DNA Replication Pg

Last Day…  DNA = 2 strands that run anti-parallel to one another –1 strand: 5’ to 3’ –2 strand: 3’ to 5’ –3’ end terminates with a hydroxyl group of the sugar –5’ end terminates with a phosphate group  Nucleotide: sugar + phosphate + base  Bases = adenine, thymine, cytosine, guanine

Bonds  Nucleotides held together with phosphodiester bonds  Nitrogen bases held together with hydrogen bonds

AS Biology. Gnetic control of protein structure and function Replication of DNA and Chromosomes  Speed of DNA replication: 3,000 nucleotides/min in human 30,000 nucleotides/min in E.coli  Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt)

AS Biology. Gnetic control of protein structure and function

Meselson and Stahl (1958)

DNA Replication: Separating the strands  Replication starts at the point of origin on the DNA strand –Protein binds  Hydrogen bonds hold the strands together, and DNA is twisted  Specific enzymes work together to unravel and separate the DNA strands

DNA Replication: Separating the strands  DNA helicase: unwinds the double helix by breaking the hydrogen bonds  Single-stranded binding proteins (SSBs): bind to the exposed DNA single strands to block hydrogen bonding  DNA gyrase: relieves tension brought by the unwinding by cutting the strands of DNA, then resealing them.

Replication  Begins in 2 directions from the origin(s) as the DNA is unwound –Complimentary strands started as soon as area of parent DNA strand exposed  Replication fork: junction where the two DNA strands are still joined –Replication proceeds towards this area on one strand of DNA, and away from it on the other

Replicating Drosophila Chromosome

Building Complementary Strands  Complementary strands are built using the unwound single DNA strand as a template  Specific enzymes help –DNA polymerase III: enzyme that builds the complementary strand in prokaryotes

Building Complementary Strands  DNA syntheses only in the 5’ – 3’ direction! –Adds free nucleotides to the 3’ end of strand –However, it requires that initial 3’ end

Building Complementary Strands  RNA primer: a group of base pairs of DNA that is annealed to the template strand –Needed at DNA polymerase cannot initiate new complementary strand by itself  RNA primer is synthesized by an enzyme “primase”  This primer is temporary, and will be removed later

Building Complementary Strands  The strand which uses the 3’ to 5’ template as its guide is called the leading strand –built towards the replication fork  The other strand is called the lagging strand –This is synthesized discontinuously in short fragments in the opposite direction of the replication fork

Building Complementary Strands Lagging Strand:  Primers are continuously added as replication fork moves along parent strand  DNA polymerase III builds short segments: Okazaki fragments  DNA polymerase I removes the RNA primers, and replaces them with the appropriate deoxyribonucleotides

Building Complementary Strands  DNA ligase: enzyme that joins the Okazaki fragments into one strand by creating a phosphodiester bond  As the 2 new strands of DNA are synthesized, two double stranded DNA molecules are produced that automatically twist into a helix.

Checking the DNA  DNA polymerase III and DNA polymerase I act as quality control checkers –They “proofread” the newly synthesized strand of DNA

Checking the DNA  If an error is detected, either enzyme can function as “exonuclease” –Enzyme backtracks past the nucleotide on the end of strand build incorrectly, removes it, and continuously adds nucleotides to the complementary strand.  Done immediately to avoid the mistake being copied in future replications.

Nature of Genetic Material  Property 1 - it must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction  Property 2 - it must replicate accurately so progeny cells have the same genetic makeup  Property 3 - it must be capable of some variation (mutation) to permit evolution

Helpful Videos:  What is DNA? (5:23) : utu.be utu.be utu.be  Animation of DNA replication: wf wf wf  DNA Structure and Replication [“Crash Course Bio”] (12:58):  DNA Replication Process [3D Animation] (5:45):  DNA Replication Animation (3:11, 1:43):

Review Questions  Pg. 223 #1, 2, 4, 5, 6, 7