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Diagram of telomerase. The top image shows a long strand of DNA with 5’ on the left and 3’ on the right. The complementary strand is much shorter and shows 3’ on the left and 5’ on the right. A circle labeled telomerase contains a complementary strand that matches the 3’ end of the upper strand and also extends past the 3’ end of the top strand. Caption: Telomerase has an associated RNA that complements the 3’ overhang at the end of the chromosome. Next, the top strand of DNA replicates using the overhang of the strand within the telomerase. Caption: The RNA template is used to synthesize the complementary strand. Next, the telomerase moves to the new 3’ end of the top strand. Caption: Telomerase shifts and the process repeats. Finally, The top DNA strand has multiple extensions. RNA primer binds near the 3’ end and builds a new strand of DNA towards the left until it meets up with the existing strand. Caption: Primase and DNA polymerase synthesize the complementary strand.
In eukaryotes, the ends of the linear chromosomes are maintained by the action of the telomerase enzyme.
Comparison of Bacterial and Eukaryotic Replication
Property Bacteria Eukaryotes
Genome structure Single circular chromosome Multiple linear chromosomes
Number of origins per chromosome Single Multiple
Rate of replication 1000 nucleotides per second 100 nucleotides per second
Telomerase Not present Present
RNA primer removal DNA pol I RNase H
Strand elongation DNA pol III pol δ, pol ε
  • How does the origin of replication differ between eukaryotes and prokaryotes?
  • What polymerase enzymes are responsible for DNA synthesis during eukaryotic replication?
  • What is found at the ends of the chromosomes in eukaryotes and why?

Dna replication of extrachromosomal elements: plasmids and viruses

To copy their nucleic acids, plasmids and viruses frequently use variations on the pattern of DNA replication described for prokaryote genomes. For more information on the wide range of viral replication strategies, see The Viral Life Cycle .

Rolling circle replication

Whereas many bacterial plasmids (see Unique Characteristics of Prokaryotic Cells ) replicate by a process similar to that used to copy the bacterial chromosome, other plasmids, several bacteriophages , and some viruses of eukaryotes use rolling circle replication ( [link] ). The circular nature of plasmids and the circularization of some viral genomes on infection make this possible. Rolling circle replication begins with the enzymatic nicking of one strand of the double-stranded circular molecule at the double-stranded origin (dso) site . In bacteria, DNA polymerase III binds to the 3’-OH group of the nicked strand and begins to unidirectionally replicate the DNA using the un-nicked strand as a template, displacing the nicked strand as it does so. Completion of DNA replication at the site of the original nick results in full displacement of the nicked strand, which may then recircularize into a single-stranded DNA molecule. RNA primase then synthesizes a primer to initiate DNA replication at the single-stranded origin (sso) site of the single-stranded DNA (ssDNA) molecule, resulting in a double-stranded DNA (dsDNA) molecule identical to the other circular DNA molecule.

Diagram of DNA replication. A circle of double stranded DNA has a region labeled SSO near a region labeled DSO. A nick forms in DSO and DNA polymerase III begins copying and displacing the nicked strand. This forms a new ring made of an old and a new strand of DNA; the second old strand of DNA is outside of this ring but eventually rejoins the nicked strand. DNA ligase then separates the dsDNA (synthesis of first strand) and the lone ssDNA. The ssDNA then has the second strand synthesized and become a ds DNA as well.
The process of rolling circle replication results in the synthesis of a single new copy of the circular DNA molecule, as shown here.
  • Is there a lagging strand in rolling circle replication? Why or why not?

Key concepts and summary

  • The DNA replication process is semiconservative , which results in two DNA molecules, each having one parental strand of DNA and one newly synthesized strand.
  • In bacteria, the initiation of replication occurs at the origin of replication , where supercoiled DNA is unwound by DNA gyrase , made single-stranded by helicase , and bound by single-stranded binding protein to maintain its single-stranded state. Primase synthesizes a short RNA primer , providing a free 3’-OH group to which DNA polymerase III can add DNA nucleotides.
  • During elongation , the leading strand of DNA is synthesized continuously from a single primer. The lagging strand is synthesized discontinuously in short Okazaki fragments , each requiring its own primer. The RNA primers are removed and replaced with DNA nucleotides by bacterial DNA polymerase I , and DNA ligase seals the gaps between these fragments.
  • Termination of replication in bacteria involves the resolution of circular DNA concatemers by topoisomerase IV to release the two copies of the circular chromosome.
  • Eukaryotes typically have multiple linear chromosomes, each with multiple origins of replication. Overall, replication in eukaryotes is similar to that in prokaryotes.
  • The linear nature of eukaryotic chromosomes necessitates telomeres to protect genes near the end of the chromosomes. Telomerase extends telomeres, preventing their degradation, in some cell types.
  • Rolling circle replication is a type of rapid unidirectional DNA synthesis of a circular DNA molecule used for the replication of some plasmids.

Fill in the blank

The enzyme responsible for relaxing supercoiled DNA to allow for the initiation of replication is called ________.

DNA gyrase or topoisomerase II

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Unidirectional replication of a circular DNA molecule like a plasmid that involves nicking one DNA strand and displacing it while synthesizing a new strand is called ________.

rolling circle replication

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True/false

More primers are used in lagging strand synthesis than in leading strand synthesis.

True

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Short answer

Why is primase required for DNA replication?

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What is the role of single-stranded binding protein in DNA replication?

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Below is a DNA sequence. Envision that this is a section of a DNA molecule that has separated in preparation for replication, so you are only seeing one DNA strand. Construct the complementary DNA sequence (indicating 5’ and 3’ ends).

DNA sequence: 3’-T A C T G A C T G A C G A T C-5’

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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