Question

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?

Short answer

Answer: The Griffith's experiment and the Hershey-Chase experiment were crucial in determining DNA as the genetic material. Griffith's experiment (1928) demonstrated transformation in bacteria, where DNA from one strain could transfer its properties to another strain. The Hershey-Chase experiment (1952) used bacteriophages with radioactive labeling to show that DNA, not protein, is responsible for transferring genetic information during infection.

Step-by-step solution

(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.

(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.

(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.

(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.

Key concepts

Hershey-Chase Experiment

The Hershey-Chase experiment, conducted in 1952 by Alfred Hershey and Martha Chase, played a crucial role in confirming that DNA is the genetic material. In their experiment, they used bacteriophages, which are viruses that specifically infect bacteria. To differentiate between DNA and protein, Hershey and Chase used radioactive isotopes to label the phage DNA with phosphorus-32 and the protein with sulfur-35.

• They allowed the labeled phages to infect bacteria, transferring their genetic material.
• After infection, they blended the samples to separate the phage coats from the bacterial cells.
• Analysis showed that only the phosphorus isotope entered the bacterial cells, indicating that DNA was injected and used to reprogram the host.

The experiment decisively proved that DNA, rather than protein, is the material responsible for genetic inheritance in these organisms. It was a landmark finding that paved the way for further DNA research.

Watson-Crick Model

The discovery of the DNA double-helix structure by James Watson and Francis Crick in 1953 revolutionized our understanding of molecular biology. Their model revealed that DNA is composed of two strands forming a right-handed double helix. This model was partly based on Rosalind Franklin and Maurice Wilkins's X-ray diffraction data.

• The X-ray images suggested a helical structure with a repetitive and orderly pattern.
• Erwin Chargaff's rules indicated that adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
• Watson and Crick proposed a model where these base pairs are held together by hydrogen bonds.

The Watson-Crick model of DNA challenged previous assumptions and provided insight into how genetic information is stored, replicated, and transferred. The complementary base-pairing ensured that DNA could be copied accurately during cell division.

Complementary Base Pairing

Complementary base pairing is a fundamental principle of DNA structure which ensures the accurate replication and transcription of genetic information. The concept was derived from Chargaff's rules which indicated that DNA from any organism has equal amounts of adenine and thymine, and equal amounts of cytosine and guanine.

• Adenine (A) forms two hydrogen bonds with thymine (T), while cytosine (C) forms three hydrogen bonds with guanine (G).
• These pairings maintain a consistent width of the DNA double helix, facilitating a uniform and stable structure.
• Complementary base pairing is crucial for DNA replication as each strand serves as a template for the new strand.

Through this mechanism, genetic fidelity is maintained, and cells can pass on identical genetic material to their progeny. This process is vital for growth, repair, and reproduction in living organisms.

Repetitive DNA Sequences

In eukaryotic genomes, repetitive DNA sequences are a significant component and contribute to a variety of biological functions. These are sequences present in multiple copies and can vary widely in length and composition.

• Repetitive sequences are classified into categories such as satellite DNA, minisatellites, and microsatellites.
• Techniques like DNA sequencing have helped identify repetitive sequences in different species.
• These sequences can contribute to chromosomal architecture, play roles in gene regulation, or act as a substrate for evolutionary processes.

The presence of repetitive DNA is crucial for genome organization and stability. They might participate in essential cellular processes like replication, repair, and recombination, revealing the intricate complexity inherent in the genomic landscapes of eukaryotes.