DNA as Genetic Material – Structure and Replication (Ch 16)
Outline of Key Info
Know the structure of DNA at the nucleotide level and polymer level
Be sure to know what the terms 3’ and 5’ refer to.
Be sure to know why the base pairing rules apply, A-T and G-C
Know the history and important experiments of
o Griffith
o Chargaff
o Avery, McCarty, and MacLeod
o Meselson and Stahl
o Hershey and Chase
o Watson, Crick and Franklin
Know that DNA replication is semi-conservative.
Know that DNA polymerase can only add nucleotides on the 3’ end of a nucleotide. So synthesis moves from the 5’ toward the
3’ end. RNA primers are laid down first so that DNA polymerase can start DNA replication. Understand why there is a leading
strand and a lagging strand and be able to explain how the lagging strand is synthesized.
Know that eukaryotes have multiple origins of replication because the DNA molecule is soooooo long and replication occurs in
both directions. Prokaryotes typically have just one origin of replication.
Be sure you know the proteins and enzymes are involved DNA replication: DNA polymerase, helicases, stabilizing proteins,
RNA primase, and DNA ligase.
Understand that DNA polymerase can make base pairing mistakes; DNA polymerase proofreads and corrects some mistakes
while others are repaired by mismatch repair and excision repair enzymes after replication has occurred. Mistakes that are not
corrected may include frame shift mutations, additions and deletions, and point mutations.
Study Questions
1. How do we know that DNA, not carbs, proteins, or lipids, is the inheritable genetic material?
2. How did Chargaff and Franklin help Watson and Crick come up with their model for DNA structure?
3. How do 3 m of DNA fit into each of our cells? (be specific!)
4. Why is a DNA molecule considered to have direction and to be “anti-parallel”?
5. What type of bonds form the sugar-phosphate backbone? What type of bonds hold the nucleotide bases together?
6. What are purines and pyrimidines? How can you tell the difference between them? Which bonds with which?
7. From what you know about the bonding between base pairs, which pairs (A-T or G-C) do you think have more breaks and
mistakes and why?
8. How did Messelson and Stahl determine the model for conservation in replication?
9. How does replication occur in both directions even though nucleotides can only be added to the 3’ end?
Scientists
Frederick Griffith
Oswald Avery, Maclyn
McCarty, and Colin
MacLeod
Alfred Hershey and
Martha Chase
Erwin Chargaff
James Watson, Francis
Crick, and Rosalind
Franklin
Matthew Meselson and
Franklin Stahl
Brief Description of Experiment/Data
Brief Conclusion from Results
Details of DNA Structure
From DNA to chromosomes:
Nitrogen Bases: Purines vs. Pyrimidines
Bases
Purines
Nucleotides are
the monomers of
nucleic acids
Structure
Pyrimidines
DNA Replication
 Coordinated by a large team of enzymes! (helicase, polymerase, ligase, primase…)
Problem: Nucleotides can only be added to the 3’ end by DNA Polymerase III…
Solution: Okazaki
 Leading and Lagging Strands
o Leading Strand  Continuous synthesis
o Lagging Strand  Okazaki fragments; joined by ligase
Priming DNA Synthesis
 DNA polymerase can only extend an existing DNA molecule; it
cannot start a new one
o Short RNA primer is built first on parent DNA by primase
o RNA primer later removed by DNA polymerase I
Editing and Proofreading DNA
 Since DNA polymerase III does 1,000 base pairs/second, it makes a lot
errors
 DNA Polymerase I (only 20 bp/sec) excises mismatched bases, repairs
DNA, and removes the primer
 DNA polymerase I reduces error from 1/10,000 bp to 1/100 million bp!
Problems at the end…
 Ends of chromosomes are “eroded” with each replication (don’t get fully
copied)
 Telomeres are expendable, non-coding sequences at the ends of the
DNA strand
o short sequence of bases repeated 1000s of times
o TTAGGG in humans
of
the
Study Help/Check for Understanding: In the diagram below, label the key structures and enzymes in DNA replication. Be sure to label 3’ and 5’ ends,
too! Then, summarize the key functions of the replication enzymes in the table below.
Enzyme
Function
Helicase
Primase
DNA Polymerase I
DNA Polymerase III
Ligase
DNA Structure – Questions and Practice
1. Label the strand of DNA below. Include phosphate groups, deoxyribose groups,
hydrogen bonds, phosphodiester bonds, adenine, thymine, cytosine, guanine, and
the 3’ and 5’ ends.
2. Which part of the DNA molecule is most likely to be broken during replication
and why?
3. Which parts of the DNA molecule are pyrimidines, and how do you know?
4. Which parts are purines, and how do you know?
5. How did Meselson and Stahl describe DNA replication and why?
6. Why does DNA replication result in a leading strand and a lagging strand?
7. What are Okazaki fragment? With which enzymes are they involved?
8. What is the function of helicase in DNA replication? What is the function of
DNA polymerase in DNA replication?
9. Why is primase essential for DNA replication?
10. If a segment of nucleic acid is CATCATTAC, what is the complementary strand?
11. The DNA of a certain organism has guanine as 30% of its bases. What
percentage of its bases would be adenine?
12. Suppose you provide an actively dividing culture of E. coli bacteria with
radioactive thymine. What would you expect to happen if a cell replicated its
DNA and divided once in the presence of the radioactive base?
a. One of the daughter cells, but not the other, would have radioactive DNA.
b. Neither daughter cell would have radioactive DNA.
c. All four bases of the DNA would be radioactive.
d. Radioactive thymine would pair with nonradioactive guanine.
e. DNA in both daughter cells would be radioactive.
13. The diagram to the right represents a student’s view of DNA synthesis
occurring in an animal cell. Arrows represent newly synthesized DNA.
This diagram is
a. Correct as shown.
b. Incorrect because DNA synthesis in animal cells is unidirectional.
c. Incorrect because DNA synthesis proceeds in a 3’ to 5’ direction.
d. Incorrect because Okazaki fragments are synthesized on both strands.
e. Incorrect because DNA synthesis is proceeding in the wrong direction on two of the strands.
14. For each of the statements below, determine if they are true or false regarding DNA replication. If they are false, rewrite or change
the statement so it is written correctly.
 (T,F) The double helix is cleaved at the phosphate bonds and new nucleotides pair with the appropriate sugars.
 (T,F) The hydrogen bonds between the bases are broken and new nucleotides pair with complementary bases on the old strands.
 (T,F) New bases are added at the 5’ carbon on the sugar so that DNA synthesis occurs in a 3’ to 5’ direction.
 (T,F) DNA is unwound by DNA polymerases and new nucleotides are assembled by ligases.
15. Which of the following statements about DNA structure and replication in prokaryotes and cell organelles is correct?
a. There is a single circular chromosome composed of DNA.
b. Chromosomes are wound around nucleosome cores.
c. Replication begins at multiple initiation sites.
d. Chromosomes line up in the center of the cell during replication.
e. Cell organelle DNA encodes all of the proteins the organelle needs.