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3.4 - DNA Replication (SL) John Thomas Didymus - DNA synthesis pictures -

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1 3.4 - DNA Replication (SL) John Thomas Didymus - DNA synthesis pictures - 18/11/ :00

2 Why DNA? Deoxy-ribose Nucleic Acid is most organisms information store. It must be very stable, able to be copied without mistake and able to be ‘read’. DNA is used to make RNA which in turn is used to make protein.

3 How does it copy itself? DNA is a double helix. Each strand is linked by hydrogen bonds between the nitrogen bases. In order to copy the DNA it must separate the strands to expose the hydrogen bonds from the nitrogen bases on the nucleotides. A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T

4 The enzyme Helicase connects to the double strand and breaks the hydrogen bonds between the nitrogen bases, leaving the hydrogen bonds ‘open’. A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = G = C = A = = A = C = G = T Helicase There are ‘free nucleotides’ in the nucleus. These free nucleotides have ‘open’ hydrogen bonds too. A = = A G = = G C = = C T = = T A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = G = C = A = = A = C = G = T A = = A = G = C T =

5 The loose nucleotides attach to their complementary pairs. They now need to be joined together to make a new strand. They nucleotides need to be polymerised! Helicas e A = = A = G = C T = A = T T = A G = C C = G A = T T = A G = C C = G A = T = A = C = G = T = A = C = G = T T = G = C = A = T = G = C A = GC = T T Polymerase This is done with another enzyme – DNA Polymerase. Polymerase

6 T = A G = C C = G A = T T = A G = C C = G A = T The DNA polymerase links the covalent bond of the phosphate from one nucleotide to the 3 rd carbon of the sugar of the above nucleotide creating the new strand. A = T T = A G = C C = G A = T Polymerase Helicase A = T T = A G = C C = G A = T T = G = C = A = T = G = C = Polymerase T = A C G On one of the strands (the leading strand) the DNA polymerase follows the helicase creating the new strand as it goes. Because the DNA polymerase can only go in one direction on the other strand (the lagging strand) it goes in the other direction, where it creates Okasaki fragments. ment ment

7 Because the original strands are used as templates for the complementary base pairing the two ‘new’ double strands are identical. Each ‘new’ strand is half ‘old’ strand. This is called Semi-conservative replication. A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T

8 Replication starts at many places on the DNA strand at once. These are called replication bubbles. 18/11/ :20http://kvhs.nbed.nb.ca/gallant/biology/replication_bubble.html

9 Some animations worth watching: A simple one to start with: A more complex one: hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/ /120076/bio23.swf::How%20Nucleotides%20are%20A dded%20in%20DNA%20Replication Somewhere in between: John Kyrk animation – good but detailed

10 Now make sure you can: Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase. Explain the significance of complementary base pairing in the conservation of the base sequence of DNA. State that DNA replication is semi-conservative.


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