The Cell Cycle and Mitosis: Chapter 12 Study Guide

The Cell Cycle and Mitosis

Overview of Genetic Material Organization

The genetic material in eukaryotic cells is organized in different forms depending on the stage of the cell cycle. Understanding the distinctions between chromatin, chromosomes, and sister chromatids is essential for grasping how cells divide and maintain genetic continuity.

• Chromatin: The complex of DNA and proteins (mainly histones) that is loosely packed in the nucleus during interphase. Chromatin allows access for transcription and replication.

• Chromosome: A single, long DNA molecule condensed and coiled during cell division. Each chromosome contains many genes.

• Sister Chromatids: Two identical copies of a chromosome, joined at a region called the centromere, formed after DNA replication in S phase.

• Non-dividing cells: DNA exists as loosely packed chromatin.

• Dividing cells: DNA condenses into visible chromosomes, each consisting of two sister chromatids after replication.

Example: In G1 phase, a human cell has 46 chromosomes (unduplicated). After S phase, each chromosome is duplicated, resulting in 46 chromosomes, each with two sister chromatids (92 chromatids total).

Phases of the Cell Cycle

The cell cycle is divided into two main phases: Interphase and M-phase (Mitotic phase). Each phase has distinct sub-phases and functions.

• Interphase: The period of cell growth and DNA replication, comprising:

  • G1 phase (First Gap): Cell grows and carries out normal functions.

  • S phase (Synthesis): DNA is replicated; chromosomes are duplicated.

  • G2 phase (Second Gap): Cell prepares for division; organelles may duplicate.

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

  • Mitosis Sub-phases:

    • Prophase: Chromosomes condense, spindle apparatus forms.

    • Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

    • Metaphase: Chromosomes align at the metaphase plate (center of the cell).

    • Anaphase: Sister chromatids separate and move to opposite poles.

    • Telophase: Nuclear envelopes reform, chromosomes decondense.

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

Example: DNA replication occurs during S phase. The shortest phase of mitosis is typically anaphase.

Identification of Mitosis Phases

Each phase of mitosis can be identified by the arrangement and appearance of chromosomes:

• Prophase: Chromosomes condense and become visible; spindle forms.

• Metaphase: Duplicated chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Chromosomes decondense; nuclear envelope reforms.

Example: In metaphase, duplicated chromosomes (each with two sister chromatids) line up in the center. In anaphase, chromatids separate and migrate to poles.

Changes During Cell Cycle Progression

As a cell progresses through the cell cycle, several key changes occur:

• During G1, the cell grows and performs normal functions.

• During S phase, DNA is replicated, resulting in duplicated chromosomes.

• During G2, the cell prepares for mitosis.

• During Mitosis, chromosomes condense, align, separate, and are distributed to daughter cells.

• During Cytokinesis, the cytoplasm divides, completing cell division.

Non-dividing state: Cells that do not receive the "go-ahead" signal enter G0 phase, a quiescent state.

Chromosome and DNA Content During the Cell Cycle

The number of chromosomes and DNA molecules changes during the cell cycle:

• In G1: Each chromosome is unduplicated (one chromatid per chromosome).

• After S phase: Each chromosome consists of two sister chromatids (duplicated).

• During Metaphase: Chromosomes are duplicated (sister chromatids attached).

• During Anaphase: Sister chromatids separate, each now considered an individual chromosome.

• After Cytokinesis: Each daughter cell has the same number of chromosomes as the original cell in G1.

Phase	Chromosomes	DNA Molecules
G1	n	n
G2	n	2n
Metaphase	n	2n
Anaphase	2n	2n
Daughter Cell (post-cytokinesis)	n	n

Example: If a cell has 5 chromosomes in G1, after S phase and during metaphase, it has 5 chromosomes (each with 2 chromatids, so 10 DNA molecules). After anaphase, there are 10 chromosomes (as chromatids separate), and each daughter cell ends up with 5 chromosomes.

Comparison of Mitosis and Binary Fission

Mitosis and binary fission are two forms of cell division, but they differ in mechanism and the organisms that use them.

• Mitosis: Occurs in eukaryotic cells (animals, plants, fungi, protists). Involves multiple linear chromosomes, spindle apparatus, and distinct phases.

• Binary Fission: Occurs in prokaryotes (bacteria and archaea). DNA is replicated and the cell splits without spindle formation or complex chromosome condensation.

Feature	Mitosis	Binary Fission
Organisms	Eukaryotes	Prokaryotes
Chromosome Structure	Linear, multiple	Circular, single
Spindle Apparatus	Present	Absent
Phases	Multiple (prophase, metaphase, etc.)	Simple (replication, division)

Properties of Cancer Cells

Cancer cells exhibit several unique properties that distinguish them from normal cells:

• Loss of Anchorage Dependence: Normal cells require attachment to a substrate to divide; cancer cells can divide without anchorage.

• Loss of Density-Dependent Inhibition: Normal cells stop dividing when crowded; cancer cells continue to divide, forming tumors.

• Defective Cell Cycle Control: Cancer cells bypass normal checkpoints, leading to uncontrolled proliferation.

Example: Cancer cells do not respond to signals that regulate the cell cycle, such as growth factors or contact inhibition, allowing them to grow uncontrollably.

Additional info:

• Spindle Apparatus: Responsible for separating sister chromatids during mitosis.

• Cytokinesis: In animal cells, occurs via cleavage furrow; in plant cells, via cell plate formation.

• G0 Phase: A non-dividing state entered by cells that do not receive the signal to proceed through the cell cycle.