BSC2010 BIOLOGICAL SCIENCE 1 SPRING 2012 STUDY GUIDE 14 DNA REPLICATION Vocabulary Arthur Kornberg tube Complementary Determined mechanism of DNA replication by studying DNA replication in a test The two strands of the DNA double helix are complimentary Means that when oriented with opposite polarity their nitrogeonous bases are in sequence to form perfect Watson Crick base pairs o Strands were sperated by breaking weak hydrogen bonds between base pairs Each strand then has information to direct synthesis of new complimentary strand to form 2 double helices Conservative replication The parental double helix remains intact and a second all new copy is made Daughter strand newly synthesized strand Contain a mixture of old and newly synthesized parts Each strand of both daughter molecules contains a mixture of old and newly Dispersive replication synthesized parts DNA Ligase repairs single stranded discontinuities in double stranded DNA molecules DNA Polymerase I II and III Cannot initatie new strand Can only add nucleotides to 3 end of growing strand new that are complementary to template strand Synthesis always in 5 to 3 direction Uses triphosphate forms of nucleotides as precursors Energy for reaction comes from the high energy phosphate bonds in the nucleotide triphosphate precursors Unwinds to two strands of DNA breaks H bonds and separates strands at the Helicase origin of replication In vitro replication In a test tube Lagging strand synthesis Leading strand synthesis template Synthesizing DNA in the opposite direction As DNA is unwound new strands must be constantly initiated New strand that primase initiated that is complementary to a single stranded DNA This strand is being synthesized continuously Messelson and Stahl Replication is semi conservative Mutation Changes in DNA sequence Nucleotide triphosphate Okazaki fragments Origin of replication Unwinding of two DNA strands Parental strand template strand Short discontinuous fragments generated on lagging strands In the conservative replication where the parental strands remains the same and a copy is made Can initiate a new strand that is complementary to a single stranded DNA o The new strand makes RNA Primase template Replication Replication bubble Replication fork Telomerase Telomeres Consist of two bidirectional replication forks a structure that forms within the nucleus during DNA replication It is created by helicases which break the hydrogen bonds holding the two DNA strands together RNA Primer It primes synthesis of the DNA Semiconservative Replication Each double helix has one old strand and one newly synthesized strand The DNA that is lost in the replication process is replaced by a special enzyme called telomerase It restores telomere sequences lost during replication The mechanism of replication suggests that DNA is lost from the very ends of linear chromosomes every time the DNA replicates Principles and Concepts Understand how the complementary nature of DNA as outlined in the Watson Crick model suggests three related models for reliable replication Know what these three models are o Watson and Crick Strands were sperated by breaking weak hydrogen bonds between base pairs Each strand then has information to direct synthesis of new complimentary strand to form 2 double helices The parental double helix remains intact and a second all new o Conservative copy is made o Semi conservative Each double helix has one old strand and one newly synthesized strand o Dispersive Each strand of both daughter molecules contains a mixture of old and newly synthesized parts Know that prokaryotic chromosomes have a single origin of replication and that replication from this origin is bi directional meaning that there are two replication forks moving in opposite directions Understand the roles of the following enzymes and structures in replication helicase DNA polymerase I and III DNA ligase primase and telomerase o Helicase Unwinds to two strands of DNA breaks H bonds and separates strands at the origin of replication o DNA polymerase I and III Cannot initatie new strand Can only add nucleotides to 3 end of growing strand new that are complementary to template strand Synthesis always in 5 to 3 direction Uses triphosphate forms of nucleotides as precursors Energy for reaction comes from the high energy phosphate bonds in the nucleotide triphosphate precursors Can initiate a new strand that is complementary to a single o DNA ligase o Primase stranded DNA template The new strand makes RNA o Telomerase The DNA that is lost in the replication process is replaced by a special enzyme called telomerase It restores telomere sequences lost during replication Know that DNA synthesis is always in a 5 to 3 direction Understand how that relates to the difference between and details of leading and lagging strand synthesis okazaki fragments telomere shortening and the need for DNA polymerase I and ligase during replication The leading strand o is the strand of DNA being replicated continuously o It is the strand that is being continuously polymerized toward the o All DNA synthesis occurs 5 3 o The original DNA strand must be read 3 5 to produce a 5 3 nascent replication fork strand o formed as a polymerase reads the template DNA and continuously adds nucleotides to the 3 end of the elongating strand The lagging strand o grows in the direction opposite to the movement of the growing fork o It grows away from the replication fork and it is synthesized discontinuously o Because the strand is growing away from the replication fork it must be replicated in fragments because the Primase that adds the RNA primer has to wait until the fork opens to be able to add the primer o These fragments of DNA produced on the lagging strand are Be able to identify the leading and lagging strands in a replication fork called Okazaki fragments o Ligase repairs the fragments o 5 3 5 is always at the top Know that DNA polymerase can t initiate its own replication but can only add nucleotides to the 3 end of an existing chain Know that eukaryotic chromosomes are much larger than prokaryotic chromosomes and how eukaryotic cells modify replication to address this size difference o The entire prokaryote chromosome is replicated by starting from a single o A single polymerase would be too slow to replicate a whole eukaryote origin of replication chromosome o The rate of replication is about 50 bases sec o In eukaryotes replication starts from as many as 1000 origins
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