Microbial genetics involves the transmission of hereditary traits in microorganisms. It plays a role in developing fields like molecular and cell biology. Bacteria contain a single circular chromosome made of DNA that is compacted. Bacteria can also contain plasmids. DNA replication copies the parental DNA. Variability in microorganisms comes from changes in genotype and phenotype from factors like mutation and recombination. Mutation rates depend on type and can be increased by mutagens. Recombination involves processes like transformation, transduction, and conjugation. Plasmids can confer traits like antibiotic resistance and are transferred by conjugation. Gene expression in bacteria is regulated through mechanisms like induction and repression that control operons.

1. Microbial Genetics
Inheritance and
Variability

2. Microbial genetics is concerned with the
transmission of hereditary characters in
microorganisms.
Microbial genetics has played a unique role
in developing the fields of molecular and cell
biology and also has found applications in
medicine, agriculture, and the food and
pharmaceutical industries.

3. Chromosome
- a dense structure inside cells that physically carries hereditary information
from one generation to the next.
Bacterial Cell - contains only one chromosome, consisting of a single molecule of
double-stranded deoxyribonucleic acid (DNA) in the form of closed circle.
*Procaryotic chromosome is:
1. Naked (lacking the nuclear membrane found in eucaryotic cell)
2. is twisted, coiled and packaged into a highly compact form (because
bacterial chromosome has length of about 1200 times of the entire
cell)
-in addition to its chromosome, a bacterial cell contains
one or more PLASMIDS (double-stranded DNA molecules
that are much smaller than the chromosome and can
replicate independently of the chromosome).
-most are circular but linear plasmids have been
found in few bacteria(e.g. Spirochete that cause
Lyme disease)
-have been used extensively in genetic engineering
techniques.

6. DNA REPLICATION
- process that copies the
nucleotide sequence of a double-
stranded parent DNA into two
double-stranded daughter molecules

7. Figure 5.14. Origin of replication in E. coli Replication initiates at a
unique site on the E. coli chromosome, designated the origin (ori)

8. Component Function
Initiator protein Binds to origin and separates strands of
DNA to initiate replication
DNA helicase Unwinds DNA at replication fork
Single-strand-binding proteins Attach to SS-DNA and prevent 20 structures
from forming
DNA gyrase Moves ahead of the replication fork ,
making and resealing breaks in the double-
helical DNA to release the torque that
builds up as a result of unwinding at the
replication fork
DNA primase Synthesizes a short RNA primer to provide
a 3’-OH group for the attachment of DNA
nucleotides
DNA polymerase III Elongates a new nucleotide strand from
the 3’-OH group provided by the primer
DNA polymerase I Removes RNA primers and replaces them
with DNA
Table 4.1 Components required for replication in bacterial cells

9. Component Function
DNA ligase Joins Okazaki fragments by sealing nicks in the sugar-phosphate
of newly synthesized DNA

10. Transcription Elongation in Eucaryotes Is Tightly Coupled To RNA Processing
Figure 6-21. Summary of the
steps leading from gene to
protein in eucaryotes and
bacteria. The final level of a
protein in the cell depends on the
efficiency of each step and on the
rates of degradation of the RNA
and protein molecules. (A) In
eucaryotic cells the RNA molecule
produced by transcription alone
(sometimes referred to as the
primary transcript) would contain
both coding (exon) and
noncoding (intron) sequences.
Before it can be translated into
protein, the two ends of the RNA
are modified, the introns are
removed by an enzymatically
catalyzed RNA splicing reaction,
and the resulting mRNA is
transported from the nucleus to
the cytoplasm.

