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Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 1
DNA Contains Cell Information
n Complete cell DNA sequence = genome
¨ Genome of bacteria usually circular n Can be linear
n Bacterial genome = 0.6–9.4 million bp n Human genome = 4 billion bp
¨ 1000x as large as E. coli ¨ But 8x genes: 30,000 (us) vs. 4,000 (E. coli) ¨ E. coli genes use less DNA sequence
n About 1000 bases in typical bacterial gene
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 2
Nucleic Acid Structure
n (Deoxy)Ribose
¨ A sugar n 3 Phosphate groups on
5ʹ′ carbon ¨ Phosphate links to
3ʹ′ OH of next base n Base attached to sugar
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 3
Nucleic Acid Information
n Information in the
order of bases ¨ DNA:
n Adenine, Guanine, Cytosine, Thymine
¨ RNA: n Adenine, Guanine,
Cytosine, Uracil
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 4
Base-pairing Combines Strands
n Strands must orient in opposite directions
n Strands wind around each other in a helix
3ʹ′
5ʹ′ 3ʹ′
5ʹ′
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 5
Both DNA Strands Have the Same Amount of Information
n Bases in 1 strand are complementary to those in the other strand
5’A-T-C-C-T-G-G-A3’
5’T-C-C-A-G-G-A-T3’ | | | | | | | |
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 6
DNA Is Packed to Fit the Cell
n Multiple loops held by anchoring proteins
n Each loop has coiled DNA
n Nucleoid of E. coli n Circle of dsDNA 1500x the
size of the cell
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 7
DNA Replication
n Semiconservative replication n Copies information from one strand to a
new, complementary strand ¨ Melt double-stranded DNA ¨ Polymerize new strand complementary to
each melted single strand
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 8
Replication Begins at oriC
n DNA melts at oriC n Polymerization
follows melting around the chromosome
Replication fork
Replication fork Origin
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 9
DNA Helicase Melts DNA
n Loader places helicase at each end of origin n One helicase moves in each direction to
copy genome
Helicase
Helicase
Origin
Loader
Loader
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 10
Polymerase Proceeds 5ʹ′→ 3ʹ′ on Each Strand
n Energy for polymerization comes from phosphate groups on added base.
¨ Must add new base to 3ʹ′OH of a chain ¨ New nucleic acids grow to extend 3ʹ′ end
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 11
Each Fork Has Two Strands
n Steady growth of new “leading” strand
¨ Leading strand follows helicase n Single DNA strand for half of chromosome
Leading Strand
Leading Strand
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 12
Lagging Strand Growth
n Polymerase continues to previous primer n Clamp loader places primase on new site n DNA present in 1000 base pieces
¨ Okazaki fragments
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 13
Both Forks Move to ter Sites
n Movement is simultaneous n Opposite directions until both meet again
at terminus n Replisomes are actually stationary
¨ DNA is threaded through the replisomes
Cell membrane
Nucleoid
Replisomes
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 14
Plasmids
n Extrachromosomal pieces of DNA n Low-copy-number plasmids
¨ One or two copies per cell ¨ Segregate similarly to chromosome
n High-copy-number plasmids ¨ Up to 50 copies per cell ¨ Divide continuously ¨ Randomly segregate to daughter cells
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 15
Plasmid Genes
n Advantageous under special conditions
¨ Antibiotic-resistance genes ¨ Genes encoding resistance to toxic metals ¨ Genes encoding proteins to metabolize rare
food sources ¨ Virulence genes to allow pathogenesis ¨ Genes to allow symbiosis
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 16
Plasmid Replication
n Bidirectional replication ¨ Similar to chromosomal replication
n Unidirectional (“rolling circle”) replication ¨ Starts at nick bound by RepA protein ¨ Provides 3ʹ′OH for replication ¨ Helicase moves around
plasmid repeatedly ¨ Complementary strand
synthesized ¨ Used by many bacteriophage viruses