1.
2.
Why are GC pairs harder to pull apart than AT pairs?
Is DNA a uniform width? What would happen to the width of DNA if two purines formed a base
pair? Two pyrimidines?
3. Scientists studying a new organism find that its DNA is made up of 21% Adenosine (A). What
percentage is Thymine? Guanine? Cytosine?
4. Write a list of at least 6 different protein enzymes needed for replication. Next to each write its job
during replication.
5. Write a list of 5-7 steps for replication.
6. T/F. DNA replication is the same for both the 5’3’ and the 3’5’ strands at the replication fork.
Explain/defend your answer.
7. The genome (sum total of all DNA) is estimated to be about 3.2 billion base pairs long. Find the rate
of DNA polymerase in your Dummies book and then use it to calculate how long it would take a single
polymerase enzyme to copy the entire genome.
8. What is the error rate of DNA polymerase before proofreading? What is the error rate after
proofreading? How many total mistakes would be found in the entire genome after proofreading?
Suppose you could redesign the replication process so that there were absolutely no mistakes. Would
this be a good or a bad thing? Explain your answer.
9. Propose a relationship between replication and the term alleles that you learned in the last class
10. What enzyme is responsible for proofreading and how does it know where the mistakes are.
11. Replication begins at special sequences in the DNA called Origins of Replication. These origins are
typically rich in AT base pairs. Can you explain why?
Summary of 10 Things You Need to Know about DNA Structure
1. A nucleotide is composed of three parts; 1) a Deoxyribose sugar, 2) a phosphate group, and 3) a nitrogen
rich base
2. The four nucleotides differ from each other in their nitrogen rich bases
a. Adenine – purine (two rings), 2 hydrogen fingers
b. Thymine – pyrimidine (one ring) 2 hydrogen fingers
c. Guanine - purine (two rings), 3 hydrogen fingers
d. Cytosine – pyrimidine (one ring) 3 hydrogen fingers
3. A single strand of DNA is made from linking the phosphate from one nucleotide with the sugar from
another nucleotide. The nitrogenous base sticks out from the chain like an arm.
4. A strand of DNA has a head and a tail.
a. The head is denoted by a phosphate end of the strand and is called the 5’ (five prime) end.
b. The tail is denoted by the sugar end of the strand and is called the 3’ (three prime) end.
5. A double helix is made from twisting two DNA stands together. These strands
a. Run in opposite directions (5’3’ & 3’5’)
b. Nitrogen- rich bases pair together by hydrogen bonding between strands
c. Sugar phosphate backbone is on the outside
6. The double helix makes one complete turn (twist) every 10 nucleotide base pairs. There are 0.34 nm
between each base pair or 3.4nm for one full twist (0.34nm x 10).
7. The double helix is the same width along its entire length (2 nm). This occurs because each base pair is
made from 1 double ring and 1 single ring of nitrogenous bases. 2 single ring pairs or 2 double ring pairs
would result in either a bump or a dip in the width. Bumps and dips are often used by repair enzymes to
identify mutations.
8. Cytosine bases always pair with Guanine bases and Thymine always pairs with Adenine
a. Base pairs always have one purine and one pyrimidine
b. Three hydrogen fingers always pair with three hydrogen fingers, two with two.
9. The hydrogen bonds holding two strands together can be pulled apart (unzipped) because hydrogen bonds
are weak. GC pairs are harder to pull apart than AT pairs.
10. DNA size is often described in terms of base pairs meaning an AT or GC pair. (ex. Chromosome 1 is
approximately 250 million base pairs or bp for short. 1000 bp = 1 Kilobase or Kb.
Summary of 9 Things You Need to Know about DNA Replication
1. Replication occurs just before cell division.
2. Replication starts simultaneously at many sites along the chromosome called Origins of Replication.
3. To replicate the two strands of the double helix must first be split. (heat or helicase)
4.
5.
6.
7.
8.
9.
Each strand provides a sequence template for building a new strand according to base pair rules.
The adding and welding of nucleotides into a new strand is carried out by a protein (enzyme) called DNA
Polymerase, which adds about 1000-5000 bp/minute depending on conditions.
Replication can only occur in one direction. DNA polymerase can only add to the 3’ end of new strand.
DNA polymerase cannot start a new strand. It requires a short primer sequence as a starting point.
Once a new strand is created it is proof-read for mistakes. Before editing the error rate is about
1/100,000.
After editing the error rate in replication is about 1/10,000,000. These errors create mutations in the
DNA from one generation to the next.