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Prokaryotic Binary Fission
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Today, we're starting with binary fission, the division process used by prokaryotic cells like bacteria. Can anyone tell me how this process begins?
Isn't it related to the origin of replication, where the DNA starts to duplicate?
That's right! It begins at the **oriC** region, where two replication forks proceed bidirectionally. What happens next during fission?
The new origins of the chromosomes move to opposite ends of the cell?
Exactly! This movement is facilitated by the **ParABS system**. Finally, how does the cell separate into two daughter cells?
The FtsZ protein forms the Z-ring at the center and helps with cytokinesis, right?
Correct! The FtsZ protein acts similarly to tubulin. Great job, everyone!
So remember, binary fission involves replication at **oriC**, segregation via **ParABS**, and septum formation led by **FtsZ**.
Eukaryotic Cell Cycle
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Now let's shift our focus to eukaryotic cells. Can anyone list the primary phases of the cell cycle?
I think they are G1, S, G2, and the M phase?
That's right! We have G1 where the cell grows, the S phase for DNA synthesis, G2 for further growth and preparation for mitosis, and finally, the M phase which is mitosis. What is important about the checkpoints we have in this cycle?
They ensure conditions are right for the cell to proceed to the next phase?
Exactly! The **G1/S checkpoint** checks for DNA integrity; the **G2/M checkpoint** ensures all DNA is replicated before mitosis; and the **spindle assembly checkpoint** confirms all kinetochores are attached. Can anyone explain why these checkpoints are crucial?
To prevent errors in cell division that could lead to problems like cancer?
Absolutely! Proper regulation is vital. Remember: checkpoints are essential for ensuring healthy cell division.
Mitosis vs. Meiosis
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Let's compare mitosis and meiosis! Who can tell me the main purpose of mitosis?
It's to create two genetically identical diploid daughter cells, right?
Correct! And what about meiosis?
Meiosis produces four haploid gametes and introduces genetic diversity!
Exactly! Meiosis includes two rounds of division and genetic recombination. What are the stages of meiosis that contribute to this diversity?
During Prophase I, crossing over occurs, which increases genetic variation!
Great point! This process results in genetic diversity. Additionally, independent assortment in Metaphase I also plays a significant role. Can anyone summarize the significance of these processes?
Theyโre key for evolution since they increase the genetic variability within populations!
Yes! Variability is essential for natural selection and adaptation. Keep this in mind as we move forward!
Regulation of Cell Division
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Now, letโs talk about how the cell cycle is regulated. Who knows what cyclins do?
Cyclins activate cyclin-dependent kinases, which are crucial for moving through the cell cycle!
That's spot on! Can anyone tell me how the activation of these kinases occurs?
The kinases get activated by phosphorylation, and they need the right cyclin present!
Exactly! The cyclin-Cdk complexes drive transitions. Why is it important for these processes to be tightly regulated?
To prevent uncontrolled cell division, which can lead to cancer?
Precisely! Regulation is key to maintain healthy cellular function. Remember: cyclins and Cdks oversee this balance!
Overall Summary
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Can anyone summarize what we discussed during our sessions today?
We talked about binary fission, the cell cycle, mitosis, meiosis, and how the processes are regulated.
And the importance of checkpoints to ensure that cells divide correctly!
Great summary! Remember that the mechanisms of both mitosis and meiosis are crucial for genetic continuity and diversity. Now, who can name one key difference between mitosis and meiosis?
Mitosis results in two identical cells, while meiosis produces four genetically diverse ones.
Excellent! Understanding these processes is vital for grasping how life continues and evolves. Let's remember the significance of these processes in biology.
Introduction & Overview
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Quick Overview
Standard
Cell and nuclear division are crucial for the propagation of life, encompassing the mechanisms of mitosis in eukaryotes, which produces two identical daughter cells, and meiosis, which yields four genetically diverse gametes. The section also outlines prokaryotic division through binary fission, highlighting the contrasting processes between eukaryotic and prokaryotic organisms.
