Exam 3: What is a Gene? Part 1

Question 1

During which century did Gregor Mendel live and perform his genetic experiments?

Answer 1

Mid 1900s

Question 2

Describe the relationship between biology and natural theology during the time of Gregor Mendel.

Answer 2

Biology was governed by the natural theory which was the development of an understanding of god through observations of nature rather than supposed “divine revelation”.

Question 3

In what way did Gregor Mendel treat science differently than virtually all of his contemporaries?

Answer 3

He started treating biology as an experimental science instead of a descriptive one.

Question 4

What is a model organism? Why are model organisms important to scientific investigation?

Answer 4

Model organism: amenable to manipulation and study and findings can be applied to numerous other species.
Important because they are easily manipulated and practical to use in a lab setting but the results have broader application.

Question 5

What model organism did Mendel choose for his genetic studies?

Answer 5

The common garden pea (Pisum sativum).

Question 6

In a single sentence, describe Mendel’s particulate nature of inheritance, focusing on the importance of the term particulate in his findings.

Answer 6

The study of heritable traits that get passed down from generation to generation through discrete physical particles.

Question 7

Why do you think the importance of Mendel’s work was not recognized during Mendel’s lifetime?

Answer 7

Biology was a descriptive science, it was based on observation and recording not understanding. Also the behavior of chromosomes during cell division had not yet been studied which is needed to understand inheritance patterns.

Question 8

Did Mendel’s discovery of the particulate nature of inheritance bring humankind closer to knowing everything there is to know? Explain.

Answer 8

At the time no because it was not widely understood. But after some of the scientific gaps were filled then the true understanding of his work and how part it brought mankind was realized.

Question 9

When was Mendel’s work rediscovered?

Answer 9

1900

Question 10

What relevant and important changes and/or discoveries in biology had occurred between the time of Mendel’s work and the rediscovery of his work? In what ways were these important?

Answer 10

Biology had developed into an experimental science and the behavior of chromosomes during cell division has been studied and was now understood.

Question 11

What were two key questions of significant importance to biology that were provoked by Mendel’s chromosome theory of inheritance?

Answer 11

1) Of what substance are these genes comprised?

2) How does a gene confer a phenotype?

Question 12

What observations did Walter Sutton make regarding chromosomes and traits?

Answer 12

The behavior of chromosomes paralleled the segregation of traits and proposed the chromosome theory of inheritance.

Question 13

In a single sentence, describe Walter Sutton’s chromosome theory of inheritance. Please include his justification for this idea within this sentence. (e.g., “A” happens because “B” is true.)

Answer 13

Genes are segregated during meiosis and chromosomes are segregated during meiosis thus genes must reside on chromosomes.

Question 14

Was the term theory used appropriately in Sutton’s chromosome theory of inheritance? Explain.

Answer 14

No, it should have been a hypothesis not theory. A theory used to explain things already shown in data while hypnosis is made before any research has been done.

Question 15

What was Thomas Hunt Morgan’s initial reaction to Sutton’s chromosome theory of inheritance? Did his view ever change? Explain.

Answer 15

He was initially skeptical of Sutton’s “chromosome theory” but ultimately his own research verified Sutton’s chromosome theory of inheritance.

Question 16

What model organism did Morgan choose for his genetic studies?

Answer 16

Fruit flies, drosophila melanogaster.

Question 17

What specific traits was Morgan following when he demonstrated that genes did, in fact, reside upon chromosomes?

Answer 17

White-eyed phenotype in fruit flies.

Question 18

Was Thomas Hunt Morgan recognized in any significant way for his experiments demonstrating the accuracy of the chromosome theory of inheritance? Explain.

Answer 18

Yes he got a Nobel Prize in 1933 for his work.

Question 19

When it was established that chromosomes carry the genetic material, did this mean that the genetic material had to be DNA? Explain.

Answer 19

No, early scientists thought the protein was the more likely candidate for holding the genetic material then DNA. DNA is made up of only 4 subunits while proteins are made of 20 amino acids. The thought was that in order to carry all that genetic material you need a more complex carrier.

Question 20

In basic terms, describe the composition of a chromosome. What different types of molecules are present within this structure?

Answer 20

DNA and protein. DNA is made up of nucleotides while protein is made of amino acids.

Question 21

What was the primary focus of Frederick Griffith’s research with Streptococcus pneumoniae? Was he successful? Explain.

