Bacterial Genetics
Bacterial Genetics
• Genetics is the study of inheritance of the
different characters from parents to offspring's
who usually have the same characters as the
parents.
• The same with bacteria where the daughter
cells inherit the same characters as the parent
forms.
Genes
• Genes are the units of inheredity.
• They are segments on the chromosome or
DNA that carry the information for a
specific character or a specific structure.
Bacterial Chromosome
Bacteria have closed, circular DNA
Genome: genetic material in an organism
E. coli
4 million base pairs
1 mm long (over 1000 times larger that actual
bacterial cell)
DNA takes up around 10% of cell volume
• DNA is a long polymer of simple units called
nucleotides, held together by a sugar phosphate
backbone.
• Attached to each sugar molecule is a molecule of one
of four bases; adenine (A), thymine (T), guanine (G)
or cytosine (C), and the order of these bases on the
DNA strand encodes information.
• In most organisms, DNA is a double-helix (or duplex
molecule) consisting of two DNA strands coiled
around each other, and held together by hydrogen
bonds between bases. Because of the chemical nature
of these bases, adenine always pairs with thymine
and guanine always pairs with cytosine.
• Genes essential for bacterial growth are carried
on the chromosome.
• Few genes associated with specialized functions
are carried on plasmids.
Chromosomal Replication
Protein Synthesis
• DNA------- mRNA------ protein
transcription
translation
Central Dogma
of Molecular Genetics
Transcription
• One strand of DNA used as a template to make a
complimentary strand of mRNA
• Promoter/RNA polymerase/termination site/5’ to
3’
• Ways in which RNA & DNA differ:
– RNA is ss
– RNA sugar is ribose
– Base pairing-A-U
Types of RNA
• Three types:
– mRNA: messenger RNA
• Contains 3 bases ( codon)
– rRNA: ribosomal RNA
• Comprises the 70 S ribosome
– tRNA: transfer RNA
• Transfers amino acids to ribosomes for protein synthesis
• Contains the anticodon (3 base sequence that is complimentary
to codon on mRNA)
Genetic Code
• DNA: triplet code
• mRNA: codon (complimentary to triplet code
of DNA)
• tRNA: anticodon (complimentary to codon)
Genetic Code
• Codons: code for the production of a specific
amino acid
• 20 amino acids
• 3 base code
• Degenerative: more than 1 codon codes for
an amino acid
• Universal: in all living organisms
Genetic Code
Translation
• Three parts:
– Initiation-start codon (AUG)
– Elongation-ribosome moves along mRNA
– Termination: stop codon reached/polypeptide released
and new protein forms
• rRNA=subunits that form the 70 S ribosomes
(protein synthesis occurs here)
• tRNA=transfers amino acids to ribosomes for
protein synthesis)
Mutations
• Changes in base sequence of DNA/lethal and
inheritable
• Can be:
– Harmful
– Lethal
– Helpful
– Silent
Genetic Transfer in Bacteria
• Genetic transfer-results in genetic variation
• Genetic variation-needed for evolution
• Three ways:
– Transformation: genes transferred from one bacterium
to another as “naked” DNA
– Conjugation: plasmids transferred 1 bacteria to another
via a pilus
– Transduction: DNA transferred from 1 bacteria to
another by a virus
From E.Coli to a Map of Our Genes
• Research on E. coli revealed
that these bacteria have a
sexual mechanism that can
bring about the combining of
genes from two different cells
• This discovery led to the
development of recombinant DNA technology
– a set of techniques for combining genes from
different sources
• DNA technology has many useful applications
– The Human Genome Project
– The production of vaccines, cancer drugs, and
pesticides
– Engineered
bacteria that
can clean up
toxic wastes
BACTERIA AS TOOLS FOR MANIPULATING
DNA
12.1 In nature, bacteria can transfer DNA in three
ways
• Transformation, the taking
up of DNA from the fluid
surrounding the cell
DNA enters
cell
Fragment of
DNA from
another
bacterial cell
Bacterial chromosome
(DNA)
Figure 12.1A
• Transduction, the
transfer of bacterial
genes by a phage
• Conjugation, the union
of cells and the DNA
transfer between them
Mating bridge
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
• The transferred DNA is then integrated into the
recipient cell’s chromosome
Donated DNA
Degraded DNA
Crossovers
Recipient cell’s
chromosome
Recombinant
chromosome
12.2 Bacterial plasmids can serve as carriers for
gene transfer
• An F factor is a DNA
segment in bacteria that
enables conjugation
and contains an origin
of replication
F factor (integrated)
Male (donor) cell
Origin of F replication
Bacterial chromosome
F factor starts
replication and
transfer of chromosome
Recipient cell
Only part of the
chromosome transfers
Recombination can occur
F factor (plasmid)
Male (donor)
cell
Bacterial
chromosome
F factor starts
replication and
transfer
• An F factor can exist as a
plasmid, a small circular
DNA molecule separate
from the bacterial
chromosome
Plasmids
Plasmid completes
transfer and
circularizes
Cell now male
Figure 12.2B, C
Plasmids
• Some genetic properties in the bacterial cell are
carried on plasmids. However, these properties
are not essential for growth.