11. Variability in Microorganisms
-associated with its genotype and its phenotype
Genotype
- represents the inheritable total potential of a cell
Phenotype
- represents the portion of the genetic potential that is actually
expressed by the cell under a given set of condition.(eg. May
be particular color or size of bacterial colony or presence of bacterial
capsules which may or may not formed by certain bacteria depending on their
environment)

12. Phenotypic changes
-both the genotype and the environment influence the phenotypeof
an organism
eg. Bacteria of the genus Azomonas form large, gummy colonies
when grown with the sugar sucrose and smaller, nongummy colonies in the
absence of this sugar
Genotypic changes
-although some phenotypic changes are the result of Environmental
influences, others are the result of changes in the DNA.
These can occur as the result of:
1. mutation – a change in the nucleotide
sequence of a gene or
2. recombination –a process that leads to new
combinations of genes on a chromosome

13. Mutation and Recombination
• Mutation is a heritable change in DNA
sequence that can lead to a change in phenotype.
By definition, a mutant differs from its parental
strain in genotype, the nucleotide sequence of
the genome.

14. • Selectable mutations are those that give the
mutant a growth advantage under certain
environmental conditions and are especially
useful in genetic research. If selection is not
possible, mutants must be identified by
screening.

15. • Although screening is always more tedious than
selection, methods are available for screening
large numbers of colonies in certain types of
mutations. For instance, nutritionally defective
mutants can be detected by the technique of
replica plating (Figure 8.2).

16. Molecular Basis of Mutation
• Mutations, which can be either
spontaneous or induced, arise because of
changes in the base sequence of the
nucleic acid of an organism's genome.

17. Mutations can be classified into various types based upon the kinds of changes
they produce in a gene.
Two common types are:
1. Point mutations –results from the substitution of 1 nucleotide for
another in a gene
a. Neutral mutation
eg. AAU to AAC still codes for
asparagine
b. missense mutation
eg. AAU become AAG asparagine
to lysine
c. nonsense mutation
eg. UAU to UAA premature
halting
2. Frameshift mutation –addition or loss of one or
more nucleotides in a gene
a. insertion
b. deletion

18. • A point mutation, which results from a
change in a single base pair, can lead to a
single amino acid change in a polypeptide
or to no change at all, depending on the
particular codon involved (Figure 8.3).

21. • Deletions and insertions cause more
dramatic changes in the DNA, including
frameshift mutations, and often result in
complete loss of gene function (Figure 8.4).

23. Figure 8-4. Different types
of mutations

24. • Table 10.1 lists various kinds of mutants.

25. Mutation Rates
• Different types of mutations can occur at
different frequencies. For a typical bacterium,
mutation rates of 10–7 to 10–11 per base pair are
generally seen.
• Although RNA and DNA polymerases make errors
at about the same rate, RNA genomes typically
accumulate mutations at much higher
frequencies than DNA genomes.

26. Mutagenesis
• Mutagens are chemical, physical, or
biological agents that increase the
mutation rate. Mutagens can alter DNA in
many different ways, but such alterations
are not mutations unless they can be
inherited.

27. • Table 8.2 gives an overview of some of the
major chemical and physical mutagens and their
modes of action.

28. • There are several classes of chemical mutagens,
one being the nucleotide base analogs (Figure
8.5).

29. • Several forms of radiation are highly mutagenic
(Figure 8.6).

30. • Some DNA damage can lead to cell death if
not repaired. A complex cellular
mechanism called the SOS regulatory
system is activated as a result of some
types of DNA damage and initiates a
number of DNA repair processes, both
error-prone and high-fidelity (Figure 8.7).

33. Mutagenesis and Carcinogenesis: The Ames Test
• The Ames test employs a sensitive bacterial assay system
for detecting chemical mutagens in the environment.

35. Recombination
•DNA rearrangements are caused by a set of mechanisms that are
collectively called genetic recombination.
•Two broad classes:
1. general recombination
2. site-specific recombination.
General recombination (also known as homologous recombination)
-genetic exchange takes place between a pair of homologous DNA sequence
The breaking and rejoining of two homologous DNA
double helices creates two DNA molecules that have
“crossed over.” In meiosis, this process causes each
chromosome in a germ cell to contain a mixture of
maternally and paternally inherited genes.

36. Homologous recombination arises when closely related DNA
sequences from two distinct genetic elements are combined in a
single element (Figure 10.9)

37. • Recombination is an important evolutionary process, and cells
have specific mechanisms for ensuring that recombination takes
place.

