Title: Unit 3 Review
Course: SAT 170 Introductory Biotechnology > Section: SAT 170 Introductory Biotechnology

Q: The nucleotides within a DNA molecule are oriented with the nitrogenous base towards the center and the sugar and the phosphate molecules of different nucleotides bonding together to form the backbone.
- True
- False
A: True

Q: What was Rosalind Franklin’s role in resolving the structure of DNA?
Feedback: Rosalind Franklin was an x-ray crystallographer who captured an image of a DNA molecule which was used by Watson and Crick to determine the twisted, double stranded nature of DNA.

Q: DNA fluctuates between the uncondensed chromatin state and the more condensed chromosome state which is much less accessible since it is very tightly coiled around different proteins.
- True
- False
A: True

Q: List and describe the features of the double helix model for DNA structure first proposed by Watson and Crick in 1953.
Feedback: DNA is composed of four types of DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T). A and G are considered to be purine nucleotides, while C and T are pyrimidines. These bases form complementary base pairs (A pairs with T and C pairs with G) in the center of the DNA molecule. The backbone of each strand is comprised of the sugars and phosphates of the nucleotides. The two strands of the DNA molecule are said to be anti-parallel, where one strand is oriented in one direction (5’ to 3’) and the parallel strand runs in the opposite direction (3’ to 5’).

Q: Explain Chargaff’s rules.
Feedback: The first of Chargaff’s rules stated that the composition of DNA varies from one species to another. The second rule is now known as complementary base pairing where the amount of adenine equaling the amount of thymine and the amount of cytosine equaling the amount of guanine.

Q: If a particular DNA sample contains 20% cytosine, how much of that DNA would be thymine? (Hint: Recall that cytosine always bonds with guanine and thymine always pairs with adenine?).
A: 30%
If right: 30%; If there is 20% cytosine, there must also be 20% guanine (since C always pairs with G). This means that 40% of the DNA is C and G; thus the remaining 60% must be A and T. Since A and T are always present in equal portions, there must be 30% of each.
If wrong: 30%; If there is 20% cytosine, there must also be 20% guanine (since C always pairs with G). This means that 40% of the DNA is C and G; thus the remaining 60% must be A and T. Since A and T are always present in equal portions, there must be 30% of each.

Q: Briefly describe the transformation experiment carried out by Frederick Griffith in the early 1900’s. Be sure to explain how this experiment was significant to our current understanding of DNA.
Feedback: 7. Griffith was studying Streptococcus pneumoniae, a pathogenic bacterium which causes pneumonia. There were two strains of this bacterium: an R strain which was non-pathogenic and an S strain which was pathogenic. When mice were injected with the R strain bacteria, they did not get sick, but when they were injected with the S strain bacteria they developed pneumonia and died. Griffith next decided to first heat kill the S strain bacteria before injecting them into mice and the mice did not get sick. However, when combining the heat-killed S cells with live R cells, he noticed that the R cells were transformed into pathogenic cells. This led Griffith to the question of what was being transferred from the heat-killed S cells to the live R cells causing their transformation. He believed that the answer to this question would lead him to identifying the genetic information.

Q: In your own words, define transformation.
Feedback: Transformation is the alteration of cells as they acquire novel genetic material from either their environment or another cell.

Q: Briefly describe the experiment performed by Hershey and Chase in the mid 1900’s. Be sure to explain how this experiment was significant to our current understanding of DNA.
Feedback: Hershey and Chase used a bacteriophage, which is a type of virus that infects bacteria. It consists entirely of DNA and protein. Knowing that a virus works is by injecting its genetic information into its host cell, Hershey and Chase used radioisotopes to label the DNA component as well as the protein component of the virus. When they traced the radioisotopes, they observed that it was the DNA fragment that ended up inside the host cell, thus, coming to the conclusion that DNA carries the genetic information.

Q: The complementary base pairs of a DNA molecule are held together by hydrogen bonds; the phosphodiester bonds are present between the sugar and the phosphate of the backbone.
- True
- False
A: False

Q: The DNA replication enzyme that is responsible for synthesizing primers is known as telomerase.
- True
- False
A: False

Q: What is a mutagen? Provide at least one example of a mutagen.
Feedback: A mutagen is an agent that can cause genetic mutations. UV radiation, x-ray radiation, and certain chemicals are all examples of mutagens.

Q: Which of the following is not true regarding DNA?
- it is double stranded
- it contains ribose sugar
- it’s structure is a double helix
- it contains A, T, C, G
- it is located within the nucleus of eukaryotic cells
A: it contains ribose sugar

Q: What were the three proposed models of DNA replication? Briefly explain each.
Feedback: The first model was known as the Conservative Model, which explained that during DNA replication the strands of the parent DNA molecule would serve as templates to produce daughter strands, but once replication was complete the result would be one DNA molecule composed of the original (parent) strands and a second DNA molecule composed of two new (daughter) strands. In contrast the semi-conservative model predicted that the strands of the parent DNA molecule would each serve as a template for a new daughter strand and the resulting two DNA molecules would each be composed on of one parent and one daughter strand. The last of the models being tested was the dispersive model. This model outlined DNA replication as a seemingly more random process, where the strands of the parent DNA molecules would serve as templates for the daughter strands but the resulting two DNA molecules would be composed of DNA strands that had both parent and daughter fragments.

Q: Compare and contrast missense and nonsense point mutations.
Feedback: Both missense and nonsense mutations are a type of point mutation which involves only a single nucleotide of DNA. A missense mutation is an alteration of a single nucleotide base within a gene that results in a change of a single amino acid of the protein product of that gene into a different amino acid. A nonsense mutation is an alteration of a single nucleotide base within a gene that results in a change of a single amino acid of the protein product of that gene into a stop signal for expressing that gene.

Q: What is the function of telomerase? Why would it be beneficial for cancer cells to produce telomerase?
Feedback: Telomerase is responsible for adding sequences of nucleotides, known as telomeres, to the ends of our chromosomes. Since telomeres shorten each time the chromosome is replicated it limits the number of times a chromosome, and therefore a cell, can be replicated. Since cancer cells divide uncontrollably and in some cases very rapidly, they would benefit from having chromosomes that would never degrade.

Q: Nucleotide excision repair is a specific type of repair mechanism that deals with DNA damage, while both mismatch repair and base excision repair can be used response to different types of damage.
- True
- False
A: True

Q: Compare and contrast the leading and lagging daughter strands of DNA replication.
Feedback: The lagging daughter strand, this is the strand that is running 5’ to 3’ parallel with the direction of replication. This strand requires only one primer and can be synthesized continuously as the parent strands are separated along the direction of replication. The opposite strand is referred to as the lagging strand. Because this strand is running 5’ to 3’ opposite to the direction of replication it must be synthesized in fragments. These fragments are known as Okasaki fragments. Each fragment requires a new primer.

Q: What is apoptosis? Why would this process be important for multicellular organisms but not unicellular organisms?
Feedback: Apoptosis is a form of programmed cell death that often follows extensive cell or DNA damage. This is beneficial for multicellular organisms because if a single cell in that organism is damaged it may become cancerous and start dividing uncontrollably. A unicellular organism does not have to worry about this risk, the DNA damage is not going to spread any further.

Q: What is a karyoptype?
Feedback: A karyotype provides information on the shape, appearance and number of chromosomes in an individual’s cells.