Cell Cycle and Division

• Cell cycle is a series of events that take place in a cell leading to its division and duplication of its DNA to produce daughter cells.
• Cell cycle has two parts: Cell growth and preparation (Inter phase) and Cell division (Mitosis for somatic cells and Meiosis for gamete cells).
• Interphase, the longest part of the cell cycle, occurs between divisions and consists of three stages: G1, S, and G2.
• G1, also known as Gap 1, is the stage where the cell just finished dividing and is recovering from mitosis.
• S, or Synthesis stage, is where DNA replicates.
• G2, also known as Gap 2, is the preparation stage for mitosis where organelles are replicated and more growth occurs.
• Cells divide for growth, repair, and reproduction.
• Cytokinesis is the actual division of the cell into two new cells.
• Mitosis is the division of the nucleus of the cell into two new nuclei.
• Sometimes cells go through mitosis without going through cytokinesis.
• Synapsis is the pairing of homologous chromosomes forming a tetrad, allowing the matching up of homologous pairs prior to their segregation and possible chromosomal crossover between them.
• Mitosis begins after G2 and ends before G12.
• The M Phase, or Prophase, is the second stage of Mitosis where the nuclear membrane breaks apart, chromosomes condense, the nucleolus disappears, and microtubules form.
• Only one pair of chromosomes differs in Meiosis, the sex chromosomes X or Y.
• Meiosis is a division of the nucleus that reduces chromosome number by half and is important in sexual reproduction.
• A diploid cell replicates its chromosomes in two stages of Meiosis, Meiosis I and Meiosis II, with only one replication.
• Telophase, the final stage of Mitosis, involves the formation of nuclei and the division of the cytoplasm, also known as Cytokinesis.
• Each chromosome in a pair are identical to the other, carrying the same trait.
• Anaphase, the third stage of Mitosis, is when the centromeres divide, precise alignment is critical to division, and spindle fibers pull one set of chromosomes to each pole.
• In Meiosis, one set of chromosomes (n) is present in gametes or sex cells, humans have 23 chromosomes.
• Meiosis causes diverse populations that over time can be stronger for survival.
• Both mitosis and meiosis involve the disappearance of the nucleus, nucleolus, and nuclear membrane.
• Both mitosis and meiosis involve the formation of spindle fibers.
• Variation refers to differences between members of a population.
• Meiosis produces daughter cells that have half the number of chromosomes as the parent.
• Daughter cells produced by meiosis are not genetically identical to one another.
• The process of sexual reproduction begins after a sperm fertilizes an egg.
• Both mitosis and meiosis involve replication.
• In meiosis, cell division takes place twice but replication occurs only once.
• Both mitosis and meiosis are forms of nuclear division.
• Meiosis results in random separation of chromosomes in gametes.
• Fertilization reduces the number of chromosomes from 2n to n, setting the stage for the union of two genomes.
• Pachytene: Synapsis was completed, crossing over between homologous chromosomes is possible.
• Meiosis is necessary for sexual reproduction.
• Zygotene: Paired threads, homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).
• Mitosis: Daughter cells undergo a second division, no additional replication occurs.
• Telophase II & Cytokinesis: Four haploid daughter cells result from one original diploid cell.
• Prophase I: Chromosomes condense, homologous chromosomes pair with each other, and each pair contains four sister chromatids (tetrad).
• Leptotene: Thin threads, replicated chromosomes condense, chromosomes become visible, homologous chromosomes are unpaired, and each chromosome begins to search its homologue.
• Diakinesis: Chromosomes condense further, tetrads are actually visible, chiasmata terminalize, and are clearly visible.