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 T m . A. Do you think T m is a constant or is dependent on other small molecules in the solution? Do you think high salt concentrations increase, decrease, or have no effect on T m ? B. Under standard conditions, the expected melting temperature in degrees Celsius can be calculated using the equation T m = 59.9 + 0.41 [%(G + C)] - [675/ length of duplex]. Does the T m increase or decrease if there are more G + C (and thus fewer A + T) base pairs? Does the T m increase or decrease as the length of DNA increases? Why? C. Calculate the predicted T m for a stretch of double helix that is 100 nucleotides long and contains 50% G + C content.

Answer 1

The melting temperature (Tm) of DNA can change based on the salt concentration and the G + C content of the DNA. More G + C content and higher salt concentration result in a higher Tm. The Tm for a 100 nucleotide long DNA with 50% G + C content is approximately 73.65°C.

Step-by-step explanation:

Melting Temperature (Tm) and DNA Properties

The melting temperature (Tm) is influenced by several factors, including base composition and the concentration of small molecules in the solution. Higher salt concentrations typically increase the Tm of DNA because they stabilize the negative charge on the DNA strands, making it harder for them to separate. Regarding base composition, the presence of more G + C base pairs increases the Tm because guanine and cytosine form three hydrogen bonds, compared to two bonds between adenine and thymine, making G-C pairs more thermally stable.

According to the provided equation, the Tm can be predicted based on base composition and length of the DNA duplex. The given equation is Tm = 59.9 + 0.41 [%(G + C)] - [675/length of duplex]. Therefore, if there is a higher G + C content, the Tm will increase because the 0.41 [%(G + C)] term will contribute a higher value to the Tm. Conversely, as the length of the DNA increases, the Tm will decrease due to the negative contribution from the -[675/length of duplex] term.

For a DNA stretch that is 100 nucleotides long with a 50% G + C content, we can calculate the Tm using the given formula: Tm = 59.9 + 0.41 * 50 - 675/100. This simplifies to Tm = 59.9 + 20.5 - 6.75 = 73.65°C. Therefore, the predicted Tm for this particular DNA sequence is approximately 73.65°C.