DNA Replication

1. DNA Replication is a process of duplication of DNA giving rise to a new molecule of DNA with the same base of sequence as the original in the cell division. This is a major "driving force" of living things.

• The process is complex:

• DNA needs a primer - RNA.
• The enzyme primase synthesizes a small piece of RNA, complementary to the template DNA.
• The Okazaki fragments are synthesized starting with this primer.
• The RNA primer is removed by DNA polymerase I and replaced with deoxynucleotides.
• The ligase enzyme joins the strands.
• DNA polymerase I also corrects the mistakes. 

2. Semi-conservative Nature of Replication

•  DNA is synthesized semi-conservatively.
• Each newly synthesized DNA molecule contains one new strand and one the old one (parental strand).
• DNA synthesized is bidirectional, starting from the origin.

  

  Source: Pearson Prentice Hall, Inc. (2006)

Source: Pearson Prentice Hall, Inc. (2006)

 3. Mechanism of Replication

• DNA is synthesized by DNA Polymerase III.
• DNA - dependent DNA Polymerase - needs a template - the old DNA strand.
• Needs all 4:

1. dTTP (deoxythymidine triphosphate)
2. dCTP
3. dATP
4. dGTP

• Requires a primer - RNA - synthesized by a primase.
• Has 3' exonuclease activity. Can remove one nucleotide from the 3' end - backspace key.
• Proofread:

1. Before proofreading one mistake in 105.
2. After proofreading, one mistake in 107 + other factors -> one mistake in 109.  

This is the replication fork, where the DNA synthesis takes place:

Source: Thomson-Wadsworth (2003)

This is the picture of DNA Polymerase:

Source: Thomson-Wadsworth (2003)

This is the picture of DNA which is synthesized from 5' to 3':

Source: Pearson Prentice Hall, Inc. (2006)

 1). One strand is formed discontinuously. Then there are leading strand and lagging strand. Then Okazaki fragments put together with a ligase.

Source: Thomson-Wadsworth (2003)

2).

Source: Pearson Prentice Hall, Inc. (2006)

3).

Source: Pearson Prentice Hall, Inc. (2006)

Source: Pearson Prentice Hall, Inc. (2006)

Source: Pearson Prentice Hall, Inc. (2006)

Source: Pearson Prentice Hall, Inc. (2006)

Click this link to watch the video about DNA replication.