GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION AND INHERITANCE OF TRAITS

19.2. MUTATIONS AND THEIR ORIGIN

19.2.2. Induced Mutations

Treating Cells with mutagenic agents can significantly increase the frequency of Mutations. In such cases, we refer to induced mutations, and the cells in which they occur are called induced mutants. Mutagens can be chemical, physical, or biological agents.

Based on their genetic Structure, mutants are classified into three categories characterized by the following defects:

1) substitution of one base pair for another (e.g., AT replaced by GC, or vice versa);

2) insertion of an extra base pair into The nucleotide sequence, or deletion of one of the existing pairs;

3) loss of a group of bases or even entire genes (deletion), relocation within the chromosome (transposition), or disruption caused by the insertion of foreign DNA (insertion).

First-Class mutations typically exhibit a high reversion rate, second-class mutants show reversions only rarely, while third-class mutations generally yield no revertants (with rare exceptions).

Let us examine some of the mechanisms underlying induced mutagenesis.

Incorporation of base analogues. Base analogues act as antimetabolites. Due to their striking structural similarity to normal purine and pyrimidine bases, they are taken up by cells and incorporated into DNA. Well-known Examples of such antimetabolites include bromouracil and 2-aminopurine.

Bromouracil is structurally analogous to thymine and can thus be incorporated into DNA in place of thymine, pairing with adenine. However, bromouracil undergoes tautomerization to its enol form more frequently than thymine. In this enol form, it pairs with cytosine, thereby causing the incorporation of guanine instead of adenine.

2-Aminopurine is incorporated into DNA in place of adenine and acts in a similar manner.

Chemical modification of bases. Nitrous acid deaminates adenine, guanine, or cytosine. As a result of substituting an amino group with a hydroxyl group, adenine is converted to hypoxanthine and pairs with cytosine instead of thymine. Similarly, cytosine is deaminated to uracil and pairs with adenine instead of guanine.

Alkylating agents. This group of agents includes ethyl and methyl methanesulfonate, dimethyl and diethyl sulfate, mustard gas, and N-methyl-N'-nitro-N-nitrosoguanidine, which rank among the most potent mutagens. For instance, ethyl methanesulfonate ethylates a nitrogen atom in the guanine molecule, yielding N-alkylguanine. This modified base is subsequently cleaved from the strand, creating a "gap." During the next Replication cycle, a "mistake" base is frequently inserted at this site.

Incorporation or loss of individual Base Pairs. Proflavin and other acridine Dyes act through a different mechanism. An acridine molecule intercalates between adjacent bases in the DNA strand, increasing the distance between them (a phenomenon known as intercalation). This triggers Two Types of errors: the loss of a nucleotide or the incorporation of an extra base pair.

Ultraviolet and ionizing radiation. Ultraviolet (UV) rays, X-rays, and Other forms of ionizing radiation exert both lethal and mutagenic effects. UV light acts on Nucleic Acids by inducing The formation of thymine dimers. An adaptive mechanism that allows a fraction of cells to survive UV and Other types of irradiation is DNA Repair. If cells are exposed to visible light (320–550 nm) immediately after UV irradiation, their survival rate increases dozens of times—a phenomenon known as photoreactivation. During photoreactivation, a specialized enzyme splits the thymine dimers and restores the normal Introduction/20.html">DNA Structure. Another light-independent DNA repair mechanism also exists, known as dark repair (or dark reactivation). In this process, defective DNA regions are excised and replaced with new NUCLEOTIDES.

Transposon-induced mutations. Transposons (Tns) are short double-stranded DNA segments consisting of more than 2,000 base pairs that confer resistance to a single antibiotic (or, exceptionally, to several Antibiotics). Transposons are capable of "jumping" from one genomic region to another, particularly between the bacterial chromosome and a plasmid, and vice versa. When a transposon invades a structural Gene within the chromosome, it disrupts the nucleotide sequence of that gene, resulting in an insertion mutant. Because transposons lack the capacity for autonomous replication, their transfer from one Cell to another requires a vector (carrier), such as a plasmid or a bacteriophage.