How do the structures of nucleic acids relate to their functions of information storage and protein synthesis?

 Structure of DNA serves as template for replication
 DNA strands separate
 Each strand serves as a template to determine the order of nucleotides for a new strand
 Nucleotides connect to form the sugarphosphate backbone

 Semi-conservative model- each of two daughter strands will have one old strand and one new strand

 Conservative Model- parent model emerges from replication intact

 Dispersive Model- Each strand of both daughter molecules contains a mixture of old and new parts

Proteins and Enzymes involved in
Replication
 Origins of replication
 Replication begins at these sites with a specific sequence of nucleotides
 Proteins recognize this sequence and attach to DNA
 DNA replication proceeds in both directions by forming a replication fork

Proteins and Enzymes involved in
Replication
 Elongating a new DNA strand
 DNA polymerase (enzyme) catalyzes elongation by adding complementary bases to the template
 DNA strands are antiparallel; backbones run in opposite directions on opposing sides

DNA replication only proceeds from the 3’ end to the 5’ end

DNA is synthesized 5’ to 3’

Proteins and Enzymes involved in
Replication
 Elongating a new DNA strand…cont.
 The strand that is synthesized toward the replication fork is the leading strand
 The strand that is synthesized away from the replication fork is the lagging strand

Lagging strand works “backwards” assembling sections of DNA called Okazaki fragments
 These fragments are joined together by DNA ligase

Proteins and Enzymes involved in
Replication

Proteins and Enzymes involved in
Replication
 Priming DNA synthesis
 Primers (short stretches of RNA and enzyme primase) must begin synthesis
 RNA nucleotides are later replaced with DNA nucleotides by DNA polymerase
 Other proteins
 Helicase untwists the double helix at the replication fork
 Single-strand binding proteins hold DNA strands apart

 DNA polymerase proofreads each nucleotide as it is added
 Mismatch repair occurs when DNA polymerase misses an incorrectly paired nucleotide; special enzymes are used to fix this

 Nucleotide excision repair
 Nuclease cuts out the damaged nucleotides and they are replaced by DNA polymerase and ligase

 5’ ends are usually not completed
 This could be a problem because repeated rounds of replication would produce shorter and shorter DNA molecules
 Prokaryotes have circular DNA strands to solve this problem
 Eukaryotes have telomeres (multiple repetitions of a nucleotide sequence that do not code for a gene)

Enzyme telomerase lengthens telomere sequence after multiple replications have shortened the telomere sequence