Detailed
Detailed Summary of Cell and Nuclear Division
Accurate segregation of duplicated genetic material is essential for growth, development, and reproduction. This section discusses the two major division processes in eukaryotic cells: mitosis and meiosis.
1. Prokaryotic Binary Fission
Prokaryotes divide via binary fission, a simpler process reflective of their circular chromosomes and lack of nuclear envelope. Key steps in this process include:
1.1 Chromosome Replication
- Begins at the oriC (origin of replication), where two replication forks progress bidirectionally until they meet at the terminus region.
1.2 Partitioning (Segregation)
- Newly replicated chromosome origins are actively moved to opposite cell poles by the ParABS system (found in many bacteria).
1.3 Septum Formation and Cytokinesis
- The FtsZ protein (a homolog of tubulin) polymerizes at midcell to form the Z-ring, necessary for cytokinesis.
- The MinCDE system prevents the formation of the Z-ring at the poles of the cell.
2. Eukaryotic Cell Cycle and Checkpoints
Eukaryotic cells progress through a defined cell cycle composed of:
- Gโ (gap 1)
- S (DNA synthesis)
- Gโ (gap 2)
- M (mitosis)
Key phases within this cycle include:
2.1 Gโ Phase
- The cell grows and evaluates environmental conditions and DNA integrity.
2.2 S Phase
- In this phase, DNA replication occurs, ensuring that each daughter cell receives an identical set of chromosomes.
2.3 Gโ Phase
- The cell continues to grow and prepares for mitosis. This phase includes checkpoint mechanisms to verify successful DNA replication and repair any damage.
2.4 M Phase (Mitosis)
Mitosis can be subdivided into several stages:
1. Prophase: Chromatin condenses, and the mitotic spindle begins to form.
2. Prometaphase: The nuclear envelope breaks down, allowing spindle fibers to attach to kinetochores.
3. Metaphase: Chromosomes align at the metaphase plate.
4. Anaphase: Sister chromatids are pulled apart toward opposite poles.
5. Telophase: Chromosomes de-condense, and the nuclear envelope re-forms.
6. Cytokinesis: The cytoplasm divides, resulting in two genetically identical daughter cells.
2.5 Cell Cycle Checkpoints
- Gโ/S Checkpoint: Assesses cell size, nutrient availability, and DNA integrity.
- Gโ/M Checkpoint: Ensures complete DNA replication before mitosis begins.
- Spindle Assembly Checkpoint: Monitors proper attachment of kinetochores to spindle fibers before anaphase commences.
3. Nuclear Division: Mitosis vs. Meiosis
3.1 Mitosis
- Produces two genetically identical diploid daughter cells for growth and repair without genetic recombination.
3.2 Meiosis
- Produces four haploid gametes through two successive divisions, introducing genetic diversity through recombination and independent assortment.
4. Regulation of Cell and Nuclear Division
Eukaryotic cell cycle regulation involves:
- Cyclin-dependent kinases (Cdks) paired with specific cyclins to drive the cell cycle forward through various checkpoints and transitions.
- The Spindle Assembly Checkpoint (SAC) monitors kinetochore attachment.
- The Cohesin and Shugoshin proteins help maintain sister chromatid integrity until the right moment in the cell cycle.
This comprehensive exploration of cell and nuclear division illustrates the processes ensuring the faithful transmission of genetic material, which is fundamental for maintaining continuity of life across generations.
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Prokaryotic Binary Fission
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- Prokaryotic Binary Fission
- Chromosome Replication
- Begins at oriC; two replication forks progress bidirectionally until meeting at the terminus region.
- Partitioning (Segregation)
- Newly replicated origins are actively moved toward opposite cell poles by the ParABS system (in many bacteria).
- Septum Formation and Cytokinesis
- FtsZ protein (homolog of tubulin) polymerizes at midcell, forming the Zโring.
