Chapter 12 (DNA Replication and Recombination) Flashcards
Compare and contrast theta, rolling circle, and linear DNA replication.
Theta replication- double-standed DNA unwinds at the replication origin… producing single-stranded templates for the synthesis of new DNA. A replication bubble forms , usually having a replication fork at each end. The forks proceed around the circle. Eventually two circular DNA molecules are produced. The products of theta replication are two circular DNA molecules.
Rolling circle- is initiated by a break in one of the nucleotide strands. DNA synthesis begins at the 3’ end of the broken strand; the inner strand is used as a template. The 5’ end of the broken strand is replaces. Cleavage releases a single-stranded linear DNA and a double-stranded circular DNA. The linear DNA may circularize and serve as a template for synthesis of a complementary strand. The products of replication are multiple circular DNA molecules.
Linear eukaryotic replication- each chromosome contains numerous origins. At each origin, the DNA unwinds, producing a replication bubble. DNA synthesis takes place on both strands at each end of the bubble as the repletion forks proceed outward. Eventually, the forks of adjacent bubbles run into each other and the segments of DNA fuse… producing two identical linear DNA molecules.The products of eukaryotic DNA replication are two linear DNA molecules.
What is a replication bubble? Replication fork?
Replication bubble- the unwinding of the double heir generates a loop
Replication fork- the point of unwinding, where the two single nucleotide strands separate from double-stranded DNA helix
What structural problems are created by progression of the replication fork. How are they relieved?
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What is a replicon? Briefly compare the number of replicons found in bacteria to eukaryotes. Why are there so many in eukaryotes?
A replicon is an individual unit of replication
In what direction does DNA replication occur (5’ to 3’ or 3’ to 5’)? Why?
DNA synthesis always occurs in the 5’ to 3’ direction… so synthesis takes place simultaneously on both template strands.
Why is an RNA primer required for initiation of DNA synthesis?
Because DNA primes is an RNA polymerase which synthesizes short stretch of nucleotides to start elongation.
Is DNA synthesis discontinuous or continuous (or both)? What are okazaki fragments?
DNA synthesis is both continuous and discontinuous. On the leading strand, where replication continuous a primer is required only at the 5’ end of the newly synthesized strand. On the lagging strand, where replication is discontinuous a new primer must be generated at the beginning of each okazaki fragment.
What are the four stages of bacterial DNA replication?
Initiation , unwinding, elongation, termination
What are the major proteins (enzymes) involved during each stage of replication ? What do each of these proteins or enzymes do?
Initiator protein- binds to origin and separates strands of DNA to initiate replication.
DNA helicase- unwinds DNA at replication fork
Single strand binding proteins- attach to single-stranded DNA and prevent secondary structures from forming.
DNA gyrase- moves ahead of the replication fork making and resealing breaks in the double-helical DNA to release the torque that builds up as a result of unwinding at the replication fork.
DNA primase- synthesizes a short RNA primer to provide a 3’-OH group of the attachment of DNA nucleotides.
DNA polymerase 3- Elongates a new nucleotide strand form the 3’-OH group provided by the primer.
DNA polymerase 1- removes RNA primers and replaces them with DNA
DNA ligase- joins Okazaki fragments by sealing nicks in the sugar-phosphate backbone of newly synthesized DNA.
Discuss three ways the fidelity of DNA replication is controlled.
- Nucleotide selection- DNA polymerases are very particular in pairing nucleotides with their complements on the template strand.
- Proofreading- 3’-5’ reading ability to correct itself
- Mismatch repair- recognized a mismatch can cut out incorrect base and replace with correct base.
What is telomerase? How does it function to replicate the ends of eukaryotic chromosomes?
a telomerase is both protein and RNA component.
it recognizes the 3’ overhang left by primer removal… new DNA is synthesized… repeats 14 to 16 nucleotides added.
