Chapter 9: The Cell Cycle

Chromosomes and Chromosome Structure

The cell cycle is a series of events that cells go through as they grow and divide. Understanding chromosome structure is fundamental to studying cell division in eukaryotes.

• Chromosome: A structure composed of DNA and proteins that contains genetic information. Humans have 46 chromosomes (23 pairs).

• Single Chromosome vs. Duplicated Chromosome: A single chromosome consists of one DNA molecule. After DNA replication, a duplicated chromosome consists of two identical sister chromatids joined at the centromere.

• Homologous Chromosomes: Chromosome pairs (one from each parent) that are similar in shape, size, and genetic content but are not identical. Sister Chromatids are identical copies formed by DNA replication.

• Example: In humans, chromosome 1 from the mother and chromosome 1 from the father are homologous chromosomes.

Haploid and Diploid

Cells can be classified based on the number of chromosome sets they contain.

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells in humans).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes: sperm and egg).

• Example: Human somatic cells are diploid (46 chromosomes), while gametes are haploid (23 chromosomes).

Phases of the Cell Cycle

The cell cycle consists of distinct phases that prepare the cell for division and ensure accurate distribution of genetic material.

• Interphase: Includes G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitosis: Division of the nucleus. Mitosis is only one phase of the cell cycle.

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

• Purpose: To produce genetically identical cells for growth, repair, and asexual reproduction.

Chromatid Attachment and Organelle Replication

During the cell cycle, chromosomes and organelles are duplicated to ensure each daughter cell receives the necessary components.

• Chromatids: After DNA replication, sister chromatids are attached at the centromere until separated during mitosis.

• Organelle Replication: Organelles such as mitochondria and the endoplasmic reticulum are replicated during interphase.

Phases of Mitosis and Chromosome Movement

Mitosis is divided into several stages, each characterized by specific events involving chromosomes.

• Prophase: Chromosomes condense, spindle fibers form.

• Metaphase: Chromosomes align at the cell's equator.

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

• Telophase: Nuclear membranes reform around separated chromosomes.

• Example: During metaphase, spindle fibers attach to centromeres to facilitate chromatid separation.

Cytokinesis

Cytokinesis is the process that divides the cytoplasm, resulting in two distinct daughter cells.

• Animal Cells: Form a cleavage furrow that pinches the cell in two.

• Plant Cells: Form a cell plate that develops into a new cell wall.

Spindle Apparatus

The spindle apparatus is essential for chromosome movement during mitosis.

• Spindle Fibers: Microtubules that attach to chromosomes and help segregate them.

• Importance: Ensures equal distribution of chromosomes to daughter cells.

Cell Cycle Structures: Cleavage Furrow and Cell Plate

Different cell types use distinct mechanisms to complete cytokinesis.

• Cleavage Furrow: Found in animal cells; a contractile ring pinches the cell membrane.

• Cell Plate: Found in plant cells; vesicles fuse to form a new cell wall between daughter cells.

Binary Fission

Binary fission is a form of asexual reproduction in prokaryotes, distinct from mitosis in eukaryotes.

• Process: The cell replicates its DNA, elongates, and divides into two identical cells.

• Example: Bacteria reproduce by binary fission.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell cycle progresses accurately.

• G1 Checkpoint: Checks for cell size, nutrients, and DNA damage.

• G2 Checkpoint: Ensures DNA replication is complete and undamaged.

• M Checkpoint: Verifies chromosome attachment to spindle fibers.

Regulation of the Cell Cycle

The cell cycle is tightly regulated by proteins to prevent uncontrolled cell division.

• Cyclins: Proteins that regulate the timing of the cell cycle.

• Cdks (Cyclin-dependent kinases): Enzymes that, when combined with cyclins, activate or deactivate other proteins by phosphorylation.

• Growth Factors: External signals that stimulate cell division.

• Equation:

Cancer and the Cell Cycle

Cancer results from uncontrolled cell division due to mutations in genes regulating the cell cycle.

• Oncogenes: Mutated genes that promote cell division.

• Tumor Suppressor Genes: Genes that inhibit cell division or cause apoptosis.

• Example: The p53 gene is a tumor suppressor that halts the cell cycle in response to DNA damage.

Benign vs. Malignant Tumors

Tumors are classified based on their ability to invade surrounding tissues.

• Benign Tumors: Non-cancerous, do not invade other tissues.

• Malignant Tumors: Cancerous, invade and destroy surrounding tissues and can metastasize.

Proto-oncogenes, Oncogenes, and Tumor Suppressor Genes

Genes involved in cell cycle regulation can contribute to cancer if mutated.

• Proto-oncogenes: Normal genes that promote cell division; can become oncogenes if mutated.

• Oncogenes: Mutated proto-oncogenes that cause excessive cell division.

• Tumor Suppressor Genes: Genes that inhibit cell division or promote cell death.

Summary Table: Cell Cycle Structures and Functions

Additional info: Academic context and definitions have been expanded for clarity and completeness.