The Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of stages that a eukaryotic cell undergoes from one division to the next. It ensures proper growth, DNA replication, and division, resulting in two genetically identical daughter cells.

• Cell Growth: The cell increases in size.

• Organelle Duplication: The number of organelles doubles.

• DNA Replication: DNA is copied to prepare for cell division.

The cell cycle consists of two main phases:

• Interphase: Includes G1, S, and G2 stages.

• Mitotic Stage: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Interphase

Interphase is the longest phase of the cell cycle, accounting for about 90% of the cycle. During this phase, the cell performs its usual functions and prepares for division.

• G1 Stage (First Gap): The cell recovers from the previous division, grows, and doubles its organelles.

• S Stage (Synthesis): DNA replication occurs. Each chromosome consists of two identical double helices called chromatids, joined at the centromere.

• G2 Stage (Second Gap): Proteins necessary for cell division are synthesized.

• G0 Stage: Some cells, such as nerve and muscle cells, exit the cell cycle and remain in this non-dividing state.

Mitotic Stage

The mitotic stage includes two key processes: mitosis and cytokinesis. These processes ensure the equal distribution of genetic material and cytoplasm to daughter cells.

• Mitosis: Nuclear division where daughter chromosomes are distributed to two daughter nuclei.

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

Regulation of the Cell Cycle

Cell Cycle Control by Signals

The cell cycle is tightly regulated by signals that can be internal or external. These signals ensure that the cell only proceeds to the next stage when conditions are appropriate.

• External Signals: Growth factors received at the plasma membrane stimulate cell division.

• Internal Signals: Cyclins are a family of proteins whose concentrations fluctuate throughout the cell cycle. Cyclins must be present for the cell to proceed past checkpoints.

Checkpoints: Critical control points where the cell cycle can be halted to allow for DNA repair or to ensure proper progression.

• G1 checkpoint: From G1 to S stage

• G2 checkpoint: From G2 to M stage

• M checkpoint: During mitosis

Example: If DNA damage is detected at a checkpoint, the cell cycle is paused to allow for repair, preventing the propagation of mutations.

The Eukaryotic Chromosome

Structure and Organization

Eukaryotic chromosomes are composed of DNA and proteins called histones. The DNA-histone complex is known as chromatin.

• Histones: Proteins that help package DNA into the nucleus.

• Nucleosome: DNA wound around a core of eight histone proteins, appearing as "beads on a string."

• Euchromatin: Loosely coiled, active chromatin.

• Heterochromatin: Tightly coiled, inactive chromatin.

During the S stage, each chromosome is replicated, forming two sister chromatids held together by a centromere.

Chromosome Number and Homologous Pairs

Each species has a specific number of chromosomes. Chromosomes come in pairs called homologous pairs, which are similar in size and shape and carry genes for the same traits.

• Karyotype: The complete set of chromosomes in a cell, organized by size and shape.

Mitosis and Cytokinesis

Phases of Mitosis

Mitosis is divided into several phases, each with distinct events that ensure accurate chromosome segregation.

1. Prophase: Chromatin condenses into visible chromosomes; nucleolus disappears; spindle begins to form.

2. Prometaphase: Nuclear envelope breaks down; kinetochores form at centromeres and attach to spindle fibers.

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

4. Anaphase: Sister chromatids separate and are pulled to opposite poles, now called daughter chromosomes.

5. Telophase: Spindle disappears; new nuclear envelopes form; chromosomes decondense into chromatin; nucleoli reappear.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two daughter cells. The process differs between animal and plant cells.

• Animal Cells: A cleavage furrow forms, and a contractile ring pinches the cell into two.

• Plant Cells: A cell plate forms, which becomes the plasma membrane and eventually the cell wall.

Example: In bone marrow, mitosis produces new blood cells for growth and repair.

The Cell Cycle and Cancer

Cancer and Cell Cycle Dysregulation

Cancer is a growth disorder resulting from uncontrolled cell division due to mutations that disrupt normal cell cycle regulation.

• Tumor: Abnormal mass of cells; can be benign (non-cancerous) or malignant (cancerous).

• Malignant Tumors: Not encapsulated, invade neighboring tissues, may metastasize (spread to distant sites).

Carcinogenesis: The development of cancer, often gradual and involving multiple genetic changes.

Genetic Basis of Cancer

• Proto-oncogenes: Genes that promote cell division. When mutated, they become oncogenes and can cause uncontrolled cell division.

• Tumor Suppressor Genes: Genes that inhibit cell division or promote apoptosis. Mutations can lead to loss of cell cycle control (e.g., p53 gene).

Telomeres: Protective ends of chromosomes that shorten with each division. The enzyme telomerase maintains telomere length; mutations can lead to continual cell division.

Characteristics of Cancer Cells

• Lack differentiation (non-specialized)

• Abnormal nuclei (enlarged, irregular)

• Abnormal chromosome number

• Do not undergo apoptosis

• Loss of contact inhibition (grow over each other)

• Undergo metastasis and angiogenesis (formation of new blood vessels)

Cancer Cells Versus Normal Cells

Prokaryotic Cell Division

Binary Fission

Prokaryotic cells, such as bacteria and archaea, divide by binary fission, a simpler process than mitosis.

• Single, circular chromosome replicates prior to division.

• Chromosome attaches to plasma membrane; cell elongates.

• Chromosomes are pulled apart; cell divides into two identical daughter cells.

• Binary fission is a form of asexual reproduction.

Functions of Cell Division

Summary Equations

• DNA Replication:

• Cell Cycle Progression:

Additional info: These notes expand on the provided outline and slides, adding definitions, examples, and tables for clarity and completeness.