Microbial Genetics

Question 1

In E. coli, the DNA is twisted by what enzyme?

Answer 1

Topoisomerase II or DNA gyrase

Question 2

Define the process of DNA replication

Answer 2

1. Two strands of parental DNA are unwound and separated from each other in one small DNA segment after another
2. Free nucleotides are matched up to the exposed bases of the single-stranded parental DNA
3. Then the parental DNA is unwound a bit further to allow the addition of the next nucleotides
4. The original strand and the newly synthesized daughter strand rewind

Question 3

What is the point at which replication occurs called?

Answer 3

Replication fork

Question 4

Why is DNA replication called semiconservative replication?

Answer 4

Because each new double-stranded DNA molecule contains one original, conserved strand and one new strand

Question 5

What enzyme is involved in the process of adding new nucleotide to the growing DNA strand?

Answer 5

DNA polymerase, which can add new nucleotides to the 3’ end only (5’ –> 3’ direction)

Question 6

Function of RNA Primer

Answer 6

RNA primer is needed to start synthesis. DNA polymerase can then add nucleotides to the 3’ end of the RNA.

Question 7

What is leading strand? What is lagging strand?

Answer 7

1. Leading strand: the DNA stand that is continuously synthesized
2. Lagging strand: since DNA polymerase can only add nucleotides to the 3’ end, one new DNA must be synthesized in pieces consisting Okazaki fragments.

Question 8

What is transcription?

Answer 8

The synthesis of a complementary strand of RNA from a DNA template

Question 9

Explain the transcription process

Answer 9

1. RNA polymerase binds to the DNA promoter
2. RNA ploymerase assembles free nucleotides into a new chain
3. RNA synthesis continues until RNA polymerase reaches DNA terminator
4. Single-stranded mRNA are released from the DNA

Question 10

Explain the transcription process

Answer 10

1. RNA polymerase binds to the DNA promoter
2. RNA ploymerase assembles free nucleotides into a new chain
3. RNA synthesis continues until RNA polymerase reaches DNA terminator
4. Single-stranded mRNA are released from the DNA (and transported to ribosomal RNA)

Question 11

There are 64 possible codons but only 20 AAs, what is this situation called? Why is it important?

Answer 11

1. It’s called degeneracy of the code
2. Degeneracy allows for a certain amount of change, or mutation in the DNA w/o affecting the protein ultimately produced

Question 12

List the 3 nonsense or stop codons

Answer 12

UAA
UAG
UGA

Question 13

List the start codon. What AA does it code for?

Answer 13

AUG

Methionine – the initiating methionine is often removed later, so not all proteins begin with Met.

Question 14

Explain the translation process

Answer 14

1. mRNA is transported to the Ribosome; a tRNA carrying the first AA is paired w/ the start codon on mRNA
   - - The place on the ribosome where the first tRNA sits is called the P site
2. A tRNA carrying the 2nd AA approaches (at A site of the Ribosome)
3. The ribosome moves along the mRNA until the wnd tRNA is in the P site, and the process continues
4. When the ribosome reaches a Stop codon, the polypeptide is released
5. Finally, the last tRNA is released, and the ribosome comes apart

Question 15

Why can’t the RNA transcrip by used for translation in Euk. cells?

Answer 15

1. Euk. genes are composed of Exons and Introns, which are transcribed to RNA by RNA polymerase
   - - Exons: The regions of DNA which can be expressed
   - - Introns: The intervening regions of DNA that do not encode protein
2. The long RNA transcript must be processed by ribozymes to remove the intron-derived RNA and splice together the exon-derived RNA, producing an mRNA
3. The mature mRNA then travels to the cytoplasm, where it is used by rRNA to direct protein synthesis

Question 16

Simply describe the transcription and translation process

Answer 16

1. DNA is transcribed into mRNA
2. mRNA attaches to ribosomes
3. rRNA delivers the AAs to the ribosome
4. The ribosome assembles the AAs into polypeptide chains

Question 17

What is a repressor?

Answer 17

1. A repressor is a regulatory protein which mediates regulatory mechanism that inhibits gene expression so to decrease the synthesis of enzymes
2. Repressors block the ability of RNA polymerase to initiate transcription from the repressed genes
   - - Repressors locate after the Promoter

Question 18

How is the repressedn gene induced to start transcription?

Answer 18

1. An inducer binds to the repressor protein and alters it so it can’t bind to the operator site
2. In the absence of an operator-bound repressor protein, RNA polymerase can transcribe the structural genes into mRNA, which is then translated into proteins

Question 19

Base substituation mutation: missense mutation, nonsense mutatuon
Frameshift mutation:

Answer 19

1. Base substituation: a single base at one point in the DNA sequence is replaced w/ a dif. base
2. Frameshift mutation: one or a few nucleotide pairs are deleted or inserted in the DNA

Question 20

Base substituation mutation: missense mutation, nonsense mutation
Frameshift mutation:

Answer 20

1. Base substituation: a single base at one point in the DNA sequence is replaced w/ a dif. base
   a. Missense mutation: incorrect AA is inserted to the protein
   b. Nonsense mutation: a stop codon is created in the middle os an mRNA molecule, preventing synthesis of a complete functional protein
2. Frameshift mutation: one or a few nucleotide pairs are deleted or inserted in the DNA

Question 21

UV induced DNA mutation

Answer 21

Adjacent Thymines in a DNA strand can cross-link to form thymine dimers.
– Such dimers unless repaired, may cause serious damage or death to the cell b/c it can’t properly transcribe or replicate such DNA

Question 22

Genetic transduction

Answer 22

1. In the proces of infection, the phage attaches to the donor bacterial cell wall and injects ins DNA into the bacterium
2. The phage DNA acts as a template for the synthesis of new phage DNA and also directs the synthesis of phage protein cots. During phage development inside the infected cell, the bacterial chromosome is broken apart by phage enzymes and,
3. At least some pieces of bacterial DNA are mistakenly packaged inside phage protein coats. The resulting phage particles then carry bacterial DNA instead of phage DNA
4. When the released phage particles later infect a new population of bacteria, bacterial genes will be transferred to the newly infected recipient cells at low frequency
5. Transduction of cellular DNA by a virus can lead to recombination b/w the DNA of the donor host cell and the DNA of the recipient host cell.