Answer 21

Primary focus was to develop a vaccine for pneumonia. No, he was not successful.

Question 22

Describe the three key differences between the S strain and the R strain of Streptococcus pneumoniae. Which of these differences are microscopic and which are macroscopic?

Answer 22

S strain: smooth, have a glycocalyx, virulent (lethal).

R strain: rough, lack glycocalyx, avirulent (harness).

Virulent vs avirulent is macroscopic while the rest are microscopic.

Question 23

How was it demonstrated that the R and S phenotypes of Streptococcus pneumoniae were genetically determined?

Answer 23

Take a colony of S bacteria and move to a streaking plate, collines grow and all of them are S strains. Same thing happens with R strain. Thus it is genetically determined.

Question 24

What is a glycocalyx? What is the connection between a glycocalyx and the genetic material?

Answer 24

Glycocalyx: dense, gel-like meshwork that surrounds the cell, constituting a physical barrier for any object to enter the cell.

Genetics determine if a cell has a glycocalyx or not .

Question 25

Why do you think the presence of a glycocalyx would make these bacteria more pathogenic?

Answer 25

Because of its ninja status, it helps hide the bacteria from the immune system making it easier to travel the body without detection. Makes it more pathogenic.

Question 26

What happened when Griffith injected live S-strain Streptococcus pneumoniae into a mouse? What can you conclude from this result?

Answer 26

Mouse died, living S-cells are virulent.

Question 27

What happened when Griffith injected live R-strain Streptococcus pneumoniae into a mouse? What can you conclude from this result?

Answer 27

Mouse survived, living R cells are avirulent.

Question 28

What happened when Griffith injected heat-killed S-strain Streptococcus pneumoniae into a mouse? What can you conclude from this result? (Think carefully about this question.)

Answer 28

Mouse survived. Inorder for S to be virulent it must be alive. Once dead, the S-strain is no longer virulent.

Question 29

What happened when Griffith mixed live R-strain Streptococcus pneumoniae with heat-killed S-strain Streptococcus pneumoniae, and injected this mixture into a mouse? What can you conclude from this result?

Answer 29

Mouse died, the combination of both was deadly. Somehow traits of the S strain were present.

Question 30

What was revealed by an autopsy of the mouse that received mixed live R-strain Streptococcus pneumoniae with heat-killed S-strain Streptococcus pneumonia?

Answer 30

There were living S cells in the blood of the dead mouse.

Question 31

Describe two hypotheses that would account for the type of bacteria found during the autopsy of the mouse that received mixed live R-strain Streptococcus pneumoniae with heat-killed S-strain Streptococcus pneumonia?

Answer 31

Living S cells came from dead S cells coming back alive.

Living R cells were modified.

Question 32

What did Griffith conclude had occurred when he injected the mixture into the mouse? What term did he use to describe this phenomenon?

Answer 32

Living R cells must have been transformed into S cells. Transformation only occurred in the presence of dead S bacteria. The transmission must have involved the uptake of genetic material from the dead S bacteria. Key word TRANSFORMATION.

Question 33

What was it about Griffiths experiment that suggested that DNA was a more likely candidate for the genetic material than protein?

Answer 33

When proteins are heated they are denatured and never work again. So in the process of killing the S strain (by boiling) the proteins were rendered useless. Something had to have the genetic material, something that was heat resistant (DNA).

Question 34

When did Oswald Avery investigate bacterial transformation, as compared to Frederick Griffith?’

Answer 34

1944, Griffith was 1928.

Question 35

How was Avery’s experiment like Griffith’s?

Answer 35

He continued to use R and S strain streptococcus mixing different modifications of the S strain with live R cells.

Question 36

How did Avery’s experiment differ from Griffith’s?

Answer 36

Purified S type components. Ran tests using purified different components that he hypothesized might hold genetic material. Proteins, RNA, or DNA. More exact way of isolating the different components.

Question 37

Describe, in terms of the steps of the scientific method, the results of Avery’s experiments prior to his incorporation of enzymatic steps.

Answer 37

Hypothesized that either proteins, RNA, or DNA held genetic material.

Purified S protein + R cells 🡪 no transformation
Purified S RNA + R cells 🡪 no transformation
Purified S DNA + R cells 🡪 transformation!

Results showed that DNA is a likely transforming agent.

