Cell Cycle, Mitosis, and Meiosis: Structure, Regulation, and Genetic Variation

Cell Division

Overview and Functions

Cell division is a fundamental biological process by which cells reproduce, enabling growth, repair, and reproduction in multicellular organisms. It ensures the continuity of life and the maintenance of genetic information across generations.

• Reproduction: Cell division allows organisms to reproduce, either asexually or sexually.

• Growth: Multicellular organisms grow by increasing their cell number through division.

• Repair: Damaged tissues are repaired by the production of new cells.

Genome and Chromosomes

Genetic Information and Organization

The genome comprises all the genetic material in a cell. In eukaryotes, DNA is packaged with proteins into structures called chromosomes.

• Human genome: 46 chromosomes (23 pairs)

• Chromosomes: DNA molecules associated with proteins, visible during cell division

Types of Cell Division

Mitotic Cell Division vs. Meiosis

There are two main types of cell division in eukaryotes: mitosis and meiosis.

• Mitotic cell division: Occurs in somatic cells, producing two genetically identical diploid daughter cells.

• Meiosis: Occurs in gametes (sperm and eggs), producing four haploid cells with half the chromosome number of the original cell.

Mitotic Cell Division

Cell Cycle Phases

The cell cycle describes the lifecycle of a cell, consisting of:

• Interphase: Accounts for 90% of the cell cycle; includes G1, S, and G2 phases.

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

During the S phase, chromosomes are duplicated, forming sister chromatids.

Mitosis Subphases

Mitosis is divided into four main subphases, followed by cytokinesis:

• Prophase: Chromosomes condense, nucleoli disappear, mitotic spindle forms, centrosomes move apart.

• Prometaphase: Nuclear envelope disintegrates, spindle microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Centromeres divide, sister chromatids separate and move to opposite poles.

• Telophase: Cell elongates, nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of cytoplasm; in animal cells, a cleavage furrow forms via microfilaments; in plant cells, a cell plate forms.

Mitosis in Plant Cells

• Centrioles: Not apparent in plant cells.

• Cytokinesis: Involves formation of a cell plate from Golgi-derived vesicles, which develops into the cell wall.

Regulation of the Cell Cycle

Checkpoints and Control Mechanisms

Cell cycle progression is tightly regulated by checkpoints, which ensure proper division and prevent errors.

• Checkpoints: Control points where stop and go signals regulate the cycle (e.g., G1, G2, M checkpoints).

• G1 checkpoint: Also called the restriction point; cells may enter a quiescent state (G0).

Cyclins and CDKs

• Kinases: Enzymes that regulate proteins by phosphorylation.

• Cyclin-dependent kinases (CDKs): Must bind cyclins to be active; phosphorylate targets for cell cycle progression.

• MPF (Maturation-Promoting Factor): A cyclin/CDK complex that triggers passage from G2 to M phase.

MPF activity peaks with cyclin concentration, leading to nuclear envelope breakdown and cyclin degradation.

Internal and External Cues

• Internal cues: Ensure proper attachment of chromosomes before separation (e.g., APC complex activation).

• External cues: Regulated by signal transduction pathways involving reception, transduction, and response.

Signal Transduction Steps

1. Reception: Signal molecule binds to receptor.

2. Transduction: Relay molecules transmit the signal, often via phosphorylation cascades.

3. Response: Cellular changes such as gene expression or cytoskeletal rearrangement.

External Growth Signals

• Growth factors and nutrients: Influence cyclin/CDK activity.

• Attachment: Cells must be attached to other cells or extracellular matrix.

• Density-dependent inhibition: Normal cells stop dividing when in contact with others.

Cancer Cells and Cell Cycle Control

• Lack of density-dependent growth control: Cancer cells do not exhibit contact inhibition.

• Immortality: Cancer cells can divide indefinitely due to altered DNA replication proteins.

• Metastasis: Cancer cells can detach and spread to other tissues.

Reproduction: Asexual and Sexual

Asexual Reproduction

• Single parent: Offspring are genetically identical to the parent.

• Mechanisms: Binary fission (bacteria, protozoa), budding (hydra).

Sexual Reproduction

• Two parents: Offspring inherit unique combinations of genes, increasing genetic variation.

• Meiosis: Produces gametes with half the chromosome number.

Meiosis

Terminology

• Somatic cell: Body cell (diploid, 2n).

• Gametes: Sex cells (haploid, n).

Human Reproduction

• Sperm and egg: Each contributes 23 chromosomes; fertilized egg has 46 chromosomes.

• All somatic cells: Contain the same DNA as the fertilized egg.

Chromosome Structure and Homology

• Pairs: 23 pairs; 22 autosomes, 1 pair sex chromosomes (XX or XY).

• Homologous chromosomes: One maternal, one paternal; same genes in same order.

Meiosis Process

Meiosis reduces chromosome number from diploid to haploid through two sequential divisions.

• Meiosis I (Reduction Division): Homologous chromosomes separate; haploid cells result.

• Meiosis II (Equatorial Division): Sister chromatids separate; haploid cells with single chromatids result.

Steps in Meiosis

• Interphase: Chromosomes replicate; centrosomes duplicate.

• Prophase I: Homologous chromosomes pair (synapsis), crossing over occurs, spindle forms, nuclear envelope disappears.

• Metaphase I: Homologous pairs align at metaphase plate.

• Anaphase I: Homologs separate to opposite poles.

• Telophase I and Cytokinesis: Haploid daughter cells form.

• Meiosis II: Similar to mitosis; sister chromatids separate.

• Result: Four haploid daughter cells.

Genetic Variation in Meiosis

Mechanisms Contributing to Variation

• Independent assortment: Random alignment of homologous chromosomes during metaphase I leads to varied combinations in gametes.

• Crossing over: Exchange of genetic material between homologous chromosomes during prophase I creates recombinant chromosomes.

• Random fertilization: Any sperm can fertilize any egg, multiplying possible genetic combinations.

Genetic Variation Calculations

• Human ovum: 8 million possible chromosome combinations.

• Human sperm: 8 million possible combinations.

• Combined: $8 imes 10^6 imes 8 imes 10^6 = 64 imes 10^{12}$ possible combinations (not including crossing over).

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Cell type	Somatic cells	Gametes
Number of divisions	1	2
Daughter cells	2 (diploid, identical)	4 (haploid, unique)
Genetic variation	None (except mutations)	High (crossing over, independent assortment)
Function	Growth, repair, asexual reproduction	Sexual reproduction

Key Terms and Definitions

• Genome: Complete set of genetic material in a cell.

• Chromosome: DNA molecule with associated proteins, visible during cell division.

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

• Homologous chromosomes: Chromosome pairs, one from each parent, with the same genes.

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes.

• Synapsis: Pairing of homologous chromosomes during prophase I of meiosis.

• Crossing over: Exchange of genetic material between homologous chromosomes.

• Independent assortment: Random distribution of homologous chromosomes during meiosis.

• MPF: Maturation-promoting factor, a cyclin/CDK complex regulating cell cycle progression.

Equations and Calculations

• Possible chromosome combinations in gametes: $2^n$, where $n$ is the haploid number (humans: $2^{23} = 8,388,608$)

• Possible combinations at fertilization: $2^n imes 2^n = 2^{2n}$ (humans: $2^{46} ext{ or } ext{about } 70 ext{ trillion}$)

Summary

Cell division, through mitosis and meiosis, is essential for growth, repair, and reproduction. The regulation of the cell cycle ensures proper division, while mechanisms such as crossing over, independent assortment, and random fertilization contribute to genetic diversity in sexually reproducing organisms.