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Once the chromosome has been completely replicated, the two DNA copies move into two different cells during cell division. The process of DNA replication can be summarized as follows:

  1. DNA unwinds at the origin of replication.
  2. Helicase opens up the DNA-forming replication forks; these are extended bidirectionally.
  3. Single-strand binding proteins coat the DNA around the replication fork to prevent rewinding of the DNA.
  4. Topoisomerase binds at the region ahead of the replication fork to prevent supercoiling.
  5. Primase synthesizes RNA primers complementary to the DNA strand.
  6. DNA polymerase starts adding nucleotides to the 3'-OH end of the primer.
  7. Elongation of both the lagging and the leading strand continues.
  8. RNA primers are removed by exonuclease activity.
  9. Gaps are filled by DNA pol by adding dNTPs.
  10. The gap between the two DNA fragments is sealed by DNA ligase, which helps in the formation of phosphodiester bonds.


[link] summarizes the enzymes involved in prokaryotic DNA replication and the functions of each.

Prokaryotic DNA Replication: Enzymes and Their Function
Enzyme/protein Specific Function
DNA pol I Exonuclease activity removes RNA primer and replaces with newly synthesized DNA
DNA pol II Repair function
DNA pol III Main enzyme that adds nucleotides in the 5'-3' direction
Helicase Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases
Ligase Seals the gaps between the Okazaki fragments to create one continuous DNA strand
Primase Synthesizes RNA primers needed to start replication
Sliding Clamp Helps to hold the DNA polymerase in place when nucleotides are being added
Topoisomerase Helps relieve the stress on DNA when unwinding by causing breaks and then resealing the DNA
Single-strand binding proteins (SSB) Binds to single-stranded DNA to avoid DNA rewinding back.

Review the full process of DNA replication here .

The previous section has demonstrated how DNA replicates by breaking existing double-stranded pieces apart, then adding a new second strand to each existing one. This may seem very logical, but scientists weren't always so certain how the process worked. All they knew was somehow a cell started with one piece of double-stranded DNA and then later it ended up with two pieces of double-stranded DNA. Scientists hypothesized three possibilities, one of which was the model described above, called semi-conservative replication. The other models (conservative replication and discontinuous replication) were rejected based on multiple experiments.

DNA replication is not a perfect process. The DNA polymerase sometimes makes mistakes. Cells have multiple ways to fix mistakes: DNA polymerase has a proof-reading ability to check its own work and different types of repair processes exist to fix different kinds of mutations. However, not every mistake is fixed. If a mistake is not fixed, we call it a mutation. Mutations are passed on because a cell can only fix a mistake as soon as it happens, so if it's not fixed right away, it stays in the cell's DNA permanently.

Section summary

DNA replication is required for cells to pass on their DNA when they divide into new cells. Multiple proteins are involved in replicating DNA, and it happens in a semi-conservative manner. The two strands are split apart, and a new, complementary copy of each strand is built. DNA polymerase can only build in the 5' to 3' direction, which means that each strand is replicated in different directions. Unfixed errors during DNA replication are mutations, and can be passed on when that cell divides.

Questions & Answers

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Eke Reply
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Joseph
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da
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da
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RAW Reply
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Professor
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Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
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LITNING Reply
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LITNING
scanning tunneling microscope
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The nanotechnology is as new science, to scale nanometric
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Source:  OpenStax, General biology part i - mixed majors. OpenStax CNX. May 16, 2016 Download for free at http://legacy.cnx.org/content/col11749/1.5
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