Topic 5-L2 - Prokaryotic Genetics Flashcards
Prokaryotes do not reproduce sexually.
Simple binary fission produces
genetically identical offspring
Gene names are
italicized – first three letters lower case, end with upper case letter (btuC)
Protein names are the same, but start with an
upper-case letter and are NOT italicized (BtuC).
Mutation -
A heritable change in the DNA sequence of a genome. Includes substitution mutations, insertions, deletions – any change.
Mutant (mutant strain):
An organism whose genome carries a mutation
Wild-type strain: –
Strain isolated from nature and/or one being used as the parental strain in a genetic study. The term “wild-type” can also be applied to a single gene
Genomic locus (plural = loci):
a specific position on the chromosome
Mapped mutations can be described using
nucleotide or amino numbers.
- Convention: WT base or amino acid, then number, then mutant base or amino acid. E.g. HisC (A77K) – residue 77 mutated from an
alanine (A) to a lysine (K).
Deletion mutations shown using the
delta (Δ) symbol (e.g. ΔbtuC)
Phenotype names have three letter (first letter = capital) designations
and strains are shown with a
plus (+) or a minus (–) for that phenotype:
- E.g. His+ strain can make histidine. His- strain is a histidine auxotroph – can’t make histidine.
mutants can be isolated by selection –
mutant grows, parent doesn’t (or grows significantly worse). E.g. antibiotic resistance.
Selection is highly efficient – can identify
single mutant with a desired
phenotype out of millions (or more) of cells
Is it easier to identify mutants that grow better than parent by selection ?
Yes
Using replica plating (plating the same colony on two different plates –
under two different conditions), you can identify
mutants that grow worse than parent (or not at all)
Mutations can be
spontaneous (naturally-occurring “mistakes”) or induced (E.g. using mutagenic chemicals or UV to damage DNA)
Point mutations
(mutations to a single base pair) within a protein
Point mutations can lead to :
- silent mutations
- missense mutation
- nonsense mutation
Silent mutations:
do not change amino acid sequence, different codon, same amino acid
Missense mutations (most common):
lead to a change in that amino acid to a different amino acid
Nonsense mutation:
lead to a change in that amino acid to a stop codon, leading to a premature end to the protein sequence (truncation)
Other mutations are not simple substitutions from one base pair to
another, but instead result in DNA being
added or lost. Insertions or deletions
Deletion mutations (DNA lost) and insertion mutations (DNA added to a specific location) can be as
small as a single bp or can be as large as thousands of bp.
Deletions/insertions within protein coding regions often result in a
frameshift mutation (highly disruptive)
Reversion:
mutant that acquires another mutation to “revert” back to wild-type. Term often applied to phenotype.
- For example – a mutant isolated with a new phenotype. That mutant strain then acquires a second mutation that changes phenotype (reverts) back to wild-type.
Suppressor mutation:
Mutations that compensate for the effects of a prior mutation. Can be to a different gene – “fixes problem” created by
initial mutation.
Horizontal gene transfer –
acquiring new genetic material from
foreign DNA via the environment, a virus (phage) or another organism - plays an even bigger role (on the whole)
Foreign DNA can enter a prokaryotic cell in 3 major ways:
1) Transfor mation
2) Transduction
3) Conjugation
Once inside the cell, this DNA can:
1) Be degraded/lost
2) Replicate as a separate entity (plasmids, phage)
3) Be integrated into the chromosome (recombination, transposition)
