Microbial Genetics Flashcards
What is the central dogma of molecular genetics?
DNA is transcribed into RNA, which is translated into RNA, which is translated to form a polypeptide
What is the exception to the central dogma of molecular genetics?
Reverse transcription
Describe transcription
Synthesis of DNA into RNA
Describe translation
Synthesis of mRNA into proteins
RNA polymerase links RNA nucleotides that are ______ to genetic sequences in DNA
Complementary
RNA polymerase binds to specific ______ to initiate transcription
Promoters
In bacteria, a polypeptide subunit of RNA polymerase called the ______ is necessary for recognition of a specific promoter
Sigma factor
Triphosphate ribonucleotides align ______ of their complements
Opposite
How, and in what orientation or direction, do all RNA polymerases synthesize or ‘write’ RNA?
5’ to 3’ direction
Like DNA polymerase, RNA polymerase links nucleotides ONLY to the ______ end of the growing molecule
3’
What are the 2 types of termination processes in bacteria?
- Self-termination
- Rho-dependent termination
When does self-termination occur?
When RNA polymerase transcribes a terminator sequence of DNA
Describe the regions present in self-termination (2)
- One region is symmetrical in guanine and cytosine
- One region is rich in adenine bases
RNA polymerase slows down during transcription of the ______ portion of the terminator
GC-rich
What is the role of the hairpin loop structure?
Puts tension on the union of RNA polymerase and DNA
What specific roles does rho protein play in rho-dependent termination? (2)
- Unzips the DNA / RNA hybrid
- Binds to a specific RNA sequence
In terms of a ‘unit of code’, how many nucleotides are necessary to encode at least 20 different amino acids?
4
If a single nucleotide encoded an amino acid, there could only be ______ different amino acids (insufficient)
4
If a pair of nucleotides encoded an amino acid, there could only be ______ different amino acids (insufficient)
16
If a triplet of nucleotides encoded amino acid, there could be ______ different amino acids (insufficient)
64
What is a codon?
mRNA molecule containing sequential nucleotides
How many sequential nucleotides are in a codon?
3
Explain why the genetic code is said to be ‘degenerate’
There are more codons than amino acids
Of the 64 possible codons, what is the start codon?
AUG
What amino acid is encoded by the start codon?
Methionine
Of the 64 possible codons, what are the stop codons? (3)
- UAA
- UAG
- UGA
What role does mRNA play in translation?
Carries genetic information from a chromosome to a ribosome
In prokaryotes, an mRNA molecule contains sequences of nucleotides that are recognized by what types of ribosomes? (3)
- Start codon
- Sequential codons
- Stop codon
What role does tRNA play in translation?
Transfers amino acids to a ribosome
A tRNA molecule has ______ ribonucleotides
75
A tRNA molecule curves back on itself to form ______
3 main hairpin loops
A tRNA molecule is held in place by ______ between complementary nucleotides
Hydrogen bonding
A tRNA molecule has ______ in its bottom loop
An anticodon triplet
A tRNA molecule has ______ at its 3’ end
An amino acid acceptor
Anticodons code for amino acid ______
Polarities
Acceptor stems code for amino acid ______
Sizes
What are the molecular components of a ribosome? (2)
- rRNA
- Polypeptides
Differentiate between 70S and 80S ribosomes in the context of prokaryotes vs. eukaryotes
- Prokaryotic cells have 70S ribosomes
- Eukaryotic cells have 80S ribosomes
70S ribosomes are composed of ______ subunits
50S and 30S
80S ribosomes are composed of ______ subunits
60S and 40S
Structural differences between prokaryotic and eukaryotic ribosomes in the context of antimicrobial drugs play a crucial role in ______
Efficacy and safety
What is the role of the A site?
Accommodates tRNA bound to an amino acid
What is the role of the P site?
Holds tRNA and the growing polypeptide
What is the role of the E site?
