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Sigma is a specificity factor. It directs RNA polymerase to the promoter and ensures that transcription is initiated only where it is supposed to be initiated. The very fact that RNA polymerase depends upon a specificity factor to direct the RNA polymerase to the correct promoter immediately offers a mechanism for controlling transcription. Different sigma factors […]
Bacterial and E. coli RNA polymerases are the best characterized RNA polymerases. There are about 13,000 RNA polymerase molecules present in an E. coli cell and the number varies with the growth conditions. Although not all the RNA polymerases are actually engaged in transcription at any one time, almost all are bound either specifically or non-specifically to DNA. The […]
RNA synthesis takes place within a ‘transcription bubble’ where the DNA is transiently separated into single strands. The unwinded template strand is used to direct the synthesis of the RNA strand. The RNA chain is synthesized from the 5′ → 3′ direction. That is, new nucleotides are added to the 3′-end of the growing chain. […]
DNA and RNA at a glance look similar; however, RNA differs from DNA in many ways, i.e., it has uracil instead of thymine; ribose sugar instead of deoxyribose; single-stranded and has the potential for structural diversity than the DNA. Thus, RNA is suited to a variety of cellular functions. RNA is the only macromolecule that […]
Inhibitors of Nucleotide Biosynthesis Reducing the supply of precursors limits the rate of nucleic acid biosynthesis and leads to mutations in DNA template. The inhibitor can function as: Inhibitors of Purine Biosynthesis Azaserine and 6-diazo-5-oxo-L-norleucine (DON)—the analogues of glutamine—inhibit three reactions in purine biosynthesis. Their activities in inhibiting cell division primarily arise from the inhibition […]
The chromosomes in eukaryotes have much complex structure than prokaryotic chromosomes. The duplication of the chromosomes of eukaryotes involves not only the replication of their DNA but also the synthesis of the associated histone and non-histone chromosomal proteins. However, at the molecular level, the replication of DNA in eukaryotes is quite similar to prokaryotes regardless […]
Whether a cell has only one chromosome or has many chromosomes, the entire genome must be replicated precisely once for ssevery cell division. Initiation of DNA replication commits the cell to further division. Once replication started, it continues until the entire genome has been duplicated. The unit of DNA in which an individual act of […]
θ Replication Autoradiography of replicating DNA was obtained by John Cairns. This indicates how DNA replicates. Autoradiogram of circular chromosome grown in medium containing 3H thymidine shows the presence of ‘replication’ ‘eyes’ or ‘bubbles’. These are called ‘Θ structures’, because of their resemblance to Greek letter theta. This indicates that double-stranded DNA replicates by the progressive […]
Prokaryotic DNA Polymerases DNA polymerases are the enzymes responsible for the synthesis of DNA during replication. There are three types of prokaryotic DNA polymerases; namely, DNA polymerase I (DNA Pol-I), DNA polymerase II (DNA Pol-II) and DNA polymerase III (DNA Pol-III). DNA Pol-I It is the first known DNA polymerase, which was discovered by Arthur […]