Chapter 14 Flashcards
Why did biologists in the early 1900’s think that proteins were the source of genetic information?
What is meant by polarity in reference to DNA?
Refers to directional orientation of DNA strands. DNA is read in the 5-3 direction and 3-5 direction
What are Chargaff’s rules?
- Amount of adenine always equals amount of thymine (A = T); amount of guanine always equals cytosine (G = C)
2.
What was Rosalind Franklins contribution to understanding the structure of DNA?
Used X-ray crystallography to study 3D structure of DNA
Describe the antiparallel configuration of the double helix.
Why is it important that DNA polymerases also have nuclease activity?
T or F. Both DNA and RNA polymerases require primers
False. Only DNA polymerase requires primers
In what way is DNA replication semidiscontinuous?
Replication in leading strand is always continuous; Lagging strand has to be replicated in small chunks - requires many primers
This together makes DNA replication semicontinuous
Why is the lagging strand replicated differently than the leading strand?
- More complicated because its discontinuous
- Primase is needed to make primers for each Okazaki fragment
- Okazaki fragments must be stitched together
What is an Okazaki fragment?
DNA fragment synthesized on lagging strands
How does the sliding clamp and clamp loader work - what is their function?
Why is replication of eukaryotic chromosomes more complicated than prokaryotic? And how do eukaryotic cells overcome these challenges?
Eukaryotes:
- multiple replication origins on each chromosome
- multiple, linear chromosomes
Prokaryotes:
- one origin of replication
- singular, circular chromosome
What DNA polymerases are involved in eukaryotic replication? How are they different than those in prokaryotes?
DNA pol epsilon - synthesizes leading strand
DNA pol delta - synthesizes the lagging strand
- Greater division of labor in eukaryotes
Compare specific and non-specific repair mechanisms.
What is a thymine dimer, and how are they repaired?
Thymine dimer - adjacent thymines become covalently linked
How does excision repair work?
A-T base pair
Adenine (A) can form two hydrogen bonds with thymine (T)
G-C base pair
Guanine (G) can form three hydrogen bonds with cytosine (C)
T or F: Strands are indentical
F: Strands are NOT identical; they are COMPLEMENTARY
What are the components of DNA replication?
Template – the parent DNA strand
Polymerase enzyme - does the actual ‘copying’
Nucleotides – the building blocks to make the new strand
What are the three stages of DNA replication?
Initiation – start of replication
Elongation – where the building blocks are added
Termination – replication ends
DNA polymerase
class of enzymes that all synthesize DNA
What are the common features that DNA polymerases share?
- add new bases to 3’ end of existing strands
- synthesize in 5’-to-3’ direction
- require a primer of RNA
RNA polymerase
enzyme that synthesizes primers
the origin of replication (oriC)
spot on chromosome where replication begins
Initiation of DNA replication in prokaryotes begins when:
- DnaA (protein) binds to oriC
- Causes the two strands at this region to separate
- Allows the enzymatic machinery for replication to attach to a strand, begin replication
- After initiation, replication proceeds bidirectionally –> elongation
Nuclease
enzyme that breaks down nucleotides
Endonuclease
enzymes capable of cleaving phosphodiester bonds between nucleotides internally in a DNA strand
Exonuclease
enzymes capable of cleaving phosphodiester bonds between nucleotides located at the ned of a DNA strand
DNA Pol I, II, and II have ________ activity
exonuclease
DNA polymerase I
acts on lagging strand to remover primers and replace them with DNA
DNA polymerase II
Involved in DNA repair
DNA polymerase III
Main replication enzyme
Helicase
enzyme that unwinds DNA (breaks hydrogen binds that hold nucleotides together)
T or F: Helicase breaks phosphodiester bonds when unwinding DNA
F: Helicase breaks HYDROGEN BONDS
Topoisomerase
enzyme that relieves torsional strain caused by unwinding
DNA gyrase
topoisomerase involved in DNA replication
Leading strand
Continuous strand
Lagging strand
discontinuous strand
uses multiple primers
Replication fork
Where DNA synthesis is occurring
DNA primase
RNA polymerase that makes the necessary primers
Primers
Short stretched of RNA (10-20 bp)
Leading strand synthesis
Lagging strand synthesis
Primase needed to make primers for each Okazaki fragment
DNA Pol I: removes primers and replaces them with DNA
DNA Pol III: synthesizes lagging strand
DNA ligase: Joins Okazaki fragments
Replicons
areas where replication is occurring on eukaryotic chromosomes
DNA pol a (and primase)
Involved in priming
DNA pol e (epsilon)
synthesize leading strand
DNA pol d (delta)
synthesizes lagging strand
Mutagen
Agent that induces changes in DNA (mutations)
Specific repair
Nonspecific repair
Mismatch repair (MMR)
removes incorrect bases and replaces them with correct base by copying template strand
Thymine dimer
Adjacent thymines become covalently linked together
What occurs during photorepair
UV light causes damage to DNA
Causes thymine dimers
Photolyase
enzyme that binds to thymine dimers
Excision repair
Damaged region is removed (excised) then replaced by DNA synthesis
What are the three steps in excision repair?
- Recognition of damage
- Removal of damaged region
- Synthesis using undamaged strand as template