Exit site for discharged tRNA
Where does the smaller ribosomal subunit of a prokaryotic initiation complex attach?
At the ribosomal-binding site
Where does the smaller initiatory tRNA of a prokaryotic initiation complex attach?
At the P site
What does the larger ribosomal subunit of a prokaryotic initiation complex attach to form?
A complete initiation complex
What the cyclical process involving the addition of amino acids to a polypeptide chain growing at the P site?
Elongation
During the elongation phase, where is the dipeptide attached to the tRNA?
At the A site
What specific role do ribosomes play during the elongation phase of translation?
Formation of a peptide bond
Where is the peptide bond formed during elongation?
Between the newly introduced amino acids and the terminal amino acid of the growing polypeptide
What are ribozymes?
RNA enzymes
Translocation uses energy supplied by GTP to …
Move the ribosome one codon down the mRNA
What does translocation accomplish?
Transfers each tRNA to the adjacent binding site
During translocation, the first tRNA moves from the ______
P site to the E site
The ribosome releases the empty tRNA from the ______
E site
How is translation terminated?
The ribosome dissociates into its subunits
Translation termination does not involve ______
tRNA
What are translation release factors?
Proteins that halt elongation
Name the 3 components that make up an operon
- Promoter
- Operator
- Series of structural genes
Transcription of the structural genes is under the control of a ______
Promoter
What role does the regulatory gene play in operon regulation?
Controls transcription of the structural genes
The regulatory gene can bind to ______
Operators
The regulatory gene is not considered part of the ______
Operon
The regulatory gene has its own ______
Promoter
The regulatory gene is transcribed / translated into a ______
Regulatory protein
Describe inducible operons
Can be turned on by a small molecule
Describe repressible operons
Can be turned off by a small molecule
What occurs during the default state of the lac operon of E. coli (absence of lactose / allolactose)?
The repressor binds to the lac operator
The lac operator is adjacent to the ______
lacZ gene
In the presence of lactose, the binding of RNA polymerase is …
No longer blocked
What conditions must first be met before E. coli transcribes its lac operon? (2)
- E. coli breaks it into glucose and galactose
- β-galactosidase converts lactose into allolactose
The lac operon is a ______ operon
Negative inducible
Why must glucose be absent before lactose is catabolized?
Ensures that bacteria only use lactose after all energy is used up
β-galactosidase is encoded by the ______ gene
lacZ
Permease is encoded by the ______ gene
lacY
What results from a high concentration of glucose compared to cAMP?
Low levels of cAMP
What results from a low concentration of glucose compared to cAMP?
High levels of cAMP
What role does CAP play in activating the lac operon of E. coli?
The concentration of cAMP is proportional to the level of available glucose
Before CAP can bind to DNA, it must form a complex with ______
cAMP
Describe negative control of the lac operon
Prevents the RNA polymerase from transcribing
Describe positive control of the lac operon
Enhances the binding of RNA polymerase to the promoter
Define mutation
A permanent change in the nucleotide base sequence
Name the 2 types of point mutations
- Base-pair mutations
- Frameshift mutations
What are base-pair mutations?
A single nucleotide is substituted for another
What are frameshift mutations?
Changes that alter the reading frame
Explain how single base insertions or deletions generate frameshift mutations
Nucleotide triplets are displaced
What do frameshift mutations typically result in? (2)
- Nonsense mutations
- Missense mutations
What type of mutation results when the substitution does not change the amino acid sequence because of the redundancy of the genetic code?
Silent mutation
What type of mutation results from changing an amino acid codon into a stop codon?
Nonsense mutation
Nonsense mutations result in ______
Nonfunctional proteins
What type of mutation results from a change in nucleotide sequence resulting in a codon that specifies a different amino acid?
Missense mutation
What is a gene’s reading frame?
Multiples of 3 nucleotides that encode amino acids
What are mutagens?
