Cell Cycle: Mitosis, Meiosis, and Biological Significance
Cell Cycle
The cell cycle is divided into two main phases: division (mitosis and cytokinesis) and interphase (the period between divisions).
Interphase
Interphase is the period between two successive cell divisions. It consists of the following phases:
- G1 Phase: Preparation for the S phase. This phase occurs at the end of the last division and involves preparation for DNA duplication. The restriction point in G1 determines whether the cell will proceed to the S phase.
- S Phase: Synthesis phase, involving the synthesis of histones and DNA replication.
- G2 Phase: Preparation for cell division. Biosynthetic processes are carried out, and a second restriction point regulates whether the cell will proceed to mitosis. Cell cytoskeletal reorganization occurs to create the mitotic spindle.
Cell Cycle Control
- Enzyme Regulation: Cyclins and kinases regulate the G1 restriction point.
- Growth: Membrane receptors stimulate the cell to pass the S phase and undergo cell division.
Mitotic Division (Mitosis)
Mitosis is the process of nuclear division, allocating complete, previously replicated genetic information into two daughter nuclei.
- Open Mitosis: The nuclear membrane disappears.
- Closed Mitosis: The nuclear membrane does not disappear (occurs in protists and fungi).
Stages of Mitosis
- Prophase: Centrioles replicate, changes occur in the nucleus, chromatin condenses, chromosomes migrate to the nuclear periphery, the nucleolus disappears, and the mitotic spindle forms.
- Prometaphase: Chromosomes attach to the mitotic spindle, and chromosome kinetochores join to the spindle.
- Metaphase: Chromosomes are most visible and align at the equator.
- Anaphase: Sister chromatids separate due to the action of the centriole, and they migrate to opposite poles as polar microtubules shorten.
- Telophase: The final phase of mitosis, where daughter chromosomes are at the poles, kinetochore microtubules disappear, chromosomes decondense, the nuclear membrane forms, and nucleoli reappear.
Cytokinesis
Cytokinesis starts during telophase. It involves the division of the cytoplasm and cell constriction at the equatorial zone. In plant cells, strangulation occurs through the accumulation of vesicles (from the Golgi apparatus). These vesicles fuse and contact the side walls, forming a cell plate (phragmoplast) that gives rise to the two daughter cell membranes.
Meiotic Division (Meiosis)
Meiosis is a special type of nuclear division that creates four haploid nuclei from a diploid nucleus (2n).
Meiotic 1st Division
- Meiotic Prophase I: (6 stages)
- Proleptotene: Chromosomes are barely visible.
- Leptotene: Chromosomes condense and attach to the nuclear membrane, forming gussets. The spindle begins to form.
- Zygotene: Homologous chromosomes pair up (synapsis).
- Pachytene: Crossing over may occur between non-sister chromatids.
- Diplotene: Homologous chromosomes begin to separate but remain joined at chiasmata. The synaptonemal complex disappears.
- Diakinesis: Chromosomes condense further, chiasmata move towards the ends of chromosomes, and the nuclear membrane and nucleolus disappear.
- Prometaphase I: The nuclear membrane and nucleolus disappear, and bivalents bind to the spindle.
- Metaphase I: Bivalents align at the equator.
- Anaphase I: Bivalents separate, and chromosomes migrate to opposite poles.
- Telophase I: Nuclear division ends with the formation of two daughter nuclei. The nuclear membrane and nucleolus reappear.
Meiotic 2nd Division
The second meiotic division is similar to mitosis.
Biological Significance
- Mitosis: Growth of the zygote and asexual reproduction.
- Meiosis: Genetic variability; gametes have only n chromosomes.
Life Cycles
- Diplontic Cycle: The individual is diploid, and the gamete is haploid.
- Haplontic Cycle: The individual is haploid, and the zygote is diploid.
- Haplo-diplontic Cycle: The individual has both diploid and haploid stages. Diploid individuals produce haploid spores, which, through fertilization, create a diploid zygote.