Cell Division:
Cell Division: The Basis of Life [1]
Cell division is the process by which cells reproduce, creating new cells from existing ones. It is essential for all living organisms.
Key Roles of Cell Division:Asexual reproduction: Single-celled organisms utilize cell division to create offspring, effectively reproducing themselves. [1]
Growth and development: Cell division is the driving force behind the development of multicellular organisms from a single fertilized egg (zygote). [1]
Tissue renewal: Cell division plays a crucial role in replacing cells that are lost due to damage or wear and tear. Examples include the continuous production of new blood cells in bone marrow and the repair of damaged skin tissue. [1]
Cellular Organization of Genetic Material [2]
The genome, a cell's complete set of genetic information, is encoded in DNA. [2]
Prokaryotic genomes typically consist of a single DNA molecule, whereas eukaryotic genomes are composed of multiple DNA molecules. [2]
Chromosomes: DNA molecules are packaged into structures called chromosomes. Each chromosome consists of a long, linear DNA molecule associated with proteins. [2]
Chromosomes organize and condense the DNA, making it manageable for replication and distribution during cell division. [2]
The combination of DNA and proteins that form chromosomes is referred to as chromatin. [2]
Chromosome Number: Each eukaryotic species has a characteristic number of chromosomes in its cells. [3]
Human somatic cells (body cells) possess 46 chromosomes, arranged in two sets of 23, one set inherited from each parent. [3]
Human gametes (sperm and egg cells) contain 23 chromosomes, representing a single set. [3]
Chromosome Duplication and Distribution [4, 5]
When a cell is not dividing, chromosomes exist as long, thin chromatin fibers. [4]
During cell division, chromosomes condense, becoming thicker and visible under a light microscope. [4]
Sister Chromatids: When a chromosome replicates, it forms two identical copies called sister chromatids. [4]
Sister chromatids are connected along their length by protein complexes called cohesins, a phenomenon known as sister chromatid cohesion. [4]
Centromere: Each sister chromatid has a centromere, a region where the chromatid is most closely attached to its sister. [4]
Proteins bind to the centromeric DNA, mediating the attachment and condensing the DNA, giving the chromosome its characteristic "waist." [4]
Separation of Sister Chromatids: During cell division, sister chromatids separate, becoming individual chromosomes. Each daughter cell receives a complete set of chromosomes, identical to the parent cell. [5]
This separation effectively doubles the chromosome number during cell division. [5]
The Cell Cycle [6, 7]
The cell cycle is the complete sequence of events from the formation of a new cell through its subsequent division into two daughter cells. [6]
Phases of the Cell Cycle: [6]
Interphase: The longest phase of the cell cycle, subdivided into:
G1 phase (first gap): The cell grows, synthesizes proteins, and performs its normal functions. [6]
S phase (synthesis): DNA replication occurs, resulting in the duplication of chromosomes. [6]
G2 phase (second gap): The cell continues to grow and prepares for cell division. [6]
Mitotic (M) phase: Consists of:
Mitosis: The division of the nucleus, resulting in two nuclei with identical genetic information. [6]
Cytokinesis: The division of the cytoplasm, producing two daughter cells. [6]
Mitosis [8, 9]
Mitosis is the process by which a single nucleus divides into two genetically identical daughter nuclei. [8]
Stages of Mitosis: [8, 9]
Prophase:Chromatin fibers condense, forming visible chromosomes. [9]
The nucleolus disappears. [9]
The mitotic spindle begins to form in the cytoplasm. [9]
The spindle is composed of microtubules and associated proteins. [9]
Centrosomes, microtubule-organizing centers in animal cells, move apart. [9]
Prometaphase:The nuclear envelope fragments. [9]
Spindle microtubules extend into the nuclear area and attach to kinetochores on chromosomes. [9]
Chromosomes are moved back and forth by the attached microtubules. [9]
Metaphase:Centrosomes are positioned at opposite poles of the cell. [9]
Chromosomes align at the metaphase plate, an imaginary plane equidistant between the poles. [9]
Kinetochores of sister chromatids face opposite poles and are attached to kinetochore microtubules. [9]
Anaphase:Cohesin proteins are cleaved, separating sister chromatids. [9]
Separated chromatids, now individual chromosomes, move towards opposite poles as their kinetochore microtubules shorten. [9]
The cell elongates due to lengthening nonkinetochore microtubules. [9]
Telophase:Two daughter nuclei form. [9]
Nuclear envelopes reform around the separated chromosomes. [9]
Nucleoli reappear. [9]
Chromosomes decondense. [9]
Spindle microtubules depolymerize. [9
Microtubules
