replication

Examples of replication in the following topics:

• DNA Replication in Eukaryotes

  • There are specific chromosomal locations called origins of replication where replication begins.
  • Because two helicases bind, two replication forks are formed at the origin of replication; these are extended in both directions as replication proceeds creating a replication bubble.
  • Eukaryotic chromosomes have multiple origins of replication, which initiate replication almost simultaneously.
  • Each origin of replication forms a bubble of duplicated DNA on either side of the origin of replication.
  • Once all the template nucleotides have been replicated, the replication process is not yet over.

• DNA Replication in Prokaryotes

  • There are specific nucleotide sequences called origins of replication where replication begins.
  • Two replication forks at the origin of replication are extended bi-directionally as replication proceeds.
  • A primer provides the free 3'-OH end to start replication.
  • The replication fork moves at the rate of 1000 nucleotides per second.
  • A replication fork is formed when helicase separates the DNA strands at the origin of replication.

• Viral Replication and Gene Expression

  • RNA viruses are classified into distinct groups depending on their genome and mode of replication.
  • Replication of viruses primarily involves the multiplication of the viral genome.
  • Replication also involves synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive sense RNA viruses), viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression.
  • Viral replication usually takes place in the cytoplasm .
  • Viruses that replicate via RNA intermediates need an RNA-dependent RNA-polymerase to replicate their RNA, but animal cells do not seem to possess a suitable enzyme.

• Chromosomes and DNA Replication in the Archaea

  • The cell division process is controlled by the cell cycle; the chromosomes within the Archaea are replicated to produce two daughter chromosomes.
  • The circular chromosomes contain multiple origins of replication, using DNA polymerases that resemble eukaryotic enzymes.
  • DNA replication, similar in all systems, involves initiation, elongation, and termination.
  • The replication of DNA, beginning at the origins of replication present on the circular chromosomes, requires initiator proteins.
  • The DNA replication system in Archaea, similar to all systems, requires a free 3'OH group before synthesis is initiated.

• Replication of Herpes Simplex Virus

  • Herpes replication entails three phases: gene transcription, viral assembly in the nucleus, and budding through the nuclear membrane.
  • The early proteins transcribed are used in the regulation of genetic replication of the virus.
  • The virion host shutoff protein (VHS or UL41) is very important to viral replication.
  • This enzyme shuts off protein synthesis in the host, degrades host mRNA, helps in viral replication, and regulates gene expression of viral proteins.
  • An enzyme shuts off protein synthesis in the host, degrades host mRNA, helps in viral replication, and regulates gene expression of viral proteins.

• Replication of Double-Stranded DNA Viruses of Animals

  • Viruses must first get into the cell before viral replication can occur.
  • However, in either cases, replication of the viral genome is highly dependent on a cellular state permissive to DNA replication and, thus, on the cell cycle.
  • Polyomaviruses, adenoviruses, and herpesviruses are all nuclear-replicating DNA viruses, each with their own specific approaches to replication.
  • Polyomaviruses replicate in the nucleus of the host.
  • Adenoviruses possess a linear dsDNA genome and are able to replicate in the nucleus of vertebrate cells using the host’s replication machinery.

• Basics of DNA Replication

  • In dispersive replication, after replication both copies of the new DNAs would somehow have alternating segments of parental DNA and newly-synthesized DNA on each of their two strands.
  • Meselson and Stahl were interested in understanding how DNA replicates.
  • Therefore, dispersive replication could also be ruled out.
  • These data support the semi-conservative replication model.
  • The three suggested models of DNA replication.

• DNA Repair

  • Most mistakes during replication are corrected by DNA polymerase during replication or by post-replication repair mechanisms.
  • DNA replication is a highly accurate process, but mistakes can occasionally occur as when a DNA polymerase inserts a wrong base.
  • Some errors are not corrected during replication, but are instead corrected after replication is completed; this type of repair is known as mismatch repair .
  • Errors during DNA replication are not the only reason why mutations arise in DNA.
  • In mismatch repair, the incorrectly-added base is detected after replication.

• Telomere Replication

  • As DNA polymerase alone cannot replicate the ends of chromosomes, telomerase aids in their replication and prevents chromosome degradation.
  • After DNA replication, each newly synthesized DNA strand is shorter at its 5' end than at the parental DNA strand's 5' end.
  • These telomeres protect the important genes from being deleted as cells divide and as DNA strands shorten during replication.
  • After sufficient rounds of replication, all the telomeric repeats are lost, and the DNA risks losing coding sequences with subsequent rounds.
  • A simplified schematic of DNA replication where the parental DNA (top) is replicated from three origins of replication, yielding three replication bubbles (middle) before giving rise to two daughter DNAs (bottom).

• Viral Identification

  • The genetic material within virus particles and the method by which the material is replicated vary considerably between different types of viruses.
  • DNA viruses: The genome replication of most DNA viruses takes place in the cell's nucleus.
  • RNA viruses: Replication usually takes place in the cytoplasm.
  • RNA viruses can be placed into four different groups, depending on their modes of replication.
  • Reverse transcribing viruses with RNA genomes (retroviruses), use a DNA intermediate to replicate, whereas those with DNA genomes (pararetroviruses) use an RNA intermediate during genome replication.