Cell Cycle and Mitosis Review
Course Introduction & Logistics
Instructor: Dr. Nikki Sharn, Course Coordinator and instructor for cycles 2, 3, 4, and 5. Dr. Beth McDougall Shackleton will teach the remainder of the term.
Seating Request: Occupy middle seats first, then expand outwards; edge seating is acceptable for early departures or frequent washroom breaks.
University Lecture Design Philosophy:
Lectures are designed (with past first-year students and teaching fellows) based on exam statistics and feedback, prioritizing difficult topics.
Slides have minimal text to discourage passive reading and prepare students for advanced studies (graduate school, professional school, conferences/presentations) where active listening and note-taking are crucial.
Support is provided: lecture outcomes, textbook readings, practice questions, and help sessions.
Secret to Success in This Course
Keep up with lectures and material consistently.
Avoid binge-watching lecture recordings before midterms; use recordings to fill in notes, not as a primary learning method.
Complete quizzes promptly after each cycle to stay on track.
Consistent effort reduces stress before midterms, allowing time for Q&A, office hours, and practice midterms (which are photocopies of previous years' exams).
Students who fall behind often panic and lack support for last-minute studying.
Support & Resources
Answering Questions:
Dr. Sharn: For general course-related questions, or anything outside of Dr. McDougall Shackleton's specific cycles/material/quizzes/midterms.
Dr. McDougall Shackleton: Only for scientific questions pertaining to her specific lecture material, quizzes, and midterm questions posted on OWL under her cycle's forum page.
Venona (Skill Development Coordinator): For all lab-related questions.
Forum and Biology Mentorship Discord page: Available, including late-night support before exams.
Office Hours (Dr. Sharn): Thursdays, 1:00 PM - 2:30 PM, North Campus Building (NCB), 3rd Floor, Room 340. Students can attend to listen even without questions.
Biology Mentorship Program:
Next Workshop: Thursday and Friday, 5:30 PM - 7:00 PM, Middlesex College Room 110.
First Study Session: Today (Tuesday), 5:30 PM - 6:30 PM, NCB 114 (down the hall).
Mentors are highly qualified (high grades, articulate, patient, love to teach) and hand-picked. Students are urged to take advantage of these resources early.
Skill Development (Labs)
Attendance: Week 1 students' skill development starts this week (or already started yesterday). Arrive on time and prepared; late arrival is recorded and can affect grades.
Grading Philosophy: Skill development and quizzes are designed to boost grades for students who follow directions.
Assignments:
Read instructions carefully (e.g., pre-lab assignments must be submitted before attending the lab).
Follow specifications (e.g., word count limits).
Submit correctly (e.g., cut and paste paragraphs into the Brightspace box, do not attach documents).
Safety Protocols:
Mandatory: Lab coat, safety glasses, long pants, closed-toe shoes, socks covering ankles.
Failure to comply results in denial of entry, impacting grades.
These rules are for health safety standards; non-compliance can lead to labs being shut down by safety officers.
Accessing Materials:
Go to Module 1, then Module 1 again, for instructions.
Pre-lab assignments are sub-tabs; click the specific assignment to see instructions.
Submit assignments under "Assessments" then "Assignments."
Quizzes
First Quiz Due: This Sunday at 11:00 PM on Brightspace.
Submission Window: Thursday to Sunday.
Confidentiality: Do not post anything about quiz questions on forums or social media. This helps maintain fair class averages and protects academic integrity.
Timed Format: Treat quizzes like a test; sit down and complete them in one sitting. The number of questions and time limits vary weekly.
Auto-Submission: Do not rely on auto-submission. Always hit the submit button before the time runs out. Glitches can occur, potentially leading to a score of zero if answers are not saved upon auto-submission.
Grades/Feedback: Returned on Monday or Tuesday.
Academic Consideration: None for quizzes. A built-in 10\% boost is applied at the end of the term. Missing a quiz results in a zero for that quiz; academic advising will not accept requests for make-ups.
Example: A 76\% quiz average becomes 86\% with the boost.
iClicker: One question per lecture; QR code provided.
The Cell Cycle and Mitosis
Why Study the Cell?
The cell is the fundamental unit of life.
Understanding cellular processes, metabolism, and environmental reactions is crucial for any life sciences career.
