Discussion of the final gene product and its connection to cDNA (complementary DNA).
Importance of the genetic code in constructing proteins and the role of the central dogma of molecular biology.
Mention of degeneracy in the genetic code: This refers to the phenomenon where multiple codons can encode the same amino acid (e.g., 64 codons for 20 amino acids). This is high-yield for the MCAT as it relates to the "wobble hypothesis" at the third codon position.
Detection of Mutations
Usage of probes to avoid single-base sequence mismatches in DNA complements.
Contrast between coding DNA (cDNA) and PCR products, which can contain thousands of bases.
Explanation of binding to targets with specific sequences to identify mutations or variations.
Mutation Detection Example: Sickle Cell (HbS)
Description of a mutation detection method using ASO (Allele-Specific Oligonucleotide) probes.
ASO hybridization is used to detect the presence of a specific point mutation in a gene.
Specific example: The mutation at the glutamate position where glutamate (Glu) is changed to valine (Val) at codon 6 of the \beta-globin gene (Missense mutation).
Explanation of DNA isolation from heterozygous (carriers, AS) versus homozygous (affected, SS) individuals.
Complementarity of patient DNA in relation to the probes allows for the distinction between the three genotypes (AA, AS, and SS).
Probes and Biotin Labeling
Transition from radioactive isotopes to non-radioactively labeled probes for safety and efficiency.
Biotin-labeled probes: Biotin is used because of its extremely high affinity for avidin or streptavidin proteins.
Visualization techniques: DNA fragments are labeled and visualized using fluorescence or enzyme-linked assays post-hybridization.
In situ hybridization (ISH): A technique used to localize specific DNA or RNA sequences within fixed tissue sections or chromosomes.
DNA Fragment Detection Techniques
Usage of restriction enzymes (restriction endonucleases) to generate specific DNA segments by cutting at palindromic sequences.
Gel Electrophoresis: A method to separate fragments based on size and charge. Since DNA is negatively charged due to its phosphate backbone, it migrates toward the positive anode (+).
Separation principle: Larger fragments move more slowly through the gel; size is determined by comparison against a molecular weight standard (DNA ladder).
Southern Blotting is often required after digestion to identify specific genes among the millions of fragments produced from genomic DNA.
Genetic Polymorphisms and Variations
Polymorphism: A sequence variant that occurs in at least 1\% of the population.
These occur frequently in non-coding regions (introns and intergenic regions) and serve as genetic markers.
Impact: While many are benign, some contribute to phenotypic differences or increased disease susceptibility.
Restriction Fragment Length Polymorphism (RFLP)
RFLP: A technique used to detect genetic variation by observing differences in the lengths of DNA fragments generated by restriction enzymes.
Mutations can cause the gain or loss of a restriction site, changing the fragment pattern on a gel.
This is a traditional method for DNA fingerprinting and identifying disease-linked alleles within families.
Conclusion – Understanding Genetic Variation
Summarization of mutation analysis (ASO, RFLP) and its implications for clinical medicine.
Genetic variation is the basis for understanding disease susceptibility, pharmacogenomics, and hereditary inheritance patterns.