In this chapter, we have focused on DNA, the molecule that stores genetic information in all living things. In particular, we discussed its structure and delved into how we analyze this molecule. Based on your knowledge of these topics, answer several fundamental questions: (a) How were we able to determine that DNA, and not some other molecule, serves as the genetic material in bacteria, bacteriophages, and eukaryotes? (b) How do we know that the structure of DNA is in the form of a right-handed double-helical molecule? (c) How do we know that in DNA G pairs with C and that A pairs with T as complementary strands are formed? (d) How do we know that repetitive DNA sequences exist in eukaryotes?

Step by step solution

01

(a) DNA as the genetic material

To determine that DNA, and not some other molecule, serves as the genetic material in bacteria, bacteriophages, and eukaryotes, several experiments were performed. Two major experiments include the Griffith's experiment and the Hershey-Chase experiment. Griffith's experiment (1928) established the concept of transformation in bacteria, where DNA from one strain of bacteria was transferred to another strain, changing its characteristics. The Hershey-Chase experiment (1952) used bacteriophages (viruses that infect bacteria) and radioactive labeling to demonstrate that DNA, and not protein, is the genetic material being passed on during infection.

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(b) Structure of DNA as a right-handed double-helix

The structure of DNA as a right-handed double-helical molecule was discovered by James Watson and Francis Crick in 1953, based primarily on the X-ray diffraction images of DNA taken by Rosalind Franklin and Maurice Wilkins. The X-ray diffraction images show a cross-shaped pattern, indicating a helical structure, and additional data from Erwin Chargaff's experiments suggest base-pairing of A with T and C with G. Watson and Crick modeled DNA with a double-helix structure, with the sugar-phosphate backbone on the outside and the bases paired on the inside, thereby resulting in a stable, right-handed double-helix structure.

03

(c) Complementary base-pairing in DNA

The knowledge that G pairs with C, and A pairs with T, in DNA was derived from Erwin Chargaff's experiments, known as Chargaff's rules. Chargaff determined that the amount of adenine (A) was nearly equal to thymine (T) and the amount of cytosine (C) was nearly equal to guanine (G) in any given DNA molecule. Watson and Crick, while working on their DNA model, discovered that the base pairings of A with T, and G with C, could establish hydrogen bonding while maintaining the uniform width of the double helix. Thus, complementary base-pairing was confirmed.

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(d) Repetitive DNA sequences in eukaryotes

The existence of repetitive DNA sequences in eukaryotes was discovered through the study of eukaryotic genome organization and comparison of DNA sequences. Advances in molecular biology techniques, such as DNA sequencing, allowed researchers to discover that some regions of the eukaryotic genome contain repetitive sequences of DNA (such as satellite DNA, minisatellites, and microsatellites). These sequences are common in various species and can have various functions, such as chromosome structure maintenance, gene regulation, and providing raw material for molecular evolution.

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