2.3 Cell cycle and division
5 min read•july 30, 2024
Cell division is the foundation of life, growth, and reproduction. The cell cycle orchestrates this process, guiding cells through stages of growth, DNA replication, and division. Understanding these mechanisms is crucial for grasping how organisms develop and maintain themselves.
and are two types of cell division with distinct purposes. Mitosis produces identical daughter cells for growth and repair, while meiosis creates diverse gametes for sexual reproduction. Both processes are tightly regulated to ensure genetic stability and prevent errors that could lead to diseases like cancer.
Cell cycle stages and characteristics
Interphase and its subdivisions
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- The cell cycle consists of and the mitotic phase (M phase)
- Interphase is subdivided into G1, S, and G2 phases
- During G1 phase, the cell grows in size, synthesizes proteins, and prepares for DNA replication
- The duration of G1 varies among cell types and can be influenced by external factors (nutrient availability, growth factors)
- S phase is characterized by DNA replication, resulting in the doubling of the cell's genetic material
- Each chromosome now consists of two sister chromatids
- In G2 phase, the cell continues to grow and synthesize proteins in preparation for cell division
- Organelles are duplicated (centrosomes, mitochondria)
- The cell ensures that DNA replication has been completed accurately
M phase and quiescent state
- M phase is composed of mitosis (nuclear division) and (cytoplasmic division)
- Mitosis is further divided into prophase, metaphase, anaphase, and telophase
- Some cells exit the cell cycle and enter a quiescent state called G0, where they remain metabolically active but do not divide
- Cells in G0 can re-enter the cell cycle under appropriate conditions (tissue damage, hormonal stimulation)
- The duration of the cell cycle varies among different cell types
- Rapidly dividing cells (embryonic cells, adult stem cells) have shorter cell cycles
- Slowly dividing or non-dividing cells (neurons, muscle cells) have longer cell cycles or remain in G0
Checkpoints in cell cycle regulation
Types of cell cycle checkpoints
- Cell cycle checkpoints are control mechanisms that ensure the proper progression of the cell through the cycle
- Maintain genomic integrity and prevent uncontrolled cell division
- The , also known as the restriction point in mammalian cells, checks for cell size, nutrient availability, and growth factors before committing to DNA replication
- The G1/S checkpoint ensures that the cell is ready for DNA replication and that the DNA is not damaged before proceeding to S phase
- The intra-S checkpoint monitors the progress of DNA replication and arrests the cell cycle if DNA damage is detected, allowing time for repair
- The G2/ assesses cell size and ensures that DNA replication is complete and error-free before the cell enters mitosis
- The spindle assembly checkpoint (SAC) in metaphase ensures proper attachment of spindle fibers to kinetochores and equal distribution of chromosomes before proceeding to anaphase
Molecular regulators of checkpoints
- Checkpoint regulation involves various proteins, such as , cyclin-dependent kinases (CDKs), and tumor suppressor proteins
- Cyclins and CDKs form complexes that drive the cell cycle progression
- Tumor suppressor proteins (p53, RB) act as negative regulators of the cell cycle
- Dysregulation of cell cycle checkpoints can lead to uncontrolled cell division and contribute to the development of cancer
- Mutations in tumor suppressor genes (p53, RB) can result in checkpoint failure and genomic instability
- DNA damage response pathways (ATM/ATR signaling) are activated at checkpoints to halt the cell cycle and initiate repair mechanisms
- If the damage is irreparable, the cell may undergo (programmed cell death) to prevent the propagation of mutations
Process of mitosis and its importance
Stages of mitosis
- Mitosis is a process of nuclear division that results in the formation of two genetically identical daughter cells from a single parent cell
- During prophase, chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the mitotic spindle begins to form
- Centrosomes, containing centrioles, migrate to opposite poles of the cell
- In metaphase, chromosomes align at the equatorial plane of the cell, with spindle fibers attached to the kinetochores of sister chromatids
- The spindle assembly checkpoint (SAC) ensures proper attachment and tension of spindle fibers
- During anaphase, sister chromatids separate and are pulled towards opposite poles of the cell by the shortening of spindle fibers
- Cohesion proteins, which hold sister chromatids together, are cleaved by separase
- In telophase, chromosomes decondense, the nuclear envelope re-forms around the separated chromosomes, and the spindle apparatus disassembles
- Nucleoli reappear, marking the end of mitosis
Cytokinesis and the importance of mitosis
- Cytokinesis, the division of the cytoplasm, occurs concurrently with telophase
- In animal cells, this involves the formation of a cleavage furrow, which pinches the cell membrane inward
- In plant cells, a cell plate forms from vesicles derived from the Golgi apparatus and grows centripetally to divide the cytoplasm
- Mitosis is essential for growth, development, and tissue repair in multicellular organisms
