THE CELL CYCLE AND ITS REGULATION

Introduction

• Overview of the Cell Cycle and Its Regulation

• Content derived from Pearson Campbell Biology, Unit 4, Topics 4.6-4.7

Short Summary

• Mitosis, or cellular division, occurs in four stages:

• prophase, metaphase, anaphase, and telophase.

• During prophase, the nuclear envelope disappears and chromosomes condense.

• Next is metaphase, when chromosomes align at the metaphase plate and mitotic spindles attach to kinetochores.

• In anaphase, chromosomes are pulled away from the center. Telophase terminates mitosis, and the two new nuclei form.

• The process of cytokinesis, which occurs during telophase, ends mitosis, as the cytoplasm and plasma membranes pinch to form two distinct, identical daughter cells.

• Interphase Once daughter cells are produced, they reenter the initial phase—interphase —and the whole process starts over. The cell goes back to its original state. Once again, the chromosomes decondense and become invisible, and the genetic material is called chromatin again.

Key Roles of Cell Division

• Distinguishing characteristic of living organisms.

• Continuity of life through reproduction of cells (cell division).

• Unicellular organisms reproduce via division of a single cell, while multicellular organisms depend on cell division for:

  • Development from fertilized cell

  • Growth

  • Repair

• Integral to the cell cycle, which encompasses the life of a cell from formation to division.

Genetic Information in Cell Division

• Most cell divisions yield daughter cells with identical DNA.

• Meiosis exception: produces non-identical gametes (sperm and egg).

• The cell's entire DNA is referred to as its genome, varying from a single DNA molecule in prokaryotes to multiple DNA molecules in eukaryotes.

• DNA is packaged into chromosomes, composed of chromatin (DNA and histone proteins).

Chromosome Characteristics

• Each eukaryotic species has a specific number of chromosomes per nucleus.

• Somatic cells have two sets (diploid, 2n).

• Gametes have half as many chromosomes (haploid, n).

Distribution of Chromosomes During Cell Division

• DNA is replicated and chromosomes condense before division.

• Duplicated chromosomes consist of two sister chromatids joined by cohesins at the centromere.

Mitosis Overview

• Mitosis involves separation of sister chromatids into two nuclei, and consists of:

  • Prophase

  • Prometaphase

  • Metaphase

  • Anaphase

  • Telophase

• Cytokinesis follows, completing cell division.

• Meiosis variation produces non-identical daughter cells with half the chromosomes of the parent.

The Cell Cycle Process

Interphase Phases

• Interphase (90% of the cell cycle): divided into:

  • G1 Phase: Initial growth and accumulation of building blocks for DNA.

  • S Phase: DNA synthesis resulting in sister chromatids; centrosome duplication occurs.

  • G2 Phase: Energy replenishment, organelle duplication, and preparation for mitosis.

Mitosis Phases

1. Prophase:

   • Breakdown of the nuclear envelope.

   • Fragmentation of membranous organelles.

   • Centrosomes move to opposite poles; microtubules form the mitotic spindle.

   • Sister chromatids condense and become visible.

2. Prometaphase:

   • Fragmentation of nuclear envelope continues.

   • Mitotic spindle develops further with chromosomes getting more condensed.

   • Kinetochores develop on sister chromatids, allowing binding to spindle fibers.

3. Metaphase:

   • Chromosomes align on the metaphase plate, maximally condensed.

   • Sister chromatids remain attached.

4. Anaphase:

   • Cohesin proteins are cleaved, allowing sister chromatids to separate.

   • Chromosomes are pulled toward opposite poles as the cell elongates.

5. Telophase:

   • Chromosomes decondense, nuclear envelopes reform, resulting in two identical daughter nuclei.

Cytokinesis

• Follows telophase for physical separation of cytoplasmic components.

• In animal cells, a cleavage furrow forms to divide the cell.

• In plant cells, a cell plate forms via Golgi vesicles, creating a new cell wall.

Cell Cycle Regulation

Control of the Cell Cycle

• Regulated by chemical signals in the cytoplasm, with checkpoints ensuring proper progression.

• Critical checkpoints during G1, G2, and M phases control the cycle's advancement.

Cyclins and Cyclin-Dependent Kinases (Cdks)

• Regulation involves cyclins and Cdks,

• Activity of Cdks fluctuates with cyclin concentration.

• Maturation-Promoting Factor (MPF) is important for G2 to M phase transition.

Checkpoints and Signals

• Checkpoints are points where the cycle halts pending signaling.

• An internal example is the assurance that all chromosomes are attached to spindle fibers before progressing to anaphase.

• External factors influencing cell division include growth factors (e.g., PDGF) and density-dependent inhibition.

Cancer and Cell Cycle Control

• Cancer cells disregard normal regulatory mechanisms, often growing without external signals or altering their control systems.

• Cells exhibit anchorage dependence and density-dependent inhibition under normal conditions; cancer cells do not.

• Transformation of normal cells into cancerous cells leads to benign or malignant tumors.

• Treatments for localized tumors include radiation; chemotherapy is used for metastatic cancers.