Discovering the Molecular Structure of Genetic Material

Introduction:

So far, we have seen how chromosomes behave in both body and reproductive cells (inside the gonads) and we have also examined how traits are passed on to offspring. We have seen that the number of chromosomes is cut in half in the creation of gametes (haploid cells, sperm and eggs) during meiosis. We also have explored the idea that when haploid cells combine to form a diploid zygote, alleles for different traits combine. These alleles are then expressed in different ways, some of which you have heard of (complete dominance, incomplete dominance, codominance) to create many different phenotypes. However, we are struck with the question: WHAT exactly are genes anyway?!?!

In this project, you will be given the opportunity to become even more familiar with a number of important discoveries about the chemical nature and composition of chromosomes, and you will attempt to create and test a model of the molecule that carries genetic information. The model should (eventually) tell you how information is passed from parent to offspring. Remember James Watson and Francis Crik? You will be undertaking the same process that resulted in their winning of the Nobel Prize in 1962.

By the end, you will read three sets of “historical clues” – these are basically timelines of important discoveries about chromosomes from 1860 all the way to 1953. You will read each set one at a time, and develop a model after reading each set. DO NOT TRY AND READ THE CLUES ALL AT ONCE! It is far too overwhelming!

Don’t be discouraged if you had difficulty understanding these readings, they are not leisure reading. Don’t get frustrated if you do not understand it right away. Read through each set a few times before you continue. It is perfectly normal to ask questions about things you don’t understand, so a large part of this project is to ASK/pose questions which you need to answer in order to proceed. Try to write your questions as clearly as possible. They might invoke a large class discussion, others may be answered on your own.

After we have completed the models, we will run through the model together.

Procedure:

1. You are trying to answer two essential questions:a.) What molecules make up the genetic material in

chromosomes?b.) What is the nature of these molecules? (How do they behave? How are they arranged?)

2. Read the six clues from clue set 1. Write five questions on the sheet labeled “THIS IS A QUESTION SHEET” that you have about the set of clues; we will be discussing these in the next class. 3. It is supposed that chromosomes carry genetic information, the question arises:

What molecule in the chromosomes carries the genetic information?

Write out two alternate hypotheses that you- as a scientist might pose after reading the clues (especially after clue #1) TO ANSWER the above question

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H2_______________________________________________________________

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4. Which of these hypotheses seems most likely to be able to be supported, based on the work done from 1900 – 1952?Give a few examples to support your idea!

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5. Try to make a model of DNA using the molecules given to

trace; or be bold and try for the chemical model shown. Sketch

your model below:

Analysis: Is your model complete? Why?

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What information do you think you need to make it complete?

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Clue Set 1

(1868- 1952)

1. In 1968, a young Swiss biochemist, Friedrich Miescher, found

that when pepsin (an enzyme known to break apart protein

molecules was added to chromosomes, atoms of oxygen, carbon,

hydrogen, and nigtrogen were detected. These were the atoms

known to be present in proteins. He also detected phosphorous

atoms. Because phosphorous atoms had never before been

found in protein molecules, he suspected that another typey of

molecule, in addition to protein, must be present in

chromosomes. He named the new molecule, nuclein.

2. During the 1880s, a German biologist named Walter Flemming

conducted the first detailed studies of the behavior of

chromosomes during cellular reproduction. His work made it

increasingly clear that chromosomes carry the genetic material-

as he showed that new individuals begin with the union of sperm

and egg cells, which always contain chromosomes and often

little else.

3. In the early 1900s, Robert Feulger, a German chemist,

discovered that all body cells of any particular organism contain

precisely the same amount of nuclein but that the amount of

protein varies from cell to cell. He also found that egg and

sperm cells contain exactly one-half the amount of nuclein

present in body cells. This was not necessarily the case for the

amount of protein.

4. Later biochemists determined that nuclein is composed of

combinations of just three different kinds of chemical

substances:

1.) a five-carbon sugar called deoxyribose

2.) an atom of phosphorous surrounded by four atoms of oxygen

and two of hydrogen, called a “phosphate group”

3.) ring like molecules that contain two nitrogen atoms in each

ring, called “nitrogenous bases”.

Four different kinds of bases were found, adenine, guanine,

cytosine, and thymine. Moreover, it was determine that each

base always occurs in combination with one phosphate group

and one deoxyribose sugar molecule. This combination was

given the name nucleotide. Thus, DNA consists of four different

kinds of nucleotides- one with each of the four nitrogenous

bases. Two representations of a nucleotide containing cytosine

are shown below:

5. During the late 1940s, English biophysicists discovered that

DNA crystallized when water is removed. This fact suggested

that the atoms of DNA must be arranged in a very orderly

fashion, perhaps with many repetitions of a fairly simple pattern.

6. In 1952, Americans Martha Chase and Alfred Hershey knew

that viruses can attack bacterial cells and inject them with virus

genetic information. They grew viruses with radioactive

phosphorus in the DNA molecules and radioactive sulphur in the

protein molecules. After these viruses attacked the bacteria and

injected their genetic information, only radioactive phosphorus

was found inside the bacteria.

CLUES #2

(1952)

1. X-ray diffraction photographs made by Rosalind Franklin and

Maurice Wilkens of Kings College, London, indicated that the DNA

molecule is shaped like a spiral helix.

An x-ray photograph of DNA in the B form, taken by Rosalind

Franklin late in 1952. (from J.D.Watson, the Double Helix,

Atheneum, New York 1968)

2. Evidence indicates that the helix contains strands of

phosphate groups and sugar molecules linked by relatively

strong chemical bonds.

3. Watson and Crick calculated that a single chain of nucleotides

would have a density only half as great as the known density of

DNA.

Study Questions

1. What type of nucleotides must be attached to each other to

make your model stable? Can you be sure your model is correct?

Why or why not?

2. What percentage of your model nucleotides is adenine,

cytosine, guanine, and thymine? Show your work! How do your

percentages compare with those presented in the table of

percentages obtained biochemically?

3. The information in question 2 was very important in the

creation of the Watson-Crick model of DNA. What does it tell you

about your modoel? Does the information from questions 1 and

2 further strengthen your hypothesized model?

4. X-rays are known to break apart the bonds that hold the DNA

molecule together. Doctors seldom X-ray patients who are in

early pregnancy. Why?

5. Genetic Engineering involves changing DNA molecules. How

might genetic engineering be used in the battle against “germs”

and disease?

Chemical structures of DNA components

(dots show where Hydrogen bonding occurs!)