Cell Cycle and Its Regulation: Study Notes for General Biology

Cell Cycle Overview

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It is essential for growth, development, and tissue repair in multicellular organisms. The cell cycle consists of interphase (G1, S, G2 phases) and the mitotic (M) phase.

• G1 Phase (First Growth Phase): Cell grows and carries out normal metabolic processes.

• S Phase (Synthesis Phase): DNA replication occurs, doubling the genetic material.

• G2 Phase (Second Growth Phase): Cell prepares for mitosis by growing and producing proteins necessary for cell division.

• M Phase (Mitotic Phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Example: Skin cells regularly undergo the cell cycle to replace dead or damaged cells.

S Phase: DNA Replication

Mechanism and Importance

During the S phase, each chromosome is duplicated to ensure that each daughter cell receives an identical set of genetic information. DNA replication is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Key Enzyme: DNA polymerase synthesizes new DNA strands.

• Replication Origin: Specific sequences where replication begins.

• Proofreading: DNA polymerase can correct errors during replication.

Equation:

$\text{DNA}_{\text{parent}} \xrightarrow{\text{Replication}} \text{DNA}_{\text{daughter}}$

M Phase: Mitosis

Stages of Mitosis

Mitosis is the process by which a cell divides its nucleus and distributes its chromosomes into two identical daughter nuclei. It consists of several stages:

• Prophase: Chromosomes condense, spindle apparatus forms.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform around the chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Example: Mitosis is crucial for wound healing and tissue regeneration.

Cell Cycle Control

Regulation and Checkpoints

Cell division is tightly regulated to ensure genomic integrity and proper cell function. Checkpoints are control mechanisms that verify whether the processes at each phase of the cell cycle have been accurately completed before progression to the next phase.

• G1 Checkpoint: Checks for cell size, nutrients, growth signals, and DNA damage.

• G2 Checkpoint: Ensures DNA has been replicated correctly and is undamaged.

• Metaphase (M) Checkpoint: Verifies spindle attachment and chromosome segregation.

Why Regulation is Important:

• Ensures enough resources for cell division.

• Prevents propagation of mutations, reducing cancer risk.

Cell Cycle Checkpoints

Detailed Checkpoint Functions

Each checkpoint serves a specific function to maintain cellular and genetic integrity.

Checkpoint	Criteria for Passing
G1 Checkpoint	Cell size adequate, nutrients sufficient, growth signals present, DNA undamaged
G2 Checkpoint	DNA successfully replicated, DNA undamaged
Metaphase Checkpoint	Chromosomes attached to spindle, chromosomes properly segregated, MPF is absent

Additional info: G0 phase is a resting state where cells exit the cycle and perform specialized functions.

Cell Signaling and Cyclins

Role of Cyclins and CDKs

Cyclins are proteins whose levels fluctuate throughout the cell cycle, activating cyclin-dependent kinases (CDKs) to drive cell cycle progression. CDKs are always present but require cyclins for activation.

• G1 Cyclins: Promote progression through G1 checkpoint.

• MPF (M-phase Promoting Factor): Cyclin-CDK complex that initiates mitosis.

Equation:

$\text{MPF} = \text{Cyclin} + \text{CDK}$

Example: Cyclin levels rise and fall, controlling the timing of cell cycle events.

DNA Damage and Repair

Mechanisms and Importance

DNA damage can result from mutagens such as UV radiation and chemicals. Cells have repair mechanisms to fix this damage and prevent mutations from being passed to daughter cells.

• P53 Protein: Signals cell cycle arrest if DNA damage is detected, allowing for repair or triggering apoptosis.

• Nucleotide Excision Repair: Removes and replaces damaged DNA segments.

Example: Elephants have multiple copies of the p53 gene, contributing to their lower cancer rates despite having many cells.

Telomeres and Cell Aging

Function and Significance

Telomeres are repetitive DNA sequences at chromosome ends that protect coding DNA during replication. Each cell division shortens telomeres, eventually leading to cell cycle arrest when they become too short.

• Sequence: TTAGGG repeats in humans.

• Buffer Function: Prevents loss of essential genes during DNA replication.

• Telomerase: Enzyme that extends telomeres, active in stem cells and cancer cells.

Equation:

$\text{Telomerase} + \text{Telomere} \rightarrow \text{Extended Telomere}$

Additional info: Telomere shortening rate can predict species lifespan.

Cancer and the Cell Cycle

Uncontrolled Cell Division

Cancer is a disease characterized by uncontrolled cell division due to failures in cell cycle regulation. Mutations in genes controlling the cell cycle, such as p53, can lead to cancer.

• Common Behaviors: Increased cell division, evasion of cell death, ability to invade other tissues (metastasis).

• Types: Lung, colon, breast, pancreatic, etc., varying in lethality and frequency.

• Mutations: Often accumulate in genes regulating cell cycle and DNA repair.

Example: Malignant tumor cells can move and invade other tissues, unlike benign tumors.

Treating Cancer

Therapeutic Strategies

Cancer treatments often target cell division and DNA synthesis to prevent tumor growth.

• DNA Synthesis Inhibitors: Drugs like norfloxacin and ciprofloxacin inhibit DNA polymerase, stopping cell division.

• Mitotic Inhibitors: Drugs that stabilize or disrupt microtubules, preventing proper chromosome segregation.

Additional info: These treatments can affect normal dividing cells, leading to side effects.