Chapter 15: Genome Replication in Eukaryotes & Archaea

Genome Replication in Eukaryotes & Archaea comprehensively analyzes the complex processes of genome replication and expression in eukaryotic and archaeal cells, drawing frequent comparisons to bacterial systems and highlighting the evolutionary hybrid nature of Archaea. Regarding DNA replication, eukaryotes manage large, linear chromosomes through multiple origins of replication (replicons) and solve the inherent "end replication problem" by utilizing the specialized reverse transcriptase enzyme, telomerase. Eukaryotic replisomes employ distinct DNA polymerases for leading (Pol ε) and lagging (Pol δ) strand synthesis, while archaea, despite having circular genomes like bacteria, utilize replisome proteins that are primarily homologous to those in eukaryotes (e.g., MCM helicases). In transcription, eukaryotes conduct RNA synthesis in the nucleus using three distinct RNA polymerases, where RNA polymerase II (RNAPII) transcribes monocistronic protein-coding genes containing noncoding introns. Eukaryotic pre-mRNAs undergo crucial modifications, including the addition of a 5′ cap and 3′ poly-A tail, alongside spliceosome-mediated removal of introns before translation. Archaea, by contrast, couple transcription and translation cytoplasmically (like bacteria), but use a single RNAPII-like polymerase and similar basal transcription factors (TBP, TFB) to eukaryotes. Translation initiation in eukaryotes is unique, involving the circularization of the mRNA facilitated by the 5′ cap and poly-A tail, allowing the ribosomal subunit (43S complex) to scan the message for the start codon. Both eukaryotes and archaea use chaperone systems for protein folding—such as TRiC/CCT in eukaryotes and the thermosome in archaea—and rely on shared Sec and Tat translocation pathways for protein localization. Finally, gene regulation in eukaryotes is intricate, often involving chromatin remodeling, remote-acting enhancers/silencers, and the sophisticated mechanism of RNA interference (RNAi) utilizing micro RNAs and small interfering RNAs.