In this chapter, we first focused on the information that showed DNA to be the genetic material and then discussed the structure of DNA as proposed by Watson and Crick. We concluded the chapter by describing various techniques developed to study DNA. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) How were scientists able to determine that DNA, and not some other molecule, serves as the genetic material in bacteria and bacteriophages? (b) How do we know that DNA also serves as the genetic material in eukaryotes such as humans? (c) How was it determined that the structure of DNA is a double helix with the two strands held together by hydrogen bonds formed between complementary nitrogenous bases? (d) How do we know that G pairs with C and that A pairs with \(\mathrm{T}\) as complementary base pairs are formed? (e) How do we know that repetitive DNA sequences exist in eukaryotes?

Step by step solution

01

(a) Evidence for DNA as the genetic material in bacteria and bacteriophages

The scientists Oswald Avery, Maclyn McCarty, and Colin MacLeod conducted experiments with Streptococcus pneumoniae bacteria in 1944, which showed that the genetic material responsible for transforming nonvirulent bacteria into virulent bacteria was DNA. Later, in 1952, Alfred Hershey and Martha Chase used bacteriophages, viruses that infect bacteria, to further support DNA's role as the genetic material. They labeled the DNA and proteins of bacteriophages with radioactive isotopes and observed that only the DNA entered the bacterial cell during infection, proving that DNA carries the genetic information.

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(b) Evidence for DNA as the genetic material in eukaryotes

The universality of DNA as the genetic material can be inferred from the experiments mentioned in part (a), as well as through observations of DNA replication and transmission during cell division (mitosis and meiosis) in eukaryotes. Additionally, genetic studies, including chromosome mapping and molecular cloning, have provided further evidence for the presence of DNA as genetic material in eukaryotes, including humans.

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(c) Discovery of the double helix structure of DNA

The double helix structure of DNA was proposed by James Watson and Francis Crick in 1953, based on the X-ray diffraction images of DNA fibers taken by Rosalind Franklin and Maurice Wilkins. The images suggested a helical structure with a repeating pattern, and Watson and Crick deduced complementary base-pairing and hydrogen bonds as essential elements of the DNA structure.

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(d) Evidence for complementary base pairing (G with C, A with T)

The evidence for the specific pairing of bases G with C and A with T comes from the work of Erwin Chargaff, known as Chargaff's rules, which states that for a given double-stranded DNA molecule, the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). This implies that A pairs with T, and G pairs with C. These base-pairing rules were incorporated into Watson and Crick's double helix model.

05

(e) Evidence for repetitive DNA sequences in eukaryotes

The existence of repetitive DNA sequences in eukaryotes can be deduced from observations of hybridization experiments that measure reassociation kinetics of single-stranded DNA molecules. The rapid reassociation of certain sequences indicates that they are repeated multiple times within the genome. Additionally, molecular cloning and DNA sequencing have directly revealed the presence of repeated sequences in eukaryotic genomes, including transposons and simple sequence repeats.

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