- MinCDE system in E. coli prevents Zโring formation at poles: MinC and MinD inhibit FtsZ polymerization; MinE oscillates, ensuring lowest MinC concentration at midcell.
- Zโring recruits division proteins (FtsA, ZipA, FtsK, FtsI [penicillinโbinding protein 3]), forming the divisome.
- Peptidoglycan Remodeling: Amidases cleave existing peptidoglycan; SEDS proteins (FtsW, RodA) synthesize new cell wall; PBP enzymes crosslink.
- Constriction proceeds inward, dividing cytoplasm and creating two daughter cells.
Detailed Explanation
Prokaryotic binary fission is how bacteria and archaea reproduce asexually. It starts with chromosome replication, where the DNA is duplicated at a specific location called oriC. Two replication forks spread out until they meet at the opposite end. Then, the two new copies of the DNA are separated and moved to opposite ends of the cell using a system called ParABS. Next, the cell prepares to divide, forming a Z-ring in the center made from the FtsZ protein, which is similar to tubulin found in eukaryotic cells. The Z-ring helps recruit other proteins that will assist in creating a new cell wall between the two future daughter cells. The process finishes with the cell constricting and dividing, forming two separate cells, each with an identical copy of the original DNA.
Examples & Analogies
You can think of prokaryotic binary fission like baking bread in a long loaf pan. Just like you might cut the dough in half to create two smaller loaves, bacteria replicate their DNA and then divide into two cells. The Z-ring is like the line where you would cut the dough, ensuring both sides get evenly baked and are identical to each other.
Eukaryotic Cell Cycle and Checkpoints
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- Eukaryotic Cell Cycle and Checkpoints
Eukaryotic cells progress through an ordered cycle: Gโ (gap 1), S (DNA synthesis), Gโ (gap 2), and M (mitosis). Quiescent cells enter Gโ. Transitions regulated by cyclinโdependent kinases (Cdks) activated by specific cyclins. - Gโ Phase
- Cell grows; evaluates environmental conditions, DNA integrity; decision point (Restriction point in mammals).
- Cdk4/6โCyclin D: Phosphorylates retinoblastoma (Rb) protein, freeing E2F transcription factors to induce Sโphase genes.
- S Phase
- DNA replication: origins licensed in Gโ (ORC, Cdc6, Cdt1 load MCM helicase), activated at Gโ/S by Cdk2โCyclin E and Cdk2โCyclin A.
- Gโ Phase
- Cell continues to grow; checks completeness of DNA replication and repairs damage; preps mitotic machinery (centrosome duplication).
- Cdk1โCyclin B: Accumulates; kept inactive by Wee1 kinase phosphorylation; activated at end of Gโ by Cdc25 phosphatase.
- M Phase
- Prophase: Chromatin condenses (condensin complexes), mitotic spindle microtubules emanate from centrosomes.
- Prometaphase: Nuclear envelope breakdown (NEBD) via phosphorylation of nuclear lamins by Cdk1; microtubules attach to kinetochores.
- Metaphase: Chromosomes align at the metaphase plate; spindle assembly checkpoint (SAC) ensures all kinetochores properly attached (Mad2, BubR1 inhibit anaphase until satisfaction).
- Anaphase:
- Anaphase A: Cohesin cleaved by separase (activated when securin is degraded via APC/C [anaphaseโpromoting complex/cyclosome] ubiquitination); sister chromatids separate; microtubules depolymerize at kinetochores.
- Anaphase B: Spindle poles move further apart as interpolar microtubules elongate, and motor proteins (kinesinโ5) push poles apart; dynein at cell cortex helps pull poles.
- Telophase: Chromosomes arrive at poles and decondense; nuclear envelope reโforms around each set; nucleoli reappear; spindle disassembles.
- Cytokinesis: Contractile actomyosin ring (microfilaments, myosin II) constricts at equator, forming cleavage furrow; in plants, a cell plate forms from Golgiโderived vesicles guided by phragmoplast (microtubule array).
