Nucleic Acids
description
Transcript of Nucleic Acids
Nucleic Acids
CH339K
Monomers: Nucleotides
Component 1: 5-Carbon Sugar
Ribose vs. Deoxyribose• Difference in components of DNA and RNA• Extra hydroxyl makes RNAs much more reactive
Sugar Pucker
• Furanoses are not planar• Can pucker out of the plane of the ring at C2 or C3• Pucker effects higher order structures (or vice-versa)
Component 2: Nitrogenous Base
Purines
Pyrimidines• Cytosine and Thymine in DNA• Cytosine and Uracil in RNA
Component 3: Phosphate
Nomenclature
Names of Base Derivatives
Base Nucleoside 5'-Nucleotide
Adenine (Deoxy)Adenosine (Deoxy)Adenosine-5'-monophosphate
Guanine (Deoxy)Guanosine (Deoxy)Guanosine-5'-monophosphate
Cytosine (Deoxy)Cytidine (Deoxy)Cytidine-5'-monophosphate
Thymine (Deoxy)Thymidine (Deoxy)Thymidine-5'-monophosphate
Uracil (Deoxy)Uridine (Deoxy)Uridine-5’-monophosphate
Syn and Anti Conformations
Syn / anti energetics
From: Neidle, S. (2008) Principle of Nucleic Acid Structure Elsevier, London, pg. 33
Condensation – Polymer Formation
Phosphodiester Linkages
Simple Condensation is Energetically Unfavorable
Go‘≈ +25 kJ/mol Keq=4.15*10-5
Synthesis is from the triphosphate
Energetics:
Phosphodiester formation - +25 kJ/molnTP cleavage – -31 kJ/molPyrophosphate cleavage - -19 kJ/mol
Keq = 24100
Tautomeric Forms of Bases-NHx groups can be in the amino or imino conformation
=O groups can be in the keto or enol conformation
The predominant form for the free base is not necessarily the predominant form in the nucleotide
Lack of basic O-Chem knowledge caused problems for Watson and Crick when they were trying to figure out the structure of DNA
Keto Enol
Base Pairing
keto
keto
amino
amino
Cytosine Guanine
Animation
Secondary Structure of Nucleic Acids
• Helical• Result of base
pairing• Defined by
– Pitch
– Rise
– In turn governed by structure of the monomers
B Helix B form
Helical Sense Right handed
Diameter ~20Å
Base pairs per helical turn 10
Helical twist per base pair 36°
Helix pitch (rise per turn) 34 Å
Helix rise per base pair 3.4 Å
Base tilt normal to the axis 6°
Major groove Wide & deep
Minor grooveNarrow &
deep
Sugar pucker C2'-endo
Glycosidic bond Anti
Typical DNA
Determination of helix parameters
Rosalind Franklin’s Diffraction Photo of B-DNA
A Helix
A form
Helical Sense Right handed
Diameter ~26 Å
Base pairs per helical turn 11
Helical twist per base pair 33°
Helix pitch (rise per turn) 28 Å
Helix rise per base pair 2.6 Å
Base tilt normal to the axis 20°
Major groove Narrow & deep
Minor groove Wide & shallow
Sugar pucker C3'-endo
Glycosidic bond Anti
RNA, DNA/RNA hybrids, dehydrated DNA
Z Helix Z form
Helical Sense Left handed
Diameter ~18 Å
Base pairs per helical turn 12 (6 dimers)
Helical twist per base pair 60° (per dimer)
Helix pitch (rise per turn) 45 Å
Helix rise per base pair 3.7 Å
Base tilt normal to the axis 7°
Major groove Flat
Minor groove Narrow & deep
Sugar puckerC2'-endo (pyrimidines)
C3'-endo (purines)
Glycosidic bondAnti (pyrimidines)
Syn (purines)Alternating Purine-Pyrimidine
Z DNA Function?
• Z DNA is antigenic• Antibodies are found in autoimmune disorders
like systemic lupus erythematosus• Antibodies bind to puffs in Drosophila polytene
chromosomes• Also bind macronuclei of ciliates• Z DNA-prone sequences found in transcription
start sites• May act as spacer between RNA polymerases• Z DNA binding proteins required for
pathogenicity by vaccinia and smallpox
Helix Parameters Summarized A form B form Z form
Helical Sense Right handed Right handed Left handed
Diameter ~26 Å ~20Å ~18 Å
Base pairs per helical turn 11 10 12 (6 dimers)
Helical twist per base pair 33° 36° 60° (per dimer)
Helix pitch (rise per turn) 28 Å 34 Å 45 Å
Helix rise per base pair 2.6 Å 3.4 Å 3.7 Å
Base tilt normal to the axis 20° 6° 7°
Major groove Narrow & deep Wide & deep Flat
Minor groove Wide & shallow Narrow & deep Narrow & deep
Sugar pucker C3'-endo C2'-endoC2'-endo (pyrimidines)
C3'-endo (purines)
Glycosidic bond Anti AntiAnti (pyrimidines)
Syn (purines)
Major and Minor Grooves
Grooves provide access to base sequence
• Telomere binding protein
• -helix fits into major groove
• Side chains can recognize bases
Another Example
cro Repressor protein of bacteriophage .
