Bio1001 Cycle 2

Virus:

• Protein shell (capsid)

• - Obligate parasitethat requires a host cell to replicate and reproduce.

Both:

• Nucleic Acid Genome

• Genes encoding proteins

• Reproduction/Replication

Cellular Life:

• Lipid membrane surrounding cytoplasm

• On the Tree of Life

• Independent metabolism

Zonotic Disease: Disesases that spill over from non-human animals to humans.

        (ex. HIV originated from SIV. SIV spilled over form nonhuman primates (simians)            to humans.)

Antivirual Drugs: Hard to design —- hard to treat viral diseases.

• Viruses use host machinery, so targeting parts of the virus cycle may harm the host cell.

Reverse Transcriptase: Viral enzyme unique to retroviruses.

• Reverse transcribes viral RNA into DNA

• Found in HIV

• Antiviral Drug Target (Ex: AZT)

  •     Nucleoside Analog of Thymine blocks DNA elongation.

HIV Evolution: Mutations are RANDOM

•    Reverse transcriptase enzyme underwent 2 nucleotide changes (mutations)

• Mutant RT has proofreading ability — AZT Resistance

Evolution by Natural Selection:

1. Mutations (ex: AZT resistant HIV)

2. Heritable Variation (ex: AZT resistant HIV, AZT susceptible HIV)

3. Selecting Agent (ex: AZT)

4. Non-random reproduction (survival of the fittest)

5. Change in population genotype

   1. Mutation proposes, selection disposes

HIV Vaccine: Hard to make:

• Viruses replicate quickly

• Viruses have high mutation rate

• Hard to make a vaccine to target all (potential) HIV variants

Evolutionary Mechanisms and Misconceptions

Natural Selection and Evolution

Natural selection and evolution are closely linked processes that drive biological change over time.

• Natural Selection: The process by which certain traits become more common in a population because they provide a survival advantage in a specific environment.

  • If a trait is better suited to the environment, individuals possessing it are more likely to survive long enough to reproduce.

  • This allows them to pass on their allelic frequency to the next generation.

• Evolution: The long-term result of natural selection. As advantageous traits are passed down through successive generations, the overall characteristics of the population change.

Gradualism and Transitional Forms

Evolutionary change does not occur instantaneously; it follows specific patterns of timing and evidence.

Gradualism

Gradualism is the concept that large evolutionary changes are the result of many, many generations of small changes accumulating over time.

• Rejection of Lamarckism: Gradualism is a primary reason why Lamarck's theory of evolution was rejected. Lamarckian evolution suggested changes could happen within a single lifetime or a single generation, whereas true evolution requires many generations.

Transitional Forms

Transitional forms are intermediate stages in the evolution of a species, providing evidence of how a species changed from one form to another.

• Fossil Record: Archaeologists and paleontologists use fossils to observe how bone structures have changed over time to result in modern forms.

• Evidence of Change: These forms represent traits that were common during specific periods in the past as species transitioned through different selective pressures.

Common Misconceptions about Evolution

The lecture clarifies several frequent misunderstandings regarding how evolution actually functions.

Summary Review Question

Question: Which statement is true based on the concept of gradualism and transitional forms as discussed in the lecture of evolution?

• Incorrect: Transitional forms are rarely observed and do not contribute significantly to fossil records.

• Incorrect: Evolutionary changes are quick and observable within a single human lifetime (Note: This may occur in microscopic organisms like bacteria, but not in large-scale observable species).

• Correct Answer: It can take many generations to produce large evolutionary changes, which can be traced through the fossil record.

Cell Cycle and Mitosis

The lecture transitions into the final topic: the cell cycle and the process of mitosis. The primary focus is on how these processes are regulated to ensure proper biological function.

Regulation of the Cell Cycle

The cell cycle is tightly regulated. This regulation is critical because failure to control the cycle leads to cancer, which is defined as uncontrolled cell division.

Key Concepts in Regulation

• Differentiation: The process by which cells become specialized for specific tissues. Without differentiation, the body could not form functional components like blood cells, liver cells, or kidney cells.

• Checkpoints: Surveillance mechanisms that ensure the cell is ready to proceed to the next phase.

Cell Cycle Checkpoints

There are three primary checkpoints discussed to ensure the integrity of cell division:

Importance of Cell Division

Cell division serves two fundamental biological purposes:

1. Multicellular Growth: Necessary for the development and growth of an organism from a young age.

2. Regeneration: Needed to replace cells lost to injury or wear (e.g., healing from a scrape or cut).

Surface Area to Volume Ratio

The lecture emphasizes why organisms are composed of many tiny cells rather than a few large ones, focusing on the surface area to volume ratio.

• High Ratio: Small cells have a high surface area relative to their volume. This is the most effective state for cellular function.

• Low Ratio: Large cells have a massive volume compared to their surface area, making them inefficient.

