Quiz 10: Molecular Structure of Chromosomes and Transposable Elements

Viral replication: Virus is a small infectious agent, but it lacks independent metabolism. The nucleic acid present in the virus is either DNA or RNA. They can replicate only in living organism (living organism are acts as hos, for viral replication process). The virus does show cell division, because they are acellular. In viral replication five steps are involved they are adsorption, penetration, uncoating, viral genome replication, maturation, and releasE.Difference between self-assembly and directed assembly: Self-assembly occurs if structure of virus is simple than spontaneous assembly of nucleic acid and coat proteins takes place. For the Self-assembly process there is no requirement of other protein machinery. Directed assembly involves the help of proteins, which are not found in the mature viral coat. For example T2 bacteriophage is directly assembled; catalysis done by the scaffolding proteins.

The two circular DNA (deoxyribonucleic acid) molecules call molecule A and molecule B, which are topoisomers to each other. When observed under electron microscope, molecule A appears more compact than molecule B. The level of gene transcription is much lower for molecule A. First possibility of molecule A having three positive supercoils and B having three negative super coils do not account for the above observations. Since, both molecules have the same level of super coiling; they have same level of compaction. Second possibility of molecule A having four positive supercoils and molecule B having one negative supercoil do account for above observations. Since, molecule A has more super coils, it is more compacted and molecule B has negative super coils, it is more transcriptionally activE.Third possibility of molecule A having zero supercoils and molecule B having three negative supercoils, would make molecule B more compact. So this does not fit well. Hence, second possibility fits well for the above observation done under the electron microscope.

Bacterial chromosomes: Bacterial chromosome is short DNA (deoxyribonucleic acid) molecule but it lacks protein coat. The DNA is double helix and closed circular in nature. In bacteria chromosome size is bp (base pairs). In bacterial chromosomal replication three major steps are involved they are initiation, elongation and termination. Eukaryotic chromosomes: Eukaryotes contain multiple pairs of linear chromosomes. In eukaryotes chromosomes are composed of DNA coiled and condensed around nuclear proteins known as histones. They consist of repeated units of chromatin known as nucleosomes. In eukaryotes chromatin not only allows large amount of DNA fit in to a small place, but also helps in regulate gene expression. Advantages and disadvantages of bacterial and eukaryotic compact structure: • Bacterial chromosomal DNA is folded in to compact structure, so it occupying relatively small part of the cell. • Bacterial chromosome is organized into independently supercoiled loops known as domains. Domain allows the chromosomal DNA to undergoing structural changes in cellular process. • In eukaryotic chromosomes consists complex made up of protein and DNA and organized in condensed manner, it permits the large amount of DNA storage in the nucleus of the cell. Functions of chromosomes: Chromosomes store the genetic code and they determine the sex. They control cell division. They are also formation of protiens and storagE.

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