DNA Replication Pg. 217 - 222. 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

DNA ReplicationDNA Replication

Pg. 217 - 222Pg. 217 - 222

Last Day…Last Day… DNA = 2 strands that run anti-parallel to DNA = 2 strands that run anti-parallel to

one anotherone another– 1 strand: 5’ to 3’1 strand: 5’ to 3’– 2 strand: 3’ to 5’2 strand: 3’ to 5’– 3’ end terminates with a hydroxyl group of the 3’ end terminates with a hydroxyl group of the

sugarsugar– 5’ end terminates with a phosphate group5’ end terminates with a phosphate group

Nucleotide: sugar + phosphate + baseNucleotide: sugar + phosphate + base Bases = adenine, thymine, cytosine, Bases = adenine, thymine, cytosine,

guanineguanine

BondsBonds

Nucleotides held together with Nucleotides held together with phosphodiester bondsphosphodiester bonds

Nitrogen bases held together with Nitrogen bases held together with hydrogen bondshydrogen bonds

AS Biology. Gnetic control of AS Biology. Gnetic control of protein structure and functionprotein structure and function

Replication of DNA and Replication of DNA and ChromosomesChromosomes

Speed of DNA replication:Speed of DNA replication: 3,000 nucleotides/min in human 3,000 nucleotides/min in human 30,000 nucleotides/min in 30,000 nucleotides/min in E.coliE.coli

Accuracy of DNA replication: Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt)Very precise (1 error/1,000,000,000 nt)




Meselson and Stahl (1958)Meselson and Stahl (1958)

DNA Replication: Separating DNA Replication: Separating the strandsthe strands

Replication starts at the point of Replication starts at the point of origin on the DNA strandorigin on the DNA strand– Protein bindsProtein binds

Hydrogen bonds hold the strands Hydrogen bonds hold the strands together, and DNA is twistedtogether, and DNA is twisted

Specific enzymes work together to Specific enzymes work together to unravel and separate the DNA unravel and separate the DNA strandsstrands

DNA Replication: Separating DNA Replication: Separating the strandsthe strands

DNA helicase: unwinds the double helix DNA helicase: unwinds the double helix by breaking the hydrogen bondsby breaking the hydrogen bonds

Single-stranded binding proteins Single-stranded binding proteins (SSBs): bind to the exposed DNA single (SSBs): bind to the exposed DNA single strands to block hydrogen bondingstrands to block hydrogen bonding

DNA gyrase: relieves tension brought DNA gyrase: relieves tension brought by the unwinding by cutting the strands by the unwinding by cutting the strands of DNA, then resealing them.of DNA, then resealing them.

ReplicationReplication

Begins in 2 directions from the Begins in 2 directions from the origin(s) as the DNA is unwound origin(s) as the DNA is unwound – Complimentary strands started as soon Complimentary strands started as soon

as area of parent DNA strand exposedas area of parent DNA strand exposed

Replication fork: junction where the Replication fork: junction where the two DNA strands are still joinedtwo DNA strands are still joined– Replication proceeds towards this area Replication proceeds towards this area

on one strand of DNA, and away from it on one strand of DNA, and away from it on the otheron the other

ReplicatingReplicating Drosophila Drosophila Chromosome Chromosome

Building Complementary Building Complementary StrandsStrands

Complementary strands are built Complementary strands are built using the unwound single DNA strand using the unwound single DNA strand as a templateas a template

Specific enzymes helpSpecific enzymes help– DNA polymerase III: enzyme that builds DNA polymerase III: enzyme that builds

the complementary strand in the complementary strand in prokaryotesprokaryotes


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


RNA primer: a group of 10-60 base RNA primer: a group of 10-60 base pairs of DNA that is annealed to the pairs of DNA that is annealed to the template strandtemplate strand– Needed at DNA polymerase cannot initiate Needed at DNA polymerase cannot initiate

new complementary strand by itselfnew complementary strand by itself RNA primer is synthesized by an RNA primer is synthesized by an

enzyme “primase”enzyme “primase” This primer is temporary, and will be This primer is temporary, and will be

removed laterremoved later


The strand which uses the 3’ to 5’ The strand which uses the 3’ to 5’ template as its guide is called the template as its guide is called the leading strand leading strand – built towards the replication forkbuilt towards the replication fork

