Unit 3c
Microbial Genetics
Microbial Genetics
Genetics: the science of heredity
Genome: the genetic information in the cell
Genomics: the sequencing and molecular characterization of
genomes
• Gregor Mendel
• Grew pea plants
from 1856-1863.
• Genetics: the science of
heredity
• Genome: the genetic
information in the cell
• Genomics: the sequencing and
molecular characterization of
genomes
A cell’s genome includes
Plasmids
• Chromosomes and _________
• Chromosomes are structures containing
the DNA
A bacterium has a single circular
chromosome consisting of a single
circular molecule of DNA
Plasmids (review)
• small loops of extrachromosomal DNA in bacteria
• often carry genes for virulence, bacteriocins (toxic proteins
that kill other bacteria) or drug resistance (codes for
enzymes that inactivate certain drugs or toxic substances)
– can recombine into new combinations
• transmitted from organism to organism
Eukaryotic DNA sites
DNA
• Fig. 2.16
Nucleotides
“Genes”
• Segments of DNA (except in some
viruses, in which they are made of RNA)
that code for functional products
DNA
• each gene could be several thousand or
more base pairs long.
– E. coli approximately 4,300 genes (4.6 million
base pairs
– Humans have approximately 20,000 to 25,000
genes.
• Based on Human Genome Project
Nucleic Acids
• DNA and RNA
• DNA: deoxyribonucleic acid
• RNA: ribonucleic acid
– Messenger RNA (mRNA)
– Ribosomal RNA (rRNA)
– Transfer RNA (tRNA)
• Nucleotides are the structural units of nucleic
acids
Nucleotides (Review)
• a nucleic acid is a long chain of nucleotides
• each nucleotide has 3 parts:
Sugar
– a 5-carbon ________
• ribose in RNA
• deoxyribose in DNA
Phosphate group
– A __________
Nitrogenous base
– a ___________
One nucleotide
RNA nucleotide with uracil
Nucleic acids
• RNA: usually a single chain of
nucleotides (may be double
in viruses)
• DNA: usually a double chain of
nucleotides (may be single
in viruses)
• 2 kinds of base pairs:
Nucleotides Complementary Base Pair
• Nucleotide bases bind to
each other in a specific
manner = complementary
base pairing.
• Specific purines
complementary base pair
with specific pyrimidines.
Complementary
base pairing in
DNA
DNA
• Double helix of James
Watson
and Frances Crick
Review of Proteins:
• long chains of amino acids: hundreds of
amino acids in complex three-dimensional
arrangements
• there are 20 naturally occurring kinds of
amino acids
• each amino acid in a protein must be
exactly the right kind of amino acid or it will
be a different protein
• the function of a gene is to determine the
sequence of the amino acids to make a
specific protein
The genetic code
• The set of rules that determine how a
nucleotide sequence is converted into
the amino acid sequence
• along a mRNA, groups of 3
consecutive nucleotides is a codon,
the genetic code for one amino acid
• e. g. —P—R—P—R—P—R—
l
l
l
U
A
C
• 64 possible mRNA codons for 20 amino acids
• there can be up to 6 codons that specify the same amino acid
• a few codons specify NO amino acid (start or stop codons),
signal the end of the protein molecule’s synthesis
The genetic
code
An overview of genetic flow
….figure 8.2
1) DNA replication
• reproduction of a molecule
• basis of continuity of life
• molecule “unzips” along the hydrogen
bonds
• each half attracts the nucleotides needed
to recreate the other half
• if successful, both new molecules are
identical to the original and to each other
DNA
Replication
DNA Polymerase
– Enzyme that 5’
connects each
nucleotide
together
3’
DNA Ligase –
Enzyme that
connects sections
of DNA together
Okazaki
Fragments
Lagging
Strand
Leading
Strand
3’
5’
Figure 8.6
DNA replication precedes cell division
2) Transcription
• = production of RNA by DNA
• DNA produces several kinds of RNA
• messenger-RNA (m-RNA) carries the genetic
code for a protein out from the chromosome
to the ribosomes
• transfer-RNA (t-RNA) carries individual amino
acids to the messenger RNA which puts them
in the proper sequence
• ribosomal-RNA (r-RNA) links up the amino
acids to form a protein
Translation
• = protein synthesis, translating the genetic
code into a specific protein
chain of
amino acids
Fig. 8.10
• Simultaneous
transcription
and
translation in
bacteria
RNA
_____________
Polymerase
____________
Connects RNA
nucleotides
together (like
DNA
polymerase)
Becomes mRNA
(messenger RNA) – this has
the code for how to build a
protein
tRNA –
transfer
RNA
anticodon
& amino
acid
Codon- A section of three
nucleotides in a row that
code for an amino acid
Polypeptide
Chain – all
the amino
acids who
together
Mutations
•
•
•
•
Can be negative, neutral, or positive!
defined as a change in the base sequence of DNA
can involve one or more nucleotides
the source of new genes (such as virulence or
drug resistance)
• about one mutation per million replicated genes
• causes:
– errors in DNA replication
– radiation
– mutagenic chemicals
The electromagnetic spectrum:
effective wave lengths:
• a. ultraviolet radiation
– damages DNA
– optimum wave length: 260 nm
– poor penetrating ability
Ames Test uses bacteria as carcinogen
indicators (figure 8.22)
• Many known mutagens have been found to
be carcinogens
Genetic Recombination
• The exchange of genes between 2 DNA
molecules to form new combinations of
genes on a chromosome.
– Vertical gene transfer
• Genes are passed from an organism to its
offspring
– Horizontal gene transfer
• Between bacteria of the same generation!
• Donor cell to recipient cell = recombinant
An overview of genetic flow
….figure 8.2
Bacterial gene transfers
• Bacteria have a number of forms of
recombination:
Conjugation
– ___________
Transformation
– ___________
Transduction
– ___________
Bacterial conjugation
(DNA transferred
through a mating process)
• 2 bacteria connected by a tube called
the sex pilus
• F = fertility factor (ability to mate)
• F+ is equal to being male (one that
grows the sex pilus)
• F– is equal to being a female
• DNA passes through the sex pilus
from the F+ to the F–
• usually just the F factor, but sometimes
other genes are carried along
• F– becomes F+
Figure 8.24: Griffith’s Transformation Experiment
Transduction:
• Transduction: host DNA
carried from cell to cell
by virus
• Figure 8.28
Biotechnology
• Restriction
Enzymes –
enzymes
found in
bacteria that
cut DNA at
specific
sequences.
Cotton Plants with Bacillus gene
inserted (left)
Bioremediation
Pharmaceuticals
Figure
9.1
DNA in diagnosis
• 4. Nucleic acid hybridization
• Basis of DNA probes
– Short segments of ssDNA that are
complementary to the desired gene
• Complementary strands of known DNA
separated by heat
• One side marked with fluorescent dye
• DNA of unknown bacteria separated by heat
• Will hybridize with fluorescent strand of
known DNA if same kind. After rinsing away
unbound DNA, a fluorescent DNA double
strand will remain
• Can hunt for complementary DNA within a
massive amount of material, such as food
DNA-DNA hybridization (fig. 10.15)
DNA probe to detect Salmonella
• Why use
E. coli ?
• Easily
grown &
researchers
are familiar
with its
genetics
• Figure 10.16
DNA probe, continued
DNA probe, continued
DNA Chips (figure 10.17)
An array of DNA probes arranged in a DNA chip can be used to
identify pathogens
• BUT should we?