Bacterial genetics can be studied through markers that allow for growth in certain conditions. Restriction enzymes cut DNA at specific sites, and plasmids are small genetic elements capable of independent replication. Transformation is the uptake of naked DNA from the environment by bacterial cells through a competent state involving cell wall permeability. Griffith's experiments showed that a virulent pneumococcal strain could be made avirulent by DNA uptake from a dead strain.

1. BACTERIAL GENETICS

2. MICROBIAL GENETICS; MARKER GROWTH
CONDITIONS OF SELECTION; AB resistance ;
genes let grow in presence or absence of drug
Expression of genes is phenotype
RESTRICTION ENZYMES; Cleave DNA at specific
sites to form DNA restriction fragments
PLASMIDS; small genetic elements capable of
independent replication in bacteria & yeasts.
Introduction of these rest.fragments in plasmids leads to
their amplification [also by PCR]

3. DNA,RNA
DNA; double stranded; double helix with bases in center
determine genetics. Each helical turn has a major/minor
groove. Major groove exposed more so binds proteins
regulating gene expression
anti parallel 5’to 3’, 3’to5’
complementary base A-T;C-G
Hydrogen bonds in centre
Template strand: coding strand
NUCLEOTIDE; 4 BASES +PHOSPHO 2’DEOXYRIBOSE

4. Length of DNA; thousands base pairs, kilobase pairs[kbp]
Viris single DNA 5 kbp; E.coli 4639 kbp 1mm length
Coiling ,super-coiling of DNA
RNA; single strand
URACIL INSTEAD OF THYMINE; A-U; C-G
mRNA; communicate gene seq.of DNA as mRNA to
ribosomes,

5. RNA
Ribosomes; ribosomal RNA [rRNA]+ proteins
tRNA ; translate mRNA to protein molecule
SIZE; tRNA few 100 bases ribosomal
RNA ;3 types 120 ,1540,2900 bases & many proteins
STABILITY; rRNA, tRNA : 95% of total RNA
mRNA gene expression alters with demand with rapid
metabolic turnover
sRNA regulators bind near 5’ end of mRNA, prevent
ribo,translation

6. GENE
UNIT OF HEREDITY
SEGMENT of DNA,whose nucleotide sequence carries
the information of a biochemical/physiological function
Genes identified by PHENOTYPE;collective
structural & physiological properties of cell or
organism eg eye colour,drug resistance
GENOTYPE is chemical basis in DNA;alteration of
sequence within a gene or organisations of genes

7. Eukaryotic genome
Genome; totality of genetic information in organism.
Carried on 2 or more LINEAR CHROMOSOMES in a
nucleus surrounded by NM
Diploid cells have 2 homologous copies of each
chromosome; evolutionarily divergent
Mutations, genetic changes are not detected in diploid
cells due to compensation of homologue.
Expressed in haploid cells with single copy of genes

8. Gene expression
Recessive genes ; no phenotypic expression bec of
homologue
Dominant genes; overrides homologue & expresses
Mitochondria/chloroplasts; circular mol. DNA few
genes encoding organelle functionmost of which is by
chromosomal genes.
PLASMIDS; small 2um circular DNA 6.3 kbp
Independent replication,found in yeasts[euk] &prok. can
be genetically manip.& introd. In cells

9. Repetitive DNA
EUKARYOTE; in extragenic regions, non coding
Large quantities
PROKARYOTES;SSR; excessive length polymorphism
SSR; short-seq.repeats
STRs; short tandemly repeat sequences
Several to thousands of dispersed copies
INTRONS; intervening seq.of DNA not transcribed on mRNA.

10. PROKARYOTIC GENOME; Haploid
1.GENES carried on chromosomes,for growth
Single circular genome;580---5220kbp
Brucella,Burkholderia, have 2 circular DNA mol.
2.Genes on plasmids; spread of drug R
Several…100kbp
REPLICONS: DNA circles [1&2] carring genetic
information for self replication

11. TRANSPOSONS; no self replication
 Genetic elements; several kbp
 Contain information for transfer from one locus to other
 In migration cause insertion mutations esp. short transposons 750…
2000bp called insertion elements or insertion seq IS
elements
 All bacteria have charcteristic ISE
 PLASMIDS also have ISE; important for Hfr strains
 Complex transposons, have genes for special function as
AB resistance flanked by ISE
 Physically attached replicon; not independantly ;copies
inserted in same or diff.replicons randomly. If plasmid
insertion can be widely disseminated

