Ch 16 DNA Replication Flashcards
Properties of DNA
- double stranded helix
- uniform diameter
- right handed twist
- chargaff’s rule A=T, C=G
- Strands are anti-parallel
semiconservative DNA replication
DNA strands separate and each one serves as a template to copy a second strand. End up with two helixes that both have one new strand and one old strand.
four requirements of semiconservative DNA replication
a. Energy to break apart helix and fuel enzymes
b. Enzymes to do the work of copying the DNA
c. Nucleotides (A, T, C, G) prepared by cells in advance
d. Sequence/DNA template
Describe how DNA acts as its own template.
Once the DNA splits apart, a copy is synthesized based on the nucleotides of the old strands (one is leading, one is lagging
antiparallel
Side by side in opposite directions, 3’ to 5’, 5’ to 3’
leading strand
runs 5’ to 3’ (DNA pol III can synthesize continuously)
lagging strand
runs 3’ to 5’ (DNA pol III must synthesize in segments)
Okazaki fragments
segments between primer on lagging strand
nuclease
DNA cutting enzyme (“nucle”, “-ase”) for nucleotide excision repair of errors
telomere
Eukaryotic cells have linear chromosomes shaped like Xes, so origin of replication is not also the end. There’s no room on the lagging strand to replace the 5’ end. Telomeres are “nonsense” DNA repeating itself over and over found at the end of chromosomes, so no information is lost when the end is not replicated.
helicase
unzipping, breaking through hydrogen bonds that hold DNA nitrogenous bases together
single stranded binding
binds to and stabilizes single stranded DNA until it can be used as a template so that they don’t reconnect
Topoisomerase
“nicks” the helix so that it doesn’t snap
Primase
makes the primer which instructs polymerase where to begin working (made of RNA)
DNA polymerase I
removes RNA nucleotides of primer from 5’ end and replaces w/ DNA. proofreads.
DNA polymerase III
the builder. adds nucleotides onto the helix one at a time
Ligase
glue, takes care of gaps between Okazaki fragments (primase has to continue to tell polymerase where to build, which leaves gaps of RNA that need to be replaced with DNA)
limitations of DNA pol III
Has to build DNA in the 5’ to 3’ direction
Cannot attach to DNA w/o a 3’ end
DNA replication - leading strand
DNA alpha helix opens up, unzipped by helicase enzyme
- Topoisomerase nicks the helix so that it doesn’t snap
- Single stranded binding proteins lay on both sides of the helix so that it doesn’t reform (strands don’t bind together again)
- Primase lays down a short segment of RNA (RNA primer is only a few nucleotides long to give the next enzyme somewhere to bond) on the leading strand
- RNA primer primes the enzyme for DNA polymerase III to add nucleotides one at a time continuously, toward replication fork
- DNA polymerase I removes the primer and replaces it with DNA
DNA replication - lagging strand
- DNA alpha helix opens up, unzipped by helicase enzyme
- Topoisomerase nicks the helix so that it doesn’t snap
- Single stranded binding proteins lay on both sides of the helix so that it doesn’t reform (strands don’t bind together again)
- Instead of one primer, there are multiple primers laid down (new strand must replicate 5’ -> 3 prime, but lagging strand replicates in opposite direction, so primers go down repeatedly at the beginning of each fragment)
- DNA pol III follows primer to start replicating segments of DNA, replicates in the direction away from replication fork
- DNA pol III completes an Okazaki fragment. When it hits primer from previous fragment, it jumps backwards (toward the replication fork) to attach to strand again
- New Okazaki fragment is created after jump
- DNA pol I removes primers between fragments
- Ligase seals up any gaps in the helix (from topoisomerase)
4 differences between strands
a. Leading is fast, lagging is slow. Replication is the same rate because of delay between fragments
b. Leading starts with one primer, lagging starts with multiple primers at each segment
c. Leading is continuous replication, lagging is Okazaki fragments
d. Leading is copied in the 5’ to 3’ direction, lagging is 3’ to 5’
telomerase
Enzyme which lengthens telomeres in germ cells. Found in large amounts in cancer cells.
bacteriophage/phage
bacteria eating virus
virus
DNA or RNA w/ protective coat (usually protein), takes over cell’s metabolic machinery. DNA enters the host cell, the protein does not