Small (66 amino acids)
Forms dimers
Binds to specific sites on DNA that activate / deactivate genes
Expression of cro results in the phage entering the lytic cycle
Ab
sorp
tio
n o
f U
V L
igh
t
UV Absorption Spectrophotometry
Beer-Lambert Law
clTA
I
IT cl
o
log :Absorbance
elyalternativor
10 :nceTransmitta
c = concentrationc = concentration
l = path lengthl = path length
= extinction coefficient= extinction coefficient
An Absorbance = 2 means that only 1% of the incident beam is getting through. An Absorbance = 2 means that only 1% of the incident beam is getting through.
Transmittance and Absorbance
Absorbance vs. Concentration Transmittance vs. Concentration
Physical Properties - Absorbance
Physical Properties - Hypochromicity
• Stacked bases in nucleic acids absorb less ultraviolet light than do unstacked bases, an effect called hypochromism
• Rules of thumb:– 280 dsDNA: 20
– 280 ssDNA/RNA: 37.5
– 280 small oligonucleotides: 50
1) Calculated spectrum of equivalent mixture of free nucleotides
2) Double stranded RNA (38% G+C)
3) Single stranded RNA (38% G+C)
From Cox, R. A. (1970) Conformation of Nucleic Acids and the Analysis of the Hypochromic Effect, Biochem. J. (1970) 120, 539-547
Denaturation: “Melting”
• Heat, alkali cause the double helix to unwind
• As H-bonds break, they form “bubbles” in the helix
• As the equilibrium shifts towards H-bonds breaking, the bubbles coalesce
• The strands come apart
As temperature increases, local denatured regions coalesce
Effect of G+C content on Tm
ss Bubbles Coalesce until Strands Separate
Effects of changing o’ and So’
Artificially generated curves
DNA Sequencing – Sanger Method
DNA Sequencing - Sequencers
Polymerase Chain Reaction(aka DNA Amplification)
Internal Structure
Palindromes and inverted repeats tend to be sites for recognition by proteins
Palindromes:
Kay, a red nude, peeped under a yak
Some men interpret nine memos
Campus Motto: Bottoms up, Mac
Internal Structure (cont.)
Replication Origin of Duck Hepatitis B
Nonstandard Base Pairs
Triplex DNA Structure
A) Duplex DNA Structure
B) Triplex DNA with 3rd Strand in Major Groove
Bissler, John J. (2007) Triplex DNA and human disease, Front. Biosci. 12: 4536-4546.
Duplex, Triplex, and Quadraplex
Quadraplexes are found in telomeres
Telomeres contain repeats of d(GGTTAG), which form quadraplexes.
Nucleic acids can form higher – order three dimensional structures…
…and it’s a good thing.
Tertiary Structures - tRNAs
tRNAs can contain a variety of modified nucleotide bases
Tertiary Structure - Viroids
• Viroids are small, naked circular, mostly double-stranded RNAs which infect plants
• Host RNA Polymerase copies the RNA many times
• Self-cleavage into individual lengths• Host ligases close into circles
Potato Spindle Tuber Viroid
African oil palm with cadang-cadang like viroid disease
Frequency of Cadang-Cadang in Coconut palms from two Phillippine provinces 1951-1976
From Zelazny, B., and Pacumbaba, E. (1982) Plant Disease 66: 547-549.
Tertiary Structures (cont.)
Examples of some specialized RNAs
E. coli 16S ribosomal RNA
Nucleases
• Nucleic acids can be hydrolyzed enzymatically by nucleases;
• Nucleases belong to the class of phosphodiesterases;– Cleavage at the 3’ side by “a” type nucleases
(leaves 5’ phosphate);– Cleavage at the 5’ side by “b” type nucleases
(leaves 3’ phosphate);– Endonucleases cleave in the middle of the NA;– Exonucleases cleave from the ends.
• DNases act on DNA; RNases act on RNA.
Examples
5’ p-A-p-G-p-G-p-T-p-C-p-C-p-T-p-A-OH 3’
a-type 3’ exob-type endo
b-type 5’ exo
Word of the Day: Processivity - The ability of an enzyme to repetitively continue its catalytic function without dissociating from its substrate.
(The exonuclease examples above are not processive)
Examples
Enzyme Substrate Type
Pancreatic RNase RNA b-type (5’) endo
Snake Venom Phosphodiesterase RNA / DNA a-type (3’) exo
Spleen Nuclease RNA / DNA b-type (5’)exo
Examples
From Smith, C., and Wood, E. J. (1991) Biological Molecules, Springer, New York, pg 188
Restriction Systems - Bacteriophage
Bacteriophage T4
Restriction Systems - Bacteriophage
RestrictionEndonucleases
E. coli R
Infectivity~1
Infectivity~1 x 10-4
E. coli K
Infectivity~1
Infectivity~1 x 10-4
Kablooey!
Kablooey!
• Phage hatched from the R strain reinfect the R strain easily.
• Phage hatched from the K strain reinfect the K strain easily.
