14.5 DNA Replication in Eukaryotes

14.5 DNA Replication in Eukaryotes

  • The replication forks are extended with the help of helicase.
  • The replication fork is covered with single-strand binding proteins to prevent it from reverting to its original state.
  • Topoisomerase prevents supercoiling by binding at the region ahead of the fork.
  • Primers are made from the DNA strand.
  • The 3'-OH end of the primer is being added to by DNA polymerase III.
  • The leading strand and lagging strand continue to be degraded.
  • exonuclease activity is the reason for the removal of RNA primers.
  • Adding dNTPs fills the gaps.
  • The gap between the two DNA fragments is sealed by a substance called DNA ligase.
  • The functions of each of the enzymes are summarized in Table 14.1.
  • Binds single-stranded DNA to prevent it from reverting back.
  • The full process of DNA replication is reviewed here.
  • Eukaryotic genomes are larger and more complex than prokaryotic ones.
    • There are a number of different linear chromosomes.
    • The human genome has 3 billion base pairs per haploid set of chromosomes, and 6 billion base pairs are replicated during the S phase of the cell cycle.
    • Humans can have up to 100,000 origins of replication across the genome.
  • The autonomously replicating sequence is found on the chromosomes in yeast.
    • The origin of replication in E. coli is similar to these.
  • Fourteen DNA polymerases are known in eukaryotes, of which five are known to have major roles during replication and have been studied.
    • They are called pol a, pol b, pol g, pol d, and pol e.
  • The same steps are used for replication in prokaryotes.
    • The DNA needs to be made available as a template.
    • Eukaryotic DNA is bound to histones and forms structures called nucleosomes.
    • Histones must be removed and replaced in order to account for the lower rate of replication in eukaryotes.
    • Some chemical modifications may be made to the chromatin so that it can slide off the proteins or be accessible to the DNA replication machinery.
    • A pre-replication complex is made at the beginning of replication.
    • The replication process begins with the recruitment of hilcase and other proteins.
  • The energy from the hydrolysis opens up the helix.
    • The forks are formed at the origin of the replication.
    • Over-winding, or supercoiling, occurs in the DNA ahead of the fork when the double helix opens.
    • The action of topoisomerases resolves these.
    • DNA pol can start synthesis with the help of the primer.
    • There are three major DNA polymerases involved: a, d and e. The lagging strand is synthesized by pol d, but the leading strand is continuously synthesised by pol e. As pol d runs into the primerRNA on the lagging strand, it takes over from the DNA template.
    • The displaced primerRNA is removed and replaced with a new one.
    • The fragments in the lagging strand are joined after the replacement of the primer with DNA.
    • The gaps are sealed by DNA ligase.
  • The chromosomes are linear.
    • The DNA pol can only add nucleotides in the 5' to 3' direction.
    • The end of the chromosomes is reached in the leading strand.
  • Each of the short stretches of the lagging strand is initiated by a separate primer.
    • There is no way to replace the primer on the 5' end of the lagging strand when the fork reaches the end of the linear chromosome.
  • repetitive sequences that code for no particular genes are tomeres.
    • Humans have a six-base-pair sequence that is repeated 100 to 1000 times.
    • The genes are protected from being deleted as cells continue to divide.
    • telomerase adds the telomeres to the ends of the chromosomes.
    • It is attached to the end of the chromosomes, and the DNA strand is 3' long.
    • Once the 3' end of the lagging strand template is long enough, DNA polymerase can add the nucleotides to the ends of the chromosomes.
    • The ends of the chromosomes are duplicated.
  • The ends of linear chromosomes are maintained by telomerase.
  • Stem cells and germ cells can be active with telomerase.
    • It isn't active in adult cells.
    • The discovery of telomerase by Elizabeth, Carol W. Greider, and Jack W. Szostak earned them the prize in 2009.
  • One of the scientists who discovered how telomerase works is Elizabeth Blackburn.