Molecular Biology & Genetics MCQ

Molecular biology earns its place in dentistry through the teeth themselves. The genes that build enamel and dentin, when mutated, produce amelogenesis and dentinogenesis imperfecta; the inheritance patterns explain who else in the family is affected; and the same translation machinery that antibiotics target is why tetracyclines both kill bacteria and stain developing teeth. Learn the flow of information (DNA to RNA to protein), then connect the breaks in it to what you see in the chair.

DNA Replication & Repair

• DNA replication is semiconservative: each new double helix keeps one old strand and one new strand. DNA polymerase synthesizes only in the 5' to 3' direction and needs a primer.
• One strand is made continuously (leading) and the other in short Okazaki fragments (lagging) that DNA ligase joins; helicase unwinds the helix and primase lays down RNA primers.
• DNA polymerase proofreads with a 3' to 5' exonuclease activity, keeping replication remarkably accurate.
• Repair systems fix errors and damage; defects cause disease, for example faulty mismatch repair in Lynch syndrome (colorectal cancer) and faulty nucleotide excision repair in xeroderma pigmentosum (UV-driven skin and lip cancers).

• Mutation types to know: a silent mutation changes no amino acid, a missense changes one amino acid, a nonsense creates a premature stop (truncated protein), and a frameshift (insertion or deletion not a multiple of three) scrambles everything downstream.

Transcription & Translation

• Transcription copies a gene from DNA into messenger RNA in the nucleus, carried out by RNA polymerase.
• Eukaryotic mRNA is processed before leaving the nucleus: a 5' cap is added, a 3' poly-A tail is added, and introns are spliced out.
• The genetic code is read in three-base codons; it is degenerate (several codons can specify the same amino acid). Translation starts at AUG (methionine).
• Translation occurs on ribosomes, where transfer RNAs match their anticodons to the mRNA codons and add amino acids to the growing chain.

Genetic Inheritance Patterns

• Autosomal dominant: the trait appears in every generation (vertical), affects both sexes, and an affected parent passes it to about half the children (examples: dentinogenesis imperfecta, cleidocranial dysplasia, Gardner syndrome).
• Autosomal recessive: the trait can skip generations, carriers are unaffected, and consanguinity raises risk.
• X-linked recessive: mainly affects males, is passed through carrier mothers, and shows no male-to-male transmission (example: hemophilia).
• Mitochondrial inheritance is maternal: an affected mother passes it to all her children. Chromosomal disorders such as Down syndrome (trisomy 21) arise from nondisjunction.

Mutations & Clinical Disorders

• Amelogenesis imperfecta is a group of enamel-gene defects affecting essentially all teeth in both dentitions, with thin or soft, discolored enamel.
• Dentinogenesis imperfecta affects dentin (a type I collagen problem) and is often part of osteogenesis imperfecta, with blue sclera and brittle bones; the teeth are opalescent with bulbous crowns and obliterated pulps.
• Cleidocranial dysplasia (RUNX2) gives multiple supernumerary and unerupted teeth with retained primary teeth, plus absent or hypoplastic clavicles.
• Gardner syndrome (APC) shows jaw osteomas, supernumerary teeth, and odontomas, and crucially marks a near-certain risk of colorectal cancer, so the dental findings can be the first clue.