38. In bacteria, gene transfer that can lead to recombination may occur in any of
three different ways:
1. transformation- simplest type of gene transfer; a recipient
cell acquires genes from “free floating” DNA molecules in the
surrounding medium
2. transduction –gene transfer in which a virus serves as
vehicle for carrying DNA from a donor bacteriumto a recipient
bacterium.
3. conjugation- a process of gene transfer that requires
cell-to-cell contact and thus differs from
transformation and trasnduction.

40. TRANSFORMATION

41. TRANSDUCTION

42. CONJUGATION

43. PART II Genetic Exchange in Prokaryotes

44. • The discovery of transformation was one of the
seminal events in biology because it led to
experiments demonstrating that DNA is the
genetic material (Figure 8.13).

45. • Certain prokaryotes exhibit competence, a state in
which cells are able to take up free DNA released by
other bacteria.
• Incorporation of donor DNA into a recipient cell requires
the activity of single-stranded binding protein, RecA
protein, and several other enzymes. Only competent cells
are transformable (Figure 8.14).

47. Transduction
• Transduction involves the transfer of host genes from
one bacterium to another by bacterial viruses.
• In generalized transduction (Figure 8.15), defective
virus particles incorporate fragments of the cell's
chromosomal DNA randomly, but the efficiency is low.

49. • In specialized transduction (Figure 8.16), the DNA of a
temperate virus excises incorrectly and takes adjacent
host genes along with it; transducing efficiency in this
case may be very high.

51. Plasmids: General Principles
• Plasmids are small circular or linear DNA molecules
that carry any of a variety of unessential genes.
Although a cell can contain more than one plasmid,
they cannot be closely related genetically.

53. • Figure 10.18 shows a genetic map of the F (fertility)
plasmid, a very well characterized plasmid of
Escherichia coli.

54. • Lateral transfer in the process of conjugation can
transfer plasmids (Figure 8.19).

55. Types of Plasmids and Their Biological
Significance

56. • The genetic information that plasmids carry is not
essential for cell function under all conditions but may
confer a selective growth advantage under certain
conditions.

57. • Examples include antibiotic resistance (Figure
8.20), enzymes for degradation of unusual
organic compounds, and special metabolic
pathways. Virulence factors of many
pathogenic bacteria are often plasmid-
encoded.

58. Types of plasmids
1. Conjugative plasmids: transmitted during
conjugation, carry a variety of information
2. R plasmids: resistance plasmids; protect against
environmental factors, MDR (multiple drug
resistance) plasmid
3. Hfr plasmids: promotes genomic recombination
4. Col-plasmids: codes for proteins that kill other
microbes
5. Degradative plasmids: contain sequencing that
allows host to digest uncommon substances (ex:
toluene, salicylic acid)
6. Virulence plasmids: codes for altering the microbe
into a pathogen

60. • Table 10.3 lists some phenotypes that plasmids confer
on prokaryotes.

64. Conjugation: Essential Features
• Conjugation is a mechanism of DNA transfer in
prokaryotes that requires cell-to-cell contact.
• Genes carried by certain plasmids (such as the F
plasmid) control conjugation, and the process
involves transfer of the plasmid from a donor cell to a
recipient cell (Figure 8.22). Plasmid DNA transfer
involves replication via the rolling circle mechanism.

66. The Bacterial Chromosome
Genetic Map of the Escherichia coli Chromosome
• The Escherichia coli chromosome has been
mapped usingconjugation, transduction,
molecular cloning, and sequencing (Figure
8.42).

68. • E. coli has been a useful model organism,
and a considerable amount of information
has been obtained from it, not only about
gene structure but also about gene
function and regulation.

69. Regulation of Gene Expression
Operon – in bacteria, the genes that code for the enzymes of a metabolic pathway are
usually arranged in a consecutive manner to form a functional unit.
*most transcriptional control mechanisms for operons involve either enzyme induction or
end-product repression
1. Induction –form of control of gene transcription, with the gene transcribed
only when appropriate substrate for the protein is present
-used mainly to control the synthesis of proteins that are used to
transport and breakdown nutrients.
2. End-product repression – transcription of an operon for a synthetic pathway is often
regulated by its end product, and not by the initial substrate of the
pathway.

70. Thank you for Listening!