MCBL 121 Exam 2 Review

Which of the following is the role of DNA Gyrase?

• The replication of the template strand DNA sequence

• The supercoiling of the DNA

• The opening of the replication bubble

• Releasing tension caused by unwinding DNA

Answer explanation

ANSWER: DNA Gyrase releases tension caused by replication!

DNA Helicase unwinds the replication bubble in order for replication to occur. Gyrase is needed to remove tension at the beginning of the replication bubble in order to protect the DNA from breaking. Topoisomerase is the enzyme for remodeling DNA supercoiling structure.

Sigma factors bind RNA Polymerase to what locations of the DNA?

• -10, -16

• -30, -10

• -10, -35

• Shine Dalgarno Sequence

Answer explanation

ANSWER: -10 and -35 positions!

Sigma factors in the RNA polymerase holoenzyme recognize and bind in at the -10 and -35 positions of the DNA. The Shine Dalgarno sequence is an RNA sequence which binds the 16S region of a ribosome in order to initiate translation.

Rolling circle replication __________________.

• Starts at one origin and proceeds in two directions at once

• Starts at several origin sites and proceeds in one direction only

• Starts at origin and proceeds in one direction only

• Starts at one origin and proceeds in two directions at once.

Answer explanation

Answer: One origin, one direction!

Rolling circle replication is one form of plasmid replication and starts at a single OriC and proceeds in one direction only. This is unlike bacterial chromosomal replication (and some plasmids) which use Theta replication which starts at one OriC but moves in two directions.

The Lac Operon is _________ in the presence of ______.

• Turned on; Lactose

• Turned off; Lactose

• Turned off; cAMP

• Turned on; Glucose

Answer explanation

ANSWER: Turned ON ; Lactose

The Lac operon is turned on in the presence of lactose or allolactose. Maximum expression also needs cAMP which is made when the cell is starved for carbon.

What are the components of an Operon in the correct order?

• Regulatory Sequence, Promoter 1, Operator, Promoter 2, Structural Genes

• Promoter 1, Regulatory Sequence, Promoter 2, Operator, Structural Genes

• Promoter 1, Regulatory Sequence, Operator, Promoter 2, Structural Genes

• Operator, Promotor 1, Regulatory Sequence, Promoter 2, Structural Genes

Answer explanation

ANSWER: Promoter 1, Regulatory Protein Sequence, Promoter 2, Operator, Structural Genes

1) Promoter 1: Promoter for the regulatory protein sequence.

2) Regulatory Protein Sequence: Encodes the regulator (activator or repressor) protein for the operon.

3) Promoter 2: Promoter for the structural genes.

4) Operator: Binds with the repressor / activator.

5) Structural Genes: The genes encoding the proteins the operon expresses.

Using the Griffith Experiment set-up, if a mouse was injected with heat-killed R strain and live S strain it would _____. If it was injected with heat-killed S strain and live R strain it would _____.

• Die ; Die

• Live ; Live

• Die ; Live

• Live ; Die

Answer explanation

ANSWER: Die ; Die

The S strain is pathogenic.

The R strain is harmless.

Any condition in which the S strain is alive will kill the mouse.

Additionally, heat killed S strain + live R strain kills the mouse because the non-pathogenic R strain uptakes the DNA containing the pathogenicity factor from the pathogenic S strain and transforms the R strain into live S strain.

In the Hershey-Chase experiment, radioactive isotope _____ was used to mark DNA because that element is found in DNA but not proteins.

• Sulfur-35

• Nitrogen-15

• Radon-223

• Phosphorus-32

Answer explanation

ANSWER: Phosphorus-32

In the Hershey-Chase experiment bacteriophage particles were radioactively labelled to determine which portion of the phage (DNA or Protein) was transmitted to the bacterial cell. Viral DNA was tagged with P-32 because phosphorus is found in DNA but not amino acids. Viral Protein was tagged with S-35 but sulfur is found in proteins but not DNA.

What is the main difference between a Plasmid and a Secondary Chromosome?

• A plasmid has its own origin of replication. Secondary chromosomes do not.

• A secondary chromosome is large and a plasmid is small.

• A secondary chromosome contains one or more essential genes. Plasmids do not have essential genes.

• Plasmids encode genes related to antimicrobial resistance.

Answer explanation

ANSWER: Secondary Chromosomes have at least one essential gene.

Plasmids contain auxiliary genes. They have their own independent origin of replication and replicate independently from the core genome. However, if that plasmid contains one or more essential genes then this is when we typically consider this plasmid a secondary chromosome.

Under which conditions would we expect transcription of the Lac Operon to be highest?

• High Glucose, Low Lactose, Low cAMP

• High Glucose, High Lactose, High cAMP

• Low Glucose, High Lactose, Low cAMP

• Low Glucose, High Lactose, High cAMP

Answer explanation

ANSWER: Low Glucose, High Lactose, High cAMP

The Lac operon needs Lactose or Allolactose in order to remove the inhibitor from the operon. Next, glucose levels need to be low in order to starve the cell for carbon. When the cell is carbon starved, cAMP levels build. The promoter for the Lac operon uses a cAMP - mediated complex.

What is the corepressor of the Trp operon?

• trpA

• Lactose / Allolactose

• tryptophan

• cAMP

Answer explanation

ANSWER: Tryptophan

When tryptophan binds to the Trp Operon aporepressor it forms the holorepressor which then binds to the Trp Operon operator site and greatly inhibits tryptophan synthesis.