12. VIRAL GENOME
 PRASITES AT GENETIC LEVEL; lytic, temperate phages
 survive but cant grow without host
 Debilitates/kills host ,lives on its energy, uses macromol.
 Bacteriophages; viruses of prok. 5000 in 140 bact.genera
 NA;DNA ds common; others RNA ss, ds; ss DNA
 Coat; protein, lipid
 REPLICATION: ds DNA linear; becomes circular at cohesive
ends, complementary tails that hybradize

13. Ligation; phosphodiester bonds form at tails
Replicates
Linear DNA formed;cleaved & packaged inside
head
Ss DNA of filamentous phages is converted to
circular double stranded replicative form.
One strand is used as a template for ssDNA
continuously a rolling circles. Ss DNA is
cleaved,packaged with protein for extrusion

14. Ss RNA PHAGES
Smallest extracellular particles with information for their
own replication
RNA phage MS2, has 4000 nucleotides, 3 genes act as
mRNA following infection;
1,coat protein
2.RNA polymerase ssRNA; ss formed from replicas

15. Template phages
Prophage stage;
1 plasmid-like existance
2.host chromosome integration at int locus shared
homology site
3.many sites of insertion like transposons
Repressed genes ; for lytic/vegetative replication
Immunity against similar phages
Derepression; triggered by mol.reaction/ uv light
vegetative burst ,lysis esp in actively dividing cells

16. Pathgenicity islands
 clusters of genes in DNA possesing specific determinants
of pathogenecity
Large; at least 200kbp
code virulence to invade higher organisms as
adhesins,invasins,toxins
Diff. G:C content than rest of genome
Linked to tRNA genes flanked by direct repeats

17. Prok genetic transfer
Widespread, genetic diversity
Small fragment transferred to recipient
Replication of recombinent
1.integration of DNA in replicon
2. independent replicon

18. Restriction to gene transfer
Retriction enzymes…endonucleases d/d self DNA from
nonself by res gene
 enzymes hydolyze DNA at specific sites with DNA seq from
4---13 bases
This specficty of fragmentation is basis of genetic engineering
Bacteria recognize sites through enzymes &modify hem by
methylation of adnine/cytosine by
Type 1 system; combined single multisubunit protein
Type 11; sparate endonucleases & methylases

19. plasmids
Wide hosr range drug resistance
Narrow host range
Coexistance of plasmids in bacteria
Compatable
Incompatable; one lost at higher rateon bacterial cell
division

20. Mechanism of recombination
DNA replicates
No replication,then find recipient DNA
RECOMBINATION
HOMOLOGOUS;close similarity in donor,recepient
common ancestrol genes. Rec gene dysfunction can give
rise to bacteria that maintain closely related genes
NON HOMOLOGOUS; enzyme-catalyzed recomb.
between dissimilar

21. Prokaryote Basics
The largest and most obvious division of living organisms is into
prokaryotes vs. eukaryotes.
Eukaryotes are defined as having their genetic material enclosed
in a membrane-bound nucleus, separate from the
cytoplasm. In addition, eukaryotes have other membrane-
bound organelles such as mitochondria, lysosomes, and
endoplasmic reticulum. almost all multicellular organisms
are eukaryotes.
In contrast, the genome of prokaryotes is not in a separate
compartment: it is located in the cytoplasm (although
sometimes confined to a particular region called a “nucleoid”).
Prokaryotes contain no membrane-bound organelles;
their only membrane is the membrane that separates the cell
form the outside world. Nearly all prokaryotes are unicellular.

22. Three Domains of Life

23. Prokaryote vs. Eukaryote Genetics
Prokaryotes are haploid, and they contain a single circular
chromosome. In addition, prokaryotes often contain small
circular DNA molecules called “plasmids”, that confer useful
properties such as drug resistance. Only circular DNA
molecules in prokaryotes can replicate.
In contrast, eukaryotes are often diploid, and eukaryotes have
linear chromosomes, usually more than 1.
In eukaryotes, transcription of genes in RNA occurs in the
nucleus, and translation of that RNA into protein occurs in
the cytoplasm. The two processes are separated from each
other.
In prokaryotes, translation is coupled to transcription:
translation of the new RNA molecule starts before transcription
is finished.

