Key points: Chromosomes are cellular structures. They carry genes & are composed of DNA, RNA & proteins. Haploid Eukaryotic Cell contains one copy of each chromosome. Diploid Eukaryotic Cell contains two copies of each chromosome. Prokaryotic Cell divide by fission. Eukaryotic Cell divide by Mitosis & Cytokinesis. Most gametes are formed by Meiosis.
‘Cell Cycle’ definition: Each time a eukaryotic cell divides, it goes through a sequence of events, that occur from the formation of a cell to its division into daughter cells, called cell cycle.
Cell Cycle have two major phases: (A) Interphase (B) Mitotic Phase
Cell growth & DNA replication occurs in this phase. It is further divided into three phases –
1) First Gap Phase (G1 Phase):
- Cell is quite active at biochemical level.
- It is the phase of initial growth of newly formed cell.
- Tenure of G1 Phase is usually variable for a given cell/cell type viz. in actively dividing cells it could be few seconds to minutes, many other cells may remain for weeks or years even.
- In many cases, cell proceeds to G0 phase (quiescent phase; cell neither divides nor prepares to divide). Special signal is required for a cell to re-enter G1 from G0 Phase.
- During this phase, a chromosome is a single, unreplicated DNA molecule with associated proteins.
- In this phase cell undergoes following processes:
- Cell accumulates chemical energy
- Prepares itself for DNA synthesis.
- Rapid synthesis of RNA and proteins takes place.
- Mitochondria, Chloroplast etc. multiply.
- ER, Golgi bodies, ribosomes etc. are formed.
2) Synthesis Phase (S Phase): Actual phase of DNA synthesis and replication. In this phase…
- Purine and pyrimidine bases are incorporated into nuclear DNA.
- Cell synthesizes additional histones.
- Replication of DNA is accompanied by duplication of each chromosome.
- Each duplicated chromosome consists of two sister chromatids, which remain joined together until mitosis.
- A diploid cell (2N) becomes tetraploid (4N) at end of S phase.
- Lasts for 6-8 hours.
- Also, in animal cells, centrosome is duplicated.
3) Second Gap Phase (G2 Phase): Period from end of DNA synthesis to beginning of mitosis. Following processes occur during this phase:
- Protein synthesis is inhibited but some RNA is synthesized.
- Cell considerably increases in volume.
- Cytoplasmic organelles such as mitochondria, plastids, ER, Golgi bodies etc. are doubled.
- Membranes are assembled and stored in small vesicles.
- Proteins for spindle are synthesized.
Mitotic Phase (M-Phase)
Eukaryotic cell division occurs in this phase. The genetic material is equally divided into daughter cells i.e. the replicated DNA molecules of each chromosome are faithfully segregated into two nuclei. Additionally, the resulting daughter cells not only possess genetic material identical to each other but also identical to their mother cell.
The cell that is about to divide is called mother cell& products of division are called daughter cells.
Mitotic phase consists of two phases/steps, namely, karyokinesis (nuclear division) & cytokinesis (cytoplasmic division) leading to formation of two daughter cells.
The karyokinesis is further subdivided into series of 5 phases:
1) Prophase: Duplicated chromosomes are prepared for segregation & mitotic apparatus is assembled. In this phase following events occur:
- Condensation of duplicated chromosomes & initiation of spindle formation marks the start of prophase.
- Chromatids are condensed using cohesin protein & become visible under light microscope as rod like structures.
- Formation of spindle is accompanied by fragmentation & dispersal of many subcellular organelles like Golgi apparatus, ER etc. However, organelles like mitochondria, chloroplast, lysosomes, peroxisomes etc. remain intact.
- Nuclear membrane dissociates into small vesicles.
- Nucleolus disappears.
- Centrosomes begin to move to opposite poles of the cell. They determine the orientation of spindle apparatus (mitotic spindle) & hence the plane of cell division.
- Microtubules that form mitotic spindle starts to arrange itself in between the centrosomes. They also push the latter farther apart as the microtubule fibers lengthen.
- Remnants of nuclear envelope fragment.
- Mitotic spindle continues to develop.
- Chromosomes become more condensed & discrete.
- Each chromatid develops kinetochore (protein structure) at centromeric region.
- Kinetochore serve as site for mitotic spindle attraction & attachment. Mitotic spindle will be attached to all sister chromatids & to their respective opposing pole.
- Mitotic spindles which do not attach to chromatids & run from one pole to other are called polar microtubules & contribute to cell elongation.
- Astral microtubules are located near poles. They help spindle orientation & mitosis regulation. In plants, these asters are absent, so, the mitosis is called anastral.
Some literatures classify ‘Prophase’ & ‘Prometaphase’ combined as ‘Prophase’ whereas some classify these separately as mentioned above.
3) Metaphase: Attachment of mitotic spindle to kinetochores is indicative that the cell is entering the metaphase. Following events occur in Metaphase:
- All duplicated chromosomes are arranged at metaphase plate (= equatorial plate) midway between the poles.
- Spindle fibers become clearly visible.
- Each sister chromatid of the duplicated chromosome is attached to different pole. This arrangement is crucial for equal distribution of genetic material in daughter cells.
- Sister chromatids are tightly attached to each other by cohesin protein.
- Mixing of nuclear matrix with cytoplasm due to complete disappearance of nuclear membrane during prophase.
4) Anaphase: Marked by separation of chromatids & their movement towards poles. Anaphase consists of following events:
- Cohesin proteins degrade & the two sister chromatids separate from each other.
- Each chromatid with its own centromere is now referred to as chromosome (daughter chromosome).
- Daughter chromosomes start migrating towards opposite poles of spindle. This movement may either be due to repulsion between centromeres or due to contraction of chromosomal fibers or both.
- Anaphase shows two types of movement:
- Anaphase A is the movement of chromosomes toward the poles.
- Anaphase B is the movement of poles farther apart.
- Cell becomes visibly elongated – oval shaped.
- Spindle Assembly Checkpoint – Mechanism that delays onset of anaphase till any misplaced chromosome(s) gets properly aligned on equatorial plate.
5) Telophase: In this phase,
- Chromosomes reach their respective poles & initiate de-condensation to chromatin form.
- Nuclear membrane forms around each set of chromosomes.
- Now, two new nuclei are formed in the cell.
- Nucleoli appear, one in each nucleus.
- Mitotic spindle depolymerize to tubulin monomers, i.e. it disappears.
Cytokinesis (Division of Cytoplasm)
- Physical separation of cytoplasmic components, forming two separate daughter cells.
- Cytokinesis differs in animal & plant cells (Key point: Plant cells have cell wall):
- In Animal Cell
- Contractile ring (made of actin & myosin) appears in late anaphase just inside the plasma membrane.
- The plane of the ring is same as that of metaphase plate.
- Ring filaments pull the equator of the cell inward, forming a fissure called cleavage furrow.
- Gradually the ring contracts, deepening the furrow & at the end during cytokinesis cleaves the membrane, dividing cell into two.
- In Plant Cell
- During interphase, Golgi apparatus accumulates enzymes, structural proteins & glucose molecules into vesicles (Golgi vesicles), prior to breaking into vesicles & dispersing during cell division.
- During telophase, the Golgi vesicles arrange on microtubules forming phragmoplast.
- From center toward cell walls, vesicles fuse & coalesce forming a structure called cell plate. More vesicles fuse & eventually it joins the cell walls of the cell. Enzymes use glucose to completely form a new cell wall.
- Golgi membrane forms plasma membrane on either side of cell wall.
- Result of the events above: Two daughter cells with complete cell walls, surrounding each.
- In Animal Cell