Chapter 12
Microbial Evolution and Systematics
Ribosomal RNAs as Evolutionary Chronometers:
The 16S rRNA or 18S rRNA Technique
Ribosomal RNAs as Evolutionary Chronometers
?Reasons:
? Ancient molecules
? Functionally constant
? Universally distributed
? Moderately well conserved in sequence
across broad phylogenetic distances
The 16S rRNA or 18S rRNA Technique
The 16S rRNA or 18S rRNA Technique
? In Prokaryotes:
? 5S rRNA is too small,contains limited info
? 23S rRNA is too large,too difficult to manage
? 16S rRNA has the right size for studies
? In Eukaryotes:
? 18S rRNA is used for phylogenetic measurements
The 16S rRNA or
18S rRNA Technique
The 16S rRNA or 18S rRNA Technique
? Ribosomal Database Project (RDP):
? http://www.cme.msu.edu/RDP
? Compare your sequences with the database to find
out the organisms you identify
Phylogenetic
Trees from DNA
Sequences
? Distance-Matrix
Method for
generating the trees
? Evolutionary
Distance (ED)
? Computer compare
the sequence
differences and
build the
phylogenetic tree
based on corrected
ED
Phylogenetic Trees from DNA Sequences
Signature Sequences,unique to certain group of organisms
Applications,Phylogenetic Probes
Questions
? Why is Ribosomal RNAs used as Evolutionary
Chronometers
? Why is 16S rRNA employed to study phylogenetics
rather than the smaller 5S rRNR and the large 23S
rRNA in Prokaryotics?
? How to sequence 16S rRNA from inside the cells?
? How to identify a prokaryotic or eukaryotic organism
based on 16S or 18S rRNA?
? How to build Phylogenetic Trees from DNA Sequences?
? Why it is said the archaea closer to eukarya than
bacteria is?
? What is signature sequence and how can it be used?