Question

In depth answer

Describe the complexity of the genomes of virus, bacteria, and eukaryotes; explain similarities and differences and how renaturation kinetics illustrates these difference

Answer 1

1) Viruses are composed of a protein coat and a core that contains its genome. Viral genomes are usually described in terms of their structure as found in the viral particle. The genomes of many viruses are composed of DNA. When packaged into the viral particle, in some viruses the DNA is double-stranded, in other cases single-stranded. Still other viruses, such as the retroviruses, have RNA genomes in the viral particle. Sometimes these RNA genomes are single-stranded, in other cases double-stranded.  Some viral genomes contain linear DNA or RNA molecules, whereas others are circular.

The genome of most prokaryotes (bacteria) is contained within a single chromosome, however there are some exceptions. For most prokaryotes, this chromosome is a single, closed, circular double helix of DNA. Bacterial genes are arranged close together with little intergenic space, and introns are extremely rare. In some regions of prokaryotic genomes, some functionally related genes are located together as a group, and one molecule of mRNA is made from the entire unit which is called as an operon.

In eukaryotic organisms, the vast majority of genes are found in the chromosomes of the nucleus. Most eukaryotic species are classified as either diploid, carrying two sets of nuclear chromosomes (two copies of the nuclear portion of the genome) in each nucleus in body cells, or haploid, with only one chromosome set per nucleus.Different chromosomes in the genome contain different sets of genes.

2) The rate of renaturing varies from organism to organism. If all the DNA fragments contain unique sequences, the rate of renaturation at a given weight concentration of DNA is proportional to the size of the genome since the concentration of any one unique fragment is inversely proportional to the size of the genome from which it originated. Thus, T4 bacteriophage DNA renatures 20 times faster than E. coli DNA at some standard condition because there is 20 times as much DNA in an E. coli bacterium as there is in a T4 phage and all nucleotide sequences in both organisms are unique. If the organism contains repeated DNA sequences, these sequences will renature more rapidly than the unique sequences. In the case of 106 copies of a repeated DNA sequence per genome, these sequences will renature 106 times more rapidly if the repeated sequence is longer than the length of helix required for stability at the standard renaturation conditions.

The genome of eukaryotes has arbitrarily been divided into three classes: unique sequences, intermediate sequences renaturing at rates suggesting 10 to 1000 copies per genome, and the very rapidly renaturing or simple sequences which renature at rates suggesting 103 to 107 copies per genome.

The key result for genome analysis is that less complex DNA sequences renature faster than do more complex sequences. Thus determining the rate of renaturation of genomic DNA allows one to determine how many kinetic components (sequences of different complexity) are in the genome, what fraction of the genome each occupies, and the repetition frequency of each component.Some viral and bacteriophage genomes have almost no repeated DNA, and Length of genome is approximately equal to its complexity. But for many genomes, repeated DNA occupies 0.1 to 0.5 of the genome.

Therefore, it is extremely dynamic how renaturation illustrates the differences between the three genomes, as each organism within the class would have different genomic composition. But it may be concluded that, more the number of repetitive copies within the DNA, faster is the renaturation.