Mutation
A change in the nucleotide sequence of DNA that results in a recognizable change in the organism; an altered mRNA sequence, which will possibly result in the formation of the incorrect protein synthesis
Substitution
A type of mutation where one nucleotide is replaced with another of a different base:
CGT (codes for alanine) —mutation—> (A)GT (codes for serine)
Removal (Deletion)
A type of mutation that occurs when one nucleotide is removed randomly and not replaced with another nucleotide:
- a gene: CGT, GCA GTT, T__,… codes for alanine, arginine, glutamine
- A removed: CGT, GCG, TTT… codes for alanine, arginine, lysine
Addition (Insertion)
A type of mutation that occurs when a nucleotide is added randomly:
- a gene: CGT, GCA, GTT… codes for alanine, arginine, glutamine
- addition: CGT, G(A)C, AGT… codes for alanine, eucine, serine
Thymine Dimers
A type of mutation that occurs when two adjacent thymine bases form a covalent bond between them and distort the double helix, damaging the DNA; caused by exposing DNA to UV or X-rays
Cellular Effects of Mutation
- Death: most are detrimental
- Benefit: some provide a benefit (increased antibiotic resistance)
- No change: some will not change the protein constructed or only affect a portion of the chromosome that is not important
Spontaneous Mutation
Occurs randomly during DNA replication (occurs 1 in 10,000 to 1 in 1,000,000,000 replications for every single gene)
Induced Mutation
DNA is exposed to mutagens
Mutagens
A physical or chemical agent that causes changes to genetic material; increase the risk of mutations by 10-1000X
Light Repair
Type of thymine dimer repair in which cells that possess the enzyme photolyase will repair these mutations when exposed to visible light; the enzyme breaks the covalent bonds between the thymines
Dark Repair
Type of thymine dimer repair in which visible light is not necessary; one enzyme compares the complementary strand of DNA and finds and removes the mutation; DNA polymerase replaces the correct base back into the DNA sequence
Proofreading by DNA Polymerase
This enzyme compares what base was incorporated to the template, if the bases are not complementary, then the correct base is incorporated into the new strand
Mismatch Repair
If DNA polymerase misses a wrong base, the another back-up system is used to repair mutations - Ultraviolet light repair system
Ultraviolet Light Repair System (Uvr)
- Two Uvr A proteins and one Uvr B (trimer) attach to the DNA molecule then move down the molecule scanning for damage
- Once damage is found, the trimer stops and the two Uvr A proteins are released; Uvr B remains
- Uvr C attaches to the Uvr B anc uts the DNA on both sides of the damaged DNA several nucleotides away
- Uvr D (a DNA helicase) releases the segment of DNA and Uvr B, C, and D are released
- The DNA sequence that now needs to be replicated is filled in with complementary nucleotides using DNA polymerase and is sealed with DNA ligase
Transformation
A method of gene transfer in which bacterial cells take up naked DNA molecules; conducted by both gram+ and gram- organisms; Griffith’s mice experiment
Competent Bacteria
Cells that have large holes that allow pieces of DNA (20 genes) to enter the cell
Natural Competence
Under certain environmental conditions or certain bacteria (soil microbes), a cell will be competent
Artificial Competence
Use of an electrical current to create large holes
Transduction
The most common type of gene transfer that occurs in every bacteria cell:
- A bacteriophage enters a bacteria cell, uses deoxyribonuclease to digest the chromosome into small pieces; the DNA or RNA of the virus is also replicated
- Some sections of the bacteria chromosome are integrated into the new viruses
- The viruses leave the infected bacterial cell and deposit the bacterial DNA section into a new bacterial cell
Bacteriophage
A virus that infects and replicates within bacteria
Conjugation
A type of gene transfer that is uncommon and requires physical contact between bacteria; occurs mostly in gram- bacteria:
- Plasmids replicate and then are transferred from donor to recipient cells (only in one direction) using pili
Plasmid
Small, circular DNA molecule that is physically separate from, and can replicate independently of, chromosomal DNA within a cell
Donor (F+) Cells
Contain the genes on the plasmid that code for the pilus and the ability to be able to transfer DNA to other cells (F stands for fertility)
Recipient (F-) Cells
No plasmid that codes for pilus; can only receive plasmid; once mixed, will become F+
