Chapter 8 Flashcards
Mutation
a change in the nucleotide sequence of a cell’s DNA that is the passed on to daughter cells
Why is mutation a problem
If the DNA sequence i s changed, then the mRNA sequence is altered, which can result in incorrect protein synthesis
Types of mutations
Substitution
Deletion
Insertion
Thymine dimers
Substitution
mutation
One or more nucleotides are randomly replaced with different nucleotides.
Example of substitution
superoxide ion and hydrogen peroxide can change guanine so it will pair with adenine
Deletion
mutation
one or more nucleotides are randomly removed and not replaced (or nucleotide becomes unreadable)
AKA frameshift mutation
Example of Deletion
ethidium bromide dye adds small molecules to a base so it is skipped during transcription or replication
Insertion
mutation
One or more nucleotides are randomly added.
AKA frameshift mutation
Types of frameshift mutations
Insertion
Deletion
Thyamine dimers
mutation
Bonding of two adjacent thymine bases
-Double helix is distorted enough that enzymes do not work properly for transcription and replication
What causes thyamine dimer mutation
Caused by exposure to UV light
Outcomes of Mutations
- Death- most mutations are detrimental
- Benefit - sometimes mutations are beneficial
- No change - mutation doesn’t change the protein or affects a part of the chromosome that isn’t important
Natural Selection
if one bacteria cell has an advantage (example antibiotic resistance) over another bacteria in the same population; it will be able to survive longer and reproduce more often.
-Can become most common type of bacteria in a population
What causes mutations?
Spontaneous Mutation
Induced Mutation
Spontaneous mutation
random changes during normal cell processes
- Rare (1/10,000 to 1/ 1,000,000 replications for a single gene)
Induced mutation
changes from influence outside of a cell.
-Mutagens - physical or chemical agent that can change genetic material; increases risk of mutation by 10 to 1000x
Examples of induced mutations
Ethidium Bromide
Superoxide ion
Hydrogen peroxide
Repair of substitution, insertion and deletion mutations
- Proofredding by DNA polymerase as DNA synthesis occurs
2. Mismatch repair - other proteins and enzymes detect and removed mismatched nucleotide
Repair of Thyamine dimers
Photoreactivation
Excision repair
Photoreactivation
thyamine dimer repair
Enzyme photolyse uses energy to break bonds in thyamine dimers
Excision repair
thyamine dimer repair
- Protein trimer (two Uvr A and one Uvr B) scan for damage
- Uvr C makes cuts around the damaged DNA
- Uvr D (helicase) releases the cut DNA piece
- DNA polymerase and DNA ligase fill in the missing part
SOS repair
- When DNA is too damaged to be repaired by other mechanisms
- approx 16 proteins including 1 type of DNA polymerase that does not proofread itself
disease resulting from faulty repair systems
Xeroderma pigmentosa
Gene Transfer
Genes are transfered between bacteria in three ways
- Transformation
- Transduction
- Conjugation
Transformation
- uptake of “naked DNA”
- Only competent cells will take in new DNA
What is “naked” DNA
a fragment of DNA not in a cell or virus
Types of Competency
Transformation
Natural competency - may be environmentally influenced
Artificial competency - from electric current or chemicals
Example of Transformation
Avery and Griffith 1944
DNA transformed non-virulent Pneumococci into virulent form. Also showed that DNA is molecule of inheritance and not proteins
Transformation process
- DNA receptor molecule bind naked DNA
- Nucleases degrade one strand of DNA
- Only one DNA strand enters cell
- New strand integrates into recipient cell’s DNA
- Replaces a similar part of recipient DNA
Transduction
Bacterial viruses (bacteriophages or phages) transfer DNA from one bacteria to another
Transduction process
- Phages inject their DNA into bacteria
- Bacteria DNA cut into pieces
- Phage DNA replicated and new phage protein coats are made
- Sometime bacteria DNA can enter phage instead of phage DNA
- New phages are released that carry bacteria DNA called transducing particles
Generalized Transduction Process
- Transducing particle attaches to bacteria
- Bacteria DNA injected into host bacteria and is integrated into host chromosome
- replaced host DNA is degraded
Conjugation
- Transfer of DNA through sex pili that joins two cells
- Plasmids and chromoses can be transferred in conjugation
Transformation=
Transduction=
Conjugation=
Transformation= naked DNA Transduction= viruses Conjugation= pilus + plasmid
Plasmid
Very small, circular pieced of DNA
- free floating in cell with a few to 1000 genes
- can replicate
- not essential, but helpful
Conjugative plasmids
Plasmids that include sequences to begin transfer and to make pili
- Resistance Plasmids
- Fertility plasmids
Resistance plasmids
R plasmids
- contain genes for resistance to antimicrobial medication and to heavy metals
- Broad host range
Fertility plasmids
F plasmids
- if F plasmid is present a bacteria cell can perform conjugation
- bacteria with F plasmid are F+ and those without are F-
Plasmid Transfer (Conjugation)
- When F plasmid is transferred the recipient cell become F+
- Plasmid is replicated as it is transferred so both cells have the F plasmid
Chromosome transfer (conjugation)
- Involves Hfr cells (High frequency recombination)
- Have genes from F plasmid in the chromosomal DNA
- When F plasmid genes are transferred part of chromosomal DNA can also be transferred
- Recipient cell is still F- because whole F plasmid is not transferred
- New chromosomal DNA will survive only if integrated into recipients DNA
Transposons
- segments of DNA that can move from one part of the cell’s DNA to another part in the same cell
- Genes on transposon and on part of the affected DNA are disrupted
Jumping Genes
Barbara McClintock 1950
- Transposons in corn kernels moved around unpredictably and changed color of the corn kernels
- McClintock recieved Novel Prize in 1983
MRSA developed vancomycin resistance from…
conjugation and transposons
