Glossary: Cell Cycle & Cell Division | Biology for ACT PDF Download

Cell Cycle

• Cell Cycle: The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. 

Phases of Cell Cycle

• Interphase: Interphase is a crucial stage in the cell cycle where the cell prepares for division. It is the longest phase and consists of three subphases: G1, S, and G2. During G1, the cell grows and synthesizes proteins and organelles. In S phase, DNA replication occurs, creating identical copies of the genetic material. G2 phase is a period of further growth and preparation for cell division. Interphase allows the cell to grow, function, and duplicate its DNA before entering the M phase.
• Karyokinesis: The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis).
• Cytokinesis: Division of cytoplasm is cytokinesis.
• S Phase: S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles.
• G₁ phase (Gap 1): G₁ phase is the first phase of interphase, where the cell grows and carries out its normal functions. It is a period of cell growth, protein synthesis, and organelle replication. In G₁ phase, the cell prepares for DNA replication and checks for any damage or errors in its genetic material before progressing to the next phase of the cell cycle.
• G₂ phase (Gap 2): G₂ phase is the final phase of interphase, occurring after DNA replication in S phase. During G₂ phase, the cell continues to grow and prepare for cell division. It synthesizes necessary proteins, organelles, and enzymes required for successful cell division. The cell also performs a final check to ensure that DNA replication was accurate before entering the M phase (mitosis phase) for cell division.
• Quiescent Stage: Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0 ) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

M Phase

• Equational Division: Mitosis is called equational division because mitosis is the process of cell division in which the chromosomes replicates and equally distribute into two daughter cells. The chromosome number in each daughter cell should be equal to that in the parent cell.
• M Phase (Mitosis phase): The M phase, also known as mitosis, is the phase in the cell cycle where cell division occurs. It consists of several stages: prophase, metaphase, anaphase, and telophase. 
• Prophase: In prophase, the chromosomes condense, the nuclear membrane dissolves, and the spindle apparatus forms. 
• Metaphase: Metaphase is when the chromosomes align in the middle of the cell. 
• Anaphase: Anaphase follows, where the sister chromatids separate and move to opposite poles of the cell. 
• Telophase marks the formation of new nuclei around the separated chromosomes, and cytokinesis begins, resulting in the division of the cell into two daughter cells.

Meiosis

• Meiosis: This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
• Meiosis I: Meiosis I is the first division in the process of meiosis, which is a type of cell division that produces gametes (sperm and eggs) with half the number of chromosomes as the parent cell. During meiosis I, homologous chromosomes pair up and exchange genetic material through a process called crossing over. The homologous chromosomes then separate, resulting in two daughter cells with a mix of genetic information.
• Meiosis II: Meiosis II is the second division in meiosis, following meiosis I. It is similar to mitosis but occurs in cells that have already undergone meiosis I. During meiosis II, the sister chromatids separate, resulting in four haploid daughter cells with unique combinations of genetic material. These cells are the final gametes that can fuse with another gamete during fertilization to form a new individual with a complete set of chromosomes.
•  Synaptonemal complex: The synaptonemal complex is a structure that forms during the prophase of meiosis I. It is a protein structure that helps align and connect homologous chromosomes. The synaptonemal complex promotes the exchange of genetic material between homologous chromosomes through a process called crossing over. This genetic exchange contributes to genetic diversity and ensures proper chromosome segregation during meiosis.
• Metaphase I: Metaphase I is a stage in meiosis I where homologous pairs of chromosomes align along the equator of the cell. These chromosome pairs, known as tetrads, are positioned randomly, leading to genetic variation in the resulting daughter cells. This alignment ensures proper separation of homologous chromosomes during the subsequent stages of meiosis.
• Prophase I: Prophase I is the longest and most complex stage of meiosis I. It involves the pairing of homologous chromosomes, known as synapsis, and the formation of the synaptonemal complex. Crossing over, the exchange of genetic material between homologous chromosomes, occurs during this stage. Prophase I prepares the cells for proper chromosome separation in later stages.
• Telophase I: Telophase I is the final stage of meiosis I, following anaphase I. During this stage, the chromosomes reach the opposite poles of the cell. Nuclear envelopes begin to form around each set of chromosomes, resulting in two daughter cells with a haploid number of chromosomes. Telophase I marks the completion of the first round of chromosome segregation.
• Anaphase I: Anaphase I is a stage in meiosis I where homologous chromosomes separate and move towards opposite poles of the cell. Unlike in mitosis, sister chromatids remain attached during this stage. The separation of homologous chromosomes ensures that each daughter cell receives one member of each homologous pair, contributing to genetic diversity.
• Metaphase II: Metaphase II is a stage in meiosis II where the chromosomes align at the equatorial plane of the cell. The centromeres of the sister chromatids attach to spindle fibers, preparing for their separation in the subsequent steps.
• Prophase II: Prophase II is the initial phase of meiosis II. During this phase, the nuclear envelope breaks down, and the chromosomes condense. The spindle apparatus forms, and the centrosomes move towards opposite poles of the cell.
• Telophase II: Telophase II is the final stage of meiosis II. The separated chromatids reach opposite poles of the cell, and new nuclear envelopes form around each set of chromosomes. The cell begins to divide, and cytokinesis occurs, resulting in the formation of four haploid daughter cells.
• Anaphase II: Anaphase II is the stage where the sister chromatids of each chromosome separate and move towards opposite poles of the cell. This ensures that each daughter cell receives a complete set of chromosomes.