Question

Describe the features of the gene organization and the gene structure between bacteria and human. How do these differences of genomic features provide evolutionary adaptations to prokaryotic and eukaryotic organismal functions.

Why are multiple specific bacterial tester strains used in the Ames test? Illustrate with a diagram, how can the toxic and mutagenic effects be distinguished from the anticipated results?

Answer 1

1. Genome of bacteria are arranged in the cell in a confined space, the nucleoid. This arrangement is dynamic allowing for DNA transactions such as replication, transcription and repair to occur at appropriate times.The genome of human is organized into 22 paired chromosomes, termed autosomes, plus the 23rd pair of sex chromosomes (XX) in the female, and (XY) in the male. These are all large linear DNA molecules contained within the cell nucleus.

Bacteria are typically haploid, usually having a single circular chromosome found in the nucleoid. Eukaryotes are diploid; DNA is organized into multiple linear chromosomes found in the nucleus.

Supercoiling and DNA packaging using DNA binding proteins allows lengthy molecules to fit inside a cell. Eukaryotes use histone proteins, and bacteria use different proteins with similar function.

Bacterial and human genomes both contain noncoding DNA, the function of which is not well understood. Some noncoding DNA appears to participate in the formation of small noncoding RNA molecules that influence gene expression; some appears to play a role in maintaining chromosomal structure and in DNA packaging.

Extrachromosomal DNA in humans includes the chromosomes found within organelles of prokaryotic origin (mitochondria and chloroplasts) that evolved by endosymbiosis.

Extrachromosomal DNA in bacteria is commonly maintained as plasmids that encode a few nonessential genes that may be helpful under specific conditions. Plasmids can be spread through a bacterial community by horizontal gene transfer.

2. Bacterial genomes increase in size by aggregating adaptive gene modules when exposed to new selective pressures, whereas human genomes increase in size by accumulating large amounts of non-functional DNA when exposed to drift.

3. The Ames test is a widely employed method that uses bacteria to test whether a given chemical can cause mutations in the DNA of the test organism. More formally, it is a biological assay to assess the mutagenic potential of chemical compounds.

Some compounds are quite specific, causing reversions in just one or two strains. The tester strains also carry mutations in the genes responsible for lipopolysaccharide synthesis, making the cell wall of the bacteria more permeable, and in the excision repair system to make the test more sensitive.

Ames test uses several strains of bacteria (Salmonella, E.coli) that carry mutation. Eg A particular strain of Salmonella Typhimurium carry mutation in gene that encodes histidine. So it is an auxotrophic mutant which loss the ability to synthesize histidine (an amino acid) utilizing the ingredients of culture media. Those strains are known as His- and require histidine in growth media.

Culturing His- salmonella is in a media containing certain chemicals, causes mutation in histidine encoding gene, such that they regain the ability to synthesize histidine (His+) This is the reverse mutation. Such chemicals responsible to revert the mutation is actually a mutagen. So, this Ames test is used to test mutagenic ability of varieties of chemicals.