Questions: DNA Structure and the Molecular Biology Revolution

Question 1

Watson and Crick published the double-helix structure of DNA in 1953. What critical data did they use, partly without adequate acknowledgment?

Model answer: Watson and Crick relied substantially on X-ray crystallography images produced by Rosalind Franklin and Raymond Gosling at King's College London. Critically, Watson was shown Franklin's best X-ray diffraction image (Photo 51) by Maurice Wilkins without Franklin's knowledge or permission — this image revealed key structural parameters. Watson and Crick's 1953 Nature paper acknowledged Franklin's work only in passing, and the Nobel Prize in 1962 was awarded to Watson, Crick, and Wilkins. Franklin had died of cancer in 1958 and was not eligible for the prize; but even absent the eligibility question, her contribution was systematically understated.

The Rosalind Franklin story has become an iconic case in the history of science illustrating both the significance of X-ray crystallography and the systematic undercrediting of women scientists. Brenda Maddox's biography 'Rosalind Franklin: The Dark Lady of DNA' is the authoritative account.

Question 2

Why did the double-helix structure of DNA 'immediately suggest' how it replicated?

• A. Because the two strands were held together by strong covalent bonds that could be cleaved
• B. Because the complementary base-pairing meant each strand could serve as a template for synthesizing a new partner strand
• C. Because DNA was shown to be made of equal amounts of all four bases
• D. Because the helix structure matched the known structure of chromosome fibers

Answer: (B)

The double helix consists of two strands, each made of a sequence of four bases (A, T, G, C), where A always pairs with T and G always pairs with C. If the two strands were separated, each strand's base sequence would specify exactly which bases should attach to form a new partner strand — a perfect copying mechanism. Watson and Crick ended their 1953 paper with the famous understatement: 'It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.'

Question 3

What is the 'central dogma of molecular biology,' and why is it called a 'dogma'?

Model answer: The central dogma, articulated by Francis Crick in 1958, states that genetic information flows in one direction: DNA is transcribed into RNA, which is translated into protein. Information does not flow backward from protein to RNA or DNA. Crick called it a 'dogma' somewhat facetiously — meaning a fundamental principle taken as given — though he later acknowledged the word choice was unfortunate since it implies religious certainty. The discovery of reverse transcription by retroviruses (including HIV) showed that RNA can be transcribed back into DNA, complicating but not overturning the basic principle.

The central dogma is foundational to molecular biology and biotechnology. Understanding it is necessary for understanding how genetic engineering, mRNA vaccines, and cancer biology work.

Question 4

The discovery of DNA's structure in 1953 immediately enabled genetic engineering and medical applications of genomics.

• T. True
• F. False

Answer: False

The structural discovery in 1953 provided a framework but not immediate applications. Decades of work were required: deciphering the genetic code (completed by 1966), developing restriction enzymes that could cut DNA (1960s-1970s), creating recombinant DNA technology (1973), polymerase chain reaction/PCR (1983), DNA sequencing methods, and eventually the Human Genome Project (completed 2003). The gap between discovering DNA's structure and practical genomic medicine spans roughly five decades of additional research.

Question 5

What revolutionary discovery about the human genome emerged from the Human Genome Project that surprised scientists?

• A. Humans have more genes than any other species
• B. Only about 1.5% of the genome codes for proteins, and there are only about 20,000 genes
• C. The human genome is 99.9% identical to chimpanzee genomes
• D. DNA replication is error-free, explaining human genetic stability

Answer: (B)

Before the Human Genome Project, many scientists predicted 100,000+ human genes. The actual number — roughly 20,000-25,000 protein-coding genes, about the same as a roundworm — was shocking. Furthermore, only about 1.5% of the human genome codes for proteins. Much of the rest was initially labeled 'junk DNA,' but subsequent research (ENCODE project) has revealed that much of this non-coding DNA has regulatory functions, complicating the simple picture of genes as the units of heredity.