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DNA Replication

Delve into the complex process of DNA replication, from unwinding and new base pairing to potential problems and error correction. Explore the Meselsohn-Stahl Experiment and discover how cells maintain genetic integrity. Unravel the mysteries of DNA synthesis with this comprehensive guide.

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DNA Replication

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  1. DNA Replication

  2. Owly Says: This lesson is probably the most “memory” heavy in this unit so don’t be afraid to ask questions.(think of it as the “glycolysis lesson” of this unit)

  3. History • How does DNA replicate? • Does it split down the middle? • If so, do those strands come back together? • Are new strands formed there?

  4. Meselsohn-Stahl Experiment

  5. THE PROCESS • relies on two basic steps, each mediated by cellular proteins and enzymes: • UNWINDING & UNZIPPING • the DNA molecule opens up to expose its bases • can happen from molecule’s end or anywhere along the length (REPLICATION BUBBLES) • Rep. Bubbles help speed up DNA replication process • NEW BASE PAIRING • once old (conservative) bases are exposed, new nucleotides will automatically pair with them • the old strands serve as a template for replication (a model, with “built – in” instructions) • the new nucleotides are found free-floating in nucleoplasm

  6. POTENTIAL PROBLEMS • Untwisting and unzipping without breaking the DNA: • HELICASE untwists the double helix (breaks H-bonds) • GYRASE works with helicase to prevent breakage; it “swivels” the DNA slightly (by cutting and re-sealing the DNA backbone) to relieve tension before Helicase does its job

  7. Keeping strands separated during new pairing • SINGLE-STRANDED BINDING PROTEINS (SSB’s) bind to each exposed strand; sort of like a doorstop. (without them, the DNA would form a helix again, since base pairing is so automatic)

  8. Forming the new strand • PRIMASE prepares a primer of RNA as a foundation • DNA POLYMERASE III links new nucleotides together to form phosphodiester bonds • DNA POLYMERASE I will eventually destroy/replace primers

  9. Dealing with directionality of DNA • DNA must be synthesized in a 5’ to 3’ direction • This means each new strand is being made in opposite directions • For the 3’-5’ template strand, its new partner is made in one continuous piece; this is the LEADING STRAND • For the 5’-3’ template strand, its new partner is made in a stop & go way: as the replication for keeps opening, more replication can occur, little by little • This constant “catching up” gives this strand the name, LAGGING STRAND • The lagging strand is composed of lots of small pieces of DNA called Okazaki Fragments which need to be joined together • The enzyme LIGASE links together these fragments

  10. Damage and errors in Replication • DNA POLYMERASE I and III “proofread” the new DNA • If a pairing error is found, they will back up, cut out the error and replace it • This is an exonuclease function of these enzymes

  11. Damage and Repair • Any damage to DNA would be lethal. Cells often spend much more energy repairing DNA than synthesizing it. • Correcting damage due to enviromental effects • Example: UV light thymine dimers. Energy in UV links thymine where it occurs side-by-side on one strand of DNA, screws up the ability of this bit of DNA to serve as template for replication or for correct reading of proteins. • One good 4-hour day at beach 10 UV-induced errors in DNA of every skin cell • Your skin cells spend lots of energy patrolling DNA, detecting such errors, cutting them out, and using the remaining good strand as a template for repair synthesis.

  12. Correcting errors during replication (proofreading) • When new DNA is synthesized, occasional errors in base pairing occur with frequency ~ 1 in 10,000 base pairs • If not corrected, could lead to mutations, loss of functions, loss of competitiveness, evolutionary weeding out. • Proofreading carried out by DNA polymerases enzymes; if base mismatch spotted, cut out new bases (keep track of which is template strand and which is new strand during replication), resynthesize copy strand from that neighborhood of template.

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