Bio 2.1.1

Cell Division

Key Concepts

• All cells come from pre-existing cells.
• New cells are created through cell division.
• Prokaryotic cells divide via binary fission.
• Eukaryotic cells divide through mitosis and cytokinesis.
• Mitosis: division of the nucleus (prophase, prometaphase, metaphase, anaphase, telophase).
• Cytokinesis: division of the remaining cell contents.
• Gamete production involves meiosis and cytokinesis, resulting in genetically unique nuclei.

1.1 Prokaryotic Cells

• Prokaryotic cells divide by binary fission, a simple and rapid process.

1.2 Eukaryotic Cells

• Eukaryotic cell division is more complex due to larger amounts of DNA.
• DNA replication is essential to provide a complete copy of genetic material to new cells.
• