DNA Replication, Transcription, and the Genetic Code

Posted on Feb 27, 2025 in Biology

DNA Replication

DNA replication is a semiconservative process that occurs on the double helix. It produces new DNA molecules, with one original strand and one new strand. The new strand comprises the polymerization of free nucleotides, complementing the template strand’s nucleobases. DNA can be bicatenary (double-stranded) and, more rarely, single-stranded.

Initiation

The initiation signal is a sequence of nucleotides in DNA called the origin of replication. Replication is circular and occurs in three steps:

  1. Unwinding and opening of the double helix: A group of enzymes and proteins, called the replisome, intervene.
    • Helicases facilitate the unwinding.
    • Gyrases and topoisomerases eliminate the tensions generated by torsion.
    • SSBP proteins bind to the template strands to prevent them from re-rolling.

Elongation

  1. Synthesis of two new DNA strands:
    • DNA polymerases synthesize the new strands in the 5′-3′ direction, reading the template strand in the 3′-5′ direction.
    • DNA polymerases I and III are involved in replication and error correction. DNA polymerase III performs the majority of the work.
    • DNA polymerase II corrects damage caused by physical agents.
    • The 3′-5′ strand is read by DNA polymerase III without any problems.
    • The 5′-3′ strand cannot be read directly. It’s read in small fragments (Okazaki fragments) in the 5′-3′ direction, which are later joined. This strand is called the lagging strand, and its synthesis is slower.
    • DNA polymerase III cannot start synthesis by itself; it needs a primer (RNA) synthesized by an RNA polymerase (primase). This primer is later eliminated.

Termination

  1. Error correction: The main enzyme that acts is DNA polymerase III, which corrects errors in replication. Other enzymes involved include:
    • Endonucleases that cut the error segment.
    • DNA polymerases that fill the gap correctly.
    • Ligases that join the corrected edges.

Transcription in Eukaryotes

Transcription is the process by which the information stored in the DNA nucleotide sequence is transferred to a complementary RNA sequence.

Key points:

  • There are three kinds of RNA polymerase: I, II, and III.
  • Genes are fragmented into areas with meaning (exons) and areas without meaning (introns). Introns must be removed before maturation.
  • Unpacking of histones occurs.

Transcription in eukaryotes has the following phases:

  1. Initiation: RNA polymerase II joins the area of DNA called the promoter.
  2. Elongation: Synthesis continues in the 5′-3′ direction. A cap (methyl guanosine triphosphate) is added to the 5′ end.
  3. Termination: It is related to the sequence TTATTT. A poly-A polymerase adds a poly-A tail to the pre-mRNA.
  4. Maturation: This occurs in the nucleus. An enzyme called RNPpn eliminates the newly formed introns. Subsequently, RNA ligases join the exons to form the mRNA.

Genetic Code Characteristics

  • It is formatted in RNA triplets called codons.
  • Reading direction: 5′-3′ in mRNA, and protein synthesis is always from the N-terminus to the C-terminus.
  • The triplet of tRNA that pairs with the codon of mRNA is called the anticodon.
  • The genetic code is not overlaid; triplets are read sequentially.
  • Of the 64 possible codons, 3 are “stop” codons that do not specify any amino acid.
  • The termination signals are degenerate; synonymous triplets exist.
  • There is colinearity between triplets and amino acids.
  • It is universal, or nearly so; most prokaryotes and eukaryotes share the same meaning.