DNA Replication

Semiconservative Model of DNA Replication

• Each parent strand of DNA is incorporated into a new double-stranded DNA molecule.
• This model was proven by experiments conducted by Meselson and Stahl.

DNA Replication Process

Step 1: Strand Separation

• DNA must "unzip" using an enzyme called DNA helicase, which breaks the H-bonds between base pairs starting at the replication origin.
• The point of separation of the two strands of DNA is called the replication fork.
• Tension in the unwinding strand is relieved by topoisomerases, which cleave one or two of the DNA strands, allowing them to untwist, then rejoin the strands.
• The strands are kept apart by single-stranded binding proteins (SSBs).

Step 2: Building Complementary Strands

• The leading strand is built in a continuous fashion by DNA polymerase III by adding nucleoside triphosphates onto the 3' end of the parent strand.
• New DNA is assembled 5' to 3'.
• Replication starts when RNA primase puts down a 'starting point' - RNA primer (about 10 to 60 RNA ribonucleotides long).
• Only one primer is needed for the leading strand, and it is built continuously toward the replication fork.
• The lagging strand is built away from the replication fork in short fragments called Okazaki fragments by DNA Polymerase III.
• RNA primers are continuously added to the 5' end of the new strand (or the 3' end of the parent strand).
• DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides (while also proofreading the strands).
• Okazaki fragments are connected by DNA ligase by creating phosphodiester bonds between nucleotides.

Step 3: Dealing with Errors during Replication

• DNA polymerase I and III proofread new strands.
• DNA polymerase II repairs damage to DNA, including damage that occurs between replication events.

DNA Replication in Prokaryotes and Eukaryotes

• Prokaryotic genomes are much smaller and usually contain just one replication bubble.
• Eukaryotic DNA is very long and linear, therefore thousands of replication bubbles can appear.

Semiconservative Model of DNA Replication

• Each parent strand of DNA is incorporated into a new double-stranded DNA molecule.
• This model was proven by experiments conducted by Meselson and Stahl.

DNA Replication Process

Step 1: Strand Separation

• DNA must "unzip" using an enzyme called DNA helicase, which breaks the H-bonds between base pairs starting at the replication origin.
• The point of separation of the two strands of DNA is called the replication fork.
• Tension in the unwinding strand is relieved by topoisomerases, which cleave one or two of the DNA strands, allowing them to untwist, then rejoin the strands.
• The strands are kept apart by single-stranded binding proteins (SSBs).

Step 2: Building Complementary Strands

• The leading strand is built in a continuous fashion by DNA polymerase III by adding nucleoside triphosphates onto the 3' end of the parent strand.
• New DNA is assembled 5' to 3'.
• Replication starts when RNA primase puts down a 'starting point' - RNA primer (about 10 to 60 RNA ribonucleotides long).
• Only one primer is needed for the leading strand, and it is built continuously toward the replication fork.
• The lagging strand is built away from the replication fork in short fragments called Okazaki fragments by DNA Polymerase III.
• RNA primers are continuously added to the 5' end of the new strand (or the 3' end of the parent strand).
• DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides (while also proofreading the strands).
• Okazaki fragments are connected by DNA ligase by creating phosphodiester bonds between nucleotides.

Step 3: Dealing with Errors during Replication

• DNA polymerase I and III proofread new strands.
• DNA polymerase II repairs damage to DNA, including damage that occurs between replication events.

DNA Replication in Prokaryotes and Eukaryotes

• Prokaryotic genomes are much smaller and usually contain just one replication bubble.
• Eukaryotic DNA is very long and linear, therefore thousands of replication bubbles can appear.