Gene Mutations
Mutation Vocabulary
Gene Mutation• Point Mutation
– Substitution– Silent– Missense– Nonsense
• Frameshift– Insertion– Deletion
Chromosomal Mutation• Duplication • Deletion• Translocation• Inversion
Chromosomal Mutations
How do genes control metabolism
• In 1909, Archibald Garrod first proposed the relationship between genes and proteins.
• “Genes dictate phenotypes through enzymes that catalyze specific chemical processes in the cell.”
• Example: Alkaptonuria – Urine appears dark red because it contains alkapton, that darkens upon exposure to air. – Wild Type Individuals have an enzyme to break the chemical
down.– Mutant Individuals do not produce the enzyme, thus unable
to metabolize alkapton.
Beadle and Tatum
One Gene – One Enzyme hypothesis : the Function of one gene is to dictate the production of a specific enzyme.
Transcription in Prokaryotes
• RNA polymerase cannot initiate transcription on its own. A protein (sigma) must bind before transcription can begin.
• RNA Polymerase + Sigma = HOLOENZYME • When a holoenzyme + DNA mix, the enzyme will attach to
only specific regions on the DNA which they now refer to as PROMOTERS. Specifically positions 10 and 35 nucleotides upstream from the gene.
• The sigma appeared to be responsible for guiding the RNA polymerase to specific locations.
• The sigma will release once initiation has commenced.
Promoter Sequence• 20 – 25 base pairs long. • Similar segment of DNA had a series of bases
identical or similar to TATAAT. – Referred to as the – 10 box.
Translation in Prokaryotes
• A large and small subunit assemble onto the mRNA. The small subunit will attach to what is called the Shine-Dalgarno Sequence on the mRNA. – A 6 base sequence upstream 8 bases from the AUG
start codon. The rRNA sequence within the small subunit will attach.
– Shine-Dalgarno Sequence – AGGAGG– Anti Shine-Dalgarno Sequence (found on the
ribosome) - UCCUCC
Translation in Prokaryotes
• The large subunit will attach and attract the first tRNA molecule. Loading in at the P-site of the ribosome.
Repairing Mistakes in DNA Synthesis
• Replication forks work at 50 bases per second• Errors = one mistake per billion• HUMAN REPLICATION– 6 billion nucleotides– Cells are replicated to create trillions of cells
DNA Polymerase Proofreading in Prokaryotes
• DNA polymerase acts as an exonuclease – (an enzyme that removes nucleotides from DNA)
• DNA polymerase III can remove nucleotides only from the 3’ end of the DNA, and only if they are not hydrogen bonded to a base on the complementary strand.
• If a wrong base is added during DNA synthesis, the enzyme pauses, removes the mismatched base that was just added, and then proceeds with synthesis.
Eukaryotic DNA polymerases
• Have the same type of proofreading ability – reduces error rate to about 1 in 10 million bases.
• At this rate there would be 600 mistakes every time a human cell replicated
Three Types of DNA Repair
• 1. Mismatch Repair• 2. Thymine Dimer Repair• 3. Excision Repair
Mismatch Repair
• When DNA polymerase doesn’t fix the problem, other enzymes spring into action. Responsible for “mismatch repair”
• The first repair enzyme is known as mutS.• “mutatorS”
Which base is right?
• Hypothesis: At the conclusion of a replication process, a methyl group is added. So the proofreading enzyme will remove the nucleotide from the unmethylated strand.
Xeroderma Pigmentosum: A Case Study (DNA Repair Disorder)
• An autosomal recessive disease in humans. • Extreme sensitivity to UV light. Skin will develop
leasion after even slight exposure to sunlight. • UV Light will cause a covalent bond to form
between adjacent Thymines on a DNA strand. • Creates a kink in the secondary structure of DNA. • Causes a stall in the replication fork during
replication.
The Study
• Cells of “normal” individuals versus cells of XP individuals.
• Exposed cells to UV radiation.• Added radioactive Thymines to the cell which
should be incorporated IF repair occurs. • High amount of radioactive Thymines in the
normal and virtual no radioactive thymines in the XP individuals.
DNA Excision Repair
• Uvr A, Uvr B, Uvr C, and Uvr D– “Ultraviolet Light Repair”
Direct DNA repair
DNA photolyase
DNA Mismatch Repair and Cancer(DNA Repair Disorder)
• Colon Cancer Variation = Hereditary nonpolyposis colorectal cancer (HNPCC) runs in families
• Tumors will develop on the colon, ovary, and other organs by the age of 50
• 1990 – researchers mapped the susceptibility to an area on chromosome 2
Do humans have mismatch repair genes?
• The research accelerated when mutS gene was identified and then research found a similar gene in a yeast genome.
• The genes were so similar, they called them homologous.
• Using the sequences from the genes, they located a similar sequence in the human gene – known as hMSH (human mutS homolog)
• Mapped to the same region on chromosome 2 as the HNPCC susceptibility gene.
Link between cancer and mismatch repair
• Cells from these patients have a mutation rate 100 times the normal.
• People who inherit a nonfunctional copy of the hMSH gene have a genetic predisposition for developing HNPCC.
• Evidence: Individuals who have this form of colon cancer have uneven repeats of sequences in their DNA (usually fixed in DNA repair).
Ataxia Telangiectasis (AT)
• Defect in the enzyme KINASE. • Cells proceed through the checkpoints. (high
mutation rate)• Radiation Sensitivity– Increased risk of breast cancer. – Any problems?
Potential Benefits to this research
• If individuals with mutant forms of hMSH can be identified early in life, dietary changes and therapy could significantly reduce their risk of developing cancer
Semi-conservative Replication
• Meselson and Stahl Experiment
Top Related