Unwinding of DNA

The interaction of proteins with ori defines the start site of replication and provides a short region of ssDNA essential for initiation of synthesis of the nascent DNA strand. This process requires the formation of a number of protein-protein and protein-DNA interactions. A critical step is provided by a DNA helicase that allows for processive unwinding of DNA. In uninfected E coli, this function is provided by a complex of dnaB helicase and the dnaC protein. Single-stranded DNA-binding proteins (SSBs) stabilize this complex. In X phage-infected E coli, the phage protein P binds to dnaB and the P/dnaB complex binds to oriX by interacting with the O protein. dnaB is not an active helicase when in the P/dnaB/O complex. Three E coli heat shock proteins (dnaK, dnaJ, and GrpE) cooperate to remove the P

Ori-binding -*

protein

Ori-binding -*

protein

Denaturation ->

Denaturation ->

Binding of factors, formation of replication fork, initiation of replication

Replication fork

O = Ori-binding protein | | = Polymerase '—~ = Nascent DNA ^m = RNA primer | | = Helicase = Primase • •• = SSB

Replication fork

O = Ori-binding protein | | = Polymerase '—~ = Nascent DNA ^m = RNA primer | | = Helicase = Primase • •• = SSB

Figure 36-13. Steps involved in DNA replication. This figure describes DNA replication in an Ecoli cell, but the general steps are similar in eukaryotes. A specific interaction of a protein (the O protein) to the origin of replication (ori) results in local unwinding of DNA at an adjacent A+T-rich region. The DNA in this area is maintained in the single-strand conformation (ssDNA) by single-strand-binding proteins (SSBs). This allows a variety of proteins, including helicase, primase, and DNA polymerase, to bind and to initiate DNA synthesis. The replication fork proceeds as DNA synthesis occurs continuously (long arrow) on the leading strand and discontinuously (short arrows) on the lagging strand. The nascent DNA is always synthesized in the 5' to 3' direction, as DNA polymerases can add a nucleotide only to the 3' end of a DNA strand.

protein and activate the dnaB helicase. In cooperation with SSB, this leads to DNA unwinding and active replication. In this way, the replication of the X phage is accomplished at the expense of replication of the host E coli cell.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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