DNA Replication and Transcription Mechanisms in Eukaryotes

DNA Replication (S phase, Bidirectional)

Replication is the process that occurs in the nucleus during S phase. DNA replication is bidirectional and proceeds from origins of replication to form replication bubbles.

Initiation (Replication)

DNA polymerases synthesize the new strand by reading the template strand in the 3'->5' direction and polymerizing in the 5'->3' direction. Replication begins at specific origin sequences (e.g., oriC in prokaryotes). Helicases unwind the double helix at the origin and break hydrogen bonds, forming a replication bubble. Topoisomerases (including gyrases in prokaryotes) relieve supercoiling and torsional stress ahead of the fork, and single-strand binding (SSB) proteins prevent the single strands from reannealing.

Elongation (Replication)

Primase synthesizes short RNA primers that provide free 3'-OH ends for DNA polymerases to extend. DNA polymerases add deoxyribonucleotides to synthesize a new strand in the 5'->3' direction. Because replication is bidirectional, one strand (the leading strand) is synthesized continuously, while the opposite (the lagging strand) is synthesized discontinuously as Okazaki fragments, each initiated by a primer. After synthesis, RNA primers are removed (by mechanisms such as RNase H and DNA polymerase I in prokaryotes or equivalent enzymes in eukaryotes), DNA polymerase fills the gaps, and ligases join the DNA fragments to produce a continuous strand.

Completion and Proofreading

DNA polymerases often have 3'->5' exonuclease activity for proofreading, correcting replication errors as synthesis proceeds. Final steps include removal of primers, replacement with DNA, and ligation of remaining nicks.

Enzymes Involved in Replication

• Helicase: Unwinds the double helix and separates strands.
• Topoisomerase / Gyrase: Relieves supercoiling ahead of the replication fork.
• SSB proteins: Prevent reformation of hydrogen bonds and protect single strands.
• Primase: Synthesizes short RNA primers.
• DNA polymerases: Read the template 3'->5' and synthesize 5'->3'; many have proofreading activity.
• RNase H / DNA polymerase I: Remove RNA primers and replace them with DNA (mechanisms differ between prokaryotes and eukaryotes).
• Ligase: Joins DNA fragments and seals nicks in the sugar-phosphate backbone.

Transcription and RNA Synthesis

Transcription is the process by which RNA is synthesized from a DNA template. It occurs in three main stages: initiation, elongation, and termination.

Initiation (Transcription)

RNA polymerase recognizes and binds to promoter sequences. Binding of RNA polymerase and associated factors opens the DNA duplex at the start site, allowing synthesis of an RNA transcript complementary to the template strand.

Elongation (Transcription)

RNA polymerase adds ribonucleotides in the 5'->3' direction as it moves along the template. Only one strand is used as the template for transcription (the template strand), so transcription is unidirectional relative to that template.

Termination (Transcription)

Termination occurs when RNA polymerase encounters termination signals. In prokaryotes, termination can be rho-dependent or rho-independent; rho-independent termination often involves formation of a hairpin structure from a palindromic sequence followed by a U-rich tract. In eukaryotes, termination involves cleavage of the pre-mRNA and addition of a polyadenine tail (poly-A tail) at the 3' end, following a specific cleavage signal. In eukaryotes, a 5' cap (a methyl-guanosine triphosphate cap) is added to the 5' end early in transcription and serves as a start signal for translation.

Maturation and RNA Processing

RNA maturation (processing) does not generally occur in prokaryotes. In eukaryotes, pre-mRNA processing includes:

• 5' capping: Addition of a methyl-guanosine cap at the 5' end.
• 3' polyadenylation: Cleavage and addition of a poly-A tail at the 3' end.
• Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons) by the spliceosome.

Introns are transcribed but not translated; exons are transcribed and translated. The spliceosome removes introns and joins exons to form the mature mRNA.

Genetic Code Characteristics

The genetic code is the relationship between RNA base triplets (codons) and amino acids. Key properties:

• It is nearly universal across known organisms.
• It is degenerate: multiple codons can encode the same amino acid (64 possible triplets code for 20 amino acids plus stop signals).
• It is unambiguous: a given codon specifies at most one amino acid (or a stop signal).