DNA REPLICATION AHL Topic 7.2 IB Biology Miss Werba

AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS 7.1 DNA STRUCTURE 7.2 DNA REPLICATION 7.3 TRANSCRIPTION 7.4 TRANSLATION 7.5 PROTEINS 7.6 ENZYMES J WERBA – IB BIOLOGY 2

THINGS TO COVER Direction of DNA replication Prokaryotic DNA replication: Enzymes Okazaki fragments deoxynucleoside triphosphates Eukaryotic DNA replication J WERBA – IB BIOLOGY 3

Ref: IB Biology, OSC

DIRECTION OF REPLICATION Command term = STATE 7.2.1 DIRECTION OF REPLICATION Command term = STATE DNA replication occurs in a 5’→ 3’ direction. This means the 5’ end of the new strand The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesized. Addition of nucleotides is catalysed by DNA polymerase. J WERBA – IB BIOLOGY 5

7.2 DNA REPLICATION The free floating nucleotides in the nucleus are called deoxyribonucleoside triphosphates These molecules are composed of a deoxyribose sugar, a nitrogen base and three phosphate groups. eg. dATP, dCTP, dTTP, dGTP C1 C2 C3 C4 C5 P BASE J WERBA – IB BIOLOGY 6

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN Review of SL information: Several processes are required for DNA replication to occur: DNA double helix “unwinds” Two strands of DNA “unzip” Free nucleotides pair up with exposed bases A new complementary strand forms Now we have to add the HL details to this very simplified version...! J WERBA – IB BIOLOGY 7

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 1) Unwinding & unzipping the DNA Enzyme involved: DNA helicase Process: DNA helicase unwinds the DNA double helix Hydrogen bonds are broken between the base pairs, allowing the strands to separate One of the old strands will be 5’3’ and the other will be 3’5’ J WERBA – IB BIOLOGY 8

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN Becomes complicated because DNA replication only occurs in a 5’ 3’ direction problems arise due to the fact that the two DNA strands run in opposite directions The two strands are referred to as: the leading strand ends in 3’ (the right way) Replication occurs continuously the lagging strand ends in 5’ (the wrong way) Replication needs to ‘leap frog’ J WERBA – IB BIOLOGY 9

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 2) Replicating the leading strand (5’ 3’) Enzyme involved: DNA polymerase III Process: DNA polymerase III binds new nucleotides to the new growing strand Uses condensation reactions to add nucleotides J WERBA – IB BIOLOGY 10

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 3) Replicating the lagging strand (3’ 5’) Enzymes involved: RNA primerase DNA polymerase III DNA polymerase I DNA ligase Process: RNA primer is added Okazaki fragments are formed RNA primer is removed Okazaki fragments are connected J WERBA – IB BIOLOGY 11

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 3) Replicating the lagging strand (3’ 5’) Adding the RNA primer Enzyme involved: RNA primerase Process: A primer is needed to create a starting point for replication of the lagging strand A few RNA nucleotides (an RNA primer) are bound to the template strand RNA primase binds the RNA nucleotides together J WERBA – IB BIOLOGY 12

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 3) Replicating the lagging strand (3’ 5’) Forming the Okazaki fragments Enzyme involved: DNA polymerase III Process: Deoxyribonucleoside triphosphates form hydrogen bonds with the exposed lagging strand DNA polymerase III binds the nucleotides together forms an Okazaki fragment (1000-2000 nucleotides) J WERBA – IB BIOLOGY 13

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 3) Replicating the lagging strand (3’ 5’) Removing the RNA primer Enzyme involved: DNA polymerase I Process: DNA polymerase I removes the RNA primer from the Okazaki fragment It replaces the RNA nucleotides with corresponding DNA nucleotides J WERBA – IB BIOLOGY 14

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 3) Replicating the lagging strand (3’ 5’) Connecting the Okazaki fragments Enzyme involved: DNA ligase Process: DNA ligase catalyses the formation of bonds between the Okazaki fragments It acts as the “glue” to “stick” the new DNA strand together J WERBA – IB BIOLOGY 15

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN J WERBA – IB BIOLOGY 16

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN J WERBA – IB BIOLOGY 17

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN J WERBA – IB BIOLOGY 18

PROKARYOTIC DNA REPLICATION Command term = EXPLAIN 7.2.2 PROKARYOTIC DNA REPLICATION Command term = EXPLAIN J WERBA – IB BIOLOGY 19 Ref: IB Biology, Oxford Study Courses

EUKARYOTIC DNA REPLICATION Command term = STATE 7.2.3 EUKARYOTIC DNA REPLICATION Command term = STATE DNA replication is initiated at many points in eukaryotic chromosomes. In order to speed up the replication process, it must start at many points along the same DNA helix at the same time. These points are known as replication bubbles. J WERBA – IB BIOLOGY 20

EUKARYOTIC DNA REPLICATION Command term = STATE 7.2.3 EUKARYOTIC DNA REPLICATION Command term = STATE DNA replication is initiated at many points in eukaryotic chromosomes. J WERBA – IB BIOLOGY 21

Sample questions Q1. What are Okazaki fragments? Short lengths of RNA primase attached to the DNA during replication Short sections of DNA formed during DNA replication Nucleotides added by DNA polymerase I in the same direction as the replication fork Sections of RNA removed by DNA polymerase III and replaced with DNA J WERBA – IB BIOLOGY 22

Sample questions Q2. Which enzyme removes the RNA primer during replication? RNA primase DNA polymerase I DNA ligase Helicase J WERBA – IB BIOLOGY 23

Sample questions Q3. Explain the process of DNA replication. [8] J WERBA – IB BIOLOGY 24

Sample questions A1. & A2. B J WERBA – IB BIOLOGY 25

Sample questions DNA replication is semi-conservative; unwinding of double helix / separation of strands by helicase; hydrogen bonds between two strands are broken; each strand of parent DNA used as template for synthesis; synthesis continuous on leading strand but not continuous on lagging strand; leading to formation of Okazaki fragments (on lagging strand); synthesis occurs in 5´ → 3´ direction; RNA primer synthesized on parent DNA using RNA primase; DNA polymerase III adds the nucleotides (to the 3´ end) according to complementary base pairing; adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) DNA polymerase I removes the RNA primers and replaces them with DNA; DNA ligase joins Okazaki fragments; as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond; Accept any of the points above shown on an annotated diagram. 8 max J WERBA – IB BIOLOGY 26