Question 38

How did Avery interpret the results of his experiment? What was the key criticism of scientists that did not accept Avery’s conclusion? How did Avery modify his experiment to address these criticisms?

Answer 38

Based on his results only purified S strain DNA resulted in the transformation of R cells. Key criticism was that data was misinterpreted, some small proteins left even after the S strain DNA was purified. Modified experiment to include enzymes that destroyed protein contaminants and another test with an enzyme that destroyed DNA contaminants.

Question 39

Describe an enzyme in terms of its structure, its function, and its specificity. What enzymes did Avery use in this experiment, and what were their specificities?

Answer 39

Enzymes are protein catalysts that execute a reaction at a faster rate then it would occur without a catalyst. He used Protease enzyme which destroys any protein contaminant. Along with DNase enzymes that destroy DNA but not protein contaminants.

Question 40

Describe Avery’s reworked experiment in terms of the steps of the scientific method. What was his conclusion?

Answer 40

Hypothesis that potentially small amounts of protein contaminants left that were the cause of genetic transfer not DNA.

S DNA + R cells 🡪 transformation

S DNA + protease + R cells 🡪 transformation
- Protease enzymes destroy any protein contaminants

S DNA + DNase + R cells 🡪 no transformation!!!
- DNase enzymes destroy DNA but not protein contaminants

The DNA test was not contaminated, no protein was present.

Question 41

Why do you think Avery’s conclusion was not widely accepted by the scientific community?

Answer 41

1) only worked with a single system, more compelling if tested on more species.

2) Scientists are not prone to jumping on bandwagons, new ideas are not easily accepted.

3) Scientists don’t want to be wrong.

Question 42

Erwin Chargaff was an American scientist. In what way is this fact is related to World War II?

Answer 42

He was a jew that fled Germany when Hitler came to power.

Question 43

Describe the nature of the DNA analyses performed by Erwin Chargaff.

Answer 43

Analyzed the base composition of DNA from a variety of organisms. Discovered the basic composition of DNA is consistent within species but differs between species.

Question 44

What did Chargaff observe with respect to the relative abundance of each of the four different DNA bases? How did these relative abundances vary within a species? How did these relative abundances vary between species?

Answer 44

The abundance of the different bases was relatively consistent between individuals of the same species. This abundance varies between different species.

Question 45

In what way did Chargaff’s observations suggest that DNA is likely to be an important component of a cell?

Answer 45

Suggested that differences in DNA is the common thread between cells but also the defining factor between cells. Acts to distinguish one cell of a species form a different species.

Question 46

What are Chargaff’s Rules?

Answer 46

DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine

Question 47

Approximately 29% of the nitrogenous bases from the DNA of the cyanobacterium Nostoc punctiforme are adenine. What are the relative frequencies of the other three nitrogenous bases?

Answer 47

Thymine should be equal to aprox 29%. Cytosine and guanine should each be equal to around 21%.

Question 48

What is a bacteriophage?

Answer 48

Viruses that infect and replicate only in bacterial cells.

Question 49

Describe the structure of T2 bacteriophage.

Answer 49

Composed exclusively of DNA and protein.

Question 50

Explain why a viral infection must constitute a genetic reprogramming.

Answer 50

The bacterial cell is converted into a virus producing factory.

Question 51

What is a radioisotope?

Answer 51

An unstable form of a chemical element that releases radiation as it breaks down and becomes more stable.

Question 52

What radioisotopes were used in Hershey and Chase’s blender experiment? Why were they used?

Answer 52

Radioactive sulfur and radioactive phosphorus were used in the experiment. Chosen because proteins have an abundance of sulfur while DNA does not and DNA has an abundance of phosphorus but proteins do not.

Question 53

When T2 bacteriophage was grown in the presence of radioactive sulfur, what viral component became radioactive? Why? Explain.

Answer 53

Proteins became radioactive as they took in the radioactive sulfur from their environment.

Question 54

When T2 bacteriophage was grown in the presence of radioactive phosphorus, what viral component became radioactive? Why? Explain.

Answer 54

DNA became radioactive because it is the only component of the bacteriophage that uses phosphorus.

Question 55

The Hershey-Chase blender experiment consisted of two parallel experiments, testing a pair of parallel hypotheses. Describe each hypothesis/experiment in terms of the scientific method. Include the results and conclusions in your description.

Answer 55

Hypothesized that the reprogramming of E.coli by T2 bacteriophage must be a genetic reprogramming. During this infection genetic material must be entered into the E.coli.

