Lecture Connections 25 | DNA Metabolism

Lecture Connections
25 | DNA Metabolism

© 2009 W. H. Freeman and Company


CHAPTER 25
DNA Metabolism
Key topics:
– DNA replication
– DNA repair
– DNA recombination


What is DNA Metabolism?
• While functioning as a stable storage of genetic
information, the structure of DNA is far from static:
– A new copy of DNA is synthesized with high fidelity before each
cell division
– Errors that arise during or after DNA synthesis are constantly
checked for, and repairs are made
– Segments of DNA are rearranged either within a chromosome
or between two DNA molecules giving offspring a novel DNA

• DNA metabolism consists of a set of enzyme catalyzed
and tightly regulated processes that achieve these tasks


The Substrate that Encodes its
Own Metabolisms

The Meselson-Stahl Experiment
• The Meselson-Stahl experiment was about the
origin of the two strands in each of the daughter
genomes
• Cells were grown on a medium containing only
15N isotope until all their DNA became fully 15N
labeled
• Cells were then switched to 14N medium and
allowed to divide once
• CsCl density gradient centrifugation was used to
determine the mass of genomic DNA before and
after each round of replication



DNA Replication is
Semiconservative
• The Meselson-Stahl experiment showed that the
nitrogen used for the synthesis of new dsDNA
becomes equally divided between the two
daughter genomes

• This suggests a semiconservative replication
mechanism



Replication of Circular DNA is

Bidirectional
• Both strands are replicated simultaneously



Synthesis of the Leading and
Lagging Strands
• Synthesis always occurs by addition of new
nucleotides to the 3’ end
• The leading strand is made continuously as the
replication fork advances
• The lagging strand is made discontinuously in
short pieces (Okazaki fragments) that are then
joined



DNA Elongation Chemistry
• Parental DNA strand serves as a template
• Nucleotide triphosphates serve as substrates in
strand synthesis
• Hydroxyl at the 3’ end of growing chain makes a
bond to the -phosphorus of nucleotide
• Pyrophosphate is a good leaving group



DNA Synthesis is Catalyzed by
DNA Polymerases
• Mg++ on the right coordinates to the -phosphate

and stabilize the negatively charged transition
state



Correct Geometry of Base Pairs
allows High Fidelity



Errors During the Synthesis are
Corrected by 3’ 5’ Exonuclease
Activity (1)




Errors During the Synthesis are
Corrected by 3’ 5’ Exonuclease
Activity (2)