Chapter 16 RQ1. What is a virus that infects

bacteria called?2. Who actually took the X-ray

diffraction photo of DNA’s structure?

3. What are the bonds between nitrogenous bases?

4. What does the “semiconservative model” describe?

5. What does “ligase” do?

1. Explain why researchers originally thought protein was the genetic material.

• Proteins are macromolecules with great heterogeneity and functional specificity

• Little was known about nucleic acids• The physical and chemical properties of

DNA seemed too uniform to account for the multitude of inherited traits

2. Describe the experiment that led to the discovery that DNA was the genetic

material in cells.

• Frederick Griffith in 1928• Trying to find a vaccine to fight pneumonia• Experimented with the two strains of

pneumococcus; smooth & rough• Smooth caused the disease, rough did not• When dead S strain was mixed with live R,

the mice DID die, indicating an acquired ability

3. Define ‘transformation’ and briefly discuss viruses and their effects on

bacteria.

• Change in phenotype due to the assimilation of external genetic material by a cell

• Viruses can inject their information into cells and cause drastic changes in behavior

4. List the three components of a nucleotide.

1. Pentose (5-C sugar)

2. Phosphate3. Nitrogenous

base

5. List the nitrogenous bases found in DNA, and distinguish between pyrimidine

and purine.

Pyrimidines• 6 membered ring

of carbon and nitrogen

• C – cytosine• T – thymine (DNA)• U – uracil (RNA)

Purines• 5 membered ring

with 6 membered ring

• A – adenine• G – guanine

6. Explain how Watson and Crick deduced the structure of DNA, and describe what

evidence they used.

• Built models to conform to x-ray data- sugar phosphate backbone- nitrogenous base interior

7. Explain the “base-pairing rule” and describe it’s significance.

• A – T : 2 hydrogen bonds

• G – C : 3 hydrogen bonds

• Suggests the mechanisms for DNA replication

• Dictates combination of complementary pairs

8. Describe the structure of DNA, and explain what kind of chemical bond connects the

nucleotides of each strand and what type of bond holds the two strands together.

• Hydrogen bonds hold the nucleotides together

• Van der Waals forces help keep helix spiral shape

9. Explain semiconservative replication and the Meselson-Stahl experiment.

10. Describe the process of DNA replication, and explain the role of helicase, single strand binding protein, DNA polymerase, ligase, and primase.

1. The helical molecule untwists while it copies its 2 antiparallel strands simultaneously

2. Very rapid – only a few hours to copy 6 billion bases of a human cell

3. Very accurate – one in a billion nucleotides are incorrect

Helicase catalyzes the unwinding of the parental double helix to expose the template

Single strand binding protein keeps the separated strands apart and stabilizes the unwound DNA

Polymerase and ligase catalyze the filling-in process

Primase the enzymes that polymerize the short segments of RNA (primers)

11. Explain what energy source drives endergonic synthesis of DNA.

• It is the hydrolysis of nucleoside triphosphates, which are nucleotides with a triphosphate covalently linked to the 5’ carbon of the pentose

• Exergonic hydrolysis of this phosphate bond drives the endergonic synthesis of DNA it provides the required energy to form the new covalent linkages between nucleotides

12. Define antiparallel, and explain why continuous synthesis of both DNA strands

is not possible.• Antiparallel the

sugar-phosphate backbones of the 2 complementary DNA strands run in opposite directions

• DNA can only elongate in the 5’ to 3’ direction due to polarity issues- 3’ end has a hydroxyl group- 5’ end has a phosphate

13. Distinguish between the leading strand and the lagging strand.

• Leading continuous DNA synthesis, it is synthesized as a single polymer in the 5’ to 3’ direction towards the replication fork

• Lagging the DNA strand that is discontinuously synthesized against the overall direction of replication

14. Explain how the lagging strand is synthesized when DNA polymerase can

add nucleotides only to the 3’ end.

• The lagging strand is produced as a series of Okazaki fragments in the 5’ 3’ direction

• Fragments are ligated by DNA ligase which catalyzes the formation of a covalent bond between the 3’ end of each fragment to the 5’ end of the chain

15. Explain the role of DNA polymerase, ligase, and repair enzymes in DNA

proofreading and repair.

• DNA polymerase and ligase catalyze the filling-in process of the new DNA strands

• Repair enzymes excise ( remove) the damaged segments and the gap is filled in by the correct nucleotides

Pictures