Question

Which DNA replication model is the correct model? Why?

Suppose that you have a chemical substance, how do you test if this substance is a mutagen?

question 2 most important

Answer 1

Ans.

1). DNA replication model ;

• After the discovery of Watson and Crick double helix DNA model, three modes of DNA replication were proposed.
• These models are conservative, semi-conservative and dispersive model of DNA replication.
• At the time of proposal, most of the biologist favored the semi-conservative model of DNA replication.
• Because of the double helix structure of DNA predicted through this model, it was the model of choice.
• So, becuase of the double helix structure theory, two DNA strands in this model are complimentary to each other and each strand act as template for the synthesis of a new DNA strand.
• The famous Meselson-stahl experiments on DNA replication using E.coli as the experiment model system also confirmed semi-conservative model of DNA replication as the most correct one.
• Their experiments showed semi-conservative replication of DNA and each DNA strand helps in the synthesis of their new partner by acting as template strand.
• Also, now it is being established that the semi-conservative DNA replication is a universal mechanism and all organisms on the earth share this trait.

2). Testing mutagenecity of a substance ;

• One of the most important test of any unknown chemical substance is to determine it's mutagenicity.
• Ames test is one of the commonly used approach by researchers to assess the mutagen potency of a substance.
• It is a biological assay that uses bacteria as model system to assess whether a compound is capable of causing mutations in the DNA of a test organism.
• It is done in accordance with the guidlines for testing of chemicals ; bacterial reverse mutation test developed by Organization for Economic Cooperation and Development (OECD).
• The principle of this test is reverse mutation or back mutation (hence, also called as bacterial reverse mutation assay).
• A positive test result suggests that the substance can be a potential mutagen and therefore, may cause cancer in most of the cases.
• Several bacterial strains that carry mutation are used in the test for eg. an auxotrophic mutant in a strain of S. typhi leads to the loss of ability in bacteria to synthsize histidine.
• Such bacterial strains are termed as His- and need histidine for their growth in the growth media.
• If, upon culturing His- bacteria in a media that contain test chemical result in a mutation in genes that encode histidine and these bacterias can again synthesize histidine, this is called as reverse mutation.
• So, such chemicals which can revert the mutation in a bacterial strain posses mutagenic capability and are actual mutagens.
• Therefore, this Ames test is a good indicator of mutagen potential of a chemical compound.