Detailed Explanation
Eukaryotic cells undergo a more complex process of division compared to prokaryotes, organized into a cell cycle with distinct phases: Gโ, S, Gโ, and M. In Gโ, the cell grows and checks for favorable conditions and DNA integrity. If all is well, it moves to the S phase, where DNA is replicated. Following DNA synthesis, the cell enters Gโ, where it checks for any errors in the DNA replication and prepares for mitosis. The decisions to progress through these phases are managed by specific proteins called cyclins, which activate cyclin-dependent kinases (Cdks). In the M phase, the actual division occurs: chromosomes condense, the nuclear envelope breaks down, and sister chromatids are pulled apart to form two new cells. Key checkpoints during the cell cycle ensure that any damage or issues are resolved before proceeding to the next stage.
Examples & Analogies
Think of the eukaryotic cell cycle as a manufacturing process in a factory. Before launching the production (mitosis), workers (cyclins and Cdks) check the quality of materials (DNA). If everything passes inspection during each phase (Gโ, S, and Gโ), only then do they proceed to assembly (M phase). This quality control prevents defects in the final product, ensuring the daughter cells are healthy and function properly.
Nuclear Division: Mitosis versus Meiosis
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- Nuclear Division: Mitosis versus Meiosis
3.1 Mitosis - Purpose: Generate two genetically identical diploid (2n) daughter cells for growth, tissue repair, and asexual reproduction.
- Key Features:
- One Division Cycle: Interphase (Gโ, S, Gโ) followed by one M phase.
- Chromosome Number Maintained: Diploid parent โ two diploid daughters.
- No Recombination (except rare somatic recombination events).
Detailed Steps:
- Prophase: Chromatin condenses via condensin complexes; nucleolus fades; centrosomes (animal cells) begin migrating to opposite poles; mitotic spindle microtubules form.
- Prometaphase:
- Phosphorylation of nuclear lamins and inner nuclear membrane proteins causes nuclear envelope breakdown (NEBD); nuclear pore complexes disassemble.
- Kinetochores assemble on centromeric DNA (bound by CENPโA histone variant). Kinetochore microtubules capture chromosomes by 'search and capture.'
- Metaphase: Chromosomes align at the metaphase plate; sister kinetochores attached to opposite spindle poles (biorientation).
- Anaphase: Cohesin (holding sister chromatids) is cleaved by separase once securin is ubiquitinated and degraded by APC/C. Sister chromatids separate and move to poles.
- Telophase: Chromatids (now called chromosomes) arrive at poles and decondense as phosphatases remove phosphorylation from histones and condensins. Nuclear envelopes reassemble around chromosome sets (lamin dephosphorylation), nucleoli reappear.
- Cytokinesis: Actomyosin contractile ring constricts at cell equator, dividing cytoplasm; midbody forms transiently before abscission. Plant cells: Vesicles from Golgi coalesce at the center (cell plate) guided by phragmoplast microtubules; cell plate becomes middle lamella separating daughter cells.
3.2 Meiosis - Purpose: Produce haploid (n) gametes (sex cells) from diploid germline cells, introducing genetic diversity via recombination and independent assortment.
- Two Division Cycles:
- Meiosis I (Reductional Division): Homologous chromosome pairs separate โ two haploid cells, each chromosome still consists of two sister chromatids.
- Meiosis II (Equational Division): Sister chromatids separate (similar to mitosis) โ four haploid cells.
Detailed Steps:
- Prophase I: Subdivided into stages:
- Leptotene: Chromosomes begin condensing; each consists of two sister chromatids attached at centromere; telomeres attach to nuclear envelope.
- Zygotene: Synapsis begins: homologous chromosomes pair up via homologous recombination machinery; formation of synaptonemal complex (lateral elements [SYCP2, SYCP3], central element [SYCP1]).
