Describe the role of DNA in relation to gene expressionDNA Replication
Contents Why replicate? The basics The enzymes Okazaki fragments Fixing errors Review Questions
Stuff that any respectfulWhy replicate? Bio student really should know… Mitosis and Meiosis What are they and where they happen! Meiosis RevisionWhen we copy this exercise100,000,000,000,000 times itturns into this DNA replicates before it forms a chromatid. I.e. when it still looks like… Over an organisms life the DNA in the zygote is copied trillions of times with minimal error. Error rates are generally 1 in 50 million
Why replicate? www.cellsalive.com Each new cell must have a copy of the entire DNA genome.Some of theDNA releasedfrom a singlehumanchromosome
DNA Replication - the very basicsThe basics of DNA REPLICATION Unwind – Unzip – Add nucleotides – wind it all back up. If only it could be that simple Two things make it a little more fiddly DNA is a VERY LONG double helix chemical molecule It has a anti-parallel structure
It’s a long Double Anti-parallelHelix structure of DNA
DNA Replication• DNA Synthesis The DNA bases on each AT strand act as a template GC to synthesize a CG complementary strand TA • Recall that Adenine (A) GC pairs with thymine (T) and guanine (G) pairs with cytosine (C) AT AT The process is GC GC semiconservative CG CG because each new TA T A double-stranded DNA GC GC contains one old strand (template) and one newly-synthesized complementary strand
DNA Replication Begins at origins of replication Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks 3’5’ Parental DNA Molecule Replication Fork3’8 copyright cmassengale 5’
T G G T A C A G C T A G T C A T CG T A C CG TAn enzyme “unzips” or opens up the DNAstrand
DNA replication forks Because the DNA chromosome is so long it needs multiple replication forks working at the same time.
Anti-parallel structure (DNA) When an enzyme travels along a strand of DNA it travels in the 3’ to 5’ direction of the original
DNA Replication3’ end has a free deoxyribose5’ end has a free phosphateAn enzyme: can only build the new strand in the 5’ to 3’ direction Thus scans the template strand in 3’ to 5’ direction
Okazaki FragmentsAn Okazaki fragment is a relatively short fragment of DNAcreated on the lagging strand. Each Okazaki fragment isjoined together by DNA ligase after the primers have beenremoved.Crash course in DNA replication:http://www.youtube.com/watch?v=teV62zrm2P0
Semi-conservative replicationA simple idea really… S.C.R is simply the formation of two double helix molecules where each molecule contains one of the original strands and one new strand of nucleotides (daughter strand). This helps to minimise the errors made in replication as each molecule contains a copy of the original nucleotide sequence.
Checking for errors You don’t need to know the detail in this box. But read it cause it is interesting ;-) In general, enzymes (DNA polymerases) are extremely accurate. Even so, some DNA polymerases also have proofreading ability; they can remove nucleotides from the end of a strand in order to correct mismatched bases.
DNA Replication video http://www.youtube.com/watch?v=teV62zrm2P0
Review Questionshttp://www.biologycorner.com DNA Quizs ENZYME FUNCTION Helicase DNA Polymerase III DNA Polymerase I RNA Primase DNA Ligase
Review Question2011 NCEA exam – Q1 (b) When DNA is replicated, each of the parent strands acts as a template. Explain why there is a difference in the way in which the parallel strands of DNA are replicated. You may use a labelled diagram to support your answer.
Review Question What words are hidden under the yellow boxes? Click to show answerClick to show answer Click to show answer Click to show answer
Review Question Add the 3’ and 5’ labels to the diagram Click for answers