Biol 103: Introductory Biology I - Lecture 12: DNA Structure and Replication

Introduction to Biology Lecture 12

Course Information
- Course Title: Biol 103: Introductory Biology I
- Lecture Number: 12
- Topic: DNA Structure and Replication
- Instructor: Dr. Michael D. Preston
- Email: michael.preston@unbc.ca
- Office Hours: 12:20-1:00 pm Mon/Wed/Fri or by appointment

Understanding Key Concepts
- Explain how DNA serves as a hereditary molecule.
- Summarize the structure of DNA.
- Outline the process of DNA replication.
- Identify mechanisms involved in DNA repair.
Required Readings
- Chapter 11: DNA Structure, Replication, and Repair

Key Question
- How do we know that DNA (chromosomes) is the source of hereditary material?

F. Griffith’s Experiments (1920s)
- Live S Cells: Injected into mouse - Mouse dies.
- Live R Cells: Injected into mouse - Mouse lives (no live R cells in blood).
- Heat-Killed S Cells: Injected into mouse - Mouse lives (no live S cells in blood).
- Controls/test with Heat-killed S cells + Live R cells:
  - Mouse dies - live S cells found in blood.
- Conclusion: Information required to form virulent strains was transferred to the live R bacterial cells.
- Pathogen involved: Streptococcus pneumoniae
  - S = Smooth capsule
  - R = Rough capsule

Objective: Identify the chemical nature of Griffith's transforming principle.

Procedure: Avery broke down heat-killed S bacteria, destroying either proteins, DNA, or RNA.
Results:
- With proteins or RNA destroyed, transformation still occurred.
- When DNA was destroyed, transformation did not occur.
Conclusion: The transforming principle was concluded to be DNA.

Used radioisotopes to differentiate between proteins and nucleotides.
Results:
- Radio-labeled 35Sulfur (incorporated into proteins) did not insert into bacteria/new viruses.
- Radio-labeled 32Phosphorus (incorporated into nucleotides) was inserted into bacteria and new viruses.
Conclusion: DNA, not protein, is the genetic material.

Overview
- Watson and Crick proposed a model for the structure of DNA in 1953, which became foundational for biological sciences.
- DNA: Deoxyribonucleic acid, forming the genetic material of all living organisms.
Key Contributors
- Maurice Wilkins and Rosalind Franklin used X-ray diffraction to elucidate DNA structure.
- Franklin’s work informed Watson and Crick’s discovery leading to shared Nobel Prize in 1962 (Wilkins and Watson/Cric)**; Franklin died in 1958 due to cancer.

Components
- Deoxyribose Sugar
- Phosphate Group
- Nucleotide Bases:
  - Thymine (T)
  - Cytosine (C)
  - Adenine (A)
  - Guanine (G)
- Base Pairing Rules:
  - [Thymine] = [Adenine]
  - [Cytosine] = [Guanine]
  - Composition: Purines (A, G) and Pyrimidines (C, T)

Structural Features:
- Each base pair is separated by 0.34 nm.
- Each full twist of the DNA double helix measures 3.4 nm.

In analyzing a DNA sample, which result adheres to the base-pairing rules?
1. A = G
2. T + C = A + G
3. A + T = C + G
4. A = C

Parent DNA molecule: Original double-stranded DNA.
Separation of Strands: During replication, the two strands separate.
Result: Each daughter DNA molecule consists of one parental strand and one newly synthesized strand.

Leading Strand:
- Continuously synthesized in the 5’ to 3’ direction.
Lagging Strand:
- Synthesized in short segments called Okazaki fragments, in the opposite direction of the unwinding.

Initiation: Helicase unwinds the DNA at the origin.
Binding Proteins: Single-stranded binding proteins stabilize the unwound strands.
Topoisomerase: Relieves tension ahead of the replication fork by snipping the backbone.
Primase: Synthesizes an RNA primer to initiate new strand.
DNA Polymerase III: Attaches new nucleotides in 5’ to 3’ direction.
Sliding Clamp: Holds DNA polymerase in place.
DNA Polymerase I: Replaces RNA nucleotides with DNA nucleotides on lagging strand.
Ligase: Joins Okazaki fragments together.

Replication starts at multiple sites along the DNA, with both strands replicated simultaneously.

Challenges with DNA Ends:
- DNA polymerase can only add nucleotides to existing strands.
- Primers, which are essential for starting replication, are typically RNA and must be replaced.
- There will be gaps after primer removal at the very ends of DNA.

Structure:
- DNA ends feature repeating sequences known as telomeres (e.g., 5’-TTAGGG-3’).
- Typically, 100-1000 telomere repetitions occur per DNA strand.
Function:
- With each replication, one telomere unit may be lost, allowing for many divisions before affecting the critical DNA sequences.

Functionality:
- Contains an RNA portion with telomeric sequences.
- Adds telomeres back onto the ends of the DNA.
- Inactive in most somatic cells, active in rapidly dividing cells (embryonic and germ cells).
- Often reactivated in cancer cells, enabling unlimited division.

Importance of Repair:
- Errors in DNA copying can potentially lead to mutations.
Proofreading by DNA Polymerase:
- Accidental mismatches are corrected during DNA replication.
Base-Pair Mismatch Repair:
- Following replication, mismatches detected by nucleases that excise the error; DNA polymerase fills in the gaps and ligase seals them.
Example of Damage:
- Thymine dimers caused by UV exposure, relevant to skin cancer risk, with approximately 100 dimers formed per second of UV exposure if uncorrected.

Base-pair mismatch repair results in maintaining very low error rates (1 in a billion base pairs), showcasing the efficiency of DNA repair mechanisms.
Conservation: The repair mechanisms have been conserved through evolutionary history, detectable across a range of organisms including bacteria, yeast, and humans.

Understanding Replication Differences:
- The leading and lagging strands are synthesized differently due to antiparallel structure and the nature of DNA polymerase's activity.
Repair and Lifespan:
- The role of telomeres and telomerase in maintaining chromosome integrity over cell lifespans plays a crucial part in aging and cancer biology.