- Pachytene: Synapsis complete; homologous chromosomes (bivalents or tetrads) are fully paired; crossing over occurs at chiasmata points (sites of genetic exchange). Crossover events create genetic recombination between non-sister chromatids.
- Diplotene: Synaptonemal complex dissolves; homologs begin to separate but remain connected at chiasmata; chromosomes further condense.
- Diakinesis: Further condensation; nucleolus disappears; nuclear envelope breaks down; spindle microtubules attach to kinetochores.
- Metaphase I:
- Paired homologs (bivalents) align at metaphase plate; orientation of each bivalent is random (maternal or paternal homolog can face either pole), resulting in independent assortment.
- Each kinetochore on one homolog is attached to microtubules from one pole; sister kinetochores are fused and function as a single unit at this stage.
- Anaphase I:
- Cohesin along chromosome arms is cleaved by separase (protected at centromeres by Shugoshin protein); homologous chromosomes (each consisting of two sister chromatids) are pulled to opposite poles.
- Telophase I and Cytokinesis:
- Chromosomes arrive at poles; partial decondensation may occur; nuclear envelope occasionally reโforms; cell divides into two haploid cells.
Interkinesis: Short interphaseโlike stage in some organisms; no DNA replication occurs.
Meiosis II (equational division):
- Chromosomes arrive at poles; partial decondensation may occur; nuclear envelope occasionally reโforms; cell divides into two haploid cells.
- Prophase II: Chromosomes condense again (if decondensed); nuclear envelope dissolves (if reformed); spindle microtubules assemble.
- Metaphase II: Chromosomes (each with two sister chromatids) align individually at metaphase plate; kinetochores of sister chromatids attach to microtubules from opposite poles (biorientation).
- Anaphase II: Cohesin at centromeres is cleaved; sister chromatids separate and move to opposite poles.
- Telophase II and Cytokinesis: Chromatids arrive at poles; nuclear envelopes re-form; chromosomes decondense; each of the two haploid cells divides, resulting in four genetically unique haploid daughter cells (gametes in animals; spores in plants and fungi).
Detailed Explanation
Nuclear division can be categorized into two primary processes: mitosis and meiosis. Mitosis is a method of division for growth and repair, resulting in two identical diploid cells (each with two sets of chromosomes). The process involves several stages from prophase, where chromosomes condense, to cytokinesis, where the cell splits. In contrast, meiosis is specifically for producing gametes: it consists of two sequential divisions (Meiosis I and II) resulting in four haploid cells, introducing genetic diversity through processes like crossing over and independent assortment. This ensures that the offspring have a mix of genetic material from both parents, which is essential for evolution and adaptability.
Examples & Analogies
Imagine mitosis like making an exact photocopy of a documentโyou have the same content and format, simply duplicated to meet a requirement. Now think of meiosis as mixing two paints together, resulting in a new color. The end productsโthe gametesโare not identical copies, but rather a unique blend of traits passed down from both 'parent' colors. This genetic variability is crucial for the adaptability and evolution of species.
Regulation of Cell and Nuclear Division
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- Regulation of Cell and Nuclear Division
- CyclinโDependent Kinases (Cdks) and Cyclins
- Sequential activation of cyclinโCdk complexes drives transitions:
- Cdk4/6โCyclin D (Gโ progression).
- Cdk2โCyclin E (Gโ/S transition).
- Cdk2โCyclin A (S phase).
- Cdk1โCyclin A/B (Gโ/M transition).
- Cyclins accumulate and degrade in a cell cycleโdependent manner; Cdks are modulated by phosphorylation (Cdk1 phosphorylated by Wee1 inhibits activity; dephosphorylation by Cdc25 activates) and by Cdk inhibitors (CKIs: p21, p27, p57).
- Spindle Assembly Checkpoint (SAC)
- Monitors attachment of kinetochores to spindle microtubules and tension generated by correct attachment.
