Prepare Questions: DNA Structure - Unit 1

Why are GC pairs harder to pull apart than AT pairs?

GC has three bonds, where as AT has two bonds. Thus the energy required to break GC bond is greater than the energy needed to break an AT bond. The answer is due to the number of bonds.

If a DNA molecule has 18% Guanine, how much Cytosine, Adenine, and Thymine does it have?

We know that there is 18% G. C always pairs with G, so that would make it the same percentage - 36%. We need to find what percentages A and T, so we subtract 36% from 100% = 64%. This means that together A and T equal 64%. If we divide 64 in half, that would be 32% for each. Adding them all up equals 100%.

Is DNA a uniform width? What would happen to the width of DNA if two purines formed a base pair? Two pyrimidines?

Yes, DNA is a uniform width. When two purines form a base pair, the DNA helix forms a bump. When two pyrimidines form a base pair, the DNA helix forms a dip. DNA strands in a double helix must have matching base pairs.

Scientists studying a new organism find that its DNA is made up of 21% Adenosine (A). What percentage is Thymine? Guanine? Cytosine?

You would basically do the same steps as you did for #4. A and T would both be 21%, because they are base pairs. Adding them up and then subtracting them from 100 would be - 21+21=42. 100-42=58%. Divide 58% by 2, to get a number for each G and C. That would come out to. 29% for each.

DNA is a polymer composed of repeating subunits called ________. Each subunit has # ____ parts. There are four different subunits that have the same __________ and ________, but have different _______________ ___________.

nucleotides, 3, phosphate and sugar, nitrogenous bases.

DNA is directional meaning each strand has a "front" and a "back." What names are given to the two ends of a DNA strand and how can you tell which is which.

DNA has a "front" which is called the head and the "back" which is called the tail. If you look at the head, there will be a phosphate, which is known by the 5' prime end. The tail has a sugar, which is known by the 3' end. You can tell which is which by looking at the number of carbons, which are the 5 and 3.

What are the base pairing rules for DNA? What units are most commonly used to describe the size of a DNA molecule? Define that unit.

The first base pair is Adenine and Thymine. There are two reasons they are base pairs: 1. Both A and T have nitrogenous bases with two fingers, which meet together to form a hydrogen bond, which keeps them together. 2. Thymine is a pyrimidine and Adenine is a purine. For Guanine and Cytosine, they are a base pair because Cytosine is a pyrimidine and Guanine is a purine and they have three fingers to make a hydrogen bond. The measurement unit for DNA is a nanometer (nm). 1 meter = 100 centimeters. Meter = 3 ft 3 in. 1 cm = 10 millimeters. 1 millimeter = 1000 micrometers. 1 micrometer = 1000 nanometers.

T/F. The DNA helix is the same width all along its length. Explain your answer.

This is true. Each DNA strand must stay parallel with one another in order to make the base pairs. These pairings are what helps to maintain the perfectly parallel position for each DNA stand. Since each single-ring structured pyrimidine always pairs with a purine with a double ring structure, this ensures that the same distance is maintained through out the strand of DNA. Because each rung will always consist of 3 rings total, we can safely say that the DNA helix will stay consistently the same width all along its length.

Name the four bases used to make a strand of DNA (full names, not just the abbreviated letters). Describe each by the number of rings in its structure and the number of hydrogen bonds it can form.

Thymine has one singular ring and two hydrogen bonds. Adenine contains two rings and can form two hydrogen bonds. Guanine has two rings in its structure and can from three hydrogen bonds. And Cytosine has one ring in its structure and three hydrogen bonds can be formed.