BPS3021 Chap.3 qPCR and Experimental Evidence

qPCR and Experimental Evidence

Calculate relative copy number only after assessing replicate behaviour, controls and product specificity.

What this chapter covers

• 01

  Cq threshold crossing

• 02

  Signed delta-Cq calculation

• 03

  Plasmid copy-number estimate

• 04

  No-template controls and melt curves

Worked example · free

Estimate plasmid copy number

Key terms

Cycle of quantification

The cycle at which PCR fluorescence crosses a defined threshold.

Melt curve

A fluorescence-temperature profile used to assess whether one principal amplification product is present.

No-template control

A reaction without target template that helps detect contamination or primer-derived signal.

qPCR and Experimental Evidence FAQ

Exam move

Write the formula, signed substitution, exponent and qualified biological conclusion as four separate lines; Quantitative PCR follows fluorescence as double-stranded DNA accumulates; The cycle of quantification, Cq or Ct, is the cycle where fluorescence crosses a defined threshold;

A lower Cq means the target crossed earlier and therefore began at a higher abundance, provided amplification efficiencies are comparable; The course uses a plasmid target and a chromosomal reference to estimate average plasmid copies per bacterial cell;

A calculation is the middle of the evidence chain, not the end.Compute with the printed relationshipThe raw slide gives CN = 2−(Ct plasmid − Ct chromosome); Suppose an original dataset has mean plasmid Cq 16.2 and mean chromosomal Cq 20.2;

Then ΔCq = 16.2 − 20.2 = −4.0, so CN = 2−(−4.0) = 24 = 16 copies per cell on average; Print the subtraction, preserve the sign, then simplify the exponent; Reversing the target order changes the conclusion drastically; Replicates should cluster, no-template controls should not show credible target amplification, and a SYBR assay should show a single melt peak consistent with one principal amplicon;

Multiple melt peaks can indicate primer dimers or nonspecific products; A late signal in a control requires investigation rather than automatic deletion; State whether the quality checks support calculation and identify any excluded replicate with a reason;

Three plasmid Cq values are 17.0, 17.2 and 17.1; chromosomal values are 21.1, 21.0 and 21.2; Means are 17.1 and 21.1; ΔCq = −4.0 and CN = 16; The answer should say “approximately 16 plasmid copies per cell on average under the assay assumptions,” not “every cell contains exactly 16 plasmids.” The target wells show two clear melt peaks;

Answer: do not treat the Cq-derived copy number as secure; Two products can contribute fluorescence, so redesign or optimise primers and confirm product identity; A no-template control crosses very late; Answer: inspect its melt peak and repeat where necessary;

The signal may reflect contamination or primer dimer; it cannot be dismissed solely because it is late; Marks are lost by reporting an exponent without the signed subtraction, by calling association “copy number” without a reference target, or by ignoring poor-quality replicates;

The stronger answer combines arithmetic with an evidence statement: what was measured, which observations support specificity, what assumptions connect ΔCq to relative abundance, and what the result does not establish about individual cells;

Average technically acceptable replicates: (17.0 + 17.2 + 17.1) ÷ 3 = 17.1 and (21.1 + 21.0 + 21.2) ÷ 3 = 21.1; Subtract in the stated order: 17.1 − 21.1 = −4.0; Apply the exponent: 24.0 = 16; Qualify the biological conclusion and report the control checks;

· qPCR EVIDENCE Quantitative evidence LAB SESSIONS Read the curve before using the number Cq, controls and melt curves decide whether a copy-number result is credible CN equals two raised to the negative delta Cq Quantitative PCR follows fluorescence as double-stranded DNA accumulates;

The cycle of quantification, Cq or Ct, is the cycle where fluorescence crosses a defined threshold; A lower Cq means the target crossed earlier and therefore began at a higher abundance, provided amplification efficiencies are comparable;

The course uses a plasmid target and a chromosomal reference to estimate average plasmid copies per bacterial cell; amplification curves Cq values ΔCq + copy number melt-curve quality A calculation is the middle of the evidence chain, not the end;

Compute with the printed relationship The raw slide gives CN = 2 −(Ct plasmid − Ct chromosome) ; Suppose an original dataset has mean plasmid Cq 16.2 and mean chromosomal Cq 20.2; Then ΔCq = 16.2 − 20.2 = −4.0, so CN = 2 −(−4.0) = 2 4 = 16 copies per cell on average;

Print the subtraction, preserve the sign, then simplify the exponent; Reversing the target order changes the conclusion drastically; Quality precedes interpretation Replicates should cluster, no-template controls should not show credible target amplification, and a SYBR assay should show a single melt peak consistent with one principal amplicon;

Multiple melt peaks can indicate primer dimers or nonspecific products; A late signal in a control requires investigation rather than automatic deletion; State whether the quality checks support calculation and identify any excluded replicate with a reason;

COURSE FORMULA ΔCq = Cq(plasmid) − Cq(chromosome); copy number = 2 −ΔCq ; Threshold placement must be consistent across compared reactions and positioned within the exponential phase; Record the rule used rather than adjusting it to obtain a preferred copy number;

Consistent analysis preserves the meaning of the Cq difference across targets and replicates; · · CALCULATE + QUALIFY Result boundaries A precise number can still support a weak claim Replicates, efficiencies and specificity determine the strength of the inference EX 3 Recompute and qualify Three plasmid Cq values are 17.0, 17.2 and 17.1;

chromosomal values are 21.1, 21.0 and 21.2; Means are 17.1 and 21.1; ΔCq = −4.0 and CN = 16; The answer should say “approximately 16 plasmid copies per cell on average under the assay assumptions,” not “every cell contains exactly 16 plasmids.” Average technically acceptable replicates: (17.0 + 17.2 + 17.1) ÷ 3 = 17.1 and (21.1 + 21.0 + 21.2) ÷ 3 = 21.1;

Subtract in the stated order: 17.1 − 21.1 = −4.0; Apply the exponent: 2 4.0 = 16; Qualify the biological conclusion and report the control checks; P1 Melt-curve check Practice The target wells show two clear melt peaks; Answer: do not treat the Cq-derived copy number as secure;

Two products can contribute fluorescence, so redesign or optimise primers and confirm product identity; P2 No-template signal Practice A no-template control crosses very late; Answer: inspect its melt peak and repeat where necessary;

The signal may reflect contamination or primer dimer; it cannot be dismissed solely because it is late; Calculate only after you can defend what generated the fluorescence; qPCR marking move Marks are lost by reporting an exponent without the signed subtraction, by calling association “copy number” without a reference target, or by ignoring poor-quality replicates;

The stronger answer combines arithmetic with an evidence statement: what was measured, which observations support specificity, what assumptions connect ΔCq to relative abundance, and what the result does not establish about individual cells;

If amplification efficiencies differ materially, the simple power-of-two relationship becomes unreliable; A careful answer identifies that assumption and proposes an efficiency check or validated standard-curve approach; Quantitative confidence comes from both correct arithmetic and a model suited to the assay; ·