In-Depth Notes on Cell Division and Chromosomes

Cell Division and Chromosomes

Key Vocabulary

• DNA: Nucleic acid; fundamental component of chromosomes.

• Gene: Basic unit of heredity; encodes traits.

• Allele: Variants of a gene that can produce different traits.

• Chromosome: Structures made of DNA that carry genes; humans have 46 chromosomes.

• Homologous Chromosome: Pairs of chromosomes containing the same genes, one from each parent.

• Genome: Complete set of genes in an organism.

• Centromere: Region of a chromosome where sister chromatids are joined.

• Karyokinesis: Nuclear division; part of the mitosis process.

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

• Mitosis: Process by which a cell divides its nucleus.

• Meiosis: Specialized form of cell division that produces gametes.

• Nucleotide: Basic building block of DNA; composed of a sugar, phosphate group, and base.

Learning Objectives

• Understand the significance of cell division in growth, repair, and reproduction.

• Describe the phases of the cell cycle and their functions.

• Differentiate between cytokinesis and karyokinesis.

• Explain the chromosomal makeup in diploid organisms using the human karyotype as a reference.

• Analyze structural and functional chromosome dynamics during the cell cycle.

• Understand the distinct events during mitosis.

• Compare animal and plant cell cytokinesis processes.

The Importance of Cell Division

• Essential for reproduction (unicellular and multicellular organisms), growth, and tissue repair.

• Involves the division of a mother cell into two daughter cells, each capable of functioning as a new mother cell.

• Regulated cell division is critical; uncontrolled division can lead to conditions like cancer or psoriasis.

The Cell Cycle

• Interphase (90% of cycle): Divided into three key phases:

  • G1 Phase: Cell growth and preparation for DNA synthesis.

  • S Phase: DNA replication occurs, resulting in duplicated chromosomes.

  • G2 Phase: Further cell growth and preparation for mitosis.

• M Phase: Includes karyokinesis (mitosis) and cytokinesis.

  • Mitosis occurs after interphase, wherein the nucleus divides.

  • Cytokinesis follows, resulting in the division of the cytoplasm.

• Resting Phase (G0): Cells that do not divide enter this phase.

Mitosis Process

1. Prophase: Chromosomes condense and become visible as paired sister chromatids.

2. Prometaphase: Nuclear envelope breaks down; spindle microtubules attach to kinetochores on the centromere.

3. Metaphase: Chromosomes align at the metaphase plate.

4. Anaphase: Sister chromatids are pulled apart toward opposite poles.

5. Telophase: Nuclear envelope re-forms; chromosomes de-condense.

6. Cytokinesis: Cytoplasmic division occurs; in animal cells, actin filaments form a cleavage furrow; in plants, a cell plate forms.

Chromosomal Structures

• Chromosomes consist of chromatin, which condenses during cell division.

• Sister Chromatids: Duplicated copies of chromosomes, joined at the centromere.

• In eukaryotic cells, most DNA is found in the nucleus (nuclear DNA), but some resides in organelles (mitochondrial and chloroplast DNA).

• Ploidy: Refers to the number of sets of chromosomes. Humans are diploid with 46 chromosomes (23 pairs).

  • Homologous Chromosomes: Pair of chromosomes from different parents, similar in structure and gene content, yet may carry different alleles.

  • Sex Chromosomes: X and Y chromosomes that determine biological sex and are not homologous in size and gene content.

Cytokinesis

• In Animal Cells: Contracting actin filaments form a cleavage furrow that pinches the cell into two.

• In Plant Cells: Golgi-derived vesicles form a cell plate, which develops into the cell wall separating the two daughter cells.