DNA and Protein Synthesis Review ANSWERS!!!
1. What are nucleotides? Describe their structure.
They are the monomers (“building blocks”) of nucleic acid. They are made up of a 5-carbon sugar (ribose in
RNA or deoxyribose in DNA), a phosphate, and a nitrogen base (A, C, G, T [DNA] or U [RNA]). The sugar
and phosphate make up the backbone of the structure. The nitrogen base is bonded to the sugar.
2. Summarize the Hershey and Chase experiment and explain what their results suggested.
The Hershey-Chase blender experiment:
- Took viruses called a bacteriophages (will infect bacteria) and labeled their protein with heavy sulfur (35S).
The bacteriophages infected the bacteria. After centrifugation, the radioactive protein was found to be in
the supernatant, NOT in the pellet with the bacteria.
- Labeled the nucleic acid of the bacteriophage with heavy phosphorus (32P) and carried out the experiment as
stated above. The radioactive phosphorus was found in the pellet, which indicated that it was injected into
the bacteria.
- Hershey and Chase concluded that the viral DNA was the genetic material, not the protein, that was taking
over the bacteria.
3. Chargaff made an important contribution to uncovering the structure of DNA. What were his findings and what do
they indicate? He found that in every species that he studied, if there were a certain number of adenines,
then there was an equal number of thymine. The same was true for guanine and cytosine. This indicated that
A must bond with T and C with G….complementary base pairing!
4. Who proposed the “double helix” model for the DNA molecule? Watson and Crick
5. Describe how DNA and RNA differ in their composition, structure, function, and location.
Nucleic Acid
Composition
Structure
Function
Location
DNA
Deoxyribose
thymine
double helix
Master “blueprint”
nucleus
RNA
Ribose
uracil
single segments or
strands
Ex. mRNA, tRNA,
primer during DNA
replication…
Mainly outside the
nucleus
6. What is meant when we say that DNA replication is complementary? A-T,C-G
7. Why is DNA replication important for every cell? Each daughter cell needs a complete set of DNA in order to
properly functionb.
8. Compare the amount of DNA in a muscle cell with that in a brain cell. It is the exact same.
9. What is meant by saying that DNA replication is semi-conservative? Each daughter molecule has one parent
(old) strand and one daughter (new) strand.
10. Briefly describe the steps in DNA replication.
Here we go…
- Helicase unwinds the DNA, and the replication fork is held open by single-strand binding proteins.
- DNA polymerase III creates a daughter strand of DNA in the 5’ to 3’ direction via complementary base
pairing.
- The leading strand is formed in a simple, continuous fashion. The lagging strand is formed via segments
called Okazaki fragments.
- To create the lagging strand, primase creates and RNA primer from which DNA polymerase III will then
create an Okazaki fragment in the 5’ to 3’ direction.
- When the DNA polymerase III reaches the RNA primer of a previous Okazaki fragment, it detaches and
DNA Polymerase I bonds to the nucleic acid and replaces the RNA primer with DNA.
- The gap between the two Okazaki fragments is then sealed by ligase.
11. Why is replication on one strand of DNA continuous, while on the other strand the replication must be
discontinuous? The strands of DNA are antiparallel and complementary strands can only be constructed in a 5’
to 3’ direction. At the 3’ end there is a free hydroxyl group and in order to bond the next nucleotide (which,
while floating in the cytosol, has three phosphates in total!!), two phosphates have to be lost so that the
energy released can be used to form a bond. This bond forms between the remaining phosphate and the
terminal sugar on the growing daughter strand.
12. What functional group is at the 5' end of a DNA molecule? Phosphate
13. Proofreading enzymes scan DNA to check for base pairing errors. Explain why these enzymes are important.
They minimize the number of mutations when replicating the DNA.
14. Why is the making of exact copies of DNA called replication rather than duplication? Because calling it DNA
duplication would make it sound like the parent DNA strands stays together and that the new DNA is entirely
made of daughter DNA. This terminology would support the CONSERVATIVE model, not the
SEMICONSERVATIVE model!
15. If human DNA contains approximately 3x10 9 base pairs, and DNA polymerase can work at the rate of about 50
nucleotides per second, how can our DNA be replicated so quickly? There are a number of origins of replication;
replication will occur in both directions within a replication bubble AND on both strands, all at the same time!
16. If 27 percent of the bases in a certain segment of DNA were adenine, what would be the percentages of thymine
(27%), cytosine (23%), and guanine (23%)?
17. A segment of chromosomal DNA which contains instructions for one protein is a ____gene_________.
18. Describe the technique of DNA fingerprinting.
Since DNA contains some sequences that are unique to individuals, this information can be used to identify
someone just like a regular fingerprint can. Restriction enzymes are used to cut the DNA of an individual at
specific sites such that they have DNA divided into varying lengths. These segments of DNA are then divided
during a process called gel electrophoresis, and the pattern of the bands in the gel is unique to one person.
19. As a research biologist, you know of a bacterium that produces an antifungal treatment that is quite effective
against a certain crop plant fungus. There would be great economic importance in enabling the plant to resist the
fungus. How might you use DNA technology to accomplish this?
Using recombinant DNA!! The bacterial DNA coding for the antifungal treatment is inserted to the plant
DNA. The resulting plant will be resistant to the fungus.