- Allows for the production of genetically identical cells to increase cell number and replace damaged or dead cells
- Mitosis also plays a role in asexual reproduction, where offspring arise from a single parent and are genetically identical to the parent
- Examples include in prokaryotes and budding in some eukaryotes (hydra, yeast)
Mitosis vs meiosis
Key differences between mitosis and meiosis
- Mitosis produces two genetically identical daughter cells, while meiosis produces four genetically diverse haploid gametes or spores
- Mitosis involves one cell division, whereas meiosis involves two consecutive cell divisions (meiosis I and meiosis II)
- Meiosis I is a reductional division, separating homologous chromosomes
- Meiosis II is an equational division, separating sister chromatids
- In mitosis, chromosomes replicate once, while in meiosis, chromosomes replicate only once, followed by two rounds of segregation
- Mitosis maintains the diploid chromosome number, while meiosis reduces the chromosome number by half, resulting in haploid cells
Genetic variation and errors in cell division
- Crossing over and independent assortment during meiosis I contribute to genetic variation in the resulting gametes or spores
- Crossing over involves the exchange of genetic material between homologous chromosomes
- Independent assortment of chromosomes leads to random combinations of maternal and paternal chromosomes
- Mitosis occurs in somatic cells for growth and repair, while meiosis occurs in germ cells or reproductive structures to produce gametes or spores for sexual reproduction
- Gametes (sperm, egg) fuse during fertilization to restore the diploid chromosome number in the zygote
- Errors in mitosis can lead to somatic mutations and contribute to cancer development
- Aneuploidy (abnormal chromosome number) and chromosomal instability are hallmarks of many cancers
- Errors in meiosis can result in chromosomal abnormalities and genetic disorders in offspring
- Nondisjunction (failure of chromosomes to separate) can lead to trisomy (Down syndrome) or monosomy (Turner syndrome)
Key Terms to Review (20)
Apoptosis: Apoptosis is a programmed process of cell death that occurs in a regulated manner, allowing for the removal of unwanted or damaged cells without causing an inflammatory response. This essential biological mechanism plays a crucial role in maintaining tissue homeostasis, regulating development, and ensuring the proper functioning of multicellular organisms. The process is characterized by distinct morphological changes and biochemical events, such as DNA fragmentation and membrane blebbing, which serve to safely eliminate cells.
Binary Fission: Binary fission is a form of asexual reproduction in which a single organism divides into two identical daughter cells. This process is crucial for prokaryotic organisms, such as bacteria, allowing them to rapidly increase their population size. Binary fission involves the replication of the organism's DNA and the division of cellular components, leading to two new cells that are genetically identical to the parent cell.
Cdks (cyclin-dependent kinases): Cyclin-dependent kinases (cdks) are a family of enzymes that play a critical role in regulating the cell cycle by phosphorylating specific target proteins. These enzymes are dependent on the binding of cyclins, which are regulatory proteins whose levels fluctuate throughout the cell cycle, ensuring that cdks are active only at the appropriate stages. This regulation is essential for proper cell division and growth, as it coordinates the progression through different phases of the cell cycle.
Cell Proliferation: Cell proliferation is the process by which cells grow and divide to produce new cells, playing a critical role in tissue growth, repair, and regeneration. This process is tightly regulated by various internal and external factors, ensuring that cells proliferate in a controlled manner, which is essential for maintaining healthy tissues and organ systems.
Cellular Senescence: Cellular senescence is a state in which cells cease to divide and grow, often as a response to stress or damage, and enter a permanent cell cycle arrest. This process serves as a protective mechanism to prevent the proliferation of damaged cells, which could lead to cancer. Senescent cells can influence their environment by secreting various factors, contributing to tissue remodeling and inflammation.
CRISPR Gene Editing: CRISPR gene editing is a revolutionary technology that allows for precise modifications to DNA sequences in living organisms. It utilizes a guide RNA to direct the Cas9 enzyme to a specific location in the genome, where it can cut the DNA, enabling scientists to add, delete, or alter genetic material. This powerful tool has opened new avenues for research and therapies, especially in regenerative medicine, by addressing genetic disorders and enhancing our understanding of cellular processes.
Cyclins: Cyclins are a family of proteins that regulate the progression of the cell cycle by activating cyclin-dependent kinases (CDKs). These proteins are essential for controlling various stages of the cell cycle, ensuring that cells divide at the right time and under appropriate conditions. Cyclins are produced and degraded in a cyclical manner, which means their levels fluctuate throughout the cell cycle, playing a crucial role in maintaining cellular homeostasis.
