Cell Cycle, Chromosomes, and Cancer: Study Guide for General Biology

Cell Cycle, Chromosomes, and Cancer

Concept 9.1: Chromosomes and Genomes

This section explores the structure and function of genomes and chromosomes, focusing on differences between prokaryotic and eukaryotic cells, and introduces key terminology.

• Genome: The complete set of genetic material in an organism. In prokaryotes, the genome is typically a single circular DNA molecule, while in eukaryotes, it consists of multiple linear chromosomes.

• Chromosome: A DNA molecule with part or all of the genetic material of an organism. Eukaryotic cells can have dozens to hundreds of chromosomes; humans have 46 (23 pairs).

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes within the nucleus of eukaryotic cells. Chromatin condenses to form visible chromosomes during cell division.

• Somatic Cells: All body cells except gametes; in humans, somatic cells are diploid (2n), containing two sets of chromosomes.

• Replication: The process by which DNA is copied before cell division. After replication, each chromosome consists of two identical sister chromatids joined at a centromere.

• Sister Chromatids: The two identical halves of a replicated chromosome.

• Centromere: The region where sister chromatids are joined and where spindle fibers attach during mitosis.

• Homologous Chromosomes: Chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location.

• Chromatid Arm: The sections of a chromatid on either side of the centromere.

Example: In humans, a diploid cell (2n) has 46 chromosomes, or 23 pairs of homologous chromosomes.

Concept 9.2: The Cell Cycle and Mitosis

This section covers the stages of the cell cycle, the process of mitosis, and the differences between mitosis and cytokinesis.

• Cell Cycle: The ordered sequence of events in the life of a cell, including interphase (G1, S, G2) and the mitotic (M) phase.

• Interphase: The phase of the cell cycle when the cell grows (G1), replicates its DNA (S), and prepares for division (G2). Most of a cell's life is spent in interphase.

• Mitosis (M phase): The process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets. Mitosis is divided into prophase, metaphase, anaphase, and telophase.

• Cytokinesis: The division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this occurs via a cleavage furrow; in plant cells, a cell plate forms.

• Mitotic Spindle: A structure made of microtubules that segregates chromosomes during mitosis. It forms during prophase and attaches to chromosomes at the centromere via the kinetochore.

• Kinetochore: A protein structure on the centromere where spindle fibers attach during cell division.

• Centrosome: The microtubule-organizing center in animal cells, which duplicates before mitosis and helps form the spindle apparatus.

• Binary Fission: The process by which prokaryotic cells divide, which is simpler than mitosis.

Example: During metaphase, chromosomes align at the cell's equator, and during anaphase, sister chromatids are pulled apart to opposite poles.

Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelopes reform; chromosomes decondense.

Table: Comparison of Mitosis and Cytokinesis

Process	Definition	Key Features
Mitosis	Division of the nucleus	Produces two genetically identical nuclei
Cytokinesis	Division of the cytoplasm	Produces two separate daughter cells

Concept 9.3: Cell Cycle Regulation and Cancer

This section discusses how the cell cycle is regulated, the role of checkpoints, and how disruptions can lead to cancer.

• Cell Cycle Checkpoints: Control points where the cell cycle can be halted until conditions are favorable. Major checkpoints include G1, G2, and M.

• G1 Checkpoint: Determines if the cell has adequate size, nutrients, and DNA integrity to proceed.

• G2 Checkpoint: Ensures DNA replication is complete and checks for DNA damage.

• M Checkpoint: Ensures all chromosomes are properly attached to the spindle before anaphase.

• Cyclins and Kinases: Regulatory proteins (cyclins) and enzymes (cyclin-dependent kinases, CDKs) that control progression through the cell cycle.

• Density-Dependent Inhibition: Normal cells stop dividing when they become crowded.

• Anchorage Dependence: Normal cells must be attached to a substrate to divide.

• Apoptosis: Programmed cell death, a normal process to remove damaged or unnecessary cells.

• Cancer Cells: Cells that divide uncontrollably due to loss of cell cycle regulation. They may not exhibit density-dependent inhibition or anchorage dependence and can become immortal.

• Benign Tumor: A mass of abnormal cells that remains at the original site.

• Malignant Tumor: A mass of cancerous cells that can invade surrounding tissues and spread (metastasize) to other parts of the body.

• Metastasis: The spread of cancer cells from the original site to other parts of the body.

Example: Cancer cells often evade apoptosis and can divide without growth factors, leading to tumor formation and metastasis.

Table: Differences Between Normal and Cancer Cells

Feature	Normal Cells	Cancer Cells
Growth Control	Regulated by checkpoints	Unregulated, checkpoints bypassed
Density-Dependent Inhibition	Present	Absent
Anchorage Dependence	Present	Often absent
Apoptosis	Normal response to damage	Often evaded
Immortality	Limited divisions	Can divide indefinitely
Metastasis	No	Yes (malignant only)

Key Equations

• Chromosome Number in Diploid Cells: $2n$ (where n = number of unique chromosomes in a set; for humans, n = 23, so 2n = 46)

• DNA Content During Cell Cycle: If G1 DNA content = 2n, then after S phase (replication), DNA content = 4n, but chromosome number remains the same until anaphase.

Additional info: The study notes above expand on the question prompts by providing definitions, context, and examples to ensure a self-contained and comprehensive review of the cell cycle, chromosome structure, and cancer biology for General Biology students.