Cell Growth and division

Cell Growth and Division

Overview

  • Human body contains approximately 37.2 trillion cells.

  • Each cell has 46 chromosomes (23 pairs).

  • Males have XY sex chromosomes; females have XX sex chromosomes.

Chromosomes and Genes

  • Chromosomes contain DNA with thousands of genes.

  • Sister chromatids are joined at the centromere.

Types of Cells

Somatic Cells

  • Somatic cells include all body cells (e.g., skin, bone).

  • Undergo mitosis to divide, producing 2 identical daughter cells (diploid - 2 sets of chromosomes).

Gametes

  • Gametes are sex cells (sperm and egg).

  • Undergo meiosis to divide, producing 4 daughter cells with half their DNA (haploid - 1 set of chromosomes).

Importance of Somatic Cell Division

  • Growth: Development from embryo to adult.

  • Maintenance: Replacing worn-out cells (e.g., skin, muscles).

  • Repair: Healing from injuries such as cuts and fractures.

Cell Cycle Stages

Interphase

  • Divided into three phases:

    • G1 Phase: Cell growth and development with a checkpoint.

    • S Phase: DNA replication.

    • G2 Phase: Preparation for mitosis with a checkpoint.

M Phase (Mitosis)

  • Nucleus divides into 2 nuclei with a final checkpoint.

  • Cytokinesis: Division of cytoplasm to form 2 identical daughter cells.

  • Cell cycle restarts in new daughter cells.

Interphase Characteristics

  • Chromosomes are not visible (chromatin form).

  • Nucleus is clearly visible.

  • Chromatin structure present.

Mitosis Phases

Prophase

  • Chromatin condenses into chromosomes.

  • Centrioles separate and move to opposite ends of the cell.

  • Nuclear envelope breaks down; longest phase of mitosis.

Metaphase

  • Chromosomes align at the cell's equatorial plane.

  • Spindle fibers connect to the centromeres of chromosomes; shortest phase.

Anaphase

  • Chromosomes split at their centromeres.

  • Spindle fibers pull chromosomes to opposite ends of the cell.

Telophase

  • Chromosomes uncoil back into chromatin.

  • Nuclear envelope reforms; nucleus becomes visible.

  • Spindle fibers disappear.

Cytokinesis

  • In animal cells, the cytoplasm pinches off.

  • In plant cells, a new cell wall (cell plate) forms.

  • Result: 2 identical cells ready to start the cycle again.