24. Bacterial Culture
Surprisingly, many, perhaps even most, of the
bacteria on Earth cannot be grown in the
laboratory today.
Bacteria need a set of specific nutrients, the
correct amount of oxygen, and a proper
temperature to grow. The common gut
bacterium Escherichia coli (E. coli) grows easily
on partially digested extracts made from yeast
and animal products, at 37 degrees in a normal
atmosphere. These simple growth conditions
have made E. coli a favorite lab organism, which
is used as a model for other bacteria.

25. More Culture
 Bacteria are generally grown in either of 2
ways: on solid media as individual
colonies, or in liquid culture.
 The nutrient broth for liquid culture allows
rapid growth up to a maximum density.
Liquid culture is easy and cheap.
 Solid media use the same nutrient broth as
liquid culture, solidifying it with agar.
Agar a polysaccharide derived from
seaweed that most bacteria can’t digest.
 The purpose of growth on solid media is
to isolate individual bacterial cells, then
grow each cell up into a colony. This is
the standard way to create a pure culture
of bacteria. All cells of a colony are
closely related to the original cell that
started the colony, with only a small
amount of genetic variation possible.
 Solid media are also used to count the
number of bacteria that were in a culture
tube.

26. Bacterial Mutants
 Mutants in bacteria are mostly biochemical in nature, because we
can’t generally see the cells.
 The most important mutants are auxotrophs. An auxotroph needs
some nutrient that the wild type strain (prototroph) can make for
itself. For example, a trp- auxotroph can’t make its own tryptophan
(an amino acid). To grow trp- bacteria, you need to add tryptophan
to the growth medium. Prototrophs are trp+; they don’t need any
tryptophan supplied since they make their own.
 Chemoauxotrophs are mutants that can’t use some nutrient (usually a
sugar) that prototrophs can use as food. For example, lac- mutants
can’t grow on lactose (milk sugar), but lac+ prototrophs can grow on
lactose.
 Resistance mutants confer resistance to some environmental toxin:
drugs, heavy metals, bacteriophages, etc. For instance, AmpR
causes
bacteria to be resistant to ampicillin, a common antibiotic related to
penicillin.
 Auxotrophs and chemoauxotrophs are usually recessive; drug
resistance mutants are usually dominant.

27. Replica Plating A common way to find bacterial mutants is replica plating,
which means making two identical copies of the colonies on a petri plate under
different conditions.
 For instance, if you were looking for trp- auxotrophs, one plate would contain
added tryptophan and the other plate would not have any tryptophan in it.
 Bacteria are first spread on the permissive plate, the plate that allows both mutants
and wild type to grow, the plate containing tryptophan in this case. They are
allowed to grow fOR a while, then a copy of the plate is made by pressing a piece
of velvet
 onto the surface of the plate, then moving it to a fresh plate with the restrictive
condition (no tryptophan). The velvet transfers some cells from each colony to an
identical position on the restrictive plate.
 Colonies that grow on the permissive plate but not the restrictive plate are
(probably) trp- auxotrophs, because they can only grow if tryptophan is supplied.

28. Replica Plating, pt. 2

30. BACTERIAL DNA unwound

32. Bacterial Sexual Processes
Eukaryotes have the processes of meiosis to
reduce diploids to haploidy, and fertilization to
return the cells to the diploid state.
 Bacterial sexual processes are not so regular.
However, they serve the same aim: to mix the
genes from two different organisms together.

33. GENETIC
TRANSFER/RECMBINATION
Exchange of genes between two DNA molecules to form
new combinations of genes on a chromosome
Contribute to genetic diversity; evolution
Better than mutation as new function beneficial to
microbe
Vertical gene transfer to offspring; plants, animals,
Horizontal: microbes via donor/recepient <1% of entire
bacterial population; vertical
transmission also in bacteria

34. No integration
Integration

35. GENETIC TRANSFER
The three bacterial sexual processes
1. Conjugation: direct transfer of DNA from
one bacterial cell to another.
2. Transduction: use of a bacteriophage
(bacterial virus) to transfer DNA between cells.
3. Transformation: naked DNA is taken up
from the environment by bacterial cells.