The other strand is called the lagging The other strand is called the lagging strandstrand– This is synthesized discontinuously in This is synthesized discontinuously in

short fragments in the opposite direction short fragments in the opposite direction of the replication forkof the replication fork


Lagging Strand:Lagging Strand: Primers are continuously added as Primers are continuously added as

replication fork moves along parent replication fork moves along parent strandstrand

DNA polymerase III builds short DNA polymerase III builds short segments: Okazaki fragmentssegments: Okazaki fragments

DNA polymerase I removes the RNA DNA polymerase I removes the RNA primers, and replaces them with the primers, and replaces them with the appropriate deoxyribonucleotidesappropriate deoxyribonucleotides


DNA ligase: enzyme that joins the DNA ligase: enzyme that joins the Okazaki fragments into one strand by Okazaki fragments into one strand by creating a phosphodiester bondcreating a phosphodiester bond

As the 2 new strands of DNA are As the 2 new strands of DNA are synthesized, two double stranded synthesized, two double stranded DNA molecules are produced that DNA molecules are produced that automatically twist into a helix.automatically twist into a helix.

Checking the DNAChecking the DNA

DNA polymerase III and DNA DNA polymerase III and DNA polymerase I act as quality control polymerase I act as quality control checkerscheckers– They “proofread” the newly synthesized They “proofread” the newly synthesized

strand of DNAstrand of DNA

Checking the DNAChecking the DNA

If an error is detected, either enzyme If an error is detected, either enzyme can function as “exonuclease”can function as “exonuclease”– Enzyme backtracks past the nucleotide on Enzyme backtracks past the nucleotide on

the end of strand build incorrectly, removes the end of strand build incorrectly, removes it, and continuously adds nucleotides to the it, and continuously adds nucleotides to the complementary strand.complementary strand.

Done immediately to avoid the mistake Done immediately to avoid the mistake being copied in future replications.being copied in future replications.

Nature of Genetic MaterialNature of Genetic Material

Property 1Property 1 - it must contain, in a stable - it must contain, in a stable form, information encoding the form, information encoding the organism’s structure, function, organism’s structure, function, development and reproductiondevelopment and reproduction

Property 2Property 2 - it must replicate accurately - it must replicate accurately so progeny cells have the same genetic so progeny cells have the same genetic makeupmakeup

Property 3Property 3 - it must be capable of some - it must be capable of some variation (mutation) to permit evolutionvariation (mutation) to permit evolution

Helpful Videos:Helpful Videos: What is DNA? (5:23) : https://www.youtube.com/watch?What is DNA? (5:23) : https://www.youtube.com/watch?

v=zwibgNGe4aY&feature=youtu.bev=zwibgNGe4aY&feature=youtu.be

Animation of DNA replication: Animation of DNA replication: http://sites.fas.harvard.edu/~biotext/animations/replication1.swhttp://sites.fas.harvard.edu/~biotext/animations/replication1.swff

DNA Structure and Replication [“Crash Course Bio”] (12:58): DNA Structure and Replication [“Crash Course Bio”] (12:58): https://www.youtube.com/watch?v=8kK2zwjRV0Mhttps://www.youtube.com/watch?v=8kK2zwjRV0M

DNA Replication Process [3D Animation] (5:45): DNA Replication Process [3D Animation] (5:45): https://www.youtube.com/watch?v=27TxKoFU2Nwhttps://www.youtube.com/watch?v=27TxKoFU2Nw

DNA Replication Animation (3:11, start @ 1:43): DNA Replication Animation (3:11, start @ 1:43): https://www.youtube.com/watch?v=dKubyIRiN84https://www.youtube.com/watch?v=dKubyIRiN84

Review QuestionsReview Questions

Pg. 223 #1, 2, 4, 5, 6, 7Pg. 223 #1, 2, 4, 5, 6, 7

Documents

DNA Replication Pg. 217 - 222. 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