Define Leading Strand
The strand of DNA that is synthesized continuously in the 5’ to 3’ direction toward the replication fork.
Define Lagging Strand
The strand of DNA that is synthesized discontinuously in short segments (Okazaki fragments) away from the replication fork.
What does ‘Semi-discontinuous’ mean in DNA replication?
It refers to the fact that one strand (leading) is made continuously while the other (lagging) is made in fragments.
Define Semiconservative Replication
The process where the resulting DNA molecules each contain one original ‘parental’ strand and one newly synthesized ‘daughter’ strand.
What are Okazaki fragments?
Short sequences of DNA nucleotides which are synthesized discontinuously and later linked together by the enzyme ligase to create the lagging strand.
What is the function of Helicase?
‘Unzips’ the DNA by breaking the hydrogen bonds between nitrogenous bases at the origin of replication.
What is the function of DNA Polymerase III?
The primary enzyme that adds new nucleotides to the growing DNA strand in the 5’ to 3’ direction.
What is the function of DNA Polymerase I?
Removes the RNA primers and replaces them with DNA nucleotides.
What is the function of Ligase?
Acts as ‘glue’ to join the sugar-phosphate backbones of Okazaki fragments together.
What is the function of Primase?
Synthesizes a short RNA primer to provide a 3’-OH group for DNA polymerase to start building.
What is the first step of DNA replication?
Helicase unwinds the double helix at the origin of replication, creating a replication bubble/forks.
What is the second step of DNA replication?
Primase adds RNA primers.
What is the third step of DNA replication?
DNA Polymerase III adds nucleotides to the 3’ end of the primer to build the new strands.
What is the fourth step of DNA replication?
DNA Polymerase I replaces RNA primers with DNA
What is the fifth step of DNA replication?
Ligase seals the DNA fragments together.
Meselson and Stahl Experiment: What was the setup?
Bacteria were grown in a ‘heavy’ nitrogen isotope (N-15) then transferred to a ‘light’ nitrogen isotope (N-14) medium.
Meselson and Stahl Experiment: What was observed after the first round of replication?
A single band of ‘hybrid’ DNA (intermediate density), which ruled out the Conservative model of replication.
Meselson and Stahl Experiment: What was observed after the second round?
Two bands—one hybrid and one light—which confirmed the Semiconservative model of replication and ruled out the Dispersive model.
In a replication fork diagram, how do you identify the 5’ and 3’ ends of the leading strand?
The 3’ end is pointing toward the replication fork.
In a replication fork diagram, how do you identify the 3’ and 5’ ends of the lagging strand?
The 3’ end is pointing away from the replication fork.
Where are Okazaki fragments located on a diagram?
On the lagging strand.
Why can DNA Polymerase only add nucleotides in the 5’ to 3’ direction?
Because it requires a free 3’-OH group to attach the phosphate of the incoming nucleotide.
If a replication fork is moving to the right, and the top template strand is 3’ to 5’ (left to right), is the top new strand leading or lagging?
Leading.
Explain the difference between DNA Polymerase I and III in terms of their roles.
Pol III does the bulk of the building; Pol I is the ‘clean-up’ crew that handles primer removal and replacement.
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11.1 Genotype, Phenotype, Punnett Squares27
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10.3/10.4 Meiosis and Errors26
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10.2 Mitosis and Differentiation23
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10.1 Cell Cycle25
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9.2 Membrane Transport23
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9.1 Cell Structure and Function33
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8.4 Translation and Mutations34
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8.3 Transcription and Central Dogma30
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8.2 DNA Replication26
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8.1 DNA39
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7.3 Cellular Respiration25
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7.2 Metabolism22
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7.1 Enzyme Structure and Function20
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6.3 Protein Structure and function29
