Microbial genetics

1. Microbial genetics
Bio 433
By
Dr. Mona Othman Albureikan

2. • Is an explanation of the flow of genetic
information within a biological system.
• It was first stated by Francis Crick in 1956.
• The informations transfer sequentially in
biological systems.
• That such informations cannot be
transferred back.
Central dogma of molecular biology
Francis Crick

3. • DNA makes RNA and RNA makes protein.
• It cannot be transferred back from protein to
either protein or nucleic acid.
• The dogma is a framework for understanding
the transfer of sequence information between
information-carrying biopolymers, in living organisms.
Central dogma of molecular biology

4. • There are 3 major classes of such biopolymers:
- DNA
- RNA (both nucleic acids)
- Protein.
• There are 3×3 = 9 possible direct transfers of
information that can occur between these.
Central dogma of molecular biology

5. General Special Unknown
DNA → DNA RNA → DNA protein → DNA
DNA → RNA RNA → RNA protein → RNA
RNA → protein DNA → protein protein → protein
Table of the 3 classes of information transfer suggested
by the dogma
• The dogma classes these into 3
groups of 3:
- 3 general transfers (believed to occur
normally in most cells).
- 3 special transfers (known to occur,
but only under specific conditions in
case of some viruses or in a
laboratory).
- 3 unknown transfers (believed never
to occur).
Central dogma of molecular biology

6. • The general transfers describe the normal flow of biological information:
- DNA can be copied to DNA (DNA replication).
- DNA information can be copied into mRNA
(transcription).
- Proteins can be synthesized using the
information in mRNA as a template (translation).
Central dogma of molecular biology
Central Dogma

7. Central dogma of molecular biology

8. DNA Replication
• The replication of a DNA
molecule involves polymerization
of special energy-carrying
nucleotides called triphosphate
deoxyribonucleotides since they
are bound to three phosphate
groups.

9. • The energy released by the enzymatic removal of two of the
phosphates is utilized in the linking of each nucleotide to its neighbor
on the growing DNA nucleoside.
DNA Replication

10. There are three possible models in DNA replication
DNA Replication

11. DNA Replication
A- Semiconservative model of DNA replication
- 1958 Matthew Meselson & Frank Stahl’s
Experiment.
- One strand of a double helix (parent strand )
passed on unchanged to each of the daughter cells
(daughter DNA) .
-This 'conserved' strand acts as a template for the
synthesis of a new, complementary strand by the
enzyme DNA polymerase.

12. • DNA replication begins at
a specific area along the
molecule called the origin
of replication (OR).
• Initiator proteins identify
specific base sequences
on DNA called sites of
origin.
DNA Replication

13. DNA Replication
The replication site in:
Prokaryotes – single origin site E.g in E. coli.
Eukaryotes – multiple sites of origin
E.g 1,000s in human.
- Begins with double-helix denaturing into
single-strands to allow replication machinery
contact with the DNA.
Many A-T base pairs because easier to break 2 H-bonds than 3 H-bonds

14. DNA Replication
- Exposes a replication bubble from which replication proceeds in both
directions.

15. • At the origin, histones are removed
to expose the DNA strand.
• Then the enzyme helicase untwists
the replicating portion of the
molecule and breaks the hydrogen
bonds between complementary
base pairs.
DNA Replication

16. - The hydrogen
bonds between
complementary base
pairs breaks, causing
the formation of
a replication fork in
the direction of
replication.
DNA Replication

17. Replication
Forks
Bidirectional movement of the DNA replication machinery
DNA Replication

18. • An enzyme called RNA
polymerase (primase)
begins the replication
process by adding RNA
nucleotides to each
template nucleoside
( RNA primer).
DNA Replication

19. DNA Replication

20. • A molecule of DNA polymerase III binds
to each of the separated strands.
• This enzyme adds nucleotide bases to
their complementary bases on the
template strand after the RNA primer
sequence and proofreads to prevent
improper nucleotides from being joined
to the template.
DNA Replication

21. DNA Replication
- DNA polymerase III
is directional so it will only
completely build and proofread
the nucleoside that moves in the
5' to 3' direction.
- This is called the
continuous (leading) strand.

22. - Following DNA
polymerase III, a new
enzyme called DNA
polymerase I removes the
RNA primers and replaces
them with DNA
nucleotides.
DNA Replication

23. DNA Replication
- The nucleotide that moves from 3' to 5' is called the discontinuous
(lagging) strand since
DNA polymerase III
cannot continuously add
nucleotides in that
direction. Instead,
primase adds RNA bases in several places along the growing strand,
enabling DNA polymerase III to add DNA nucleotides between them.

