Molecular genetics is the study of molecules and mechanisms involved in genetic inheritance. Archival information molecules are long polymers of deoxyribonucleic acid (DNA) comprising bases in specific sequence. The bases adenine (A), thymine (T), cytosine (C), and guanine (G), function as codon triplets – sequences of three bases that code for specific amino acids or for translation initiation (start codons) or termination (stop codons). Uracil is substituted for thymine in RNA.
The segments of DNA that contain protein-coding instructions are called genes, and these gene sequences comprise a portion of the total genome of a cell. The genome includes both the genes (coding-sequences, domains) and the non-coding sequences – both exons, which include open reading frames, and introns.
Because the 64 possible combinations of GATC code for only the 20 amino acids commonly found in proteins, the code is redundant with more than one triplet combination coding for each amino acid. (This code reduncancy provides hereditary stability by reducing mutation mistakes.) The double helix of DNA comprises paired nucleotide strands with bases hydrogen bonded to complementary bases in the adjacent chain. Adenine pairs with thymine or uracil (A-TU), and cytosine pairs with guanine (CG).
During cellular reproduction, strands of archival DNA are copied or replicated. In prokaryotic cells – without a nuclear membrane – translation may begin prior to termination of transcription. Molecular genetics of eukaryotic cells is more complicated than that of prokaryotes. Various molecules of ribonucleic acid (RNA) participate in the transcription of the DNA code into processed mRNA in a series of RNA processing stages including capping, polyadenylation, pre-mRNA splicing. Following pre-mRNA processing, RNAs undergo extranuclear transfer (tRNA). Mature RNAs may undergo post-transcriptional modulation (via miRNAs) before translation of the archival DNA instructions into specific sequences of amino acids in the polypeptides and proteins that participate in cellular function and structure. Ribosomal RNAs participate in assembly of polypeptides and proteins at cytoplasmic ribosomes along the rough endoplasmic reticulum. Here RNAs serve as ribozymes – non-protein enzymes.
A number of process are involved in control of cellular function through the maintenance of accuracy of genetic inheritance. DNA damage may result from replication errors, incorporation of mismatched nucleotides (substitution errors – transitions and transversions), oxygen radicals, hydroxyl radicals, ionizing or ultraviolet radiation, toxins, alkylating agents, and chemotherapy agents. A number of vital mechanisms repair DNA damage, to bases (including C to T, C to U, and T U mismatch) and to strands, including double strand breaks. All organisms, prokaryotic and eukaryotic, utilize at least three enzymatic excision-repair mechanisms for damaged bases: base excision repair, mismatch repair, and nucleotide excision repair.