When double-stranded DNA is heated, the two strands separate into single strands

Question

When double-stranded DNA is heated, the two strands separate into single strands in a process called melting or denaturation. The temperature at which half of the duplex DNA molecules are intact and half have melted is defined as the Tm.

A. Do you think Tm is a constant, or can it depend on other small molecules in the solution? Do you think high salt concentrations increase, decrease, or have no effect on Tm?

B. Under standard conditions, the expected melting temperature in degrees Celsius can be calculated from the equation Tm = 59.9 + [0.41 × %(G + C)] – (675/length of duplex). Does the Tm increase or decrease if there are more G + C (and thus fewer A + T) base pairs? Does the Tm increase or decrease as the length of DNA increases? Why?

C. Calculate the predicted Tm for a stretch of double helix that is 100 nucleotides long and contains 50% G + C content.

Explanation / Answer

A).The Tm is not constant as it is the melting temperature and it depends on the identity and concentration of other molecules in the solution.Moreover as the High salt concentrations which are present are more effective at shielding the two negatively charged sugar–phosphate backbones in the double helix from each other,This implies that the two strands end up repelling each other less strongly as compared to a normal situation..A high salt concentration ends up stabilizing the duplex and this in turn helps to increase the melting temperature

B. The Tm usually increases as soon as the proportion of G + C bases increases along with an increase in length .This means that the overall thermal energy which is required for melting depends on the number of hydrogen bonds that are present between the strands and these bonds must be broken.Finally, each of the G-C base pair leads to three hydrogen bonds.On the other hand each of an A-T base pair contributes only two hydrogen bonds.

C.Let us Insert values into the equation which is given in part B

=>Tm = 59.9 + (0.41 × 50) – (675/100) = 73.65°C,

This temperature is usually twice the normal temuperature of the human body and its extremely high to ytouch.

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.