Average adult human body contains approximately 30 imes 10^{12} cells, mostly prokaryotic (microbiome), but humans are eukaryotic.
All external stimuli (chemicals, food, sun rays, social interactions) impact your genome and are processed by cells, dictating development and evolution.
Phases of the Cell Cycle
All cells are at some point in the cell cycle. Advanced reading on this topic was expected.
G0 Phase (Quiescent/Dormant Stage):
Cells are not replicating and halt growth.
Some cells, like mature neurons in the brain, function in G0 throughout life and never re-enter the cell cycle.
Stem cells can sit in G0 and enter the cell cycle to differentiate upon signal.
G1 Phase (Growth 1/First Gap):
Main function: Carrying out normal cellular functions, growing in preparation for S phase.
Producing enough energy, proteins, and RNA.
Differentiation (cells becoming different cell types, crucial in embryogenesis) also occurs.
Contains a G1 checkpoint.
S Phase (Synthesis):
DNA synthesis: The genome is doubled.
For humans (starting with 2 copies from parents), the genome duplicates to 4 copies.
Duplication is necessary for mitosis to produce two equal daughter cells (each with 2 copies).
G2 Phase (Growth 2/Second Gap):
Preparatory phase: Cells prepare for M phase.
Ensures the cell is large enough to divide, and has sufficient nutrients and energy (e.g., ATP from mitochondria, needed proteins for division).
Cells generally do not perform their primary function in G2 due to the doubled genome.
Contains a G2/M checkpoint.
M Phase (Mitosis):
Cell division.
Consists of Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis (prerequisite reading).
Example: Fluorescently labeled cell showing chromosomes lined up at the metaphase plate.
Why Cells Divide
Multicellular Organisms: For growth (e.g., embryo development from a single zygote).
Tissue Repair and Regeneration: Stem cells (quiescent in G0) differentiate and divide to replace damaged tissue (e.g., liver regeneration).
Stem cell research: Promising for diseases like Parkinson's, where differentiated stem cells can form dopaminergic neurons and synaptic connections, unlike adult neurons.
Varying Turnover Rates: Different cell types have different lifespans and division rates:
Red/White Blood Cells: Fast turnover (every 1-5 days) from stem cells in bone marrow (hematopoiesis).
Liver Cells: High regenerative capacity (a third of the liver can regrow in 1-2 months).
Lens Cells (eye): No division or regeneration; born with existing cells.
Oocytes (female gametes): No replication; females are born with a fixed number of eggs, arrested in prophase I meiosis during embryogenesis. This correlates with a woman's age and healthy baby outcomes.
The Fundamental Scientific Reason: Surface Area to Volume (SA:V) Ratio.
Analogy: A balloon where volume increases faster than surface area when inflated.
Cellular Context: The cell membrane (surface area) is responsible for nutrient import and waste export, essential for cell growth (volume).
Limitation: As a cell grows, its volume increases faster than its surface area. This causes the SA:V ratio to decrease.
Problem: A low SA:V ratio means the surface area cannot meet the metabolic demands of the growing volume.
Solution: The cell divides to restore a high SA:V ratio, which is optimal for function.
Key points:
High SA:V ratio \implies Small cell.
Low SA:V ratio \implies Large cell.
Avoid overcomplication: Think of fractions (1/2 > 1/4); as the denominator (volume) increases relative to the numerator (surface area), the fraction's value (ratio) decreases.
Cell Cycle Checkpoints: Critical for Regulation
Purpose: Prevent uncontrolled cell division (cancer) and ensure accurate DNA replication and chromosome segregation.
G1-S Checkpoint:
Function: Ensures the cell is ready for DNA replication (S phase).
Key role: Prevents replication of damaged DNA.
Significance: DNA damage, if replicated, leads to mutations that are difficult for cells to fix, potentially causing disease.
G2-M Checkpoint:
Function: Ensures the cell is properly prepared for mitosis (M phase).
Key role: Verifies cell size, nutrient availability, and energy reserves are sufficient for this demanding process.
Mitotic Spindle Checkpoint (Metaphase Checkpoint):
Location: Occurs during metaphase of mitosis.
Function: Ensures that all sister chromatids are properly attached to the mitotic spindle fibers (at the kinetochore).
