Mitosis: Chromosomes vs Chromatids

Chromosomes vs. Chromatids - Structure

• A chromatid is one of the two identical strands of DNA that make up a chromosome.
• Chromatids are attached to each other at the centromere and are separated during cell division.

Pairs of Chromatids - Formation

• During DNA replication, the DNA double-helix is copied to produce two identical copies of each chromosome.
• These two copies are called sister chromatids.
• Sister chromatids are attached to each other at the centromere.

Mitosis

• Mitosis is a type of cell division in which one cell divides into two identical daughter cells.
• Before mitosis begins, the DNA in the cell is replicated so that each daughter cell will have its own set of chromosomes.
• Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase.

Mitosis Stages

• Prophase: chromosomes become visible, and the nuclear membrane begins to break down.
• Metaphase: chromosomes line up in the center of the cell called the metaphase plate.
• Anaphase: sister chromatids separate and move to opposite ends of the cell.
• Telophase: two new nuclear membranes form around the chromosomes, and the cell membrane pinches in the middle to divide the cell into two daughter cells.

Cytokinesis

• Cytokinesis is the final stage of cell division.
• In cytokinesis, the cytoplasm of the cell divides into two parts, each of which contains one of the two daughter cells.

Spindle Fibers - Chromatids on the Move

• The mitotic spindle is a structure that forms during mitosis and helps to separate the chromosomes.
• The spindle is made up of microtubules, which are long, thin fibers.
• The microtubules of the spindle attach to the centromeres of the chromosomes and pull them apart.
• In animal cells, the spindle fibers are organized by the centrosomes.
• In plant cells, the cell wall is rigid and prevents microtubules from forming asters. Instead, spindle fibers extend between the centriole pairs.

Microtubule Formation - A Drug Target

• Microtubules play an essential role in mitosis by forming the mitotic spindle, which is responsible for separating the chromosomes during cell division.
• Microtubules are potential targets for cancer drugs.
• Taxanes are a class of drugs that inhibit the formation of microtubules, which prevents the mitotic spindle from forming, leading to cell death.

Differences between Mitosis and Meiosis

• Mitosis is a type of cell division in which one cell divides into two identical daughter cells.
• Meiosis is a type of cell division in which one cell divides into four non-identical daughter cells.
• Meiosis involves two rounds of cell division, whereas mitosis involves only one round.
• Meiosis results in four daughter cells, each with half the number of chromosomes as the parent cell, whereas mitosis results in two daughter cells with the same number of chromosomes as the parent cell.

Meiosis

• Meiosis is a type of cell division in which one cell divides into four non-identical daughter cells.
• Meiosis involves two rounds of cell division.
• Meiosis results in four daughter cells, each with half the number of chromosomes as the parent cell.
• Meiosis is preceded by interphase, which includes chromosome replication.
• Meiosis is a reductive division, meaning that the number of chromosomes is reduced by half.
• The original cell is diploid, and the four daughter cells produced are haploid.

Meiosis Stages

• Meiosis I: homologous pairs separate, resulting in two haploid cells.
• Meiosis II: sister chromatids separate, resulting in four haploid cells.

Tetrad Formation

• Tetrad formation occurs in prophase I of meiosis.
• Homologous chromosomes (each with sister chromatids) join to form a tetrad.
• Synapsis occurs in prophase I, where homologous chromosomes pair and exchange segments.

Meiosis I

• Interphase I: centrosomes (with centriole pairs) form.
• Prophase I: chiasmata, spindle, and tetrad formation occur.
• Metaphase I: microtubules attach to the metaphase kinetochore plate.
• Anaphase I: sister chromatids remain attached, and nuclear envelope and chromatin form.
• Telophase I: homologous chromosomes separate.

Meiosis II

• Prophase II: centrosomes (with centriole pairs) form.
• Metaphase II: microtubules attach to the metaphase kinetochore plate.
• Anaphase II: sister chromatids separate, and nuclear envelope and chromatin form.
• Telophase II: haploid daughter cells form.

Cytokinesis in Meiosis

• Cytokinesis occurs in telophase II, resulting in the formation of four haploid daughter cells.
• Cleavage furrow forms in telophase II, separating the cytoplasm into two parts.