DNA Replication. 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). History. How does DNA replicate? Does it split down the middle? If so, do those strands come back together? - PowerPoint PPT Presentation
Text of DNA Replication
Owly Says:This lesson is probably the most memory heavy in this unit so dont be afraid to ask questions. (think of it as the glycolysis lesson of this unit)
HistoryHow does DNA replicate?Does it split down the middle?If so, do those strands come back together?Are new strands formed there?
THE PROCESS relies on two basic steps, each mediated by cellular proteins and enzymes:UNWINDING & UNZIPPINGthe DNA molecule opens up to expose its basescan happen from molecules end or anywhere along the length (REPLICATION BUBBLES)Rep. Bubbles help speed up DNA replication processNEW BASE PAIRINGonce old (conservative) bases are exposed, new nucleotides will automatically pair with themthe old strands serve as a template for replication (a model, with built in instructions)the new nucleotides are found free-floating in nucleoplasm
POTENTIAL PROBLEMSUntwisting 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
Keeping strands separated during new pairingSINGLE-STRANDED BINDING PROTEINS (SSBs) 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)
Forming the new strandPRIMASE prepares a primer of RNA as a foundationDNA POLYMERASE III links new nucleotides together to form phosphodiester bondsDNA POLYMERASE I will eventually destroy/replace primers
Dealing with directionality of DNADNA must be synthesized in a 5 to 3 directionThis means each new strand is being made in opposite directionsFor the 3-5 template strand, its new partner is made in one continuous piece; this is the LEADING STRANDFor 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 littleThis constant catching up gives this strand the name, LAGGING STRANDThe lagging strand is composed of lots of small pieces of DNA called Okazaki Fragments which need to be joined togetherThe enzyme LIGASE links together these fragments
Damage and errors in ReplicationDNA POLYMERASE I and III proofread the new DNAIf a pairing error is found, they will back up, cut out the error and replace itThis is an exonuclease function of these enzymes
Damage and RepairAny 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.
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.