Microtubules are a dynamic component of the cytoskeleton and are crucial for cell division. [1]
They are made of tubulin dimers, composed of alpha and beta tubulin monomers. [1]
Microtubules have a plus (+) end and a minus (-) end. [1]
Dynamic instability is a key feature of microtubules. They can switch between periods of slow growth and rapid disassembly. [2]
This dynamic instability is influenced by the presence of GTP-bound tubulin, which stabilizes the plus end of microtubules. [2]
When the GTP cap is lost, the microtubule undergoes rapid depolymerization. [2]
The mitotic spindle forms from microtubules in the cytoplasm during prophase. [3]
The spindle is made up of fibers of microtubules and associated proteins. [3]
The spindle forms as other microtubules in the cytoskeleton disassemble. [3]
In animal cells, spindle microtubule assembly begins at the centrosome, a region containing material that organizes the cell’s microtubules. [3]
A pair of centrioles is located at the center of the centrosome. [3]
Centrioles are not essential for cell division and are not present in plant cells. [3]
During interphase, the single centrosome duplicates, forming two centrosomes that move apart during prophase and prometaphase. [3]
By the end of prometaphase, the two centrosomes are at opposite poles of the cell. [3]
Three types of microtubules make up the mitotic spindle: [4, 5]
Kinetochore microtubules: attach to the kinetochores of sister chromatids. [4, 5]
Nonkinetochore microtubules: interact with those from the opposite pole of the spindle, helping to elongate the cell. [4-7]
Aster microtubules: extend from each centrosome. [6]
Motor proteins play an important role in chromosome movement during mitosis. [5]
These proteins carry chromosomes directionally along microtubules, a process that requires ATP. [5]
Two mechanisms are at play during anaphase: [8]
Motor proteins on the kinetochores “walk” the chromosomes along the microtubules. The microtubules depolymerize at their kinetochore ends after the motor proteins pass. [8]
Chromosomes are “reeled in” by motor proteins at the spindle poles. The microtubules depolymerize after they pass by these motor proteins. [8]
Both mechanisms contribute to chromosome movement, and their relative importance varies among cell types. [8]
The drug Taxol stabilizes microtubules and blocks depolymerization, which prevents mitotic spindle formation. [4]
This property makes Taxol useful as an anti-cancer drug as it causes actively dividing cells to arrest. [4]
Cytokinesis [10-12]
Cytokinesis is the division of the cytoplasm, resulting in two daughter cells. It usually overlaps with the later stages of mitosis. [10]
Animal Cells:Cleavage furrow: A shallow groove forms on the cell surface near the former metaphase plate. [10]
A contractile ring of actin microfilaments and myosin proteins contracts, deepening the furrow and pinching the cell in two. [10]
Plant Cells:Cell plate formation: Vesicles from the Golgi apparatus move to the middle of the cell, coalescing to form a cell plate. [11]
Cell wall materials accumulate within the cell plate, which grows outward until it fuses with the plasma membrane, dividing the cell in two. [11]
Binary Fission in Bacteria [13-15]
Prokaryotes, such as bacteria, reproduce asexually through binary fission, a process of cell division that results in two genetically identical daughter cells. [13]
Process of Binary Fission:DNA replication begins at a specific site called the origin of replication, producing two origins. [13]
As replication continues, one origin migrates to the opposite end of the cell, and the cell elongates. [14]
When replication is complete, the plasma membrane pinches inward, dividing the cell into two daughter cells, each with a complete genome. [14]
Proteins Involved in Binary Fission:Actin-like proteins contribute to chromosome movement during binary fission. [15]
Tubulin-like proteins assist in pinching the plasma membrane inward. [15]
Evolution of Mitosis [16, 17]
It is hypothesized that mitosis evolved from simpler prokaryotic cell division mechanisms like binary fission. [16]
Support for this hypothesis comes from the observation that certain proteins involved in binary fission are related to eukaryotic proteins that function in mitosis. [16]
Some unicellular eukaryotes exhibit unusual nuclear division mechanisms that might represent intermediate stages in the evolution of mitosis. These mechanisms often involve the nuclear envelope remaining intact during division. [16, 17]
Cell Cycle Control System [18-20]
The cell cycle is regulated by a complex network of signaling molecules known as the cell cycle control system. [18, 19]
This system ensures that the cell cycle progresses in an orderly manner, with each phase completed before the next begins. [18, 19]
