1) One more Replication Question: How much energy does it take the replication D

Question

1) One more Replication Question: How much energy does it take the replication DNA? Here are some AG's of cleavage that may be useful: NTPH20NDPPi NTP + H2O-) NMP + PPi PPi+ H2O2 Pi ?G°,--7.3 kcal/mol AG10.9 kcal/mol AG--4.6 kcal/mol A). What processes use energy in DNA replication? How much energy does each use? B). How much Energy is needed to replicate a bacterial chromosome that is 4,000,000 nucleotides long? C). How much Energy is needed to replicate a eukaryotic chromosome that is 10,000,000,000 nucleotides long?

Explanation / Answer

Ans. Energetics of phosphodiester bond formation:

Step 1: (dNMP)n + dNTP ---------> (dNMP)n+1 + PPi     ; dG0’= +0.478 kcal mol-1    

Step 2: PPi -----------------------> 2 Pi                             ; dG0’= -4.6 kcal mol-1

            --------------------------------------------------------------------------------

Net Reaction: (dNMP)n + dNTP ---------> (dNMP)n+1 + 2 Pi   

                                                                                                ; dG0’net= -4.122kcal mol-1

# The formation of phosphodiester bond is step has a slightly positive dG0’ of +0.478 kcal/mol (= 2.0 kJ/mol). So, the first step is not thermodynamically favourable on its own.

# Hydrolysis of pyrophosphate releases 4.6 kcal/mol energy. Step 2 is coupled to step 1, thus making the overall coupled reaction of phosphodiester bond formation thermodynamically favourable with a net dG0’ of – 4.122 kcal/mol.

# A: The following hydrolysis reactions are needed to form a phosphodiester bond-

            NTP + H2O ---------> NMP + PPi              ; dG0’ = -10.9 kcal/mol

            PPi + H2O ----------> 2 Pi                           ; dG0’ = -4.6 kcal/mol

            ----------------------------------------------------------------------

Net Rxn: NTP ---------> NMP + 2 Pi                     ; dG0’ = -15.5 kcal/mol        

Therefore, formation of one mol phosphodiester bond requires 15.5kcal/mol energy.

#B. Number of phosphodiester bonds = Number of nucleotides – 1

                                                            = 4000000 – 1 = 3999999

# During replication, two strands of DNA are synthesized.

So, total number of phosphodiester bonds (PB) = 2 x 3999999 = 7999998

Moles of phosphodiester bond = No. of PB / NA

                                                = 7999998 PB / (6.022 x 1023 PB/mol)

                                                = 1.3285 x 10-17 mol

# Now, Total required energy = Moles of PB x Energy required for one mol PB                                                       = 1.3285 x 10-17 mol x (15.5 kJ/ mol)

                                                = 2.059 x 10-16 kJ

#C. Number of phosphodiester bonds = Number of nucleotides – 1

                                                            = 10000000000 – 1 = 9999999999

# During replication, two strands of DNA are synthesized.

So, total number of phosphodiester bonds (PB) = 2 x 9999999999 = 19999999998

Moles of phosphodiester bond = No. of PB / NA

                                                = 19999999998 PB / (6.022 x 1023 PB/mol)

                                                = 3.3212x 10-17 mol

# Now, Total required energy = Moles of PB x Energy required for one mol PB                                                       = 1.3285 x 10-14 mol x (15.5 kJ/ mol)

                                                = 5.1478 x 10-13 kJ

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