Grew one batch of phage in the presence of radioactive sulfur. Reactively labeled phage proteins not DNA since sulfur is present in proteins but not DNA

Grew a second batch of phages in the presence of radioactive phosphorus. Reactively labeled DNA not proteins since phosphorus abundant in DNA but not proteins.

Infected E.Coli with the radiolabeled phages. Left together for a short amount of time so infection could be established.

Blended mixture to remove phage parts loosely associated with the bacteria surface. Parts of the phage that entered the bacteria are maintained.

Separated cells from the liquid in a centrifuge. Portions of virus not inside cells remained in liquid, cells were collected and pelted.

Measure radioactivity in the pellet and liquid. In batch 1 radioactive sulfur found in the liquid, protein did not enter the cell. In batch 2 radioactive phosphorus found in the pellet, DNA did enter the cell.

Since macrophages are only made of DNA and protein one of the two have to be responsible for genetic transfer. Based on the test results only DNA enters the cell not proteins.

Question 56

What did Hershey and Chase conclude to be the genetic material? In a single sentence, provide their rationale for this conclusion.

Answer 56

DNA holds the genetic material because it was shown to enter the bacterial cell while the proteins did not.

Question 57

Why do you think the conclusions of Hershey and Chase were accepted by the scientific community much more quickly than the results of Griffith or Avery?

Answer 57

Time, almost 10 years had passed so more scientists were accepting and had opened up to the idea that DNA held genetic material not proteins.

Question 58

Were Alfred Hershey and Martha Chase recognized in any significant way for their experiments demonstrating the identity of the genetic material? Explain.

Answer 58

Hershey received a Nobel Prize in 1969. Chase did not receive the Nobel Prize nore did Hershey acknowledge her contributions during his acceptance speech.

Question 59

How may experiments did Crick and Watson perform in their quest to determine DNA’s three-dimensional structure?

Answer 59

None, their entire determination was based on modeled DNA structure based experimental evidence on actual experiments of their own.

Question 60

In what way did Erwin Chargaff’s work help Crick and Watson build an accurate three-dimensional model of DNA?

Answer 60

Analyzed DNA base composition of several species. Discovered variation between but not within species. Also discovered “chargaff’s rules” which state A and T residues are always equal and G and C residues are always equal. Whatever the D structure is it must abide by “chargaff’s rules”.

Question 61

In what way did Rosalind Franklin’s work help Crick and Watson build an accurate three-dimensional model of DNA?

Answer 61

Used x-ray crystallography. Purified DNA is crystalized which diffract x-rays from a pattern of exposure on x-ray film. Determined various characteristics of DNA; phosphate groups on the outside of the molecule, DNA is helical, DNA is two stranded, determined the shape and size of the double helix. Could not not determine how the bases are paired.

Question 62

Were Crick and Watson recognized in any significant way for their contributions in elucidating the three-dimensional structure of DNA? Explain.

Answer 62

Nobel Prize in 1962

Question 63

What was Rosalind Franklin’s research focus?

Answer 63

X-ray crystallography, specifically DNA crystallography.

Question 64

In a single sentence, describe the process of X-ray crystallography in basic terms.

Answer 64

Crystals diffract x-rays which form a pattern of exposure on x-ray film.

Question 65

What features of DNA’s three-dimensional structure did Rosalind Franklin infer from her X-ray crystallography images?

Answer 65

Phosphate groups are on the outside of the molecule

DNA is helical, and has two strands

Determined the shape and size of the double helix

Could not yet determine how the bases paired

Question 66

Was Rosalind Franklin recognized in any significant way for her contributions in elucidating the three-dimensional structure of DNA? Explain.

Answer 66

No, not only because she was a woman but because she died before the Nobel Prize was awarded so was never elliable.

Question 67

Was Maurice Wilkins recognized in any significant way for his minimal contributions in elucidating the three-dimensional structure of DNA? Explain.

Answer 67

Yes, the 1962 Nobel Prize awarded to crick and Watson was shared with Wilkins.

Question 68

Using whatever combination of words and pictures you desire, describe the three-dimensional structure of DNA.

Answer 68

Two antiparallel strands pointed in opposite directions.

5’ and 3’ ends are distinguished by which carbon is not attached to another sugar.

The base sequence of the first strand dictates the base sequence of the second strand.

Second strand is important for DNA replication.