Chapter 12 Study Guide: The Cell Cycle

The Cell Cycle

Introduction

The cell cycle is the series of events that cells go through as they grow and divide. It is fundamental to both unicellular and multicellular life, ensuring continuity, growth, and development. This chapter covers the purpose of cell division, the organization of genetic material, the phases of the cell cycle, mitosis, cytokinesis, and the regulation of the cell cycle.

Concept 12.1: The Purpose of Cell Division

Cell Division in Different Organisms

• Unicellular Organisms: Cell division serves as a means of reproduction, producing two genetically identical offspring from one parent cell. Example: Escherichia coli reproducing by binary fission.

• Multicellular Organisms: Cell division is essential for growth, development from a fertilized egg, tissue renewal, and repair. Example: Human skin cells dividing to replace damaged tissue.

Organization of Genetic Material

• Genome: The complete set of an organism's genetic material.

• Chromosomes: Structures that organize and package DNA; each chromosome contains many genes.

• Genes: Units of heredity made up of DNA that code for proteins.

• Chromatin: The complex of DNA and proteins that makes up chromosomes; exists in a relaxed state during interphase and condenses during mitosis.

• Chromosome Condensation: The process by which chromatin fibers become tightly coiled to form visible chromosomes during cell division.

• Sister Chromatids: Two identical copies of a chromosome connected by a centromere, formed during DNA replication.

• Centromere: The region where sister chromatids are most closely attached.

• Mitosis: The division of the nucleus, resulting in two genetically identical daughter nuclei.

• Cytokinesis: The division of the cytoplasm, producing two separate daughter cells.

Types of Cells

• Somatic Cells: All body cells except reproductive cells; diploid (contain two sets of chromosomes).

• Gametes: Reproductive cells (sperm and egg); haploid (contain one set of chromosomes).

Homologs and Sister Chromatids

• Homologous Chromosomes (Homologs): Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.

• Sister Chromatids: Identical copies of a single chromosome, joined at the centromere, produced during DNA replication.

Concept 12.2: The Eukaryotic Cell Cycle

Phases of the Cell Cycle

• Interphase: The period of cell growth and DNA replication; consists of G1, S, and G2 phases.

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

Sub-phases of Interphase

• G1 Phase (First Gap): Cell grows and carries out normal functions.

• S Phase (Synthesis): DNA is replicated, forming sister chromatids.

• G2 Phase (Second Gap): Cell prepares for mitosis; additional growth and organelle duplication occur.

Sequence of the Cell Cycle

• G1 → S → G2 → M (Mitosis and Cytokinesis)

Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form.

• Prometaphase: Nuclear envelope fragments; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

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

• Telophase: Nuclear envelopes reform around chromosomes; chromosomes decondense.

Chromosome States During the Cell Cycle

• Interphase: Chromosomes are in a relaxed, uncondensed state (chromatin).

• Mitosis: Chromosomes condense and become visible.

Mitotic Spindle and Kinetochore

• Mitotic Spindle: Structure made of microtubules that segregates chromosomes during mitosis.

• Kinetochore: Protein complex on the centromere where spindle fibers attach.

Metaphase Plate Formation

• Chromosomes align at the metaphase plate due to the tension from spindle fibers attached to kinetochores.

Cytokinesis Timing

• Cytokinesis typically begins during telophase, completing shortly after mitosis ends.

Cytokinesis in Animal Cells

• Cleavage: The process of dividing the cytoplasm in animal cells.

• Cleavage Furrow: A contractile ring of actin microfilaments forms at the cell's equator, pinching the cell into two.

• Cytoskeleton Involved: Actin microfilaments and myosin.

Cytokinesis in Plant Cells

• Cell Plate Formation: Vesicles from the Golgi apparatus coalesce at the center of the cell, forming a cell plate that develops into a new cell wall.

• Organelles Involved: Golgi-derived vesicles.

• Cytoskeleton Involved: Microtubules guide vesicles to the center.

Prokaryotic Cell Division

Binary Fission

• Prokaryotes (e.g., bacteria) reproduce by binary fission, a simpler process than mitosis.

• DNA replicates, and each copy attaches to the plasma membrane. The cell elongates and divides into two identical cells.

Concept 12.3: Regulation of the Cell Cycle

Cell Cycle Checkpoints

• Checkpoints: Control points where the cell cycle can be halted until conditions are favorable.

• Major Checkpoints: G1 (restriction point), G2, and M (metaphase-anaphase transition).

• Functional Importance: Ensure proper division, prevent damaged DNA from being passed on, and coordinate cell growth with division.

• Relation to Cancer: Loss of checkpoint control can lead to uncontrolled cell division and cancer.

G0 Phase

• Cells may exit the cell cycle and enter a non-dividing state called G0 if they do not receive the proper signals at the G1 checkpoint.

• Factors influencing entry into G0 include lack of growth factors, cell density, and differentiation signals.

Supplemental Materials

• The Cell Cycle (YouTube)

• Mitosis (YouTube)

• Mitosis Rap (YouTube)

Summary Table: Key Terms and Definitions

Term	Definition
Genome	Complete set of genetic material in an organism
Chromosome	DNA molecule with associated proteins, carrying genetic information
Gene	Segment of DNA coding for a protein
Chromatin	DNA-protein complex in relaxed state
Sister Chromatids	Identical copies of a chromosome joined at the centromere
Centromere	Region joining sister chromatids
Somatic Cell	Non-reproductive body cell
Gamete	Reproductive cell (sperm or egg)
Homologous Chromosomes	Chromosome pairs with similar structure and gene content
Mitosis	Nuclear division producing identical nuclei
Cytokinesis	Division of the cytoplasm
Binary Fission	Prokaryotic cell division
Checkpoint	Control point in the cell cycle
G0 Phase	Non-dividing state

Key Equations and Concepts

• DNA Replication: Each chromosome duplicates to form two sister chromatids during S phase.

• Cell Cycle Sequence:

$\mathrm{G1 \rightarrow S \rightarrow G2 \rightarrow M}$

• Chromosome Number in Mitosis: Daughter cells have the same number of chromosomes as the parent cell.

Additional info: The cell cycle is tightly regulated by cyclins and cyclin-dependent kinases (CDKs), which are not covered in detail here but are essential for checkpoint control. Cancer cells often have mutations in genes that regulate these checkpoints, leading to uncontrolled proliferation.