Question

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 T as complementary base pairs are formed?

Short answer

#tag_title# Question Based on the discoveries and experiments that led to our understanding of DNA, explain how DNA was determined to be the genetic material in bacteria and bacteriophages, as well as eukaryotes. Additionally, describe how the structure of DNA as a double helix was determined, and how we know that G pairs with C and A pairs with T in DNA.

Step-by-step solution

(a) Determination of DNA as the genetic material in bacteria and bacteriophages

The discovery that DNA serves as the genetic material in bacteria and bacteriophages came from a series of experiments and observations. The most important experiment was the Hershey-Chase experiment, which showed that DNA is the substance that enters the host bacterial cells during bacteriophage infection and provides the genetic information necessary for the production of new bacteriophages. The Hershey-Chase experiment used radioactive isotopes to label DNA and proteins in bacteriophages and showed that only the radioactive DNA was found inside the bacterial cells, proving that it is the genetic material.

(b) Determination of DNA as the genetic material in eukaryotes

The discovery that DNA also serves as the genetic material in eukaryotes, such as humans, came from several lines of evidence, including the presence of DNA in the nuclei of eukaryotic cells, the observation that DNA is replicated during cell division, and that DNA mutations can lead to inheritable changes in an organism's traits. Additionally, experiments involving the transformation of eukaryotic cells with foreign DNA provided further evidence that DNA is responsible for carrying the genetic information in eukaryotes.

(c) Discovery of DNA's double helix structure

The double helix structure of DNA was determined by James Watson and Francis Crick, who used several pieces of information to build their model. One of the most important pieces of information was the X-ray diffraction data obtained by Rosalind Franklin, which suggested that DNA has a helical shape. Watson and Crick also used the knowledge of DNA's chemical composition, including the presence of four nitrogenous bases (adenine, guanine, cytosine, and thymine), the sugar-deoxyribose, and a phosphate group. By combining this information with the concept of base pairing and Chargaff's rules, they were able to propose the double helix structure with antiparallel strands and complementary base pairing, which explained how DNA can store genetic information and be replicated.

(d) Complementary base pairing in DNA

The idea that guanine (G) pairs with cytosine (C) and adenine (A) pairs with thymine (T) in DNA came from Erwin Chargaff's observation of base ratios in different organisms. Chargaff found that the amount of A was always equal to the amount of T, and the amount of G was always equal to the amount of C. Watson and Crick used this observation to propose that in the double helix structure, A always pairs with T, and G always pairs with C, forming complementary base pairs that are held together by hydrogen bonds. This base pairing provides a mechanism for DNA replication, as each strand can serve as a template for the synthesis of a new complementary strand.

Key concepts

Hershey-Chase Experiment

The Hershey-Chase experiment stands as a cornerstone in molecular biology, marking the moment when DNA was confirmed as the carrier of genetic information in organisms. Before this revelation, proteins were the prime suspects due to their complexity and abundance in cells. In 1952, Alfred Hershey and Martha Chase performed experiments on bacteriophages, which are viruses that infect bacteria.

By cleverly using radioactive isotopes to tag DNA and proteins of bacteriophages (with phosphorus-32 for DNA and sulfur-35 for proteins), they could track which of these molecules entered the bacterial cells upon infection. Their results strikingly indicated that only the radiolabeled DNA, not protein, made its way into the bacterial cells. This concrete evidence placed DNA in the spotlight as the substance carrying genetic blueprints, a discovery essential for understanding heredity and the functioning of all living organisms.

Double Helix Structure

The recognition of DNA's double helix structure represents another monumental moment in science, brought to light by James Watson and Francis Crick in 1953. This structure is elegantly simple yet incredibly intricate, having two intertwined strands that form the helical shape of DNA. Imagine a twisted ladder, with sugar-phosphate backbones as the side supports and the base pairs as the rungs.

Watson and Crick built on previous work, including Rosalind Franklin's critical X-ray diffraction images that hinted at DNA's helical form, as well as the known compositions of DNA's bases, sugar, and phosphate groups. The double helix is not only aesthetically pleasing but functional, with its structure enabling the reliable storage and transfer of genetic instructions across generations.

Complementary Base Pairing

At the heart of DNA's structure are the rules of complementary base pairing, a system ensuring that DNA strands are properly aligned and that the genetic code is accurately copied. DNA contains four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Complementary base pairing refers to the consistent attraction between specific bases: adenine pairs with thymine, and cytosine pairs with guanine.

The ability of these bases to form hydrogen bonds with their complements (A with T and C with G) is pivotal. The rules dictate that the two strands of the double helix are complementary to each other. So, for every adenine in one strand, there is a thymine across from it on the other strand, and for every cytosine, a guanine. This meticulous pairing is the key to the reliability of DNA replication and central to the transmission of genetic information.

Chargaff's Rules

Erwin Chargaff's discovery, known as Chargaff's Rules, provided the critical piece of the puzzle that led to the understanding of DNA's double helix structure and its complementary base pairing. In the early 1950s, Chargaff found a regularity in the ratios of the four nucleotide bases of DNA: the amount of adenine equals that of thymine, and the amount of guanine equals that of cytosine. This rule applied irrespective of the species from which DNA was extracted, suggesting a biological universality.

Chargaff's Rules helped confirm the base pairing relationships that were crucial for Watson and Crick's double helix model. They indicated how nucleotide bases interact with one another and hinted at the mechanism by which DNA could be faithfully replicated. Chargaff's empirical observations advanced the understanding of DNA's function as the hereditary material, solidifying the foundation for modern genetics.