Molecular Genetics
Overview
Molecular genetics examines how genetic information is stored, copied, and expressed at the DNA and RNA level. This chapter bridges the gap between chromosome-level genetics and the molecular laboratory techniques used in clinical testing. Molecular genetics concepts underpin variant interpretation, genetic testing methodology, and the molecular basis of genetic disorders.
Understanding DNA replication and repair is essential for explaining why certain types of mutations arise and how defects in these processes cause disease, from cancer predisposition syndromes caused by mismatch repair deficiency to conditions caused by nucleotide excision repair defects. Gene expression, the flow of information from DNA to RNA to protein, provides the framework for understanding how variants disrupt protein function and why different types of variants (missense, nonsense, splice site, frameshift) have different clinical consequences.
These concepts recur across clinical genetics practice. Interpreting a variant, predicting its effect on protein function, or explaining why a particular laboratory test was chosen all draw on the molecular genetics fundamentals covered in this chapter.
Key Concepts
- Central dogma: DNA replication, transcription, RNA processing, and translation
- Types of mutations: point mutations, insertions, deletions, and their effects on protein coding
- Splice site variants: how disruption of donor/acceptor sites or creation of cryptic splice sites affects mRNA
- DNA repair pathways: mismatch repair, nucleotide excision repair, base excision repair, and double-strand break repair
- Epigenetic regulation of expression: promoter methylation, histone modification, and chromatin remodeling (connecting to the Epigenetics chapter)
DNA Maintenance
DNA Replication and Repair covers the molecular machinery of DNA synthesis and the major repair pathways: mismatch repair, base excision repair, nucleotide excision repair, and double-strand break repair (HR and NHEJ). Inherited defects in these pathways cause well-known genetic syndromes (Lynch syndrome from mismatch repair, xeroderma pigmentosum from nucleotide excision repair, and Fanconi anemia from interstrand crosslink repair), and the pathway logic also explains why certain tissues are more vulnerable to specific types of DNA damage.
Gene expression
Transcription and gene regulation zooms in on the regulatory layer: how RNA polymerase II is recruited, how cis- and trans-acting factors tune expression, and how disruption of regulatory elements produces disease. Translation mirrors this on the protein-synthesis side: ribosome biology, the genetic code, post-translational modification, and the variant types (nonsense, frameshift) that derail translation.
Genome architecture
DNA Structure covers the chemical and architectural features of DNA: nucleotide composition, the antiparallel double helix, chromatin packaging, telomeres, centromeric and repetitive DNA, and mitochondrial DNA. These features explain why CpG sites are mutational hotspots, why GC-rich regions cause sequencing dropouts, and why repeat-rich regions drive structural variation and expansion disorders.
Gene Structure and Organization covers how a transcription unit is built (promoters, exons, introns, UTRs, polyadenylation signals) and the distal regulatory landscape (enhancers, silencers, insulators, TADs). Gene families, pseudogenes, alternative splicing, and HGNC/HGVS nomenclature also live here. This is the foundation for interpreting non-coding and splicing variants and for understanding why pseudogenes (PMS2, SMN1, CYP21A2) demand allele-specific clinical assays.
RNA biology
Non-coding RNA covers the functional RNA species that do not encode proteins: microRNAs, long non-coding RNAs, snRNAs, snoRNAs, and the imprinting-relevant ncRNAs (XIST, KCNQ1OT1, H19). Their dysregulation contributes to imprinting disorders, cancer, and many neurodevelopmental phenotypes.
Together, these topics provide the molecular foundation for the Laboratory section's coverage of sequencing technologies and variant interpretation.
Transcription and translation
Gene Expression is the integrated leaf on the central dogma: transcription, RNA processing (capping, splicing, polyadenylation), and translation. It provides the vocabulary needed to read HGVS variant nomenclature and to predict the functional consequences of missense, nonsense, splice-site, and frameshift variants. Regulatory elements (promoters, enhancers, UTRs) also enter here, since their disruption can cause disease without altering the coding sequence.