DNA Replication(in Ten easy steps)Biology 12Mr. Teed

Overview

• In preparation for mitosis or meiosis, a cell must replicate its entire DNA so that each daughter cell has a complete set of 46 chromosomes .

• DNA replication is semi-conservative

• Replication begins at a specific site on the DNA known as the replication origin.

• DNA of eukaryotes has multiple origins of replication.

Multiple Sites Accelerate Replication• On the leading strand, DNA polymerase III adds nucleotides at a rate of 50/sec.

• At this rate, it would take approximately 700 days to replicate the 3 billion base pairs of the human genome, yet the whole process takes place in approximately 5 to 10 hours.

• The accelerated rate indicates that replication is occurring at many points at once.

Two parent DNA strands must be unravelled and kept separate.

DNA helicase unwinds the double helix by breaking the hydrogen bonds between the complementary base pairs.

Strands are kept apart by single-stranded binding proteins (SSBs). SSBs block hydrogen bonding.

Separating the DNA Strands

• Replication begins in two directions from the origin(s) as a region of the DNA is unwound.

• DNA replication proceeds toward the direction of the replication fork on one strand and away from the fork on the other strand.

DNA polymerase III functions only under certain conditions• It synthesizes DNA in the 5’ to 3’ direction and therefore

adds free deoxyribonucleoside triphosphates to a 3’ end.

• It requires an initial starting 3’ end to commence elongation. Since DNA polymerase III cannot initiate a new complementary strand by itself, an RNA primer of 10 to 60 base pairs is annealed to the template strand. This primer is synthesized by the enzyme primase.

Deoxyribonucleoside triphosphate

• The free bases in the nucleoplasm used by to build complementary strands are deoxyribonucleoside triphosphates.

• DNA polymerase III uses the energy derived from breaking the bond between the first and second phosphate to drive the dehydration synthesis

• Since DNA is always synthesized in the 5’ to 3’ direction and the template strands run antiparallel, only one strand is able to be built continuously.

• This strand, which uses the 3’ to 5’ template strand as its guide, is called the leading strand and is built toward the replication fork.

• The other strand is synthesized discontinuously in short fragments in the opposite direction to the replication fork and is known as the lagging strand.

• Primers are continuously added as the replication fork forms along the DNA parent strand, and DNA polymerase III builds in short segments known as Okazaki fragments.

Meanwhile, on the Lagging strand

• DNA polymerase I removes the RNA primers from the leading strand and fragments of the lagging strand and replaces them with the appropriate deoxyribonucleotides.

• DNA ligase, joins the Okazaki fragments into one strand by creation of a phosphodiester bond.

The proofreading efficiency of DNA polymerase I and polymerase III reduces the error rate to one in 1 billion base pairs

• DNA replication animation