• Biological Significance:

  • Communication: Small cells can communicate and transport materials more quickly across the organism.

  • Nutrient/Waste Exchange: A high surface area allows for efficient exchange of nutrients and waste products between the cell and its environment. In a massive cell, moving nutrients from one side to the other would be too slow to sustain life.

Mechanisms of Regulation: Positive and Negative Regulators

The cell cycle is controlled by two types of regulatory proteins:

• Positive Regulators: Stimulate the progression of the cell cycle.

• Negative Regulators: Inhibit the cell cycle (often acting as "brakes" to stop division if errors are detected).

Molecular Mechanism of Activation

The lecturer describes the specific biochemical interaction required to activate the cell cycle machinery:

1. Binding: The protein CDK (Cyclin-Dependent Kinase) binds to a protein called Cyclin.

2. Complex Formation: This binding creates a Cyclin-CDK complex.

3. Phosphorylation: An enzyme called Kinase adds a phosphate group to the Cyclin-CDK complex.

4. Activation: The addition of this phosphate group (phosphorylation) activates the complex, allowing the cell cycle to proceed.

Regulation of the Cell Cycle: Positive vs. Negative Control

The cell cycle is controlled by a balance between signals that promote progression and signals that halt it.

Types of Regulators

• Positive Regulators: These act as "accelerators." They signal the cell to "keep going" and proceed through the various phases of the cell cycle.

• Negative Regulators: These act as "brakes." They signal the cell to "stop right now."

Negative Regulation and DNA Damage

Negative regulators are primarily activated when a cell is damaged. When damage is detected, the cell follows one of two scenarios:

1. Repairable Damage: The cell cycle halts to allow time for the cell to fix the damage. Once repaired, the cell can proceed.

2. Irreparable Damage: If the damage is too severe to be fixed, the cell undergoes apoptosis.

Apoptosis

Apoptosis is defined as programmed cell death or "self-destruction." This mechanism ensures that cells with significant genetic errors do not continue to divide and potentially become cancerous.

Key Regulatory Proteins: p53 and p21

The lecturer emphasizes the critical connection between these two proteins and how they interact with the Cyclin-CDK complex.

p53: The "Guardian of the Genome"

• Function: p53 is a transcription factor.

• Role: It acts as a master controller of the cell cycle in response to DNA damage.

• Mechanism: When DNA damage is detected, p53 binds to the promoter of the p21 gene.

p21: The Inhibitor

• Mechanism of Action: Once p53 binds to the p21 promoter, the p21 gene is transcribed and translated into the p21 protein.

• Interaction with Cyclin-CDK: The p21 protein binds directly to the Cyclin-CDK complex.

• Result: p21 acts as an inhibitor. When it binds to the complex, it inactivates it, effectively halting the cell cycle.

Summary of the p53-p21-CDK Pathway

The logical flow of the negative regulation pathway is as follows:

1. DNA Damage occurs $\rightarrow$ 2. p53 is activated $\rightarrow$ 3. p53 binds to p21 promoter $\rightarrow$ 4. p21 is produced $\rightarrow$ 5. p21 binds to Cyclin-CDK $\rightarrow$ 6. Cell Cycle Stops.

Application: Practice Question Analysis

Question: Which of the following can lead to increased cell division?

Detailed Breakdown of Options

• Loss of p53 (Correct Answer): Since p53 is a negative regulator that halts the cycle to fix damage, losing it means there is no way to stop the cell. This leads to uncontrolled cell division, which is a hallmark of cancer.

• Loss of p21: Similar to p53, p21 is a negative regulator. If p21 is lost, the Cyclin-CDK complex cannot be inhibited, leading to increased division.

• Loss of CDK or Cyclin: Because these are positive regulators (essential for progression), losing them will prevent the cell cycle from moving forward, resulting in decreased cell division.

Additional Details on Regulators

• Loss of p21: As a negative regulator, the loss of p21 removes a crucial inhibitory signal, which subsequently leads to increased cell division.

• Loss of Cyclins and CDKs:

  • Cyclins and CDKs function as a pair to drive the cell cycle forward.

  • If either the CDK or the Cyclin is lost, the "engine" of the cell cycle is missing.

  • This results in the cell being unable to move through the cycle, leading to stopped cell movement rather than increased division.

Exam Strategy: Process of Elimination

The professor highlighted a specific logical approach for tackling multiple-choice questions regarding these regulators:

• Certainty-Based Elimination: When you are 100% certain that a specific option (e.g., Option 1) is correct and another (e.g., Option 2) is definitively incorrect, you can use that certainty to narrow down the possible combinations in multiple-choice formats.

• Logical Deduction: Use the known function of the regulator (positive vs. negative) to immediately discard options that suggest the wrong direction of cell division (increase vs. decrease).