36. TRANSFORMATION
Transfer of “naked” DNA between bacteria
Active process; needs specific proteins called
“competence factors”
Fredrick Griffith in 1928 worked on 2 strains of S
pneumoniae
Oswald Avery and associates 1944 proved the chemical
material transferred was DNA.
Recombinant or hybrid; new cell transfers to descendants
that are identical

37. (transformation)
Discovered by Fredrick Griffith in 1928 while working with
Streptococcus pneumoniae
Griffith realized S. pneumoniae existed in two forms
Encapsulated, virulent form (smooth in appearance)
Nonencapsulated, avirulent form (Rough in appearance)
Griffith hypothesized that injections with the smooth
strain could protect mice from pneumonia
Griffith injected mice with the two different strains

38. Griffith’s Results

39. Transformation
Nature: different genera of Niesseria, Haemophilus
Streptococcus, Staphylococcus, Acinetobacter
Best between closely related cells
DNA is a large molecule, passes only when cell wall in a
physiological competent state.
Competence involves alterations in cell wall that make it
permeable to large DNA molecule.
Occurs in late log and early stationary phase in nature

40. Dying cells rupture during the stationary and death
phases. The chromosome breaks into small
pieces and explodes through the ruptured cell
wall
Recipient cells absorb pieces of “naked” DNA
Enzymes cleave recipient DNA
The naked DNA is integrated into the recipient
cell’s DNA at that site
Naked DNA integrates at a homologous site on the
recipient’s chromosome

41. Transformation
Recombinant DNA work.
 remove DNA from cells, manipulate it in the test
tube, then put it back into living cells.
 In the case of E. coli, cells are made “competent”
to be transformed by treatment with: calcium
chloride ions
heat shock.
 E. coli cells in this condition readily pick up DNA
from their surroundings and incorporate it into their
genomes.

42. Conjugation

44. High frequency of
recombination – Hfr strains

46. Conjugation
Conjugation is mediated by a plasmid
R plasmids
F plasmids
Conjugation requires direct contact between cells
Cells must be of opposite mating types
Donor cells carry a plasmid that codes for fertility factor or “F
factor”
This cell is designated F+
Recipient cell does not carry a plasmid
This cell is designated F-

50. CONJUGATION BY PLASMID
The ability to conjugate is conferred by the F plasmid.
can spontaneously be lost
 A plasmid is a small circle of DNA that replicates
independently of the chromosome. Bacterial cells
that contain an F plasmid are called “F+”. Bacteria that
don’t have an F plasmid are called “F-”.
F+ cells grow special tubes called “sex pilli”
from their bodies. When an F+ cell bumps into
an F- cell, the sex pilli hold them together, and a
copy of the F plasmid is transferred from the F+
to the F-. Now both cells are F+.

51. When it exists as free plasmid, the F plasmid can only
transfer it self; no use in genetics.
However if F plasmid can become incorporated into
bacterial chromosome by a cross-over between F plasmid
and the chromosome, the resulting bacterial cell is called
“Hfr” ie High frequency of recombination”
Hfr bacteria conjugate like F+ do but they drag
a copy of entire chromosome into F- cell

52. Hfr Conjugation

53. Interrupted Mating
 Chromosome transfer from the Hfr
into the F- is slow: it takes about
100 minutes to transfer the entire
chromosome.
 The conjugation process can be
interrupted using a kitchen
blender.
 By interrupting the mating at
various times you can determine
the proportion of F- cells that have
received a given marker.
 This technique can be used to
make a map of the circular E. coli
chromosome.

54. Different Hfr Strains
The F plasmid can
incorporate into the
chromosome in almost
any position, and in either
orientation. Note that the
genes stay in fixed
positions, but the genes
enter the F- in different
orders and times, based on
where the F was
incorporated in the Hfr.
Data are for initial time of
entry of that gene into the
F-.
gene Hfr 1 Hfr 2 Hfr 3
azi 8 29 88
ton 10 27 90
lac 17 20 3
gal 25 12 11

55. Intracellular Events in Conjugation
The piece of chromosome that enters the F- form the
Hfr is linear. It is called the “exogenote”.
The F- cell’s own chromosome is circular. It is called
the “endogenote”.
Only circular DNA replicates in bacteria, so genes on
the exogenote must be transferred to the endogenote
for the F- to propagate them.
This is done by recombination: 2 crossovers between
homologous regions of the exogenote and the
endogenote. In the absence of recombination,
conjugation in ineffective: the exogenote enters the
F-, but all the genes on it are lost as the bacterial cell
reproduces.

