Cell Division and the Cell Cycle: Structure, Function, and Regulation

Unit 5: Cell Division

Overview

This unit covers the fundamental processes of cell division, including the cell cycle, DNA replication, mitosis, cytokinesis, and meiosis. Understanding these processes is essential for grasping how organisms grow, develop, and reproduce.

The Cell Cycle

Definition and Importance

• Cell Cycle: The series of stages through which a cell passes from one division to the next, including interphase and mitosis/cytokinesis.

• Ensures that parent cells provide daughter cells with hereditary instructions (encoded in DNA) and sufficient metabolic machinery for independent function.

• Key quote: "Omnis cellula e cellula" (Every cell from a cell) – Rudolf Virchow, 1855.

Phases of the Cell Cycle

• Interphase: The longest phase, consisting of three subphases:

  • G1 phase (Gap 1): Cell grows and synthesizes proteins and organelles.

  • S phase (Synthesis): DNA replication occurs; chromosomes and centrioles are duplicated.

  • G2 phase (Gap 2): Further growth and preparation for division.

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

• G0 phase: Resting state where cells may exit the cycle temporarily or permanently.

Diagram: The Cell Cycle

The cell cycle is often depicted as a circular process, with checkpoints at key transitions (G1, S, G2, and M phases).

Cell Division Mechanisms

Eukaryotic vs. Prokaryotic Division

• Eukaryotic organisms:

  • Mitosis: Produces two genetically identical diploid daughter cells.

  • Meiosis: Produces four genetically unique haploid gametes (sex cells).

• Prokaryotic organisms:

  • Binary Fission: A simpler process resulting in two identical cells.

Roles of the Cell Cycle

• Growth: Increase in cell number for organismal development.

• Cell Replacement: Renewal of old or damaged cells.

• Repair: Regeneration of tissues (e.g., lizard tail regeneration).

• Asexual Reproduction: Production of genetically identical offspring (mitosis, binary fission).

• Sexual Reproduction: Formation of gametes (meiosis) for genetic diversity.

Mechanisms of Reproduction

Asexual Reproduction	Sexual Reproduction
Formation of new individuals without genetic material union; offspring are clones.	Genes from two individuals combine to produce genetically unique offspring.
Examples: Mitosis (eukaryotes), Binary Fission (prokaryotes)	Example: Gametes form via meiosis

Diploid vs. Haploid

• Diploid (2n): Two sets of chromosomes (one from each parent); typical of somatic cells.

• Haploid (n): One set of chromosomes; typical of gametes (sperm and egg).

During mitosis, cells remain diploid. During meiosis, cells become haploid to ensure genetic diversity upon fertilization.

Chromosomes and DNA Organization

Chromosome Structure

• Chromosome: A DNA molecule with attached proteins, duplicated during S phase.

• Centromere: Central region where sister chromatids are attached.

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere.

Forms of DNA

• DNA: Nucleic acid composed of A, T, C, G bases, deoxyribose sugar, and phosphate groups.

• Chromatin: DNA with associated proteins in a relaxed form.

• Chromosomes: Condensed chromatin visible during cell division.

Organization within the Nucleus

• DNA wraps around histone proteins to form nucleosomes, further compacted into chromatin and chromosomes.

Histones and Chromatin Types

• Histones: Basic proteins that help condense DNA; their positive charge allows association with negatively charged DNA.

• Euchromatin: Loosely packed, transcriptionally active (acetylated histones).

• Heterochromatin: Tightly packed, transcriptionally inactive (methylated histones).

Modifications to histone tails regulate gene expression and cell specialization.

Chromosome Number and Karyotypes

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

• Somatic cells: Diploid (2n), two sets of chromosomes.

• Germ cells: Diploid (2n), specialized for gamete production.

• Gametes: Haploid (n), one set of chromosomes.

Human Chromosome Number

• Humans: 2n = 46 chromosomes (23 pairs; one set from each parent).

• Mitosis produces two identical daughter cells (2n = 46).

Karyotypes

• Karyotype: The number and appearance of chromosomes in a cell.

• Used to determine sex, chromosome number, and large-scale chromosomal abnormalities.

• Most chromosomes are autosomes (not related to sex); X and Y are allosomes (sex chromosomes).

Examples of Karyotypes

Species	Chromosome Number (2n)
Human	46
Dog	78

Homologous Chromosomes and Alleles

• Homologous chromosomes: Pairs of chromosomes with the same length, centromere position, and gene loci; one from each parent.

• Alleles: Alternative forms of a gene found at the same locus on homologous chromosomes.

Control and Regulation of the Cell Cycle

Checkpoints

• G1 Checkpoint: Checks for nutrients, growth factors, and DNA damage.

• S Checkpoint: Ensures DNA has been replicated correctly.

• G2 Checkpoint: Checks for cell size and DNA replication completion.

• Metaphase Checkpoint: Ensures chromosomes are properly attached to the spindle.

Regulation

• Cyclin-dependent kinases (CDKs): Enzymes that regulate the cell cycle in conjunction with cyclins.

• Loss of checkpoint control can lead to uncontrolled cell division (cancer).

Summary Table: Cell Cycle Phases and Key Events

Phase	Main Events
G1	Cell growth, protein synthesis
S	DNA replication, chromosome duplication
G2	Further growth, preparation for mitosis
M (Mitosis & Cytokinesis)	Nuclear and cytoplasmic division

Key Terms

• Chromosome

• Chromatin

• Centromere

• Sister Chromatids

• Homologous Chromosomes

• Allele

• Karyotype

• Diploid (2n)

• Haploid (n)

• Interphase

• Mitosis

• Meiosis

• Binary Fission

• Checkpoints

• Cyclin-dependent kinase

Additional info: This guide covers the foundational aspects of the cell cycle and cell division, as outlined in General Biology college courses (Chapters 9 and 10). For further details on mitosis and meiosis, refer to subsequent lessons or textbook chapters.