The Cell Cycle: Structure, Phases, and Mechanisms of Cellular Replication

The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It is fundamental to the growth, development, and reproduction of all living organisms.

• Cell division: The process by which new cells are formed by the splitting of pre-existing cells.

• Observations in embryos show that plants and animals:

  • Start life as single-celled embryos

  • Grow through a series of cell divisions

  • Meiosis produces reproductive cells called gametes

  • Mitosis produces all other cell types, known as somatic cells

Cell Division: Mitosis and Meiosis

• Both mitosis and meiosis are usually accompanied by cytokinesis (division of cytoplasm into two daughter cells).

• Mitosis results in daughter cells that are genetically identical to the parent cell and to each other.

• Meiosis produces daughter cells that are genetically different from each other and have half the hereditary material of the parent cell.

How Do Cells Replicate?

For life to exist, cells must replicate. Cellular replication involves three basic steps:

1. Copying DNA

2. Separating copies

3. Dividing cytoplasm to create two complete cells

Chromosome Structure and Replication

What Is a Chromosome?

• Chromosome: A single long double helix of DNA wrapped around proteins called histones.

• DNA encodes the cell's genetic information.

• Gene: A region of DNA in a chromosome that codes for a specific RNA and, therefore, a specific protein.

Chromosome Replication and Condensation

• Before mitosis, each chromosome is replicated and condenses into compact structures.

• Each double-stranded DNA copy is called a chromatid.

• Chromatids are attached along their length by proteins called cohesions and, once mitosis begins, only at the centromere.

• Sister chromatids: Chromatid copies that remain attached at the centromere; two attached sister chromatids are still considered a single chromosome.

Changes in Chromosome Morphology

• Unreplicated chromosome: Consists of a single, long DNA double helix wrapped around histones.

• Replicated chromosome: Consists of two identical DNA double helices (sister chromatids).

• Condensed replicated chromosome: Chromatin condenses for mitosis, making chromosomes visible under a microscope.

Phases of the Cell Cycle

Alternation between M Phase and Interphase

• M phase (mitotic or meiotic): The dividing phase where cells separate their chromosomes.

• Interphase: The non-dividing phase where chromosomes are uncoiled, and cells grow, prepare to divide, or fulfill specialized functions. Most of the cell's time is spent in interphase.

The S Phase and Gap Phases

• S (synthesis) phase: The stage in which DNA replication occurs.

• The cell cycle is an orderly sequence of events from the formation of a eukaryotic cell, through chromosome duplication, to cell division.

• There are two gap phases (G1 and G2) during which no DNA synthesis occurs, separating the S phase from the M phase.

The Four Phases of the Cell Cycle

• M phase

• G1 phase: Cell performs its functional roles and decides to begin replication.

• S phase: DNA synthesis/replication.

• G2 phase: Preparation for mitosis.

M Phase: Mitosis and Cytokinesis

Overview of M Phase

• Mitosis: Division of replicated chromosomes to form two daughter nuclei with identical chromosome genes.

• Cytokinesis: Division of cytoplasm to form two daughter cells.

Chromatin and Chromosome Replication

• Chromatin: DNA-histone complex; relaxed during interphase.

• During S phase, chromosomes replicate, resulting in two sister chromatids per chromosome.

Events in Mitosis

• Mitosis begins when chromatin condenses.

• Two sister chromatids separate to form independent daughter chromosomes.

• Each daughter cell receives a copy of the genetic information.

• Mitosis is a continuous process with five subphases:

  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase

Detailed Subphases of Mitosis

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

• Prometaphase: Nuclear envelope breaks down; microtubules attach to chromosomes at kinetochores (structures at the centromere).

• Metaphase: Chromosomes align at the metaphase plate; spindle formation is complete.

• Anaphase: Cohesions split; sister chromatids are pulled to opposite poles, creating two identical sets of chromosomes.

• Telophase: New nuclear envelopes form; chromosomes decondense; mitosis is complete when two nuclei have formed.

Structures Involved in Mitosis

Structure	Definition
Chromosome	A structure containing genetic information in the form of genes
Chromatin	The material that makes up eukaryotic chromosomes; consists of a DNA molecule complexed with histone proteins
Sister chromatids	Double-stranded DNA copies of a replicated chromosome, genetically identical
Centromeres	Specialized regions where sister chromatids are most closely joined
Kinetochores	Structures on sister chromatids where microtubules attach
Centrosome	Microtubule-organizing center in animals and some fungi
Microtubules	Cytoskeletal filaments forming the spindle apparatus
Microtubule motor proteins	Proteins that move chromosomes and the poles of the spindle apparatus

Mechanisms of Chromosome Movement

• Kinetochore microtubules remain stationary during anaphase but shorten as tubulin subunits are lost from their plus ends.

• Proteins from the kinetochore attach to a ring that surrounds the microtubule; as the plus end disassembles, the ring moves along the microtubule, pulling the chromosome toward the pole.

Cytokinesis

• In plants: Vesicles from the Golgi apparatus bring membrane and cell wall components to the middle of the cell, fusing to form a cell plate.

• In animals and many eukaryotes: A ring of actin and myosin filaments contracts inside the cell membrane, pinching inward to form a cleavage furrow until division is complete.

Example: Binary Fission in Bacteria

• Bacteria divide by binary fission, a process similar to eukaryotic M phase but without mitosis.

• Bacterial chromosomes are replicated, and protein filaments pull chromosomes apart before the cytoplasm divides.