56. Transduction
Transduction is the process of moving bacterial
DNA from one cell to another using a
bacteriophage.
Bacteriophage or just “phage” are bacterial
viruses.
 They consist of a small piece of DNA inside a
protein coat. The protein coat binds to the
bacterial surface, then injects the phage DNA.
The phage DNA then takes over the cell’s
machinery and replicates many virus particles.

57. types
Two forms of transduction:
1. generalized: any piece of the bacterial
genome can be transferred
2. specialized: only specific pieces of the
chromosome can be transferred.

58. Transduction

59. General Phage Life Cycle
1. Phage attaches to the
cell and injects its DNA.
2. Phage DNA replicates,
and is transcribed into RNA,
then translated into new
phage proteins.
3. New phage particles are
assembled.
4. Cell is lysed, releasing
about 200 new phage
particles.
Total time = about 15
minutes.

60. Generalized Transduction
Some phages, such as phage P1, break up the bacterial
chromosome into small pieces, and then package it into some
phage particles instead of their own DNA.
These chromosomal pieces are quite small: about 1 1/2 minutes
of the E coli chromosome, which has a total length of 100
minutes.
A phage containing E coli DNA can infect a fresh host, because
the binding to the cell surface and injection of DNA is caused by
the phage proteins.
After infection by such a phage, the cell contains an exogenote
(linear DNA injected by the phage) and an endogenote (circular
DNA that is the host’s chromosome).
A double crossover event puts the exogenote’s genes onto the
chromosome, allowing them to be propagated.

61. Transduction Mapping
Only a small amount of chromosome, a few genes,
can be transferred by transduction. The closer 2
genes are to each other, the more likely they are to
be transduced by the same phage. Thus, “co-
transduction frequency” is the key parameter
used in mapping genes by transduction.
Transduction mapping is for fine-scale mapping
only. Conjugation mapping is used for mapping
the major features of the entire chromosome.

62. Mapping Experiment
Important point: the closer 2 genes are to each other, the
higher the co-transduction frequency.
We are just trying to get the order of the genes here, not put
actual distances on the map.
Expt: donor strain is aziR
leu+
thr+
. Phage P1 is grown on the
donor strain, and then the resulting phage are mixed with the
recipient strain: aziS
leu-
thr-
. The bacteria that survive are then
tested for various markers
1. Of the leu+
cells, 50% are aziR
, and 2% are thr+
. From this
we can conclude that azi and leu are near each other, and that
leu and thr are far apart.
But: what is the order: leu--azi--thr, or azi--leu--thr ?

63. Mapping Experiment, pt. 2
2. Do a second experiment to determine the order.
Select the thr+
cells, then determine how many of
them have the other 2 markers. 3% are also leu+
and
0% are also aziR
.
By this we can see that thr is closer to leu than it is
to azi, because thr and azi are so far apart that they
are never co-transduced.
Thus the order must be thr--leu--azi.
Note that the co-transduction frequency for thr and
leu are slightly different for the 2 experiments: 2%
and 3%. This is attributable to experimental error.

64. Larger Experiment
A few hints:
1. There are 3 experiments shown. In each, 1 gene is
selected, and the frequencies of co-transduction with
the other genes is shown.
2. start with 2 genes that are selected and that have a
non-zero co-transduction frequency. Put them on the
map.
3. Then locate the other genes relative to the first 2.

65. sele
cted
co-
tran
sdu
ced
freq sele
cted
co-
tran
sdu
ced
freq sele
cted
co-
tran
sdu
ced
freq
e a 0 f a 90 c a 74
e b 85 f b 2 c b 32
e c 29 f c 41 c d 0
e d 62 f d 0 c e 21
e f 0 f e 0 c f 39

66. Intro to Specialized Transduction
Some phages can transfer only particular genes to
other bacteria.
Phage lambda (λ) has this property. To understand
specialized transduction, we need to examine the
phage lambda life cycle.
lambda has 2 distinct phases of its life cycle. The
“lytic” phase is the same as we saw with the general
phage life cycle: the phage infects the cell, makes
more copies of itself, then lyses the cell to release the
new phage.

