The “Main Things” for
DNA as Genetic Material – Structure and Replication
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 Thomas Hunt Morgan o Griffith o Avery, McCarty, and MacLeod o Hershey and Chase o Chargaff o Meselson and Stahl o Watson, Crick and Franklin
Know that DNA replication is semi-conservative.
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.
Eukaryotes have multiple origin of replication sites because the DNA molecule is soooooo long and the
DNA replication occurs in both directions. Therefore it would appear that only one side is replicated but that is not the case. One side is replicated in one long strand the leading strand. The other side is replicated in bits and pieces forming Okazaki fragments that are later joined together by DNA ligase.
Be sure you know the proteins and enzymes are involved DNA replication
DNA polymerase, helicases, stabilizing proteins, RNA primase, DNA ligase,
Understand that DNA polymerase can make base pairing mistakes
Frame shift mutations
Additions versus deletions
Point mutations

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
Use your textbook or the internet to find the ONE or TWO most important facts about these scientists’ experiments with respect to the study of heredity and DNA.
Scientists Diagrams Key Contributions to Study of
Heredity
Thomas Hunt
Morgan
Frederick
Griffith

From DNA to chromosomes Nucleotides are the monomers of nucleic acids
Nitrogen Bases: Purines vs. Pyrimidines
Purines
Pyrimidines
Bases Structure

Key Questions
1.
How do 3 m of DNA fit into each of our cells? (be specific!)
2.
Why is a DNA molecule considered to have direction and to be “anti-parallel”?
3.
What type of bonds form the sugar-phosphate backbone?
4.
What type of bonds hold the nucleotide bases together?
5.
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?
Models of DNA Replication
 How does base-pairing ensure that the daughter strands will be identical to the parent strands?

DNA Replication
 Coordinated by a large team of enzymes
DNA Enzymes
 Helicase o
 DNA Polymerase III o o o
Problem: Nucleotides can only be added to the 3’ end by DNA Polymerase…
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

Summary of Replication Enzymes
Enzyme
Helicase
Primase
DNA Polymerase I
DNA Polymerase III
Ligase
Function
Editing and Proofreading DNA
 Since DNA polymerase III does 1,000 base pairs/second, it makes a lot of errors
 DNA Polymerase I (only 20 bp/sec) excises mismatched bases, repairs the DNA, and removes the primer
 DNA polymerase I reduces error from 1 in 10,000 bp to 1 in 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
In the diagram below, label the key enzymes and structures in DNA replication. Be sure to label 3’ and 5’ ends, too!

DNA Structure – Questions and Practice
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.
1.
Which part of the DNA molecule above is most likely to be broken during replication and why?
2.
Which parts of the DNA molecule are pyrimidines, and how do you know?
3.
Which parts are purings, and how do you know?
4.
How did Meselson and Stahl describe DNA replication and why?
5.
Why does DNA replication result in a leading strand and a lagging strand?
6.
What are Okazaki fragments and which enzyme is important in their replication role?
7.
What is the function of helicase in DNA replication?
8.
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 DNA 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 below 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.