Chapter 16.2 - DNA Replication Flashcards
Important points about DNA replication (4)
- DNA replication occurs in the same way in all organisms
- DNA must be copied accurately
- Genetic info (DNA) must be transmitted to daughter cells
- DNA directs its own replication
Actual Process of DNA Replication (5)
- DNA strands separate
- Enzymes bind and use each strand as a template to make a new strand
- Semiconservative replication
- Each new double helix consists of one old and one new strand
- DIAGRAM
Semiconservative model
The two strands of the parental molecule separate, and each functions as a template for synthesis for a new, complimentary strand
Origin of Replication (4)
- Replication of each DNA molecule begins at sites with a specific sequence of nucleotides
- A-T rich sequence at origin of replication
- Specific proteins attach and separate strands
- Check diagrams!
Replication Fork
At the end of each replication bubble. This is a Y shaped region where the parental strands of DNA are being unwound by several types of proteins.
Origins of replication in a Eukaryote
DNA helix must unwind to expose template strand. See page 338!
3 Proteins involved in unwinding DNA
- Helicase
- Single-stranded bonding protein
- Topoisomerase
Helicase
- unwinds strands
- breaks hydrogen bonds between nitrogenous bases
Single-stranded binding protein
-binds to the unpaired DNA strands, keeping them from re-pairing after the parental strands separate
Topoisomerase
-an enzyme that helps relieve the strain by breaking, swivelling and rejoining DNA strands
Enzymes involved in DNA synthesis (2)
- DNA Polymerase
2. Primase
DNA polymerase
-catalyzes the synthesis of new DNA by adding nucleotides to the 3’ end of a preexisting chain
Primase
-makes the RNA primer and provides initial 3’ OH
Directional Polymerization of Nucleotides (4)
- DNA polymerase catalyzes addition of nucleotide triphosphate to 3’ end of growing strand
- Synthesis of Nucleic Acid is always 5’3’ and antiparallel to template
- Nucleotide added is determined by base on template strand
- Replication proceeds in both directions and occurs simultaneously in both strands
Lagging Strand
Discontinues synthesis
Leading strand
Continuous synthesis
Okazaki fragments
Fragments of the lagging strand (separately primed)
Proofreading (4)
- DNA polymerase can detect and remove non-complementary nucleotides during replication
- If incorrectly paired nucleotides are missed, other enzymes will later remove and replace the mismatched nucleotide
- These DNA repair systems also repair damaged DNA
- Result in mutation if not corrected. Mutations are perpetual
DNA ligase
An enzyme that joins all the sugar-phosphate backbones of the Okazaki fragments into a continuous strand
A summary of bacterial DNA Replication
Diagram on 341
Mismatch Repair
Other enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors
Termination of Replication in prokaryotes
- chromosomes are circular
- replication proceeds in both directions to end around 180 degrees
Termination of Replication in eukaryotes
- chromosomes are linear
- replication terminates at nucleotide sequences known as telomeres
- no genes present
Telomeres (3)
Telomeres do not contain genes and instead consists typically of repetitions of one short nucleotide sequence
Telomerase
An enzyme that catalyzes the lengthening of telomeres in eukaryotic germ cells, thus resorting to their original length and compensating for the shortening.
*produced in 90% of cancer cells