Cytokinesis: Cytokinesis is the process during cell division where the cytoplasm of a parental cell is divided into two daughter cells. This crucial phase follows mitosis and ensures that each new cell receives the necessary organelles and cytoplasmic components to function properly. It plays a key role in maintaining cellular integrity and is essential for the growth, development, and repair of tissues.
Differentiation: Differentiation is the process by which unspecialized cells develop into specialized cells with distinct functions and characteristics. This critical process is essential for the formation of tissues and organs during development, as well as for maintaining the functionality of adult tissues through regenerative processes.
DNA repair mechanisms: DNA repair mechanisms are a set of biological processes that identify and correct damage to the DNA molecules that encode an organism's genetic information. These mechanisms are essential for maintaining genomic integrity and preventing mutations, which can lead to diseases such as cancer. The efficiency of DNA repair processes is closely linked to the cell cycle, particularly during specific phases where DNA is replicated or repaired, highlighting their crucial role in cell division and overall cellular health.
G1 Checkpoint: The G1 checkpoint is a critical regulatory point in the cell cycle, occurring at the end of the G1 phase before the cell transitions into the S phase. It serves as a control mechanism that assesses whether the cell is ready to proceed with DNA replication, ensuring that conditions are favorable for division and that any DNA damage has been repaired. This checkpoint plays a key role in maintaining genomic integrity and preventing uncontrolled cell proliferation, which can lead to cancer.
G2 Checkpoint: The G2 checkpoint is a critical regulatory point in the cell cycle that occurs at the end of the G2 phase, just before the cell enters mitosis. This checkpoint ensures that the cell has successfully completed DNA replication and checks for any DNA damage, allowing the cell to proceed to division only if conditions are favorable. The G2 checkpoint is essential for maintaining genomic integrity and preventing the division of damaged or incomplete DNA.
Induced pluripotent stem cells (iPSCs): Induced pluripotent stem cells (iPSCs) are a type of stem cell that can be generated from adult cells through the introduction of specific genes and factors, reprogramming them back into a pluripotent state. This unique ability allows iPSCs to differentiate into virtually any cell type in the body, making them a powerful tool for regenerative medicine, cell sourcing, and tissue engineering applications.
Interphase: Interphase is the stage of the cell cycle where a cell spends the majority of its life, preparing for division by growing and replicating its DNA. This phase is crucial because it consists of three subphases: G1 (gap 1), S (synthesis), and G2 (gap 2), which collectively ensure that the cell is ready to divide successfully. During interphase, the cell also performs its regular functions and metabolic processes, making it a vital period for cellular health and function.
M checkpoint: The m checkpoint, also known as the mitotic checkpoint, is a crucial regulatory point in the cell cycle that ensures proper chromosome alignment and segregation during mitosis. It serves to prevent cells from progressing to anaphase until all chromosomes are correctly attached to the spindle apparatus, thus maintaining genomic stability. This checkpoint is vital for preventing aneuploidy, where cells have an abnormal number of chromosomes, which can lead to cancer and other diseases.
Meiosis: Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the formation of gametes—sperm and eggs. This process is crucial for sexual reproduction, ensuring genetic diversity through recombination and independent assortment during gamete formation, which plays a significant role in the life cycle of sexually reproducing organisms.
Mitosis: Mitosis is a type of cell division that results in two genetically identical daughter cells, each having the same number of chromosomes as the original cell. This process is crucial for growth, development, and tissue repair in multicellular organisms. Mitosis ensures that when cells divide, the genetic material is accurately replicated and distributed, maintaining genetic stability across generations of cells.
Pluripotency: Pluripotency refers to the ability of a stem cell to differentiate into any type of cell in the body, except for those needed to develop a fetus. This unique capability allows pluripotent stem cells to give rise to cells from all three germ layers: ectoderm, mesoderm, and endoderm. Understanding pluripotency is crucial as it relates to cellular reprogramming, stem cell therapy, and tissue engineering, all of which hold potential for regenerative medicine.
Synthesis phase: The synthesis phase is a critical part of the cell cycle where DNA replication occurs, leading to the duplication of the cell's genetic material. This phase ensures that each daughter cell will have an identical set of chromosomes after cell division. During this period, not only is the DNA copied, but associated proteins and organelles may also be synthesized, setting the stage for successful cell division.
Tumorigenesis: Tumorigenesis is the process by which normal cells transform into cancerous cells, leading to the formation of tumors. This complex process involves multiple stages, including initiation, promotion, and progression, and is influenced by genetic mutations, environmental factors, and alterations in cellular signaling pathways. Understanding tumorigenesis is essential for developing targeted therapies and interventions in cancer treatment.