AP Biology A A A A T C G C G T G C T
Macromolecules: Nucleic Acids Examples: RNA (ribonucleic acid) single helix DNA (deoxyribonucleic acid) double helix Structure: monomers = nucleotides RNADNA
AP Biology Nucleotides 3 parts nitrogen base (C-N ring) pentose sugar (5C) ribose in RNA deoxyribose in DNA phosphate (PO 4 ) group Are nucleic acids charged molecules? Nitrogen base I’m the A,T,C,G or U part!
AP Biology Types of nucleotides 2 types of nucleotides different nitrogen bases purines double ring N base adenine (A) guanine (G) pyrimidines single ring N base cytosine (C) thymine (T) uracil (U) Purine = AG Pure silver!
AP Biology Nucleic polymer Backbone sugar to PO 4 bond phosphodiester bond new base added to sugar of previous base polymer grows in one direction N bases hang off the sugar-phosphate backbone Dangling bases? Why is this important?
AP Biology Pairing of nucleotides Nucleotides bond between DNA strands H bonds purine :: pyrimidine A :: T 2 H bonds G :: C 3 H bonds Matching bases? Why is this important?
AP Biology DNA molecule Double helix H bonds between bases join the 2 strands A :: T C :: G H bonds? Why is this important?
AP Biology Copying DNA Replication 2 strands of DNA helix are complementary have one, can build other have one, can rebuild the whole Matching halves? Why is this a good system?
AP Biology When does a cell copy DNA? When in the life of a cell does DNA have to be copied? cell reproduction mitosis gamete production meiosis
AP Biology But how is DNA copied? Replication of DNA base pairing suggests that it will allow each side to serve as a template for a new strand
AP Biology Replication Video Replication Video
AP Biology DNA Replication Issues 1. DNA strands must be unwound during replication DNA helicase unwinds the strands Single stranded binding proteins (SSB) prevent immediate reformation of the double helix Topoisomerases “untying” the knots that form
AP Biology DNA Replication DNA has directionality 5’ end End with the hanging phosphate group 3’ end Has an unbonded –OH group on the sugar
AP Biology Replication Issues 2. A new DNA strand can only elongate in the 5’ 3’ direction DNA polymerase can add only at the 3’ end Replication is continuous on one strand Leading Strand discontinuous on the other Lagging strand Okazaki fragments
AP Biology Okazaki fragments Synthesis of the leading strand is continuous The lagging strand (discontinuous) is synthesized in pieces called Okazaki fragments
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Replication Issues 3. DNA polymerase cannot initiate synthesis because it can only add nucleotides to end of an existing chain Requires a “primer” to get the chain started RNA Primase can start an RNA chain from a single template strand DNA polymerase can begin its chain after a few RNA nucleotides have been added
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Summary At the replication fork, the leading strand is copied continuously into the fork from a single primer Lagging strand is copied away from the fork in short okazaki fragments, each requiring a new primer
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Learning Check 1. What is the purpose of DNA replication? 2. How is the new strand ensured to be identical to the original strand? 3. How is replication on one side of the strand different from the other side?
AP Biology Replication Issues 4. Presence of RNA primer on the 5’ ends of daughter DNA leading strand leaves a gap of uncopied DNA Repeated rounds of replication produce shorter and shorter DNA molecules Telomeres protect genes from being eroded through multiple rounds of DNA replication
AP Biology Telomeres Ends of eukaryotic chromosomes, the telomeres, have special nucleotide sequences Humans - this sequence is typically TTAGGG, repeated ,000 times Telomerase adds a short molecule of RNA as a template to extend the 3’ end Room for primase & DNA pol to extend 5’ end
AP Biology Summary Explain how the cell overcomes each of the following issues in DNA replication 1. DNA strands must be unwound during replication 2. A new DNA strand can only elongate in the 5’ 3’ direction 3. DNA polymerase cannot initiate synthesis and can only add nucleotides to end of an existing chain 4. Presence of RNA primer on the 5’ ends of daughter DNA leading strand leaves a gap of uncopied DNA