- Unattached or tensionless kinetochores generate the Mitotic Checkpoint Complex (Mad2, Bub3, BubR1, Cdc20) that inhibits APC/CโCdc20, preventing anaphase onset until all kinetochores are properly attached.
- Cohesin and Shugoshin
- Cohesin (heterotetrameric complex of SMC1, SMC3, SCC1/RAD21, SCC3/SA) holds sister chromatids together.
- Shugoshin (Sgo1) protects cohesin at centromeres during Meiosis I and early Mitosis, preventing premature separation.
- Separase (cysteine protease) cleaves SCC1/RAD21 to trigger chromatid separation.
Detailed Explanation
The regulation of cell and nuclear division is crucial to ensure that cells divide accurately and efficiently. This regulation is largely governed by cyclin-dependent kinases (Cdks) which function together with cyclins at different stages of the cell cycle. For example, the Cdk4/6โCyclin D complex is essential for initiating the cell cycle, while Cdk1โCyclin B is key for the transition into mitosis. The Cyclins increase and decrease in concentration throughout the cycle, and Cdks can be activated or inhibited by them and other regulatory proteins. Additionally, at key transitions, checkpoints like the Spindle Assembly Checkpoint (SAC) ensure that cells only progress if they are ready, preventing errors that could lead to cancer or other diseases.
Examples & Analogies
Think of the regulation of cell division like a well-organized conveyor belt in a factory. The cyclins are like workshops working on different parts of the product, and the Cdks are the supervisors ensuring each section completes its task before passing it to the next. If a piece doesn't meet the quality check (like an undetected flaw), the process stops until everything is confirmed ready to proceed, just like in a factory, to prevent defective products before they reach customers.
Definitions & Key Concepts
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Key Concepts
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Binary Fission: Prokaryotic process of cell division that produces two identical cells.
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Mitosis: Nuclear division resulting in two diploid daughter cells.
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Meiosis: A two-step process of cell division that produces four genetically diverse haploid gametes.
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Cell Cycle Checkpoints: Regulatory points in the cell cycle that ensure proper division.
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Cohesin: A protein responsible for holding sister chromatids together.
Examples & Real-Life Applications
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Examples
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In binary fission, a bacterium like Escherichia coli replicates its DNA and divides into two identical offspring.
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During meiosis, a human germ cell divides to produce gametes (sperm or eggs) with half the chromosome number.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
๐ต Rhymes Time
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In prokaryotes, fission is the key, two identical cells, that's the decree.
๐ Fascinating Stories
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Once upon a time, in a small bacterial kingdom, a queen cell needed to replicate. Using her ancient scrolls, she activated her magical oriC and danced her way to sepation, creating two identical daughters through binary fission.
๐ง Other Memory Gems
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For the cell cycle, remember G1, S, G2, M - 'Get Set Go, Mate!'
๐ฏ Super Acronyms
Cdks
- Cyclin-dependent kinases regulate the cell with Cyclins' aid to prevent division mistakes.
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Binary Fission
Definition:
A simple form of asexual reproduction in prokaryotes where a single cell divides into two identical cells.
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Term: Cohesin
Definition:
A protein complex that holds sister chromatids together until they are separated during division.
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Term: Cyclin
Definition:
Proteins that regulate the cell cycle by activating cyclin-dependent kinases (Cdks).
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Term: Cytokinesis
Definition:
The physical process of cell division that divides the cytoplasm of a parental cell into two daughter cells.
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Term: Mitosis
Definition:
A type of nuclear division that results in two genetically identical daughter cells.
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Term: Meiosis
Definition:
A specialized form of cell division that produces four haploid gametes from one diploid cell.
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Term: OriC
Definition:
The origin of replication in prokaryotic cells where DNA begins to replicate.
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Term: ParABS system
Definition:
A system in prokaryotes responsible for partitioning replicated chromosomes during cell division.
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Term: Segregation
Definition:
The process of separating replicated chromosome copies during cell division.