Download Chapter 8: Microbial Genetics

Chapter 9: Microbial Genetics
Mutation – a change in the base sequence of DNA
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Harmful ( disadvantageous)
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Lethal
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Beneficial ( advantageous)
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Neutral
A. Types of Mutations
1. Point Mutations – change at 1 base
a. Base Substitution – incorrect base is substituted at one position
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Missense – an incorrect amino acid is incorporated into the protein
o E.g., normal hemoglobin vs. sickle cell hemoglobin
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Nonsense – creates a stop codon in the middle of an mRNA molecule
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Silent – doesn’t change the amino acid ( genetic code is degenerate)
2. Frameshift Mutations
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Deletions
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Insertions
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Altered reading frame  terminated or non-functional protein
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E.g., Huntington’s Disease
B. Causes of Mutations
1. Spontaneous
2. Induced by mutagens
IV. Mutagens – agents in the environment that chemically or physically interact with DNA
A. Chemical Mutagens
1. Nitrous Acid (HNO2) – alters an “A” so it base-pairs with “C” instead of “T”
2. Base Analogs
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2 –aminopurine – will get placed into DNA inplace of normal “A”
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5-bromouracil – will incorporate into DNA instead of a normal “T”
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basis of some antiviral drugs ( e.g., AZT used to treat HIV)
3. Agents that cause deletions ( frameshifts )
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Benzpyrene ( smoke & soot)
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Aflatoxin – from mold, Aspergillus flavus; carcinogen; associated with grains & peanut
butter
B. Radiation
1. Ionizing Radiation – X-rays & gamma rays
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Highly penetrating
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Forms reactive ions
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Breaks covalent bonds of the sugar-phosphate backbone of DNA
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Physical breaks in chromosomes
2. Ultraviolet (UV) Radiation
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Non-ionizing
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Most mutagenic wavelength ( 260 nm) ; absorbed by DNA; screened out by ozone layer
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Forms Thymine dimers
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DNA repair enzymes
C. Frequency of Mutation
1. spontaneous – 1 / billion base pairs ( 1/ 1 million genes)
2. low rate ; random location – necessary for evolution
3. mutagens increase the spontaneous rate 10-1000X
D. Identifying Mutants
1. select or test for altered phenotype
2. Positive selection – looks directly for mutated cells
3. Negative selection – selects for a cell that cnnot perform a certain functin
E. Identifying Chemical Carcinogens
1. faster & less expensive to use bacterial model
2. Ames Test – identifies chemicals that are mutagens ( About 90% of substances found to be
mutagens by the Ames test are also carcinogenic in animals)
V. Genetic Transfer and Recombination
A. Genetic recombination – exchange of genes between 2 molecules of DNA to form new combinations of
genes on a chromosome
1. Crossing Over – during meiosis –
a. homologous chromosomes line up;
b. chromosomes break and rejoin  gene recombination occurs ( eukaryotes)
2. Meiosis – random combinations of genes go into each egg & sperm
3. Sexual reproduction - fertilization between egg & sperm is a random event
B. Genetic Recombination in Bacteria
1. Involves transfer of DNA from a donor cell to a recipient cell
2. Recombinant cell – has incorporated the donor DNA into its own genome
3. Genetic Recombination in Bacteria – due to
a. Transformation
b. Conjugation
c. Transduction
C. Transformation – genes are transferred from one bacterium to another as “naked” DNA in solution
1. 1928 – Griffith showed that a “transforming factor” could transfer genetic material from one bacterial
cell to another.
2. 1944 – Avery, MacCleod & McCarty – showed that the “transforming factor “ was DNA
3. Occurs naturally ; bacteria die & release their DNA into environment
4. Recipient cells can take up fragments of the DNA across their cell membrane.
5. The DNA gets incorporated into the chromosome  all descendants of this organism will be identical
“recombinants”
6. Only some species can do this – works best if the 2 species are closely related.
7. “Competent” cell – is able to take up DNA across its cell membrane

naturally competent cells

chemical or electrical treatment in lab can make cells competent ( e.g., E. coli or yeast)
D. Conjugation
1. plasmids – extrachromosomal DNA – transferred during conjugation
2. needs cell to cell contact
3. needs cells of opposite mating type; donor (F+) ; recipient (F-)
4. in gram negatives – transfer occurs via a “sex pilus” that forms a bridge between the two cells.

The gene to make the sex pilus is called the “F factor” and is carried on a plasmid.
5. a copy of the plasmid is transferred from donor to recipient
6. If the plasmid gets incorporated into the recipient’s chromosome, then the recipient can transfer
some of its chromosomal genes to another recipient cell.
E. Transduction
1. bacteriophage (phage) – a virus that infects bacteria
2. transduction – bacterial DNA is transferred from a donor cell to a recipient cell inside a phage.
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The phage attaches to the bacterial cell ; injects viral DNA into the cell
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Phage DNA is replicated ; new viral protein coats are made
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Viral enzymes break up host cell DNA
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As phage assembles – phage DNA gets packaged inside the protein coats
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Randomly – some bacterial DNA fragments also get trapped inside the viral protein coats
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Assembled phage particles are released from the cell ; infect new bacterial cells
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Bacterial genes get transferred to the new hosts
3. generalized transduction – random event – all genes in a phage-infected bacteria are equally likely
to get packaged in the phage particle and transferred.
4. specialized transduction – only certain bacterial genes get transferred.
5. phage-infected strains of bacteria often produce toxins ( e.g., diphtheria, E.coli, Streptococcus
pyogenes)
F. Plasmids
1. self-replicating; small pieces of circular DNA
2. found in bacteria, some yeast
3. carry 10-100 genes – genes that are not necessary for survival
4. dissimilation genes

catabolism of unusual sugars or hydrocarbons

bacteria survive in diverse environments

bioremediation
5. toxin production – increases bacterial virulence

E.coli, Staphylococcus aureus, Clostridium tetani, Bacillus anthracis
6. R-factors- plasmids that carry genes for antibiotic resistance, resistance to heavy metals or
resistance to toxins

Bacteria acquire resistance by transfer of these plasmids ( usu. by conjugation)
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Multidrug resistance

Antibiotic usage selects for the survival of bacteria with R-factors
7. Capsule production – increases bacterial virulence
8. F-factors – genes for making a sex pilus
G. Transposons – mobile genetic elements – “jumping genes”
1. small ( 700-40,00 b.p.)
2. can move from one region of a DNA molecule to another
3. 1950’s – Barbara McClintock – corn
4. effect depends on where they insert on the new piece of DNA – can activate or inactivate a gene
5. can sometimes carry other genes e.g., genes for toxins or antibiotic resistance
6. can be carried from cell to cell by plasmids or viruses
Genes and Evolution
 Any regulation of genes, mutation or recombination of genes contributes to genetic diversity.
 Genetic diversity ( variation) is necessary for evolution.
 The ability to adapt to different habitats can affect survival & reproductive success.
 Reproductive success is the measure of Darwinian fitness.
 The basis for Natural Selection