Chapter 8: Microbial Genetics

Question 1

Initiation (step 1)

Answer 1

30S and 50S ribosomal subunits assemble to form 70S around mRNA and fMet-tRNA binds P site
The AUG start codon of the mRNA is at the P site; anticodon of fMet-tRNA binds AUG codon

Question 2

Translation

Answer 2

mRNA is translated in codons (three nucleotides)

Question 3

Elongation (step 2)

Answer 3

(repeats until mRNA stop codon reaches A site)
Amino acyl tRNAs (tRNA+amino acid) are brought to the A site of the ribosome (anticodons binding mRNA codons)

Question 4

Transpeptidation

Answer 4

the amino acids in the P site join the new amino acid in the A site by a peptide bond.

Question 5

Translocation

Answer 5

movement of the ribosome along the mRNA; what was in the A site moves to P, the uncharged tRNA in P moves to E and dissociates, A site is empty

Question 6

Termination (step 3)

Answer 6

occurs when ribosome reaches a stop codon (on mRNA in A site)
Release factors (RF): recognize stop codon
Completed polypeptide is cleaved from tRNA, and ribosome subunits dissociate
Ribosomal subunits free to form new initiation complex and make another polypeptide (on same mRNA or on different mRNA)

Question 7

Polysomes

Answer 7

a complex formed by ribosomes simultaneously translating mRNA

Question 8

Constitutive genes

Answer 8

are expressed at a fixed rate (are not regulated)
Other genes are expressed only as needed (are regulated)

Question 9

Other Constitutive genes that are expressed only as needed (are regulated)

Answer 9

Repressible genes (turn OFF by a repressor)
Inducible genes (turned ON by an inducer)

Question 10

Mutation

Answer 10

A change in the genetic material
Mutations may be neutral, beneficial, or harmful

Question 11

Spontaneous mutations

Answer 11

Occur in the absence of a mutagen (like mistakes by DNA polymerase during replication)

Question 12

Point mutation

Answer 12

a single base change

Question 13

Missense mutaton

Answer 13

mutation that results in a different amino acid code

Question 14

Silent or neutral mutation

Answer 14

mutation that results in the same amino acid code

Question 15

Nonsense mutation

Answer 15

results in a stop or nonsense codon

Question 16

Frameshift mutation

Answer 16

loss/gain of base results in a change in reading frame, all downstream amino acids are changing

Question 17

Horizontal gene transfer

Answer 17

acqusition of DNA from an external source

Question 18

Recombination

Answer 18

movement of segments of DNA within the same cell

Question 19

Vertical gene transfer

Answer 19

occurs during reproduction
DNA passes from parent to progeny

Question 20

Horizontal gene transfer

Answer 20

The acqusition of DNA from an external, unrelated source.
Plasmid conjugation (typical and atypical)
Transformation
Transduction

Question 21

Recombination

Answer 21

Exchange of DNA between two molecules within a cell
DNA moves from one place to another within the same cell

Question 22

Homologous recombination

Answer 22

requires flanking sequences of homology (matching DNA on the left and right side, switching out the middle)

Question 23

Transformation

Answer 23

Outside naked DNA taken up into cell
DNA must recombine into chromosome by homologous recombination (for successful HGT)
Or, it will be degraded (unsuccessful HGT), used as a source of nucleotides only

Question 24

Plasmids

Answer 24

Plasmids are small circles of DNA that carry non-essential genes.

Question 25

Conjugative plasmid

Answer 25

Carries genes for conjugative transfer of the plasmid (sex pilus)

Question 26

F plasmid or F factor

Answer 26

best-studied conjugative plasmid (in E. coli and relatives)

Question 27

Bacterial Conjugation (aka mating)

Answer 27

Requirements for conjugation:
Cell-to-cell contact (mediated by sex pilus and mating bridge)
tra genes (encode the pilus and other apparatus necessary for conjugation)
oriT (origin of transfer, where rolling-circle replication begins to make a copy of the plasmid DNA)