Consequence of failure: Uneven chromosome separation leads to aneuploidy (cells having an abnormal number of chromosomes).
Example: Trisomy 21 (Down syndrome), where cells have 3 copies of chromosome 21 instead of the normal 2.
Regulation of Cell Cycle Checkpoints
Positive Regulators (Green Light - Proceed)
Cyclins and Cyclin-Dependent Kinases (CDKs):
Both are proteins encoded by separate genes.
They form an active Cyclin-CDK Complex.
CDK has an associated phosphate-donating protein.
Once activated by cyclin, the Cyclin-CDK complex phosphorylates (adds a phosphate group to) specific target proteins.
Phosphorylation activates these target proteins, signaling the cell to proceed through the cell cycle phases.
Negative Regulators / Tumor Suppressor Proteins (Red Light - Stop)
p53 (The Guardian of the Genome):
A key transcription factor and protein.
Function: Senses DNA damage.
Actions upon damage detection:
Halts the cell cycle (G1 arrest): Allows time for DNA repair enzymes.
Activates transcription of p21: A gene whose protein product halts the cell cycle.
Induces Apoptosis: If DNA damage is irreparable, p53 triggers programmed cell death to prevent the propagation of mutations and potential disease (e.g., cancer).
Relevance to Cancer: Cancer cells often target and mutate p53 to enable uncontrolled proliferation (rapid division), as regulating the cell cycle is essential for tumor formation.
p21:
A protein whose transcription is driven by p53 (binds to p21's promoter).
Mechanism: p21 protein binds directly to the active Cyclin-CDK complex.
Effect: This binding inactivates the Cyclin-CDK complex, preventing it from phosphorylating target proteins.
Outcome: Cell cycle progression is halted, acting as a
1. The distinct stages of the cell cycle & Main characteristics of each stage in the cell cycle
G0 Phase (Quiescent/Dormant Stage): Cells are not replicating and halt growth. Examples include mature neurons. Stem cells can enter and exit G0.
G1 Phase (Growth 1/First Gap): Cells carry out normal functions, grow, produce energy, proteins, and RNA in preparation for S phase. Differentiation occurs here.
S Phase (Synthesis): DNA synthesis occurs, doubling the genome (from 2 to 4 copies in humans) in preparation for mitosis.
G2 Phase (Growth 2/Second Gap): Cells prepare for M phase, ensuring sufficient size, nutrients, and energy. Cells generally don't perform primary function due to doubled genome.
M Phase (Mitosis): Cell division, consisting of Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis. Chromosomes line up at the metaphase plate for division.
2. The reason why the G1 and G2 phases exist
G1 Phase: Exists for the cell to perform its normal cellular functions, grow in size, and produce necessary energy, proteins, and RNA needed before DNA replication in S phase. It's also a crucial time for cell differentiation.
G2 Phase: Exists as a preparatory phase for M phase. During G2, the cell ensures it is large enough, has sufficient nutrients, energy (e.g., ATP), and all proteins required for the demanding process of cell division.
3. Location of actively cycling cells in multicellular animals
Stem Cells: Actively divide for tissue repair and regeneration (e.g., liver regeneration).
Bone Marrow: The site of hematopoiesis, where stem cells rapidly divide to produce red and white blood cells (fast turnover every 1-5 days).
Liver Cells: Have a high regenerative capacity, able to regrow significantly within 1-2 months.
Exceptions (non-cycling): Mature neurons, lens cells of the eye, and oocytes (female gametes) generally do not actively cycle after differentiation or birth.
4. Purpose of cell division and risk if cells grow too large in size
Purpose of Cell Division:
Growth: For multicellular organisms, such as embryo development from a single zygote.
Tissue Repair and Regeneration: Stem cells differentiate and divide to replace damaged tissue.
Risk of Cells Growing Too Large: Surface Area to Volume (SA:V) Ratio
As a cell grows, its volume increases faster than its surface area.
This leads to a decrease in the surface area to volume (SA:V) ratio.
A low SA:V ratio means the cell membrane (surface area), responsible for nutrient import and waste export, cannot adequately meet the metabolic demands of the growing cell volume.
The cell must divide to restore a high SA:V ratio, which is optimal for efficient function and nutrient exchange.
