Microbial Genetics Questions and Answers

Answer: A

The technique is Replica plating.

An auxotroph grows on rich medium like the wild type, so it cannot be told apart on the rich plate. Colonies are transferred with a velvet pad from a master plate onto a complete medium and onto a minimal medium in the same positions. A colony that grows on the complete medium but is missing from the minimal plate is an auxotroph, and it can be picked from the master plate.

Streaking and pour plating isolate single colonies but give no way to tell which colony is the mutant, and a microscope cannot see a nutritional requirement.

Answer: C

Resistance arises mainly from a missense mutation in the gyrase gene.

DNA gyrase is essential, so a resistant cell must keep a working enzyme. Nalidixic acid resistance usually comes from single amino-acid substitutions in the GyrA subunit (in the 'quinolone resistance-determining region', for example at serine 83 in E. coli), which lower the drug's binding while leaving the enzyme active.

A nonsense or deletion mutation would destroy the enzyme and kill the cell, and degrading the gyrase protein would do the same, so none of them can be the usual route to resistance.

Answer: B

The order of replication is AGBFCDE.

Replication starts at Ori and two forks move away from it in opposite directions at the same speed. A gene is therefore copied at a time proportional to its distance from Ori, measured along the shorter way round. Reading the map: A lies just clockwise of Ori and G just anticlockwise; B is next on the clockwise side, then F on the anticlockwise side; the clockwise fork then reaches C and D, and E, lying almost opposite Ori, is copied last.

ABCDEFG follows only one fork round the whole circle, as if replication were unidirectional. GAFBECD and GAFEBCD put G before A and bring E in far too early, although E is the gene farthest from the origin.

Answer: A

Genetic recombination is the formation of a new genotype by reassortment of genes following an exchange of genetic material between two different chromosomes that have similar genes at corresponding sites. These are called homologous chromosomes and are from different individuals.

Answer: A

Correct answer: structural genomics

Structural genomics is concerned with the physical organisation of the genome - sequencing it, mapping it, and determining the structures of what it encodes. Functional genomics asks what the genes do, and comparative genomics what the differences between genomes signify.

Answer: D

Correct answer: the study of the entire collection of proteins expressed by an organism

The proteome is what the genome actually produces, and it differs between cells, conditions and moments in a way the genome does not. It also reflects processing and modification that the sequence cannot predict, which is why proteomics is a separate study rather than a corollary of genomics.

Answer: A

Correct answer: proteins

Bacteriocins are proteins produced by one strain that kill closely related strains, usually by forming pores or degrading nucleic acid. They are typically plasmid-encoded, and the producing cell carries an immunity protein to protect itself - which is what makes the weapon usable.

Answer: C

Correct answer: DNA through RNA to proteins

This is the central dogma: DNA is transcribed to RNA and RNA translated to protein. Retroviruses reverse the first step with reverse transcriptase, but no mechanism translates protein back into nucleic acid, and it is that last impossibility which gives the dogma its force.

Answer: D

Correct answer: Because it has many copies of genes encoding DNA repair

Deinococcus radiodurans survives doses that shatter its chromosome into hundreds of fragments, and reassembles it within hours. Multiple genome copies provide the templates and an unusually efficient repair apparatus does the work - the resistance lies in repair rather than in any shielding.

Answer: C

Correct answer: E. coli has acquired many genes via horizontal transfer

Comparison of the two genomes shows a substantial fraction of the E. coli chromosome differing in base composition and codon usage from the rest, which marks it as foreign in origin. Several hundred kilobases appear to have arrived by horizontal transfer since the two lineages separated, and much of what distinguishes the two species lies in that acquired material.