Unit 11 - DNA and Its Role in Heredity

Unit 11 - DNA and Its Role in Heredity

Review

• Macromolecules: Identify monomers and polymers, functions/types.

  • Carbohydrates: Energy storage, structure (e.g., cellulose).

  • Lipids: Membrane structure, energy storage, signaling.

  • Proteins: Catalyze reactions, structure, signaling.

  • Nucleic Acids: Store genetic information (DNA, RNA).

• Monomer Drawings: Draw for protein and nucleic acid.

Historical Background

Griffith's Experiment (1928)

• Studied Streptococcus pneumoniae.

  • Smooth Strain: Pathogenic, lethal to mice.

  • Rough Strain: Nonpathogenic, nonlethal to mice.

• Key Observations:

  • Mouse lives with rough strain.

  • Mouse dies with smooth strain.

  • Mouse dies with rough + heat-killed smooth strain.

• Conclusion: Transformation principle - a chemical substance from the dead smooth bacteria transformed the live rough bacteria into a pathogenic form.

Avery, McCarty, and McLeod (1944)

• Built on Griffith’s findings to identify DNA as the transforming agent.

• Conducted experiments using macromolecule treatments (each macromolecule was inactivated in each); transformation only occurred when DNA was present.

• Conclusion: DNA is the genetic material.

Hershey-Chase Experiment (1952)

• Focused on bacteriophages composed of DNA and protein.

• Key Finding: Only DNA enters the bacterial cell.

  • DNA vs. Protein

    • How are DNA and proteins different?

      • DNA contains the elements C, H, O, N and P

      • Proteins contain the elements C, H, O, N and S

    • Scientists could use radioactive elements to trace the movement of both types of molecules

      • ³²P and ³⁵S

• Conclusion: DNA is the genetic information, not protein.

DNA vs. RNA

• Structure:

  • DNA: Double-stranded helix. 

  • RNA: Single-stranded.

• Sugar:

  • DNA: Contains deoxyribose sugar. 

  • RNA: Contains ribose sugar.

• Bases:

  • DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). 

  • RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

• Function:

  • DNA: Stores genetic information. 

  • RNA: Converts genetic information from DNA into proteins.

• Location:

  • DNA: Found in the nucleus and mitochondria. 

  • RNA: Found in the nucleus, cytoplasm, and ribosomes

Structure of DNA

Basic Composition

• DNA nucleotides consist of:

  • Deoxyribose sugar

  • Phosphate group

  • Nitrogenous bases: Purines, two ring structure (A, G), Pyrimidines, single ring structure (C, T)

• Chargaff's Rule: purines = pyrimidines

  • Amount of A = Amount of T

  • Amount of C = Amount of G

DNA Structure

• DNA is a double helix formed by:

  • Antiparallel strands

    • The 5’ end of each strand has the free phosphate group.

    • The 3’ end of each strand has the free hydroxyl

  • Complementary base pairing (A with T, C with G)

  • Phosphodiester bonds in the sugar-phosphate backbone

  • DNA has four important features—double-helical structure is essential:

    1. Storage of genetic information—millions of nucleotides; base sequence encodes huge amounts of information

    2. Precise replication during cell division by complementary base pairing

    3. Expression of the coded information as the phenotype—nucleotide sequence is transcribed into RNA and determines sequence of amino acids in proteins

    4. Susceptibility to mutations—a change in information—possibly a simple alteration to a sequence

• Stability: Three hydrogen bonds between C and G; two between A and T.

DNA Replication

• Semiconservative Model: Each new DNA molecule consists of one original (parental) strand and one new (daughter) strand.

• Key Enzymes:

  • Helicase: Unzips DNA helix.

  • Primase: Synthesizes RNA primer.

  • DNA Polymerase: Adds nucleotides and proofreads.

  • Ligase: Joins Okazaki fragments in lagging strand.

• Replication Process:

  • DNA unwinds (helicase).

  • Template strands are exposed for new base pairing.

  • New nucleotides are added in the 5’ to 3’ direction.

• Phosphodiester bonds

  • Deoxyribonucleoside triphosphates (dNTPs), or deoxyribonucleotides, are the building blocks—two of their phosphate groups are released and the third bonds to the 3′ end of the DNA chain.

Difference Between Leading and Lagging Strand

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the fork.

Telomeres and Replication Problems

• End Replication Problem: Linear chromosomes shorten with each replication because RNA primers cannot be replaced at the end.

• Telomeres: Repetitive sequences at the ends of chromosomes to protect genetic data (TTAGGG).

• Telomerase: Enzyme that extends telomeres in certain cells.

Mutations

Definition and Types

• Mutations: Changes in nucleotide sequences passed to offspring.

  • Types by Cell: Somatic vs. Germline (affects future generations).

  • Types by Effect: Silent, coding, loss/gain of function

  • Size of Mutation: Point mutation (single nucleotide change), Chromosomal mutation (larger structural changes).

Causes of Mutations

• Spontaneous: Errors during DNA replication, natural changes.

• Induced: Environmental factors (e.g., radiation, chemicals).

Repair Mechanisms

• Proofreading: DNA polymerase checks and corrects errors during replication.

• Mismatch Repair: Enzymatic correction of mismatched bases after replication.

Conclusion

Summary of DNA Function

1. Information Storage: Encodes genetic information.

2. Replication: Ensures fidelity during cell division.

3. Expression: Directs synthesis of proteins via RNA.

4. Mutability: Allows for evolution and adaptation through variations in the genetic code.