24. - These completed DNA portions
are called Okazaki fragments.
- DNA polymerase I replaces the
RNA with DNA along the chain,
filing the gaps between Okazaki
fragments, but leaves unconnected
"nicks" (unjoined regions) in the
sugar-phosphate backbone.
DNA Replication

25. • To complete the strand,
a new molecule
called DNA ligase links
the nicks together.
• DNA gyrase twists the
new double helix back
into a supercoiled form
in the bacterial cell.
DNA Replication

26. DNA Replication
Anti parallel strands replicated simultaneously
Leading strand synthesis continuously in 5’– 3’
Lagging strand synthesis in fragments in 5’-3’

27. Segments of single-stranded DNA are called template strands.
Gyrase (a type of topoisomerase) relaxes the supercoiled DNA.
Initiator proteins and DNA helicase binds to the DNA at the
replication fork and untwist the DNA using energy derived from ATP
(adenosine triphosphate).
 Primase synthesizes a short RNA primer of 10-12 nucleotides, to
which DNA polymerase III adds nucleotides.
DNA Replication

28. DNA Replication
Polymerase III adds nucleotides 5’ to 3’ on both strands beginning at
the RNA primer.
The RNA primer is removed and replaced with DNA by polymerase I,
and the gap is sealed with DNA ligase.
Single-stranded DNA-binding (SSB) proteins (>200) stabilize the
single-stranded template DNA during the process.

29. DNA Replication

30. DNA Replication

31. The mechanism of DNA replication
Arthur Kornberg, a Nobel prize winner and other biochemists deduced steps of
replication
 Initiation
 Proteins bind to DNA and open up double helix
 Prepare DNA for complementary base pairing
 Elongation
 Proteins connect the correct sequences of nucleotides into a
continuous new strand of DNA
 Termination
 Proteins release the replication complex
DNA Replication

32. • Leading strand synthesized 5’ to 3’ in the direction of the
replication fork movement.
• continuous
• requires a single RNA primer
• Lagging strand synthesized 5’ to 3’ in the opposite direction.
• semidiscontinuous (i.e., not continuous)
• requires many RNA primers , DNA is synthesized in short fragments.
DNA Replication

33. • Bacterial DNA replication is bidirectional since the chromosome is
circular.
• It begins from a central origin and proceeds around the chromosome
until the two polymerase enzymes meet.
DNA Replication

34. • The torsion placed on the separated strands by the untwisting activity
of helicase is relaxed by the enzyme topoisomerase by cutting the
twisting sections and re-joining them opposite to the direction of the
supercoil.
DNA Replication

35. • Eukaryote DNA replication proceeds in a manner very similar to that of
bacteria, with the following exceptions:
• A. Eukaryotes utilize four different DNA polymerase molecules, α2 which
initiates synthesis and places primers (bacteria use primase for this), σ which
elongates the leading strand, ε which elongates the lagging strand and γ that
replicates mitochondrial DNA (note - mitchondrial DNA is circular and
naked in the mitochondrial matrix).
DNA Replication

36. B. Eukaryote DNA requires many points of replication owing to its
large size.
C. Eukaryote Okazaki fragments are far shorter than those of
prokaryotes.
D. Methylation of plant and animal DNA occurs only on cytosine
molecules (usually adenine, seldom cytosine in bacteria).
DNA Replication

37. Five common DNA polymerases from mammals.
1.Polymerase  (alpha): nuclear, DNA replication, no proofreading
2.Polymerase  (beta): nuclear, DNA repair, no proofreading
3.Polymerase  (gamma): mitochondria, DNA repl., proofreading
4.Polymerase  (delta): nuclear, DNA replication, proofreading
5.Polymerase  (epsilon): nuclear, DNA repair (?), proofreading
DNA Replication

38. DNA Replication

40. References
 Molecular Genetics of Bacteria ( 4th Edition ) (2013), Larry Snyder , Joseph E. Peters , Tina M. Henkin , Wendy
Champness ISBN 10: 1555816274 ISBN 13: 9781555816278.
 Molecular Genetics of Bacteria, 5th Edition, by Jeremy W. Dale, Simon F. Park ,April 2010, ©2010.
 Genetics of Bacteria, Sheela Srivastava,(2013) ISBN: 978-81-322-1089-4
 Microbial Genetics. (1994). Jones and Bartlett Series in Biology. Jones and Bartlett Publishers, Inc.; 2nd
edition, ISBN-10: 0867202483, ISBN-13: 978-0867202489, 484 pages.
 Microbial genetics. (2008). Jones and Bartlett series in biology
Series of books in biology. David Freifelder, publisher, Jones and Bartlett, 1987. 601 pages.
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