Discovering Dna Structure Worksheet Answer Key

Discovering DNA Structure Worksheet Answer Key: A Comprehensive Guide

Unlocking the secrets of DNA's structure is a pivotal moment in the history of biology. This worksheet explores the journey of discovery, from early experiments to the groundbreaking double helix model. This comprehensive guide provides detailed answers and explanations, enriching your understanding of this fundamental molecule of life.

Section 1: Early Experiments and Discoveries

1.1 Griffith's Transforming Principle:

• Question: Describe Griffith's experiment and its significance. What did he conclude?
• Answer: Griffith's experiment involved injecting mice with different strains of Streptococcus pneumoniae. The smooth (S) strain, possessing a polysaccharide capsule, was virulent and killed the mice. The rough (R) strain, lacking the capsule, was non-virulent. He found that heat-killed S strain bacteria, when mixed with live R strain bacteria, still killed the mice. This indicated that a "transforming principle" from the heat-killed S strain transformed the harmless R strain into a virulent form. This experiment suggested that genetic information could be transferred between bacteria. The significance lies in demonstrating that genetic material could be passed on, paving the way for future investigations into the nature of that material.

1.2 Avery, MacLeod, and McCarty's Experiment:

• Question: How did Avery, MacLeod, and McCarty build upon Griffith's work? What did they discover?
• Answer: Avery, MacLeod, and McCarty aimed to identify Griffith's "transforming principle." They purified different components (proteins, DNA, RNA, etc.) from the heat-killed S strain and tested each for its ability to transform R strain bacteria. They found that only DNA was capable of causing this transformation. This provided strong evidence that DNA, not protein, is the genetic material. This landmark experiment significantly advanced the understanding of genetics.

1.3 Hershey-Chase Experiment:

• Question: Describe the Hershey-Chase experiment and how it confirmed DNA as the genetic material.
• Answer: Hershey and Chase used bacteriophages (viruses that infect bacteria) to definitively prove that DNA is the genetic material. They labeled the phage DNA with radioactive phosphorus (³²P) and the phage protein coat with radioactive sulfur (³⁵S). After allowing the phages to infect bacteria, they separated the phage ghosts (empty protein coats) from the infected bacteria. They found that the radioactive phosphorus (³²P) was inside the bacteria, while the radioactive sulfur (³⁵S) remained outside. This demonstrated that DNA, not protein, entered the bacteria and carried the genetic information needed for phage replication. This provided conclusive evidence that DNA is the genetic material.

Section 2: The Structure of DNA: Clues from Chemistry and X-ray Diffraction

2.1 Chargaff's Rules:

• Question: State Chargaff's rules and explain their significance in understanding DNA structure.
• Answer: Chargaff's rules state that in DNA:
  • The amount of adenine (A) always equals the amount of thymine (T).
  • The amount of guanine (G) always equals the amount of cytosine (C). These rules suggested a pairing mechanism between the bases, crucial for understanding the double helix structure. They indicated a specific pairing pattern A-T and G-C, hinting at a complementary structure.

2.2 Rosalind Franklin's X-ray Diffraction Images:

• Question: What information did Rosalind Franklin's X-ray diffraction images provide about DNA's structure?
• Answer: Franklin's X-ray diffraction images of DNA revealed several key features:
  • The DNA molecule is helical (spiral-shaped).
  • The helix has a diameter of approximately 2 nanometers.
  • The helix has a repeating structure with a spacing of 3.4 nanometers along its axis.
  • The DNA molecule has a regular pattern of subunits. This crucial information was instrumental in Watson and Crick's deduction of the double helix model.

Section 3: The Watson-Crick Model of DNA

3.1 The Double Helix:

• Question: Describe the Watson-Crick model of DNA structure. Include details about the components and their arrangement.
• Answer: The Watson-Crick model describes DNA as a double helix, resembling a twisted ladder. The sides of the ladder are made up of alternating sugar (deoxyribose) and phosphate molecules, forming the sugar-phosphate backbone. The rungs of the ladder are formed by pairs of nitrogenous bases: adenine (A) pairing with thymine (T) via two hydrogen bonds, and guanine (G) pairing with cytosine (C) via three hydrogen bonds. The two strands are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5'). The double helix is stabilized by hydrogen bonds between base pairs and hydrophobic interactions between stacked bases.

