Rucete ✏ AP Biology In a Nutshell
9. The Cell Cycle
This chapter explains the stages of the cell cycle, how cell division is regulated, and how its malfunction can lead to cancer. It also covers the roles of oncogenes, tumor suppressor genes, and apoptosis in maintaining healthy tissue growth and development.
Overview
• The cell cycle enables growth, repair, and reproduction of cells.
• Proper regulation ensures division occurs at the right time and prevents tumors.
Phases of the Cell Cycle
• The cycle includes: – Interphase (G1, S, G2) – M phase (mitosis) – Cytokinesis
• Nondividing cells may exit the cycle into G0 phase.
Interphase
• Longest part of the cycle; prepares the cell for division.
– G1 phase: cell grows and duplicates organelles like centrioles.
– S phase: DNA is replicated; each chromosome now has two identical chromatids joined by a centromere.
– G2 phase: cell grows again and synthesizes proteins for mitosis (e.g., spindle fibers).
Mitosis (M phase)
• Ensures accurate distribution of identical genetic material to two daughter cells.
• Four stages:
– Prophase: • Nuclear membrane dissolves • Chromosomes condense • Spindle fibers form
– Metaphase: • Chromosomes align at the cell’s center (metaphase plate) • Spindle fibers attach to centromeres
– Anaphase: • Sister chromatids separate at centromeres • Chromatids (now individual chromosomes) move to opposite poles
– Telophase: • New nuclear membranes form around chromosome sets • Identical nuclei are formed
Cytokinesis
• Divides cytoplasm and cellular contents.
• In animals: cleavage furrow forms.
• In plants: cell plate forms to divide the cell and build new walls.
G0 Phase
• Some cells exit the cycle after G1 into a resting/non-dividing state called G0.
• Cells in G0 can remain quiescent (e.g., nerve, muscle cells) or reenter the cycle later (e.g., liver cells).
• G0 is important for controlling growth and conserving resources.
Regulation of the Cell Cycle
• Controlled by internal and external signals to prevent uncontrolled division.
• Internal checkpoints: – G1 checkpoint: is DNA intact? is the cell large enough? – G2 checkpoint: was DNA replicated correctly? – M checkpoint: are chromosomes properly attached to spindle?
• Cyclins: proteins that regulate the cycle by activating kinases.
• CDKs (cyclin-dependent kinases): enzymes activated by specific cyclins to trigger cell cycle progression.
• When cyclin levels drop, CDKs become inactive, halting progression.
Growth Factors
• External signals that stimulate division.
• Examples: Platelet-Derived Growth Factor (PDGF), hormones, density-dependent inhibition, anchorage dependence.
Cancer and Uncontrolled Cell Growth
• Cancer = loss of cell cycle control → cells divide uncontrollably.
• May form benign (non-spreading) or malignant (spreading) tumors.
Key Genes Involved
• Proto-oncogenes: normal genes that promote division (e.g., growth factors).
• Oncogenes: mutated proto-oncogenes → permanently active → overstimulate division.
• Tumor suppressor genes: inhibit division or repair DNA (e.g., p53).
• If mutated → cannot stop damaged cells from dividing → cancer risk increases.
Apoptosis (Programmed Cell Death)
• Controlled process that removes damaged or unnecessary cells.
• Prevents cancer by eliminating abnormal cells.
• Also important in development (e.g., shaping fingers, removing tadpole tails).
In a Nutshell
The cell cycle controls how and when cells divide, with checkpoints ensuring accurate DNA replication and division. Proteins like cyclins and CDKs regulate progression, while apoptosis and tumor suppressors prevent harmful growth. Disruptions in these controls can lead to uncontrolled cell division and cancer.