67. Lysogenic Phase
The “lysogenic”: the lambda phage binds to the bacterial cell
and injects its DNA.
 Once inside the cell, the lambda DNA circularizes, then
incorporates into the bacterial chromosome by a crossover,
similar to the conversion of an F plasmid into an Hfr.
Once incorporated into the chromosome, the lambda DNA
becomes quiescent: its genes are not expressed and it
remains a passive element on the chromosome, being
replicated along with the rest of the chromosome. The
lambda DNA in this condition is called the “prophage”.

68. reproducing itself, then lysing the cell.
After many generations of the cell, conditions might get
harsh. For lambda, bad conditions are signaled when
DNA damage occurs.
When the lambda prophage receives the DNA damage
signal, it loops out and has a crossover, removing itself
from the chromosome.
Then the lambda genes become active and it goes into
the lytic phase,

69. More Lysogenic Phase

70. Specialized Transduction
Unlike the F plasmid that can incorporate anywhere in the E coli
genome, lambda can only incorporate into a specific site, called
attλ.
The gal gene is on one side of attλ and the bio gene (biotin
synthesis) is on the other side.
Sometimes when lambda come out of the chromosome at the
end of the lysogenic phase, it crosses over at the wrong point.
This is very similar to the production of an F’ from an Hfr.
When this happens, a piece of the E coli chromosome is
incorporated into the lambda phage chromosome

71. These phage that carry an E coli gene in addition to the
lambda genes are called “specialized transducing
phages”. They can carry either the gal gene or the bio
gene to other E coli.
Thus it is possible to quickly develop merodiploids (partial
diploids) for any allele you like of gal or bio.
 Note that this trick can’t be used with other genes, but
only for genes that flank the attachment site for lambda
or another lysogenic phage.

72. PLASMIDS
GENETIC ELEMENTS; 1/5size of bacterial DNA
Additional mechanism of genetic exchange; selective
advantage in an environment
Present in prokaryotes and rarely eukaryotes
Self replicating autonomously called Replicons;
 horizontal transmission by conjugation, tran, trans
Used as vectors for molecular cloning recombining
sequences; Gene therapy in humans

73. PLASMIDS

74. 2 TYPES Integrated/Non

76. TYPES OF PLASMIDS
1.F-PLASMID :
CONJUGATIVE PLASMID
 WITH GENE FOR SEX PILUS
WITH GENE FOR TRANSFER TO OTHER CELL
2.DISSIMILATION PLASMID:
Code for enzymes that trigger catabolism of unusual sugars
and hydrocarbons

77. PSEUDOMONAS
Toluene
Camphor
Hydrocarbons of petrolium
Survival value in adverse conditions
USE: Cleanup of environmental wastes

78. 3.Pathogenicity of Bacteria
Eg E coli: harmless commensal of large gut
Strains causing infant diarrhea & traveler’s diarrhea
Code for: 1 .toxin production
 2. intestinal attachment
S aureus: Exfoliative toxin
Cl tetani: neurotoxin
B anthrax: toxin

79. Bacteriocins synthesis genes in plasmids
RESISTANCE FACTORS: R factors
Discovered in Japan in1950 in dysentry cases
Resistance to one or >anti-biotics
Resistance in normal flora too eg E coli
Spread of plasmid mediating transfer is called R factors

80. AMP & TETR R GENES

81. R FACTORS
Resistance to AB, Heavy metals, Cellular toxins
2 components:
R transfer factors: genes for plasmid replication and
conjugation
R determinant: resistant genes
 code for enzymes inactivating AB & toxins
Multiple R factors in a bacterium can combine giving
new combinations of r determinants

82. AB Resistance
Widespread use in industry, agriculture, animal feed
Preferential selection of AB R bacteria
R bacteria grow and expand within same species
And other species eg Neisseria acquired pencillinase-
producing plasmid from Streptococcus and
Agrobacterium
Non conjugative plasmid can insert in
conjugative plasmid or chromosome; or by
transformation

83. TRANSPOSON
Small DNA segments; 700-4000 base pairs long.
Can ”Transpose” from one DNA region to another of
wide host range; bacteria….humans
Discovered in 1950 in corn but now seen in all
microorganisms by Barbra McClintok
They move within one chromosome from one site to
another, or to another chromosome or to a plasmid.
Rare phenomenon like mutation at frequency of 10
-1
-10-7;
Role in evolution