3.2 Base Pairing and Complementarity:

• Question: Explain the concept of base pairing and its importance in DNA replication and information storage.
• Answer: Base pairing refers to the specific pairing of A with T and G with C. This complementarity is crucial for DNA replication because each strand can serve as a template for the synthesis of a new complementary strand. It also allows for the accurate transmission of genetic information from one generation to the next. The sequence of bases along a DNA strand determines the genetic code, making base pairing essential for storing and transmitting genetic information.

3.3 Antiparallel Strands:

• Question: Why is the antiparallel orientation of the DNA strands important?
• Answer: The antiparallel orientation (5' to 3' and 3' to 5') is essential for DNA replication and stability. The antiparallel arrangement allows for the proper alignment of bases during replication and ensures that the hydrogen bonds between base pairs are optimally positioned for stability. The enzymes involved in DNA replication work in specific directions along the strands; the antiparallel nature accommodates this process.

3.4 Major and Minor Grooves:

• Question: What are the major and minor grooves in the DNA double helix, and what is their significance?
• Answer: The double helix has two grooves, a major groove and a minor groove, resulting from the unequal spacing of the sugar-phosphate backbone. These grooves are important because proteins that interact with DNA (such as transcription factors) often recognize specific base sequences by binding to the major or minor groove. The different widths and chemical environments of the grooves allow for specific protein-DNA interactions.

Section 4: DNA Replication and the Significance of the Double Helix Structure

4.1 Semi-conservative Replication:

• Question: Explain the semi-conservative model of DNA replication.
• Answer: The semi-conservative model of DNA replication proposes that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This means that during replication, the double helix unwinds, and each strand serves as a template for the synthesis of a new complementary strand. The resulting two DNA molecules each contain one strand from the parent molecule and one newly synthesized strand.

4.2 Enzymes Involved in Replication:

• Question: Briefly describe the roles of key enzymes involved in DNA replication (e.g., helicase, DNA polymerase, ligase).
• Answer:
  • Helicase: Unwinds the DNA double helix.
  • DNA polymerase: Synthesizes new DNA strands by adding nucleotides to the 3' end of the growing strand. It also proofreads the newly synthesized strand.
  • Ligase: Joins Okazaki fragments (short DNA fragments synthesized on the lagging strand) together. These enzymes work in a coordinated manner to ensure accurate and efficient replication.

Section 5: Beyond the Double Helix: Variations and Considerations

5.1 DNA Supercoiling:

• Question: What is DNA supercoiling, and why is it important?
• Answer: DNA supercoiling refers to the further twisting of the DNA double helix upon itself. It's important because it allows for the compact packaging of DNA within the cell's nucleus. Supercoiling also influences the accessibility of DNA to enzymes involved in replication and transcription. The level of supercoiling is regulated by enzymes called topoisomerases.

5.2 Histones and Chromatin Structure:

• Question: How do histones contribute to the organization of DNA in eukaryotic cells?
• Answer: Histones are proteins that play a crucial role in organizing DNA into chromatin. DNA wraps around histone octamers (clusters of eight histone proteins), forming nucleosomes. Nucleosomes are further packaged into higher-order structures, ultimately condensing the DNA into chromosomes. This packaging allows for the efficient storage and regulation of the vast amount of DNA in eukaryotic cells.

5.3 Non-B DNA Forms:

• Question: Briefly discuss the existence of alternative DNA structures, such as Z-DNA.
• Answer: While the B-form DNA double helix is the most common structure, alternative forms exist, such as Z-DNA. Z-DNA is a left-handed double helix with a zigzag appearance. The significance of these alternative forms is still being investigated, but they are thought to play roles in gene regulation and other cellular processes.

This comprehensive guide provides detailed answers and explanations related to discovering DNA's structure. This understanding underpins our modern approach to genetic research, medicine, and biotechnology. Further research and exploration of these topics will continuously enrich our understanding of this fundamental molecule of life. Remember to consult your textbook and other reliable sources for further information.