Chapter 7A

Chapter 7A: DNA Structure and Gene Function

Learning Outcomes

  • Cells and DNA: Understand how cells utilize DNA to produce proteins.

  • Evidence for DNA: Explain the evidence that DNA serves as genetic material.

  • Components of DNA: Identify the basic components of double-stranded DNA.

  • Complementary Strands: Create a complementary DNA strand using a strand of nucleotide sequence.

  • Hydrogen Bonds: Describe how hydrogen bonds contribute to DNA's structure.

  • Central Dogma: Explore the roles of DNA, RNA, and proteins in genetic expression.

  • Transcription Participants: Describe the participants and events involved in transcription.

  • mRNA Creation: Use DNA nucleotides to create a complementary mRNA strand.

  • Post-Transcription Processing: Explain how mRNA is processed in eukaryotic cells.

  • Translation Process: Describe the participants and events in the translation process.

  • Genetic Code Mapping: Utilize the genetic code to translate nucleic acid sequences into polypeptides.

  • Codon-Amino Acid Relationship: Understand the relationship between codons and amino acids.

  • Protein Folding: Elucidate why errors in polypeptide folding result in nonfunctional proteins.

What is DNA?

  • Definition: DNA is a nucleic acid molecule composed of nucleotides.

  • Function: DNA contains instructions for protein synthesis.

Key Experiments Leading to the Discovery of DNA

Frederick Griffith (1930s)

  • Experiments with Mice: Investigated the effects of different bacterial strains.

    • Live Rough strain: Mice lived.

    • Live Smooth strain: Mice died.

    • Killed Smooth strain: Mice lived.

    • Killed Smooth and live Rough: Mice died, indicating transformation by live Smooth bacteria.

Avery, MacLeod, and McCarty (1940)

  • Isolation of Transforming Principle: Separated the components of lethal S cells.

    • DNA was the only substance that transformed R cells to S cells.

  • Conclusion: DNA is the transforming principle.

Evidence of DNA as Genetic Material

  • Hershey and Chase: Demonstrated that DNA, not protein, carries genetic information using viruses that infect bacteria.

Structure of DNA

Components of DNA Nucleotides

  • Consists of:

    • Phosphate group

    • Deoxyribose sugar

    • Nitrogenous base (Adenine, Guanine, Cytosine, Thymine)

Nucleotide Structure and Arrangement

  • Nucleotide Sequence: The linear order of nitrogenous bases is crucial for genetic information.

  • Double Helix Formation: DNA consists of two strands of nucleotides coiling into a helix.

Base Pairing and Hydrogen Bonds

  • Complementary Base Pairing: Hydrogen bonds form between specific bases.

    • Adenine pairs with Thymine

    • Guanine pairs with Cytosine

  • Chargaff’s Rule: %A = %T and %G = %C.

Antiparallel Orientation

  • Strands Orientation: DNA strands run in opposite directions, with 5-prime and 3-prime ends.

Gene Function and Protein Production

Definition of a Gene

  • Gene: A segment of DNA coding for a specific protein.

RNA's Role in Protein Production

Stages of Protein Synthesis

  • Transcription: RNA synthesis from the DNA template occurs in the nucleus.

  • Translation: Protein synthesis happening in cytoplasm involving ribosomes.

Transcription Process

  1. Initiation: RNA polymerase binds to the promoter region.

  2. Elongation: RNA polymerase synthesizes RNA from the DNA template.

  3. Termination: RNA polymerase reaches a terminator sequence, completing RNA synthesis.

RNA Processing in Eukaryotic Cells

  • Post-Transcription Modifications:

    • Addition of a 5' cap.

    • Addition of a poly-A tail to the 3' end.

    • Removal of introns, splicing exons together.

Translation Process

  1. Initiation: Small ribosome unit binds to mRNA; initiator tRNA binds to start codon.

  2. Elongation: tRNAs bring amino acids, forming peptide bonds.

  3. Termination: Release factors bind to stop codons, completing the protein synthesis.

Molecular Adaptors in Translation

  • mRNA: Carries genetic information from DNA.

  • tRNA: Brings amino acids and matches them to mRNA codons.

  • Ribosomes: Site of protein synthesis with rRNA and proteins supporting its structure.

Genetic Code and Its Usage

  • Codons: Three-nucleotide sequences coding for specific amino acids.

  • Translation Efficiency: Multiple ribosomes can translate a single mRNA, allowing rapid protein production.

Key Queries and Further Reflection

  • Comparison of DNA vs. RNA specifications.

  • Understanding transcription, critical steps, and translation processes.

  • Functions of different RNA types involved in protein synthesis.