Dna unwinding, such as that occurring in replication, affects the superhelical density of dna. in the absence of topoisomerases, the dna would become over wound ahead of a replication fork as the dna is unwound behind it. a bacterial replication fork will stall when the superhelical density (sigma) of the dna ahead of the fork reaches sigma = 0.14. bidirectional replication is initiated at the origin of a 6,000 bp plasmid in vitro, in the absence of topoisomerases. the plasmid initially has a sigma of 0.06. how many base pairs will be unwound and replicated by each replication fork before the forks stall? assume that each fork travels at the same rate and that each includes all components necessary for elongation except topoisomerase.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.

energy per photon is e = h*c / λ  

e = 6.63*10^-34 * 3*10^8 / 6*10^-7 j = 3.315*10^-19 j  

nr. of photons = 10j / 3.315*10 = ^-19 j per photon = 3.02*10^19 photons