Chapter 9 - The Cell Cycle

Overview of the Cell Cycle

The continuity of life is fundamentally based on the reproduction of cells, a process also known as cell division. This mechanism is vital for organisms, as it distinguishes living entities from nonliving matter by enabling growth, development, and maintenance of cellular structures.

Importance of Cell Division

Cell division serves crucial functions in both unicellular and multicellular organisms:

• Unicellular Organisms: In unicellular organisms, reproduction occurs through the division of a single cell, leading to the formation of new organisms.

• Multicellular Eukaryotes: In multicellular organisms, cell division allows for growth from a single fertilized egg (zygote) into a fully developed organism. Furthermore, it facilitates the renewal, repair, or replacement of cells necessary for maintaining tissue health and functionality. This process is essential for healing wounds and regenerating lost or damaged tissues.

Cell division is a critical component of the cell cycle, which outlines the life span of a cell from its formation until its division into daughter cells.

Concept 9.1: Genetic Identity in Cell Division

Most instances of cell division result in genetically identical daughter cells, ensuring that the genetic material is passed on without alteration across generations. This is essential for maintaining the integrity of genetic information in organisms.

• DNA Distribution: The distribution of DNA to daughter cells occurs through a highly regulated process that guarantees each new cell inherits the complete genome of the parent cell.

Cellular Organization of Genetic Material

• Genome: The complete set of DNA within a cell is referred to as its genome. In prokaryotes like bacteria, the genome typically consists of a single circular DNA molecule, while eukaryotes contain multiple linear DNA molecules organized into structures known as chromosomes.

• Chromatin Composition: Eukaryotic chromosomes are made up of chromatin, a complex of DNA and histone proteins that not only package DNA but also regulate its accessibility for transcription and replication.

• Chromosome Number: Each species of eukaryote has a characteristic number of chromosomes. Somatic (body) cells are diploid (2n), containing two sets of chromosomes, whereas gametes (sex cells) are haploid (n), containing only one set. Additionally, some organisms can have polyploid cells, such as tetraploid cells that contain four sets of chromosomes (4n). This condition is common in certain plants and can result in

Chromosome Distribution During Division

Prior to cell division, the cell undergoes DNA replication, leading to the condensation of chromosomes in preparation for mitosis.

• Sister Chromatids: Each duplicated chromosome consists of two sister chromatids that are joined at a region called the centromere. This connection is key for the accurate separation of genetic material during cell division.

• During the division process, sister chromatids are pulled apart and distributed into two newly formed nuclei, resulting in independent chromosomes.

Eukaryotic Cell Division

Eukaryotic cell division comprises two major phases:

1. Mitosis: The process through which the nucleus divides, ensuring that each daughter cell receives an identical set of chromosomes.

2. Cytokinesis: The subsequent division of the cytoplasm, leading to the physical separation of the two daughter cells.

• Meiosis: A specialized form of division that produces nonidentical daughter cells, which have half the chromosome count of the parent cell, essential for sexual reproduction.

Concept 9.2: The Cell Cycle Structure

The cell cycle is a carefully orchestrated series of events that can be divided into alternating phases:

• Mitotic (M) Phase: This phase encompasses both mitosis and cytokinesis, culminating in the division of a parent cell into two daughter cells.

• Interphase: This preparatory phase includes three distinct subphases:

  • G1 Phase (First Gap): The cell grows and synthesizes proteins necessary for DNA replication.

  • S Phase (Synthesis): DNA replication occurs, resulting in the formation of paired chromosomes.

  • G2 Phase (Second Gap): The cell continues to grow and prepares for mitosis by producing microtubules and other necessary components.

Mitosis Phases

Mitosis can be broken down into five noteworthy phases, each with specific functions:

1. Prophase: Chromatin condenses into visible chromosomes, and the mitotic spindle begins to form.

2. Prometaphase: The nuclear envelope breaks down, and spindle microtubules attach to kinetochores at the centromeres of chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate, ensuring they are positioned for accurate separation.

4. Anaphase: The cohesion holding sister chromatids together is cleaved, pulling them towards opposite poles of the cell.

5. Telophase: Chromosomes decondense back into chromatin, and nuclear envelopes reform around each set of chromosomes.

• Cytokinesis begins during telophase and involves the division of the cytoplasm to form two distinct cells.

The Mitotic Spindle

The mitotic spindle is a specialized structure made up of microtubules and proteins that is essential for the movement and segregation of chromosomes during mitosis.

• Origin: The spindle originates from the centrosome, which duplicates during interphase and organizes microtubules that attach to chromosomes.

Prometaphase to Metaphase Transition

During prometaphase, spindle microtubules attach to kinetochores located at the centromeres of chromosomes, facilitating chromosome movement. At metaphase, the alignment of centromeres at the metaphase plate ensures the fidelity of genetic segregation.

Anaphase and Cytokinesis

• During anaphase, sister chromatids are separated and pulled toward opposite poles of the cell, a process powered by the shortening of microtubules.

• Cytokinesis follows mitosis, where animal cells form a cleavage furrow that pinches the cell into two, while plant cells develop a cell plate that eventually forms a new cell wall between the daughter cells.

Binary Fission in Prokaryotes

Prokaryotic organisms, such as bacteria, undergo binary fission, a simpler form of reproduction involving replication of a single circular chromosome and division of the cytoplasm into two daughter cells.

Regulation of the Cell Cycle

The eukaryotic cell cycle is tightly regulated by a series of checkpoints and signaling pathways, ensuring various conditions are met before a cell progresses through the cycle:

• G1 Checkpoint: a key regulatory point determining whether the cell should proceed to division; it assesses cell size, nutrient availability, and DNA integrity.

• Cells that do not receive the necessary signals to move forward enter a non-dividing state known as the G0 phase, where they remain metabolically active but no longer divide.

Cancer and Cell Cycle Regulation

Cancer cells are characterized by their ability to bypass normal regulatory mechanisms, resulting in uncontrolled division. These cells may:

• Produce their own growth factors.

• Respond abnormally to external growth signals, leading to the formation of tumors.

• Undergo metastasis, where cancer cells spread to other tissues.

Advances in Cancer Treatment

Recent advancements in our understanding of cell cycle regulation have facilitated the development of personalized cancer treatments, focusing on targeting specific characteristics of tumors to improve treatment efficacy and minimize side effects.