• Plasmids are extrachromosomal double
straned , cercular DNA molecules smaller than
the chromosome.
They replicate autonomously
Plasmids
• Multiple copies of the same plasmid may be
present in each bacterial cell.
• Different plasmids may co-exist within the
same bacterium.
• They are inherited by daughter cells.
• Some plasmids can transfer to other bacteria of
the same or different species.
Plasmids
• Transfer takes place normally by conjugation.
• Since plasmids can transfer from cell to cell,
they are widely used as vectors for cloning DNA
in yeast and bacteria.
Bacterial properties carried on plasmids Include:
• Drug resistance
• Virulence
• Production of antimicrobial agents
• Sex factor plasmids
Transposons “Jumping genes”
• These are genitic elements that contain several
Kbp of DNA.
• Can move extremly readly from plasmid to
plasmid or plasmid to chromosome (and vice
versa), hence the “jumping genes”.
Bacterial variation
• Genotype is the set of genitic
determinants within the cell.
• The changes is heritable.
• Genitic variation occurs through:
• 1-Mutation
• 2-Gene transfer
Bacterial variation
Bacterial variation may be phenotypic or
genotypic variation.
• Phenotype is the observable structural and
physiologic properties of the cell.
• The changes is not heritable
12.3 Plasmids are used to customize bacteria: An
overview
• Plasmids are key tools for DNA technology
– Researchers use plasmids to insert genes into
bacteria
1
Bacterium
Plasmid
isolated
2
DNA
isolated
3 Gene
Bacterial
chromosome
Cell containing gene
of interest
inserted
into plasmid
Plasmid
Gene of
interest
Recombinant DNA
(plasmid)
4
DNA
Plasmid put into
bacterial cell
Recombinant
bacterium
5 Cell multiplies with
gene of interest
Copies of gene
Gene for pest
resistance
inserted into
plants
Copies of protein
Clones of cell
Gene used to alter bacteria
for cleaning up toxic waste
Protein used to
make snow form
at higher
temperature
Protein used to dissolve blood
clots in heart attack therapy
Figure 12.3
12.4 Enzymes are used to “cut and paste” DNA
• Restriction enzymes
cut DNA at specific
points
• DNA ligase “pastes”
the DNA fragments
together
Restriction enzyme
recognition sequence
1
DNA
Restriction enzyme
cuts the DNA into
fragments
2
Sticky end
Addition of a DNA
fragment from
another source
3
Two (or more)
fragments stick
together by
base-pairing
• The result is
recombinant DNA
4
DNA ligase
pastes the strand
5
Figure 12.4
Recombinant DNA molecule
12.5 Genes can be cloned in recombinant plasmids:
A closer look
• Bacteria take the recombinant plasmids and
reproduce
• This clones the plasmids and the genes they
carry
– Products of the gene can then be harvested
E. coli
1 Isolate DNA
Human cell
from two sources
2 Cut both
Plasmid
DNAs with
the same
restriction
enzyme
DNA
Gene V
Sticky ends
3 Mix the DNAs; they join
by base-pairing
4 Add DNA ligase
to bond the DNA covalently
Recombinant DNA
plasmid
Gene V
5 Put plasmid into bacterium
by transformation
6 Clone the bacterium
Bacterial clone carrying many
copies of the human gene
Figure 12.5
12.6 Cloned genes can be stored in genomic
libraries
• Recombinant DNA
technology allows
the construction of
genomic libraries
Genome cut up
with restriction
enzyme
Recombinant
plasmid
OR
– Genomic libraries are
sets of DNA fragments
containing all of an
organism’s genes
• Copies of DNA fragments
can be stored in a cloned
bacterial plasmid or phage
Bacterial
clone
Plasmid
library
Recombinant
phage DNA
Phage
clone
Phage
library
Figure 12.6
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