3.4 - DNA Replication (SL)

3.4 - DNA Replication (SL) John Thomas Didymus - DNA synthesis pictures - http://www.dnassequencing.com/wp-content/uploads/2010/10/DNA-Synthesis.jpg 18/11/2010. 09:00

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.

How does it copy itself? T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T A = T T = A G = C C = G A = T • 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.

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

= A = C = G = T C G Polymerase A = A = = C T = = G = A = C = G = T A = T T = A G = C C = G A = T T = A G = C C = G A = T • 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! T = G = C = A = Helicase = A T = G = C A = = T T Polymerase This is done with another enzyme – DNAPolymerase.

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. http://en.wikipedia.org/wiki/Okazaki_fragment Polymerase T = A G = C C = G A = T T = A T = A G = C C = G A = T • The DNA polymerase links the covalent bond of the phosphate from one nucleotide to the 3rd carbon of the sugar of the above nucleotide creating the new strand. A = T T = A G = C C = G A = T Helicase T = G = C = A = T = G = C = Polymerase A = T T = A G = C C = G A = T C On one of the strands (the leading strand) the DNA polymerase follows the helicase creating the new strand as it goes. G

T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T A = T T = A G = C C = G A = T • 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. T = A G = C C = G A = T T = A G = C C = G A = T T = A G = C C = G A = T A = T T = A G = C C = G A = T

Replication starts at many places on the DNA strand at once. These are called replication bubbles. http://kvhs.nbed.nb.ca/gallant/biology/replication_bubble.html 18/11/2010 09:20

Some animations worth watching: A simple one to start with: http://www.youtube.com/watch?v=z685FFqmrpo&feature=related A more complex one: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio23.swf::How%20Nucleotides%20are%20Added%20in%20DNA%20Replication Somewhere in between: http://www.youtube.com/watch?v=Luw5_z8mIrI&feature=related John Kyrk animation – good but detailed http://www.johnkyrk.com/DNAreplication.html

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.

3.4 - DNA Replication (SL)