Molecular Biology and Genetics

Benzo[a]pyrene, found in smoke is a mutagenic agent that intercalates in the DNA causing frameshift mutation.

Correct answer. The International Nucleotide Sequence Database Collaboration (INSDC) consists of a joint effort to collect and disseminate databases containing DNA and RNA sequences. It consists of the following databases: DNA Data Bank of Japan (Japan), GenBank (USA) and the European Nucleotide Archive (UK).

6-mercaptopurine is a well known example of hypoxanthine analog that blocks nucleic acid synthesis.

Insertion and deletion mutations add or remove one or more DNA bases. Insertion and deletion mutations cause frameshift mutations which change the grouping of nucleotide bases into codons. This results in a shift of reading frame during protein translation.

A frameshift mutation is generally much more serious and will cause a change all the way down a DNA sequence making each codon a different sequence not just in one point or base like a point mutation that would only slightly change the sequence of a single codon. For example consider our DNA sequence is GTA CCT AGG. In a frameshift mutation a whole base would be added somewhere in that sequence making it look something like GTAT CCT AGG. Since it is impossible though to have 4 bases in a codon our body would automatically shift every letter down one even to the next codon in the sequence. In the end the final product would look something like GTA TCC TAG (with the left over G being the first base in the next codon and so on).

It is a point mutation involving substitution of one base pair for another by replacement of one purine by another purine and of one pyrimidine by another pyrimidine but without change in the purine-pyrimidine orientation. Here the GC base pair is changed to AT that is the purine G is substituted by another purine adenine and the pyrimidine C is substituted by another pyrimidine thymine. So it is a transition type of mutation. Hydroxylating agents can make such type of transition mutation.

Methylnitrosoguanidine is a biochemical tool used experimentally as a carcinogen and mutagen. Specific mispairing is caused when a mutagen changes a base's structure and therefore alters its base pair characteristics. An example of this type of mutagen is methylnitrosoguanidine which preferentially reacts with some bases and produce a specific kind of DNA damage. Methylnitrosoguanidine is an alkylating agent that adds methyl groups to guanine causing it to mispair with thymine.

This type of mutation is originated from lesions in DNA as well as from replication errors. For example it is possible for purine nucleotide to be depurinated that is to lose their base. This results in the formation of an apurinic site which will not base pair normally and may cause transition type mutation after the next round of replication. Cytosine can be deaminated to uracil which is then removed to form an apyrimidinic site.

Ultraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds. These radiations with wavelength above 260 nm are absorbed strongly by bases producing pyrimidine dimers which can cause error in replication if left uncorrected. Two common UV products are cyclobutane pyrimidine dimers (CPDs including thymine dimers) and 6, 4 photoproducts. A cyclobutane pyrimidine dimer (CPD) contains a four membered ring arising from the coupling of the C=C double bonds of pyrimidines. Such dimers interfere with base pairing during DNA replication leading to mutations.

A transition is a point mutation that changes a purine nucleotide to another purine (A ↔ G) or a pyrimidine nucleotide to another pyrimidine (C ↔ T). So here in the given DNA sequence the transition mutation makes changes in the third position where the pyrimidine cytosine is present. As a result this cytosine is substituted by another pyrimidine that is thymine. The DNA base sequence GTC which codes for the amino acid valine is changed to GTT that also codes for the same amino acid valine.

A base analog is a chemical that can substitute for a normal nucleobase in nucleic acids. A common example would be 5-bromouracil (5BU), the abnormal base found in the mutagenic nucleotide analog BrdU. When a nucleotide containing 5-bromouracil is incorporated into the DNA and it is most likely to pair with adenine. It is because 5BU is a base analog of thymine it base pair with cytosine as thymine does.

Induced mutation on the molecular level can be caused by ultraviolet radiation. Two nucleotide bases in DNA that is cytosine and thymine are most vulnerable to radiation that can change their properties. UV light can induce adjacent pyrimidine bases in a DNA strand to become covalently joined as a pyrimidine dimer.

Methotrexate works by inhibiting an enzyme known as dihydrofolic acid reductase,  which is important for transforming an inactive form of folic acid into the active form, which is necessary to make some of the building blocks needed for DNA production. It inhibits the synthesis of DNA, RNA and proteins.

The complementary base of guanine is cytosine. Hence, guanine content is equal to cytosine content in DNA molecule.

Adenine is present in both DNA and RNA.