DNA Contains Cell Information - faculty.weber.edu slides/DNA Replication and Plasmids.pdfDNA Contains Cell Information ! Complete cell DNA sequence = genome " Genome of bacteria usually

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


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


Nucleic Acid Information

n  Information in the

order of bases ¨ DNA:

n  Adenine, Guanine, Cytosine, Thymine

¨ RNA: n  Adenine, Guanine,

Cytosine, Uracil


Base-pairing Combines Strands

n Strands must orient in opposite directions

n Strands wind around each other in a helix

3ʹ′

5ʹ′ 3ʹ′

5ʹ′


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’ | | | | | | | |


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


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


Replication Begins at oriC

n DNA melts at oriC n Polymerization

follows melting around the chromosome

Replication fork

Replication fork Origin


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


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


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


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


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


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


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


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

Documents

DNA Contains Cell Information - faculty.weber.edu slides/DNA Replication and Plasmids.pdfDNA Contains Cell Information ! Complete cell DNA sequence = genome " Genome of bacteria usually