Cell Cycle: Mitosis and Meiosis

The Cell Cycle: Mitosis and Meiosis

• A newborn baby has approximately 26,000,000,000 cells.
• The average adult male has around 36 trillion cells, while females have 28 trillion.
• A cell cycle is a series of events that happen in a cell as it grows and divides.
• The main stages in the cell cycle are interphase and cell division (karyokinesis and cytokinesis).
• Interphase is further divided into gap 1, synthesis, and gap 2.

Interphase (I)

• This is the growth period in the cell cycle and is divided into three parts:
  • Gap 1 (G1): The first part of the cell cycle, normal metabolic functions occur inside the cell. Cells increase in size and organelles increase in number during this phase. Cells cannot grow beyond a certain size, even with plenty of nutrients.
  • Synthesis (S): The second stage, the cell makes a copy of its nuclear DNA. Copies of chromosomes are made, each containing exact copies of the parent's genetic material.
  • Gap 2 (G2): The cell continues normal functions, and grows further in this stage. A critical checkpoint ensures proper transitioning to the next phase.

Checkpoints

• A checkpoint is a control point in the cell cycle where signals determine if the cell can proceed to the next phase or not.
• There are three important checkpoints.
  • The first checks if the cell is growing enough and if the DNA is undamaged. If DNA is damaged, it will not replicate and the cell will die.
  • The second checks if the DNA is replicated correctly.
  • The third checks if the cell has enough resources to continue and if the cell is ready for mitosis.

Mitosis

• Mitosis is the division of the nucleus into two identical nuclei with the same full set of DNA (Karyokinesis).
• It occurs in body cells (somatic cells) except for egg and sperm.
• Mitosis prepares the cell for cytokinesis.
• Mitosis is divided into four phases: Prophase, Metaphase, Anaphase, and Telophase.

Prophase

• In early prophase, centrioles move to opposite cell poles, chromosomes appear as thin threads, the nucleolus becomes less distinct, and the nuclear membrane is visible.
• In mid-prophase, centrioles organize spindle fibers. Sister chromatids form with a centromere at their point of attachment.
• In late prophase, centrioles move to opposite cell poles, the nuclear membrane breaks down, spindle fibers attach to the centromere of each chromosome, and chromosomes move towards the equator, and the nucleolus is gone.

Metaphase

• The nuclear membrane has completely disappeared.
• The centromere of each double-stranded chromosome attaches to a spindle fibril at the cell's center.
• Centrioles are at opposite poles of the cell.
• The chromosomes align at the cell's center forming the metaphase plate.

Anaphase

• In early anaphase, sister chromatids separate and start moving towards opposite poles of the cell.
• In late anaphase, the two sets of new, single-stranded chromosomes migrate towards their respective poles in the cell. Cytokinesis begins (division of cytoplasm).

Telophase

• The new nuclear membrane starts forming in each new cell.
• Chromosomes become longer, thinner, and less distinct.
• The nucleolus reappears.
• Centrioles are replicated. Cytokinesis is nearly complete.
• Spindle fibers and asters disappear. (In plant cells, the primary cell wall or division plate forms a boundary between the two daughter cells.)
• The cleavage furrow deepens and the cells divide into two parts.

Meiosis

• Meiosis is cell division that creates reproductive cells (gametes) like sperm and egg cells, or spores.
• Meiosis produces four haploid daughter cells, which are analogous to mitosis where sister chromatids are segregated with four haploid daughter cells.
• Meiosis is divided into phases:
  • Meiosis I (reductional division): Ploidy is reduced from diploid to haploid.
    • Prophase I, Metaphase I, Anaphase I, Telophase I
  • Meiosis II (equational division): Sister chromatids are separated creating haploid daughters.
    • Prophase II, Metaphase II, Anaphase II, Telophase II

Ploidy

• Ploidy is the number of chromosome sets in a nucleus.
• There are two prominent types of ploidy:
  • Haploid (containing one set of chromosomes).
  • Diploid (containing two sets of chromosomes, one from each parent).

Meiosis I - Prophase I

• Chromosomes coil and shorten.
• The nuclear envelope disintegrates.
• Homologous chromosomes pair via a process called synapsis.
• Genetic material may exchange between homologous chromosomes.

Meiosis I - Metaphase I

• Paired homologous chromosomes align at metaphase.
• The chromosomes in pairs are attached to spindle fibers.

Meiosis I - Anaphase I

• Homologous chromosomes move towards opposite poles of the cell.

Meiosis I - Telophase I

• Chromosomes reach opposite poles.
• The nuclear membrane forms; cytokinesis follows.

Meiosis II - Prophase II

• Nuclear membrane disintegrates.
• New spindle fibers form around chromosomes.

Meiosis II - Metaphase II

• Chromosomes align at the metaphase plate; attached to spindle fibers.

Meiosis II - Anaphase II

• Chromosomes divide into two sister chromatids and they move to opposite poles.

Meiosis II - Telophase II

• Nuclear membrane forms around each set of chromosomes.

• Spindle fibers disintegrate.

• Cytokinesis occurs creating four haploid daughter cells.

• Gamete formation in males is called spermatogenesis and in females is oogenesis.

• A sperm cell has 23 chromosomes; a human body cell has 46 chromosomes.

• Sex cells have half the usual number of chromosomes so a fertilized egg receives a normal number of chromosomes.

This quiz covers the cell cycle, focusing on the process of mitosis and meiosis. Explore the different stages of cell division, including interphase and its subphases: Gap 1, Synthesis, and Gap 2. Test your understanding of how cells grow and divide, fundamental concepts in biology.