Physical or chemical agents that cause mutations
Describe the origin of pyrimidine dimers
UV light causes adjacent pyrimidine bases to bond
Describe the consequences of pyrimidine or thymine dimer mutations (3)
- Prevents hydrogen bonding
- Distorts the sugar phosphate backbone
- Prevents replication and transcription
Nucleotide analogs are structurally similar to ______
Normal nucleotides
Nucleotide analogs inhibit ______
Nucleus polymerases
Nucleotide analogs result in ______
Mismatched base pairing
How does 5’-bromouracil cause point mutations?
Replaces thymine resulting in a wrong complement
What is an enzyme that is activated by visible light to break pyrimidine dimers?
DNA photolyase
What role does DNA photolyase play in the repair of pyrimidine dimers?
Reverses mutations
How does dark repair of pyrimidine dimers differ from light repair?
Involves a different repair enzyme that doesn’t require light
What roles do DNA polymerase I and DNA ligase play in dark repair?
They fix the gap created dark repair enzymes
During mismatch repair, old strands are ______
Methylated
What are homologous sequences?
Identical nucleotide sequences
Describe vertical gene transfer
The passing of genes to the next generation
Describe horizontal (lateral) gene transfer
Prokaryotes acquire genes from other microbes of the same generation
When the recipient cell inserts part of the donor’s DNA into its own chromosome, it becomes a ______
Recombinant cell
What are the 3 types of horizontal gene transfer?
- Transformation
- Transduction
- Conjugation
When a recipient cell takes up DNA from the environment, it is called ______
Transformation
The transfer of DNA from one cell to another via a replicating virus is called ______
Transduction
______ requires physical contact between cells
Conjugation
Describe the significance of Frederick Griffith’s experiments with Streptococcus pneumoniae
Discovered the process of transformation in 1928
What structural feature of Streptococcus pneumoniae conferred virulence in Griffith’s transformation studies?
The polysaccharide capsule
Describe strain S bacteria (2)
- Smooth colonies
- Encapsulated cells
Describe strain R bacteria (2)
- Rough colonies
- CANNOT make capsules
Describe what happened when Griffith injected heat-killed strain S bacteria into mice
The mice lived (no strain S cells were recovered)
Describe what happened when Griffith injected both heat-killed strain S and living strain R bacteria into mice
The mice died - even though neither strain was harmful when administered alone
Streptococcus pneumoniae / strain S cells are ______
Virulent
Strain R cells are ______
Non-virulent
What substance was ultimately shown to be responsible for the ‘transformation’ that Griffith had discovered?
DNA
What role did Avery, McCarty, and MacLeod investigators play in identifying the chemical nature of Griffith’s transforming principle?
Showed that the transforming principle was DNA
What term is used to describe viruses that only infect bacteria?
Bacteriophage
Describe the ‘life cycle’ of a lytic bacteriophage (4)
- Injects its genome into the host cell
- Synthesizes new DNA and proteins
- The host cell lyses
- Daughter and transducing phages are released
When bacteriophages mistakenly incorporate remaining fragments of bacterial DNA, it is caused by ______
Transducing phages
What are the bacterial structures that mediate conjugation?
Conjugation pili (sex pili)
Where are the genes responsible for conjugation located within the cell?
F plasmid
What are F+ cells?
Cells that serve as donors during conjugation
F+ cells contain ______
F plasmid
What are F- cells?
Recipient cells that lack F plasmid and conjugation pili
In what state is the F plasmid transferred from donor to recipient?
Conjugation
What happens when the F- recipient synthesizes a complementary strand of F plasmid?
Becomes an F+ cell
What happens when the donor cell synthesizes a complementary plasmid DNA strand?
Remains an F+ cell
What are Hfr donors?
Cells that can conjugate with an F- cell
How is the outcome of conjugation between an F+ donor and an F- recipient different from that of an Hfr donor and an F- recipient?
Remains an F- cell
Hfr donor cells are considered to be ______
Fertile