Checkpoints: The cell cycle control system includes checkpoints, which are control points where stop and go-ahead signals can regulate the cycle. [19]
Checkpoints ensure that critical cellular processes have been completed accurately before the cell proceeds to the next phase. [19]
Key checkpoints exist in the G1, G2, and M phases of the cell cycle. [19]
Cyclins and Cyclin-Dependent Kinases (Cdks): The cell cycle control system is primarily driven by two types of proteins: cyclins and cyclin-dependent kinases (Cdks). [20]
Cyclins: Proteins whose concentration fluctuates cyclically throughout the cell cycle. [20]
Cdks: Kinases (enzymes that phosphorylate other proteins) that are activated when they bind to cyclins. [20]
The activity of Cdks rises and falls with the concentration of their corresponding cyclins. [20]
Stop and Go Signals: Internal and External Signals at the Checkpoints [21-23]
Stop and Go-Ahead Signals: Checkpoints in the cell cycle are regulated by stop and go-ahead signals that originate from both inside and outside the cell. [21]
Internal signals: Monitor the completion of crucial cellular processes, such as DNA replication and chromosome attachment to the spindle. [21]
External signals: Include growth factors that stimulate cell division and density-dependent inhibition that stops cell division when cells become crowded. [21]
Key Checkpoints: [22, 23]
G1 checkpoint: One of the most important checkpoints, determining whether a cell will proceed through the cell cycle or exit into a non-dividing state called the G0 phase. [22]
Cells that do not receive a go-ahead signal at the G1 checkpoint may enter the G0 phase, where they remain until stimulated to re-enter the cell cycle. [22]
M checkpoint: Ensures that all chromosomes are properly attached to the spindle at the metaphase plate before anaphase begins. [23]
This mechanism prevents daughter cells from inheriting an incorrect number of chromosomes. [23]
S phase checkpoint: Prevents cells with DNA damage from continuing the cell cycle. [24]
Anaphase/Telophase checkpoint: Ensures that anaphase is completed and chromosomes are properly separated before cytokinesis begins. [24]
Growth Factors and Density-Dependent Inhibition [24-27]
Growth factors: Proteins released by certain cells that stimulate other cells to divide. [24]
Different cell types respond to specific growth factors. [24]
Example: Platelet-derived growth factor (PDGF), released by platelets at injury sites, stimulates fibroblast division, promoting wound healing. [24, 25]
Density-dependent inhibition: A phenomenon where crowded cells stop dividing. [26]
Cells typically divide until they form a single layer, at which point they cease dividing due to contact inhibition. [26]
This mechanism helps regulate cell growth and tissue density. [26]
Anchorage dependence: Most animal cells require attachment to a surface or the extracellular matrix to divide. [27]
This dependence ensures that cells divide only in appropriate locations. [27]
Density-dependent inhibition and anchorage dependence function both in cell culture and within the body, controlling cell growth and preventing uncontrolled proliferation. [28]
Loss of Cell Cycle Controls in Cancer Cells [29, 30]
Cancer cells are characterized by uncontrolled cell division. [29]
They disregard the normal signals that regulate the cell cycle. [29]
Key characteristics of cancer cells: [29, 30]
Lack of density-dependent inhibition and anchorage dependence: Cancer cells continue to divide even when crowded or unattached. [29]
Indefinite division potential: Cancer cells can divide indefinitely in culture if provided with nutrients, making them "immortal." [29]
Evasion of apoptosis: Cancer cells can evade programmed cell death (apoptosis) even when significant damage or errors occur. [29]
Tumor Formation: Uncontrolled cell division can lead to the formation of tumors, masses of abnormal cells. [30]
Benign tumors: Remain at the original site and typically do not pose serious health threats. [30]
Malignant tumors (cancer): Can invade surrounding tissues and spread to other parts of the body through metastasis. [30]
Metastasis: The spread of cancer cells to distant locations from the original tumor. [31]
Cancer Treatments [31-33]
Radiation therapy: Uses high-energy radiation to damage DNA in cancer cells, often targeting localized tumors. [31]
Chemotherapy: Employs drugs that are toxic to actively dividing cells, often used to treat metastatic cancers. [31]
Chemotherapeutic drugs interfere with specific stages of the cell cycle. [31]
Example: Taxol prevents microtubule depolymerization, halting cell division at metaphase. [31]
Targeted therapies: Aim to disrupt specific proteins or signaling pathways involved in cancer cell growth and proliferation. [34]
Example: Herceptin targets the Her2 receptor tyrosine kinase, and tamoxifen targets estrogen receptors, both of which can be overexpressed in certain breast cancers. [34]