Understanding DNA replication – and the resulting transmission of genetic information from cell to cell, and generation to generation – lays the groundwork for understanding the principles of heredity

Understanding DNA structure and replication is a prerequisite for understanding some of the principal tools of molecular biology

DNA structure

The structure of DNA makes it an ideal genetic material

• Faithful replication
• Information content
• Capable of change
  
  

The building blocks of DNA

Nucleotides _____________________

Polynucleotides _____________________

Complementary base pairing _____________________

Semiconservative replication

DNA replication is semiconservative : _______________________

Requirements for DNA synthesis 

• dATP, dGTP, dTTP, and dCTP
• template DNA (a pre-existing single strand)
• DNA polymerase (NEXT LEVEL)

The essentials of DNA synthesis: replication is 5' 3'

Details of DNA replication  

• Replication begins at a fixed point, called the origin, and proceeds bidirectionally. There is one origin in E. coli, which has some unique properties. In a higher plant, there are thousands of origins. 
• The DNA helix needs to be opened up. This is accomplished by helicase enzymes, which break the hydrogen bonds holding the two strands of the helix together.
• The unwound DNA is stabilized by a protein (single strand binding protein (SSB)).
• Priming: DNA polymerases can extend a chain, but they cannot start one.  Accordingly, DNA synthesis start with a primer.  For example, in E. coli, the primase enzyme synthesizes a short piece of RNA complimentary to the DNA. The RNA primers are later removed by exonuclease activity of a polymerase and replaced with DNA.
• Leading and lagging strands: DNA polymerases synthesize new chains only from 5' to 3', yet the DNA molecule is antiparallel and DNA synthesis is semi-conservative. DNA synthesis is continuous on one strand (the leading strand) and discontinuous on the other strand (the lagging strand). The DNA on the lagging strand is thus formed in fragments, called Okazaki fragments.
• Editing: Some polymerases have 3'-5' exonuclease activity. This allows these enzymes to "search" for mismatched bases that were incorrectly added during polymerization and removes them. This proofreading function occurs at the 3' end of the growing strand and proceeds 3' 5'.

Telomeres and telomerase

The replication problem at chromosome ends: __________________

The telomerase solution: _________________________

Text:  Chapter 7