How do mutagens alter the structure of the DNA? Give examples of mutagenic agents.

Question

How do mutagens alter the structure of the DNA? Give examples of mutagenic agents.

Answer 1

Mutagens can alter the structure of DNA by altering the sequence of nucleotide bases. Four types.

Substitution where one of the four bases is replaced by another.

Inversion where the position of two or more bases is reversed.

These are called point mutations and only alter the genetic code in respect for the reading of one amino acid in the genetic code for protein synthesis

Frame shift mutations have a greater effect and alter the reading of all the genetic code after the point where the occur.

Deletion is where a single nucleotide base is removed and alters the triplet code from that point on.

Insetion is where a single nucleotide isinserted into the DNA strand also altering the reading of the genetic code during protein synthesis.

This results in the incorrect amino acids being assembled and production of the wrong or non functioning protein.

Types of radiation are mutagenic eg X rays, UV light gamma rays etc.

Also some chemicals are known mutagens such as bromine, colchicine and nitrous acid.

Hope this helps!

Answer 2

A mutagen is a natural or human-made agent (physical or chemical) which can alter the structure or sequence of DNA.

A. Chemical mutagens

It is possible to distinguish chemical mutagens by their modes of action; some of these cause mutations by mechanisms similar to those which arise spontaneously while others are more like radiation (to be considered next) in their effects.

1. Base analogs

These chemicals structurally resemble purines and pyrimidines and may be incorporated into DNA in place of the normal bases during DNA replication:

    * bromouracil (BU)--artificially created compound extensively used in research. Resembles thymine (has Br atom instead of methyl group) and will be incorporated into DNA and pair with A like thymine. It has a higher likelihood for tautomerization to the enol form (BU*)

    * aminopurine --adenine analog which can pair with T or (less well) with C; causes A:T to G:C or G:C to A:T transitions. Base analogs cause transitions, as do spontaneous tautomerization events.

2. Chemicals which alter structure and pairing properties of bases

There are many such mutagens; some well-known examples are:

    * nitrous acid--formed by digestion of nitrites (preservatives) in foods. It causes C to U, meC to T, and A to hypoxanthine deaminations. [See above for the consequences of the first two events; hypoxanthine in DNA pairs with C and causes transitions. Deamination by nitrous acid, like spontaneous deamination, causes transitions.

    * nitrosoguanidine, methyl methanesulfonate, ethyl methanesulfonate--chemical mutagens that react with bases and add methyl or ethyl groups. Depending on the affected atom, the alkylated base may then degrade to yield a baseless site, which is mutagenic and recombinogenic, or mispair to result in mutations upon DNA replication.

3. Intercalating agents

acridine orange, proflavin, ethidium bromide (used in labs as dyes and mutagens)

All are flat, multiple ring molecules which interact with bases of DNA and insert between them. This insertion causes a "stretching" of the DNA duplex and the DNA polymerase is "fooled" into inserting an extra base opposite an intercalated molecule. The result is that intercalating agents cause frameshifts.

4. Agents altering DNA structure

We are using this as a "catch-all" category which includes a variety of different kinds of agents. These may be:

    * --large molecules which bind to bases in DNA and cause them to be noncoding--we refer to these as "bulky" lesions (eg. NAAAF)

    * --agents causing intra- and inter-strand crosslinks (eg. psoralens--found in some vegetables and used in treatments of some skin conditions)

    * --chemicals causing DNA strand breaks (eg. peroxides)

What these agents have in common is that they probably cause mutations not directly but by induction of mutagenic repair processes.

B. Radiations

(including Ionizing radiations, UV (ultraviolet) radiations etc..

hope that helps..