Question 1

Genotype:

Accepted Answer

the precise genetic makeup of an organism. It is designated by three lowercase letters followed by a capital letter (in all italics). An example is hisC, which indicates the gene encoding for an enzyme in the biosynthetic pathway of histidine.

Question 2

Phenotype:

Accepted Answer

visible characteristics. Designated by a capital letter followed by two lowercase letters (not italicized), with either a + or - superscript to indicate the specific phenotype. Ex: His+ means strain can synthesize its own histidine, while - cannot.

Question 3

Mutation:

Accepted Answer

a heritable change in the base sequence of nucleic acid of the genome of an organism.

Question 4

Mutant:

Accepted Answer

a strain carrying a mutation. A mutant differs from its parental strain (wild type) in genotype. Its phenotype may also be altered. Ex: a mutation in the hisC gene may lead to change from His+ to a His- phenotype

Question 5

Autotroph:

Accepted Answer

a mutant that has an additional nutritional requirement for growth (ex. His-). This is in contrast to a wild-type organism, or prototroph, which can synthesize all the necessary organic compounds for its growth from a minimal medium.

Question 6

Prototroph:

Accepted Answer

parental strain (usually wild type). Replica plating can find mutant strains.

Question 7

Spontaneous Mutations:

Accepted Answer

occur without external intervention, mostly from occasional replication errors by DNA polymerase (extremely rare).

Question 8

Induced Mutations:

Accepted Answer

made environmentally and deliberately, result from exposure to natural radiation or DNA-modifying chemicals.

Question 9

Spontaneous Mutation Example:

Accepted Answer

often occurs due to issues with tautomerization (tautomers are isomers of a compound which differ only in the position of the protons and electrons. The carbon skeleton is unchanged).

Question 10

Simple Definition of Tautomerization (**):

Accepted Answer

a chemical shift within a molecule where protons and double bonds relocate, converting amino (–NH₂) to imino (–NH) groups or oxo (C=O) groups to enol (C–OH) forms. This alters bonding patterns without changing the overall composition.

Question 11

Mutations Arising from Tautomerization Require Two Rounds of Replication to Become Permanent. Why?

Accepted Answer

A tautomeric base during the first replication causes a mismatched pair (e.g., C:A) that may go unrepaired. In the second replication, the mismatch separates, producing one DNA molecule with the original sequence and one with the mutation (e.g., T:A).

Question 12

Mutagens:

Accepted Answer

chemical, physical, or biological agents that increase the mutation rate. Mutagens can alter DNA in many different ways, but such alterations are not mutations unless they can be inherited.

Question 13

Base Analogs:

Accepted Answer

Structurally similar to normal bases. DNA polymerase incorporates them into DNA. But pair incorrectly after incorporation

Question 14

DNA Reactive Chemicals:

Accepted Answer

alters the base in DNA so that it accidentally pairs with the wrong base when replicated.

Question 15

Alkylating Agents:

Accepted Answer

adds a methyl group causing mispairing or crosslinks DNA.

Question 16

Intercalating Agents:

Accepted Answer

“stretches” the DNA and causes insertions or deletions (frameshifts).

Question 17

Radiation:

Accepted Answer

UV Radiation (ultraviolet light) → Causes pyrimidine dimers (usually thymine dimers, T-T). Ionizing Radiation (X-rays, gamma rays, cosmic rays) → Breaks DNA backbone (strand breaks).

Question 18

Point Mutation:

Accepted Answer

which results from a change in a single base pair, can lead to a single amino acid change in a polypeptide or to no change at all, depending on the particular codon involved.

Question 19

Silent Mutations (PM):

Accepted Answer

do not affect the sequence of the encoded polypeptide (due to the degenerate genetic code, almost always in the third base).

Question 20

Missense Mutations (PM):

Accepted Answer

the sequence of amino acids in the ensuing polypeptide is changed, often resulting in an inactive protein or one with reduced activity (not all missense mutations alter activity).

Question 21

Nonsense Mutations (PM):

Accepted Answer

results in premature stop codon and typically results in a truncated (incomplete) protein that lacks normal activity.

Question 22

Frameshift Mutations:

Accepted Answer

deletions or insertions that result in a shift in the reading frame and scrambles the polypeptide sequence downstream. Insertion/deletion of three base pairs adds/deletes an amino acid, which is usually not as bad.

Question 23

Proofreading:

Accepted Answer

correction of errors in base pairing made during replication. Errors corrected by DNA Polymerase.

Question 24

Many types of DNA repair mechanisms exist…

Accepted Answer

repair enzymes scan the DNA for mismatched pairs. Mismatched pairs are removed and replaced by DNA polymerase and DNA ligase.

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Mutations & Repair