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DNA Replication Mechanism and Proofreading

← Builds on DNA Structure and Nucleotide Components

Terms you will need

  • dna polymeraseDNA polymerase is an enzyme that builds new DNA strands by adding nucleotides to the 3' end of an existing strand, using the original DNA as a template. It helps copy DNA during cell division.
  • replication forkThe replication fork is a Y-shaped structure in DNA where the two strands are separated during replication. It allows new DNA strands to be built on each side.
  • leading strandThe leading strand is the DNA strand that is copied continuously in the same direction as the replication fork moves. It is built without stopping.
  • lagging strandThe lagging strand is the DNA strand that is copied in short pieces called Okazaki fragments, in the opposite direction of the replication fork. It is built in sections.
  • okazaki fragmentsOkazaki fragments are short pieces of DNA made on the lagging strand during replication. They are later joined together to form a complete strand.
  • proofreadingProofreading is when DNA polymerase checks for mistakes in the new DNA strand and fixes them by removing incorrect nucleotides before continuing replication.

How this topic runs

  1. 1Start here
  2. 2How it works
  3. 3At competition depth

DNA replication is the process by which a cell makes exact copies of its DNA. This process occurs in all organisms and is essential to biological inheritance, cell division, and repair of damaged tissues. DNA replication ensures that each of the newly divided daughter cells receives its own copy of each DNA molecule. DNA most commonly occurs in double-stranded form, made up of two complementary strands held together by base pairing of the nucleotides comprising each strand. The two linear strands of a double-stranded DNA molecule typically twist together in the shape of a double helix. During replication, the two strands are separated, and each strand of the original DNA molecule then serves as a template for the production of a complementary counterpart strand, a process referred to as semiconservative replication. As a result, each replicated DNA molecule is composed of one original DNA strand as well as one newly synthesized strand. Cellular proofreading and error-checking mechanisms ensure near-perfect fidelity for DNA replication. DNA replication usually begins at specific locations known as origins of replication which are scattered across the genome.

Figures (8)

Formation of the Okazaki fragments
A circular chromosome, showing DNA replication proceeding bidirectionally, with two replication forks generated at the "origin". Each half of the chromosome replicated by one replication fork is called a "replichore". (Graphic computer art by Daniel Yuen)
DNA polymerase moves along the old strand in the 3'–5' direction, creating a new strand having a 5'–3' direction.
DNA polymerase with proofreading ability
A slight mutation in the matched nucleotides can lead to chromosomal aberrations and unintentional genetic rearrangement
Bidirectional replication in a circular chromosome.
oriC motifs in bacteria

DNA replication is the process of making exact copies of DNA before a cell divides. DNA is a double-stranded molecule, with two strands twisted into a shape called a double helix. During replication, the strands separate, and each becomes a template for a new complementary strand.

This means one original strand pairs with a new strand, creating two identical DNA molecules. This process is called semiconservative replication. Replication starts at specific points in the DNA called origins.

Enzymes like helicase unwind the DNA, and other enzymes, like DNA polymerase, build new strands. DNA polymerase can only add new nucleotides in one direction, so one strand (the leading strand) is copied continuously. The other strand (the lagging strand) is made in short pieces called Okazaki fragments.

These fragments are later joined together by an enzyme called DNA ligase. DNA polymerase also has a proofreading function. It checks for errors as it builds the new strand and can remove incorrect nucleotides.

This helps keep DNA replication nearly perfect. Additional repair systems fix any mistakes that slip through, ensuring the new DNA is almost identical to the original. Replication is a highly coordinated process involving many proteins and enzymes.

These work together to unwind DNA, build new strands, and ensure accuracy. This process is essential for passing genetic information to new cells and for maintaining the integrity of an organism's DNA.

Key Points

  • DNA polymerase is an enzyme that synthesizes DNA from deoxyribonucleotides, adding nucleotides to the 3' end of a growing DNA strand during replication.
  • The replication fork is a structure formed during DNA replication where the DNA double helix is unwound into two single strands, creating a Y-shaped region that serves as a template for new DNA synthesis.
  • The leading strand is the DNA strand synthesized continuously in the same direction as the replication fork moves, allowing DNA polymerase to add nucleotides without interruption.
  • The lagging strand is the DNA strand synthesized discontinuously in the opposite direction of the replication fork, requiring the formation of short segments called Okazaki fragments.
  • Okazaki fragments are short, discontinuous segments of DNA synthesized on the lagging strand during replication, which are later joined together by DNA ligase to form a continuous strand.
  • Proofreading is a mechanism used by some DNA polymerases to detect and remove mismatched nucleotides during DNA replication, using their 3'–5' exonuclease activity to correct errors and maintain replication fidelity.

Terms

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