Chromosomes and the Cell Cycle Guided Notes - Bio I Dudley 2025

Chromosomes and the Cell Cycle

Overview of the Cell Cycle

• The cell cycle consists of distinct phases where cells grow, duplicate their DNA, and prepare for division.

  • Interphase: The cell grows and copies its DNA.

    • G1 Phase: Cellular growth occurs.

    • S Phase: DNA synthesis takes place, where chromosomes are replicated.

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

Mitosis

• Mitosis is the process in which a cell divides its copied DNA and cytoplasm into two new daughter cells. The stages of mitosis include:

  • Prophase: Chromatin condenses into visible chromosomes.

  • Metaphase: Chromosomes line up at the cell's equatorial plane.

  • Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

  • Telophase: The chromosomes begin to de-condense, and the nuclear envelope re-forms.

G0 Phase

• Some cells may enter a resting state known as the G0 Phase, where the cell performs its functions but is not preparing to divide.

Nucleobases and Nucleic Acids

DNA

• DNA (Deoxyribonucleic Acid) consists of nucleobases that form a double helix structure:

  • Bases include:

    • Adenine (A)

    • Thymine (T)

    • Cytosine (C)

    • Guanine (G)

• Each DNA strand is made up of sugar-phosphate backbone with nucleobases attached.

RNA

• RNA (Ribonucleic Acid) is another crucial nucleic acid, consisting of:

  • Bases include:

    • Adenine (A)

    • Uracil (U)

    • Cytosine (C)

    • Guanine (G)

• RNA is primarily involved in protein synthesis.

Investigative Phenomenon

• Key Questions:

  • How do organisms grow from a single cell?

  • What processes produce new cells, and why is the sequence of events important?

  • Why do somatic cells divide?

  • Why does DNA quantity change during the cell cycle, and why is this important?

  • How are specialized cells formed in organisms?

Chromatin and Chromosomes

Chromatin

• Chromatin is the uncondensed form of DNA found in the nucleus, composed of DNA and proteins.

Chromosomes

• During cell division, chromatin condenses into chromosomes, which are structures that carry genes.

  • A chromosome contains genes that regulate protein production and influence inherited traits.

• A typical human somatic cell contains 46 chromosomes (23 pairs).

Homologous Chromosomes

• Each pair of chromosomes is homologous, meaning they are similar in size, shape, and gene content (same genes in the same locations).

The Cell Cycle Defined

• The cell cycle is the complete process from one cell division to the next, divided into two main phases:

  • Interphase: The cell is not actively dividing (includes G1, S, and G2 phases).

  • Cell Division: Involves mitosis and cytokinesis where the cell divides.

Stages of the Cell Cycle

• Interphase: Cell growth and preparation phases.

  • G1 Phase: Cells grow, doubling in size and copying organelles.

  • S Phase: DNA replication occurs.

  • G2 Phase: Preparation for mitosis is completed, and further growth happens.

• Cell Division: The process where the nucleus and cytoplasm divide:

  • Mitosis and Cytokinesis.

Importance of Mitosis

1. Produces new somatic cells essential for growth and repair (e.g., skin and muscle cells).

2. Provides a mechanism for asexual reproduction in some unicellular eukaryotes (e.g., some protists and fungi).

3. Facilitates the specialization of cells, leading to diverse cell types such as liver, kidney, and brain cells.

Expanded Phases of Mitosis

1. Prophase:

   • Chromatin Condensation: The DNA in the nucleus condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere.

   • Spindle Apparatus Formation: The mitotic spindle, composed of microtubules, begins forming from the centrosomes, which move toward opposite poles of the cell.

   • Nuclear Envelope Breakdown: The nuclear envelope begins to disintegrate, allowing the spindle fibers to access the chromosomes.

2. Metaphase:

   • Chromosome Alignment: Chromosomes align along the cell's equatorial plane, also known as the metaphase plate. This ensures that each sister chromatid will be separated equally into the daughter cells.

   • Spindle Fiber Attachment: The spindle fibers attach to the centromeres of the chromosomes, securing them for the next phase.

3. Anaphase:

   • Separation of Sister Chromatids: The proteins holding the sister chromatids together are cleaved, allowing them to be pulled apart toward opposite poles of the cell.

   • Cell Elongation: The cell begins to elongate as the spindle fibers pull the chromatids apart, preparing for division.

4. Telophase:

   • Chromosome De-condensation: The separated sister chromatids, now individual chromosomes, begin to de-condense back into chromatin.

   • Nuclear Envelope Reformation: The nuclear envelope re-forms around each set of chromosomes at the poles, creating two distinct nuclei within the cell.

   • Spindle Disassembly: The spindle fibers break down as the cell prepares for cytokinesis, the final separation of the cytoplasm into two cells.