• Phage from the R strain are restricted on K
• Phage from the K strain are restricted on R.
Restriction due to endonuclease / methylase system
The endonuclease and the methylase recognize the same sequence
The endonuclease will not cut the methylated DNA
Host can discriminate its own DNA from that of a virus if the virus is raised in a bug with a different restriction system
Protective Role of Restriction Systems
Example of a restriction modification system
EcoR1 (first restriction system from E. coli strain R) recognizes a 6-base palindrome:
5'-GAATTC-3'3'-CTTAAG-5'
The methylase puts a methyl group on the underlined adenosines if the sequence is not methylated. The nuclease clips each strand between the 5' G and A of the unmethylated recognition site
5'-G AATTC-3'3'-CTTAA G-5'
The resulting overhangs are "sticky ends" - they will base pair with a complementary sequence.
Cloning using Restriction Endonucleases
There are a zillion REs for just about any palindrome
(Enzymes for 234 recognition sites available from New England Biolabs as of March 2010)
DNA Modifying Agents as Drugs
Intercalating Agents
• Stack between base pairs
• Ethidium is used as a fluorescent DNA stain
• Acridine is also used as a stain for DNA (green) and RNA (red)
• Actinomycin inhibits transcription by binding at the start site
Ethidium Bromide Intercalated into DNA
DNA stained with ethidium bromide
Reactivity of Ribonucleic Acid Due to the 2’-Hydroxyl
Base1
O
OHO
HHHH
PO
O
O
O- Base2
O
OH
HHHH
O
PO
-O
O-
Base1
O
OO
HHHH
PO
O
O
O- Base2
O
OH
HHHH
O
PO
-O
O-
base
Base1
O
OO
HHHH
PO
O
Base2
O
OH
HHHH
O
PO
-O
HO
O-
O-
H2O
Base1
O
OHO
HHHH
PO
O
Base2
O
OH
HHHH
O
PO
-O
HO
O-
O-O-
Supercoiling
• DNA in “relaxed” state - 10.4 bp/turn• If DNA is twisted, the strands become more tightly or
more loosely wound: supercoiling– in direction of helix = “positive supercoiling”
– in the opposite direction = “negative supercoiling”
• In nature, most DNA has a slight negative supercoiling that is introduced by enzymes called topoisomerases (counteract effect of transcription and replication)
Supercoiling
• Can open helix• Overwind or underwind• Changes Linking Nbr• L = T + W• Underwound DNA is
primed to uncoil– Transcription
– Replication
– Recombination
– Z-DNA formation
Linking Numbers
Negative and Positive Supercoils
Supercoiling in a viral DNA
Different levels of supercoiling in Simian Virus 40 (SV40) DNA.. SV40 may (or may not) be involved in causing human tumors. Those of us inoculated for polio prior to 1962 were probably exposed to SV40 as a contaminant of the polio vaccine.
Supercoiling of Constrained Linear DNA
Supercoiling: Energetic Considerations
• Because there are ~10.4 bp/turn in B-helical DNA, the relaxed Linking Number is
Lo = bp / 10.4
• Upon supercoiling, the change in L is
ΔL = L − Lo
• We can define superhelical density as
σ = ΔL / Lo
• The free energy involved in supercoiling is related to
ΔG / N = KRTσ2 (usually shown as ΔG = KNRTσ2)
where N = number of (constrained) base pairs and
K depends on the solution (ionic strength, concentration, etc.)
Supercoiling (cont.)
• Wrapping of DNA around nucleosome requires ≈ -0.05
Type 1 TopoisomerasesCut 1 strand of the DNAChange L by 1Involved in protein synthesis control
Type 2 Topoisomerases
Cut both strands simultaneouslyChange L by 2Most well known is DNA GyraseIn presence of ATP can induce supercoilingUnwinds DNA ahead of replication fork
Topoisomerase Action
Simian virus 40 (SV40) DNA incubated w/ a human topoisomerase for 0, 1, 3, 6, 10, and 30 minutes, going from an average of 25 superhelical turns to 0 (relaxed)
Riddle me this, Doc…
…why does DNA use Thymine instead of uracil? Seems like a waste of complexity.
Spontaneous Deamination
Asn Pro Gly CysAAT CCT GGC TGTTTA GGA CCG ACA
Asn Ser Gly CysAAT TCT GGC TGTTTA AGA CCG ACA
Frequency: 100 – 500 times per human cell per day.
That’s about 1 - 5 * 1015 per person per day.
DNA Mismatch Repair
DNA has a system to recognize uradines in the DNA strand
Glycosylase clips of the uracil base
An endonuclease clips out the sugar phosphate
Polymerase fills the gap
Expectations• Know structures of nucleotides and components.• Understand the linear, directional, backbone / base
structure of the polymer.• Understand base pairing.• Understand the properties of the helix types and what
types of nucleic acids assume which forms.• Beer’s law• Difference between primary, secondary, and tertiary
structure.• Different types of nucleases; what are restriction
systems?• Meaning and significance of supercoiling;
topoisomerases.• What the heck is spontaneous deamination?