5. Main role of cell cycle checkpoints
Primary Role: To prevent uncontrolled cell division (a hallmark of cancer) and ensure the accuracy of DNA replication and proper chromosome segregation during cell division. They act as regulatory mechanisms to pause the cycle if conditions are not met.
6. The difference between positive and negative regulation at cell cycle checkpoints
Positive Regulation (Green Light - Proceed): Involves proteins that, when active, signal the cell to continue through the cell cycle phases. They promote progression.
Negative Regulation (Red Light - Stop): Involves proteins (tumor suppressor proteins) that halt cell cycle progression, especially in response to damage or unfavorable conditions. They inhibit progression.
7. What proteins are involved in positive/negative regulation pathways and how these proteins operate
Positive Regulators:
Cyclins and Cyclin-Dependent Kinases (CDKs):
Cyclins (proteins) bind to CDKs (kinases) to form an active Cyclin-CDK Complex.
This complex then phosphorylates (adds a phosphate group to) specific target proteins.
Phosphorylation activates these target proteins, triggering the cell to proceed to the next phase of the cell cycle.
Negative Regulators (Tumor Suppressor Proteins):
p53 (The Guardian of the Genome):
A transcription factor that senses DNA damage.
Upon damage detection, it can:
Halt the cell cycle at G1: Provides time for DNA repair.
Activate transcription of p21: A gene whose protein product stops the cell cycle.
Induce Apoptosis: If DNA damage is irreparable, p53 triggers programmed cell death.
p21:
A protein whose production is driven by p53.
Binds directly to and inactivates the active Cyclin-CDK complex.
This prevents the phosphorylation of target proteins, thereby halting cell cycle progression.
8. What is the risk of any protein involved at the cell cycle checkpoints not functioning properly (be specific)?
Risk of positive regulators (Cyclin-CDK) overactivity or negative regulators (p53, p21) malfunction:
Uncontrolled Cell Division (Cancer): If checkpoints fail due to non-functional negative regulators (e.g., mutated p53) or uncontrolled positive regulators, cells can divide rapidly without proper checks, leading to tumor formation and proliferation.
Propagation of Damaged DNA: If the G1-S checkpoint fails (e.g., due to non-functional p53/p21), damaged DNA can be replicated, leading to mutations that are difficult to fix and contribute to disease.
Aneuploidy: If the Mitotic Spindle Checkpoint fails (e.g., due to issues with proteins ensuring kinetochore attachment), sister chromatids may segregate unevenly. This results in daughter cells with an abnormal number of chromosomes (aneuploidy), such as Trisomy 21 (Down syndrome), which has 3 copies of chromosome 21.
9. Why p53 is considered the guardian of the genome
p53's role as "Guardian": It actively monitors the integrity of the genome. Upon detecting DNA damage, it employs several crucial mechanisms to protect the cell from propagating mutations:
Pauses the cell cycle: Giving repair enzymes time to fix the damage.
Activates repair genes: By upregulating p21 to halt cell cycle progression.
Initiates Apoptosis: If the damage is too severe to repair, p53 triggers programmed cell death, preventing damaged cells from dividing and potentially becoming cancerous. This prevents the transmission of errors to daughter cells.
10. Examples of situations in which cells would be programmed to die by apoptosis
Irreparable DNA Damage: As triggered by p53, if a cell accumulates DNA damage that cannot be fixed, apoptosis ensures it doesn't become cancerous or pass on harmful mutations.
Infected or Damaged Cells: Cells infected by viruses or displaying significant damage are often eliminated through apoptosis to protect the organism.
11. The stages of mitosis
Mitosis consists of:
Prophase
Metaphase (chromosomes lined up at the metaphase plate)
Anaphase
Telophase
Followed by Cytokinesis (division of the cytoplasm).
12. What the process of mitosis ensures in subsequent generations
Ensures genetically identical daughter cells: Mitosis ensures that when a parent cell divides, it produces two daughter cells that are genetically identical to each other and to the parent cell.
Maintains chromosome number: The DNA duplication in S phase (from 2 to 4 copies) followed by precise segregation during M phase ensures that each daughter cell receives a full and correct set of chromosomes (back to 2 copies), maintaining the species' characteristic chromosome number across generations of cells.
Growth and tissue repair: Through this process, multicellular organisms can grow and repair damaged tissues