes through the neck and body of the phage and is then injected into the bacterial through the baseplate
• bacterial replication machinery will duplicate the phage genome while the transcription and translation machinery will generate viral mRNAs and proteins

Question 3

Fredrick Griffith (Transforming Principle)

Answer 3

• published the results of an experiment that demonstrated that a substance within one strain of bacterial cells could alter the character of a second bacterial strain
• used a pathogenic strain of bacteria (S) and a non-pathogenic strain (R). Mice injected with a living S strain would die of a respiratory infection while mice injected with the R strain were unaffected
• heat killed and fragmented the S bacteria and performed two key experiments. Injection of the heat killed/fragmented S strain, on its own, had no effect. However, mice would contract and die of a respiratory infection if they were injected with a mixture of heat killed/fragmented S strain and living population of the R strain
• concluded that the R strain was transformed by a substance from the heat killed/fragmented S strain–>”The Transforming Principle”
• results confirmed Mendel’s theories on inheritance and were the foundation for later experiments the proved that the transforming substance was DNA.
• demonstrated for transformation of bacteria for the first time

Question 4

X-ray Crystallography

Answer 4

• the structure of complex molecules can be determined by bombarding a crystal (of the subject of interest) with a beam of x-rays.
• when the beam hits the crystal, it is diffracted towards a piece of film.
• resulting image corresponds to atomic structure of substance in question.
• used to determine atomic structure of DNA, proteins, and viruses, and molecular nature of protein-nucleic interactions
• information derrived from structure is used to make drugs/antibodies/vaccines/theraputic anti-cancer drugs.

Question 5

Rosalind Franklin

Answer 5

• used x-ray crystallography to generate diffraction pattern for crystal of DNA
• in process of solving the structure when Watson and Crick stole her research and solved it themselves.

Question 6

Watson and Crick

Answer 6

• their structure of DNA was based on Franklin’s diffraction pattern and gave her no credit when the won the nobel prize
• at the time of solving, they knew DNA contained nucleotide bases, sugar molecules, and phosphate groups
• they determined that it is composed of two polynucleotide chains that are wound into a double helix.

Question 7

DNA Double Helix Properties

Answer 7

• comprised of 2 polynucleotide chains
• within in single strand individual nucelotides are joined together through phosphodiester bonds
• the two strands that comprise the double helix are joined together by hydrogen bonding between purine and pyrimidine bases.
• major groove: where DNA binding proteins interact with DNA here
• minor groove: histone proteins interact here

Question 8

Major Groove

Answer 8

-place where many DNA binding proteins and transcription factors sit down

Question 9

Minor Groove

Answer 9

-not much chemical information so non-sequence specific proteins sit here

Question 10

DNA Nucleotide Sturcture

Answer 10

• DNA comprised of 4 building blocks called nucleotides. each consists of purine or pyrimidine base, a sugar moiety and phosphate group.
• purine bases are A and G
• pyrimidine bases are T and C
• based on x-ray diffraction it was determined that the purine bases form hydrogen bonds with pyrimidines.

Question 11

Erwin Chargaff (Base Pairing and Phosphodiester Bonds)

Answer 11

• isolated nucleic acids from variety of tissues and organisms and determined the percentage of each nucleotide.
• amount of adenine equaled amount of thymine and amount of cytosine was equal to amount of guanine.
• Watson and Crick ripped him off to to postulate that adenine bases formed hydrogen bonds with thymine and cytosine formed hydrogen bonds with guanine.
• amount of each nucleotide differs amongst tissues and organisms but A:T and C:G ratios is always 1:1
• polynucleotide chain is held together by phosphodiester bonds

Question 12

Double Helix Dynamics: Antiparallel

Answer 12

• poly nucleotide strands oriented anti-parallel to each other
• 3’ means that #3 carbon will be bound to phosphate group of the next nucleotide
• 5’ means the #5 carbon will be bound to the deoxyribose sugar in the adjacent nucleotide

Question 13

Interaction of Bases in Double Helix

Answer 13

• A:T bases interact through 2 hydrogen bonds
• C:G bases interact through 3 hydrogen bases
• amount of energy it takes to separate nucleotides is directly proportional to the number of hydrogen bonds with G:C pairs taking more energy/heat to separate than A:T pairs

Question 14

Oswald Avery

Answer 14

• repeated Griffity’s experiments but conducted set of controls that proved nucleic acid is the hereditary material
• heat killed and fragmented pathogenic S bacterial strain
• employed biochemical methods to separate nucleic acid, protein and lipid fractions
• mixed each fraction with non pathogenic R strain and injected each mixture into mice
• mixtures containing R bacterial strain and either the protein or lipid had no effect
• mice died with mixtures of living R bacteria and nucleic acid from heat killed/fragmented pothegenic S strain were injecte
• transforming agent=DNA

Question 15

Hershey and Chase

Answer 15

• proved nucleic acid was transforming agent and hereditary material with differen approach
• parasiic interaction between bacteriophage and bacteria.
• phage coat proteins labeled with radioactive sulfur and genome labled with radioactive phosphorus.
• bacteriophage allowed to attach to bacterial cell wall and inject genome into cytoplasm
• empty phage particles removed
• new phage particles later isolated from bacterial cells
• some contained radioactively labeled genomes derived from original phage particles
• others contained non-radioactive genomes-generated by bacterial replication machinery.
• both types of tenomes packaged within non-radioactive phage particles-generated by host transcription translation machinery (not radioactive so not original particles)
• show phage genome was transforming agent and hereditary material