The Double Helix quiz

The two strands of a DNA molecule are held together by hydrogen bonds between the

nitrogenous bases on each strand

The instructions for the traits of an organism are determined by

the order of nucleotides in DNA molecules

Why did this other type of molecule seem like a likely candidate?

there are many proteins, they come in different shapes and have many different functions. Also, proteins are made of 20 different amino acid components, while DNA is composed of only four basic subunits.

In the diagram below, strands I and II represent the two complementary strands of a portion of a DNA double helix. The sequence of strand I is indicated below. What is the sequence of strand II? Strand I -----------C-T-A-C----------- Strand II -----------?-?-?-?-------------

GATG

Which of the following observations can be supported by the data in the table?

In each animal, the percentage of adenine is the same as the percentage of thymine in the DNA.

In one or two sentences, explain how these observations helped Watson and Crick develop their model of DNA

It suggests that in the double helix, adenine always pairs with thymine and guanine always pairs with cytosine.

What was the other type of molecule that some scientists thought might carry genetic information?

Protein

Although their model was wrong, what assumption made it reasonable to build a model with the bases projecting to the outside?

The bases contain genetic information which vary in amount between species and in their arrangement within the molecule.

The image on the right shows the famous photo B51 taken in May 1952 by Rosalind Franklin and her student Raymond Gosling. This x-ray diffraction pattern provided information about the positions of atoms in a DNA molecule. Identify the clue in this photo that revealed that DNA is a helix.

The clue is the X shape in the image.

What evidence caused Watson and Crick to revise their model?

The dimensions of the double helix from Franklin's images, as well as Chargaff's pairing rules for the bases, led them to a revised model. Watson had misremembered some key measurements from Franklin's work. Her images showed there was much more water in DNA than the triple-helix model allowed. Further, the triple-helix model could not explain how the three phosphate backbones could be held together.

Even before the structure of DNA was solved, studies indicated that the genetic material must have the following properties: • be able to store information; • be faithfully replicated and be passed on from generation to generation; and • allow for changes, and thus evolution, to occur. Explain how the structure of the double helix showed that DNA had these properties. Write one or two sentences per point.

The order of the bases, A, T, G, and C, contained information. Because A is always paired with T and G with C, the order of bases on one strand determines the order on the other strand. Thus, if a DNA molecule were unwound, each strand could be copied into a complementary strand, producing an exact replica of the original molecule. Errors in the copying mechanism could result in mutations, or changes in the DNA sequence, that could be inherited by future generations.

In one or two sentences, explain why the proportions of nitrogenous bases in the DNA of two different human tissues (thymus and sperm) are about the same.

All body cells and tissues in a particular organism contain the same DNA.

Measurements revealed that the distance between the two strands was always equal. Explain how this information helped Watson and Crick build a successful model of DNA.

Watson and Crick concluded that a large purine molecule (adenine or guanine) always bonds with a smaller pyrimidine molecule (thymine or cytosine)—that way, the distance between the two strands of DNA is always the same.

In 1928, Frederick Griffith conducted an experiment in which he injected mice with different kinds of bacteria. When bacteria that cause disease (pathogenic) were injected in healthy mice, these mice got sick and died. Other types of bacteria (nonpathogenic) did not cause the mice to die. Griffith took the DNA from dead pathogenic bacteria and transferred it into living nonpathogenic bacteria. These altered bacteria were then injected into healthy mice. The mice died of the same disease caused by the pathogenic bacteria. Based on this information, which statement would be a valid conclusion?

When DNA from one organism is transferred to another organism, it can give new traits to the second

Was this information consistent with the data obtained by Chargaff (question 9)? Explain your answer.

Yes. Chargaff discovered that in the DNA of an organism, the proportion of adenine (a purine) is the same as the proportion of thymine (a pyrimidine). The same is true for guanine (a purine) and cytosine (a pyrimidine). These data are consistent with a model in which a purine on one strand always bonds with a pyrimidine on the other strand.

What are the chemical components of a DNA nucleotide?

a phosphate, a sugar, and a nitrogenous base