Determining the Structure of DNA

http://gslc.genetics.utah.edu Tour of the Basics Web Quest - Answer Key

Log on to: http://gslc.genetics.utah.edu/units/basics/tour/. Explore this activity to ﬁ nd the answers to the questions below. 1. What is DNA? The instructions for building parts of the cell.

2. What does “DNA” stand for? DeoxyriboNucleic Acid 3. What is the four-letter DNA alphabet and what are the special rules by which the alphabet pieces bind together? A, C, T, and G. A binds with T, C binds with G.

4. What is a gene? Genes are instruction manuals for our bodies.

5. What are genes made of? Genes are made of DNA.

6. How many genes do humans have? Humans have 25,000 genes.

7. For what molecule do genes contain the instructions for building? Genes contain the instructions for building proteins.

8. What is a chromosome? Chromosomes are packages of compact DNA.

9. How many chromosomes does a human cell hold? Each human cell holds 46 chromosomes.

10. How are the human sex chromosomes labeled? Sex chromosomes are labeled “X” and “Y”.

11. How many different kinds of proteins does one cell contain? Each cell contains thousands of different proteins.

12. Why do scientists use computer programs to model protein structure and function? Proteins are very small and hard to see.

13. What provides the “blueprint” for making a protein? Genes provide the blueprint for making a protein.

14. What is heredity? The passing of traits from parent to child.

15. Why aren’t children identical to either one of their parents? Each parent contributes one set of chromosomes to each child. The set of chromosomes is passed on randomly, so each child receives a unique combination.

16. In humans, how many chromosomes does each parent pass on to their offspring? Each parent passes on 23 chromosomes to their offspring.

17. Does the second baby in the What is Heredity? animation inherit the exact same chromosomes as the ﬁ rst? Do both babies have a complete set? No, the second baby’s chromosomes are different from the ﬁ rst baby’s. Yes, both babies have a complete set.

18. What is a trait? A trait is a notable feature or quality in a person.

19. List the types of traits that exist. There are: physical traits, behavioral traits, and predispositions to medical conditions.

20. Give an example of how an environmental factor can inﬂ uence a trait. Answers may include one of the following: exposure to sun or hair dyes can change hair color, you can train retrievers to roll over and play dead instead of fetch, eating healthy foods and exercising can decrease the risk of heart disease.

21. Brieﬂ y explain how the Hitchhiker’s Thumb trait is determined using the following words: allele, dominant, recessive, homozygous, heterozygous. You may draw pictures if you wish.

Answers will vary. Example answer: If two dominant alleles are inherited, the person is homozygous and will have a hitchhikers thumb. If two recessive alleles are inherited, the person is homozygous and will have a straight thumb. If a person is heterozygous, one dominant and one recessive allele are inherited and that person will have a hitchhiker’s thumb.

1) Pauling proposed an alternative structure for DNA. What was this structure that Pauling proposed and how did it differ from that proposed by Watson and Crick.

Pauling's proposed three-stranded helix had the bases facing out.

2) Watson and Crick believed that Pauling would soon recognise the error in his proposed DNA structure. How did Watson and Crick finally derive their structure.

Watson and Crick’s rapport led them to speedy insights as well. They incessantly discussed the problem, bouncing ideas off one another. This was especially helpful because each one was inspired by different evidence. When the visually sensitive Watson, for example, saw a cross-shaped pattern of spots in an X-ray photograph of DNA, he knew DNA had to be a double helix. From data on the symmetry of DNA crystals, Crick, an expert in crystal structure, saw that DNA’s two chains run in opposite directions.

The last hurdle for Watson and Crick was to figure out how to arrange DNA’s four bases (adenine, thymine, guanine, and cytosine) inside the double helix without distorting the molecule. To visualize the answer, Watson built cardboard cutouts of the bases. Early one morning, as Watson moved the cutouts around on a tabletop, he found that the overall shape of an adenine molecule paired with a thymine molecule was similar to the overall shape of a guanine-cytosine pair. He immediately realized that arranging the bases in these pairs made a DNA structure without bulges or strains. Watson solved the puzzle "not by logic but serendipity," Crick recalled in his book What Mad Pursuit.

Watson and Crick picked up this model-building approach from eminent chemist Linus Pauling, who had successfully used it to discover that some proteins have a helical structure.

3) What arguments existed between the vitalists and the reductionists. How did the determination of DNA’s structure help to overcome these differences?

According to science historian Victor McElheny of the Massachusetts Institute of Technology, this date was a turning point in a longstanding struggle between two camps of biology, vitalism and reductionism. While vitalists studied whole organisms and viewed genetics as too complex to understand fully, reductionists saw deciphering fundamental life processes as entirely possible—and critical to curing human diseases. The discovery of DNA’s double-helix structure was a major blow to the vitalist approach and gave momentum to the reductionist field of molecular biology.

4) It is felt, at least by the Americans, that Linus Pauling would have been the first to deduce the structure of DNA if it had not been for which event.

Historians wonder how the timing of the DNA race affected its outcome. Science, after years of being diverted to the war effort, was able to focus more on problems such as those affecting human health. Yet, in the United States, it was threatened by a curb on the free exchange of ideas. Some think that American researcher Linus Pauling would have beaten Watson and Crick to the punch if Pauling’s ability to travel had not been hampered in 1952 by the overzealous House Un-American Activities Committee.

5) Rosalind Franklin is often considered to be a somewhat unsung hero of the whole DNA structure debate. What contribution did she make? The effort to discover the structure of DNA was a race among several players. They were world-renowned chemist Linus Pauling at the California Institute of Technology, and X-ray crystallographers Maurice Wilkins and Rosalind Franklin at King’s College London, in addition to Watson and Crick at the Cavendish Laboratory, Cambridge University.

In February 1953 Francis Crick and James D. Watson of the Cavendish Laboratory in Cambridge University had started to build a model of the B form of DNA using similar data to that available to the team at King's. Model building had been applied successfully in the elucidation of the structure of the alpha helix by Linus Pauling in 1951,but Rosalind Franklin was opposed to building theoretical models, taking the view that building a model was only to be undertaken after the structure was known. Watson and Crick then indirectly obtained a pre-publication version of Franklin's DNA X-ray diffraction data (possibly without her knowledge), and a pre-publication manuscript by Pauling and Corey, giving them critical insights into the DNA structure.

Rosalind Franklin was probably never aware that her work had been used during construction of the DNA model.