basics of hybridization
DESCRIPTION
Basics of hybridization. What is hybridization?. Complementary base pairing of two single strands of nucleic acid double strand product DNA/DNA RNA/RNA DNA/RNA. What holds the two strands together?. Hydrogen bonds between the base pairs. What holds the two strands together?. - PowerPoint PPT PresentationTRANSCRIPT
Basics of hybridization
What is hybridization? Complementary base pairing of two single strands
of nucleic acid double strand product DNA/DNA RNA/RNA DNA/RNA
What holds the two strands together? Hydrogen bonds
between the base pairs
What holds the two strands together? Hydrophobic
interactions of stacked bases
van der Waals forces between stacked bases
Factors affecting the strength of strand pairing
Number of GC pairs vs. AT pairs Mismatch Length of hybridizing strands [Salt] of hybridization solution Temperature Concentrations of denaturants
Factors affecting the strength of strand pairing
Number of GC pairs vs. number of AT pairs The more H-bonds between
strands, the more strongly they are held together 3 H-bonds between G and C 2 H-bonds between A and T
So…the more GC pairs, the more H-bonds between strands
Factors affecting the strength of strand pairing
% Mismatch the greater the lack of complementarity,
the fewer hydrogen bonds the lower the strength of the hybrid
Factors affecting the strength of strand pairing
Length of hybridizing strands the longer the strands,
the more hydrogen bonds and the more hydrophobic interactions, so
the greater the strength of the hybrid
Factors affecting the strength of strand pairing [salt] of solution [salt] strength of the hybrid
negative charges of the phosphate moieties of the sugar-phosphate backbones repel each other
+ ions from salts in solution act as counterions to reduce repulsion Monovalent cations (Na+) Divalent cations (1 mM Mg++ = 100 mM Na+)
– Why does [Mg++] affect specificity of PCR priming?
Factors affecting the strength of strand pairing
Temperature heat increases the kinetic energy of each of the two
strands sufficient heat makes kinetic energy > H-bond energy strands separate
Factors affecting the strength of strand pairing
pH [OH- ], ~pH 12
enolic hydroxyl groups on bases ionize keto-amino H-bonds disrupted
Concentration of denaturants formamide, urea
Combined effects of these factors can be expressed as equations for the Tm What is Tm? Equation to estimate Tm for DNA oligonucleotides Equation to estimate Tm for polynucleotides
What is Tm?
Tm = temperature of melting or separation of strands Tm is a function of the DNA fragment or RNA strand
under consideration and the solution in which the hybridization is occuring. Changing the temperature does not change the Tm!
What is Tm? For complementary oligonucleotides (10 - 23 nt)
Temp at which 50% of complementary molecules exist as single strands
50%
5’ - - - - - - - - - - - - - 3’
3’ - - - - - - - - - - - - - 5’
50%5’ - - - - - - - - - - - - - 3’
3’ -
- - -
- - -
- - -
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5’
What is Tm?
For complementary polynucleotides (>~25nt) Tm is the temp at which 50% of hydrogen bonds
within any one hybrid are broken
Combined effects of factors contributing to strength of a hybrid can be expressed as equations for Tm
for DNA oligonucleotides in 1.0M Na+
Tm (oC) = 4 (G+C) + 2 (A+T)
Note: how does this equation account for length? % GC? The conditions of the solution
Combined effects can be expressed as equations for Tm
for DNA polynucleotides and oligos as short as 14 nt
Tm = 81.4 + 16.6 log [(M+)/1+0.7(M+)]
+ 0.41 (%G+C) - 600/L - %mismatch
- 0.65 (% formamide)
M+ = monovalent cation concentration
L = length of probe sequence
Tm for polynucleotides (cont’d)
How does the equation on the previous slide account for length? % GC? The conditions of the solution
Membrane hybridization One nucleic acid component is affixed to membrane; the
other is in solution probe(s) affixed; sample in solution
HLA-DQalpha samples affixed; probe(s) in solution
14;18 translocation Membrane material binds DNA or RNA
nylon charged nylon nitrocellulose
Steps in membrane hybridization
blocking or prehybridization hybridization wash or rinse visualization
Blocking/prehybridization
Why? Remember, membrane binds nucleic acid, so
labeled nucleic acid in hybridization solution can bind everywhere on membrane background
Blocking/prehybridization
How?
Membrane with affixed nucleic acid is bathed in blocking solution at hybridization temperature
Components of blocking solution bind non-specifically to membrane to prevent labeled nucleic acid from binding except to complementary strands
Blocking/prehybridization common blocking agents
sodium dodecyl sulfate (SDS) nonfat dry milk bovine serum albumin Ficoll
(carbohydrate polymer)
polyvinylpyrollidone (PVP)
Hybridization What?
Labeled nucleic acid in solution is allowed to anneal to affixed complementary strands
Conditions Must be determined empirically Hybridization solution includes
[Salt] determined from Tm formulas Membrane blocking agents Denatured labeled nucleic acid; denatured by
• High temperature (95oC) or
• Alkaline (high pH) conditions
Hybridization Conditions (cont’d)
Temp set below Tm to optimize rate of hybridization oligonucleotides: 15o below Tm polynucleotides: 15-35o below Tm
Wash/rinse
Why? To remove labeled probe/sample that is
in excess non-specifically bound bound with loose complementarity
Wash/rinseHow?
Bathe membrane in solution lacking labeled probe/sample
Use stringency conditions that minimize non-specific hybridization stringency = likelihood that two strands will separate
Be aware that wash conditions for oligonucleotide and polynucleotide hybridizations differ because: oligonucleotide hybrids are not in equilibrium polynucleotide hybrids are in equilibrium
Choosing wash conditions
To wash polynucleotide hybridizations (equilibrium) raise stringency conditions to make it harder for
imperfect hybrids to remain annealed perform washes just below the Tm
stringency likelihood that two strands will separate Lower the salt concentration Raise the temperature Include denaturants
Choosing wash conditions (cont’d)
To wash oligonucleotide hybridizations Use stringency similar to or lower than hybridization
condtions Same or lower temperature Same or higher salt concentrations
Short time periods
In the HLA-DQ alpha assay, is the hybridization oligo or poly?
How do the hybridization and wash conditions compare?
Visualization
requires a visible signal radioactive non-radioactive, enzyme linked non-radioactive, non-enzymatic
e.g., use of fluorescent label
for enzyme-linked signal generation additional block and rinse steps required
avoid conditions which will disrupt hybrids
How were conditions for HLA-DQ determined?
Use buffer recipes, probe sequences, and Tm equations to estimate Tm for one or more oligonucleotide probes for the
hybridization solution wash solution citrate buffer
Compare calculated Tms to temps used for each step. How does the Tm change if there is a single
nucleotide allelic difference?
How were conditions for HLA-DQ determined? (cont’d)
What do your answers tell you about the design of the protocol?
How do you think the intensity of color at the control probe is made to be less than the intensities of positive hybridization at shorter probes?