85. Transpose mechanism
Directly: cut paste
Make copies: these transpose
Effects:
Interrupt the normal spelling of DNA
Interrupt protein formation by putting oFF or increase by
putting ON
Gene mutation
Survival value: AB resistance, make new proteins

86. TYPES
Contain information of their own transposition
SIMPLEST: Insertion sequences contain a gene for
enzyme transposase….catalyzes cutting and resealing
of DNA
Recognition sites are short inverted repeat sequences
that the enzyme recognizes as recombination sites
between chromosome and transposon

87. Complex transposons
Carry genes other than transpositioning eg
Endotoxin gene
AB resistance gene
Plasmids as R factors are made of a collection of
transposons
Function: natural mechanism of gene movement from
one chromosome to other
From one organism to another via plasmids, viruses

88. MUTATIONS
A change in base sequence of DNA
It may alter a product encoded by that gene
EFFECT:
Disadvantage: eg enzyme may be rendered inactive
Lethal: may be lethal mutation
Beneficial: give enhanced activity to organism

89. TYPES
SILENT: neutral ie no effect on activity of product
encoded by the gene
Eg one nucleotide substitution in DNA for another at
position 3 of mRNA codon
A nucleotide substitution may still encode for same aa or
even change in aa may not bring a change
May not alter the structure, function of gene product or
a minor alteration in nonfunctional part may occur

90. BASE SUBSTITUTION: point mutation
Single base at one point of DNA seq is substituted with a
different base eg AT for GC, or GC for CG;
If protein is encoded mRNA will transcribe an incorrect
base, hence incorrect aa translated
This is MISSENSE MUTATION
Effect: dramatic as in sickle cell disease. A missense
change A to a T results in aa valine instead ofglutamic
acid

91. Shape of Hb changes esp under low O2 , shape of RBC
changes, movement of RBC in capillaries is impeded
A STOP (non sense) codon may be created in the
middle of mRNA molecule; some base substitutions
prevent creation of functional protein; Only a fragment is
made.
A base substitution ending in a NONSENSE CODON
is called a nonsense mutation

92. FRAMESHIFT MUTATION
Few nucleotide seq are added or deleted in DNA
Huntington’s chorea: many bases added to a gene.
This alteration shifts the “translational reading frame” ie
the 3 by 3 nucleotide grouping read as CODONS by
tRNAs during translation.
Eg deleting a nucleotide pair in the mid gene may change
many aa downstream from site of original mutation. So
long stretch of altered aa made resulting in inactive
protein at site beyond mutation. usually a nonsense
codon is encountered that terminates the translation.

93. MUTATIONS
Spontaneous: mistake during DNA replication
Mutagens: chemical: household
 radiations: X rays, UV light
 physical
Bacteria: AB resistance, altered cell membrane, capsule
are mutations

94. CHEMICALS
1.Nitrous acid: Random base substitution
A does not pair T but C. so in progeny AT is replaced by
CG
2.Nucleoside analog: structurally similar to bases but
base pairing different
5 bromouracil, substitutes thymine and pairs cytosine
2 aminopurine (substitutes adenine but may pair with
guanine

95. Such analogs when added to growing cells, they are
incorporated in DNA, substitute bases AND MISPAIR.
Passed onto daughter cells as mutations
Antiviral and anti-tumor drugs are nucleoside analogs
Frameshift mutagens: often potent carcinogens
Benzpyrene in smoke and soot causes
Aflatoxin made by Aspergillus flavus in peanuts

96. RADIATIONS
X rays
Gamma rays
Ionize atoms; electrons pop out from shells, bombard
more molecules to cause more damage resulting in
reactive ions and free radicles (molecular fragments with
unpaired electron)
Bind,damage DNA bases, erors in replication/repair
Physical breaks in backbone: covalent bonds broken

97. UV LIGHT
Non-ionizing component of ordinary light
Mutagenic component is 260nm screened by ozone layer
Harmful covalent bonds made between based
Adjacent thymine dimers form which unrepaired can
cause mutation.
REPAIR: Light repair enzymes
 Nucleotide excision repair

98. Enzymes cut out distorted cross-limked thymines by
opening wide gap; excision repair defect in xeroderma
pigmentosa; inherited. UV light sensitivi
Fill gap by complimentary strand
Restore original base pair sequence
DNA ligase seals it
If error remains…..it is mutation
Sun tann: large no of thymine dimers in skin; cancers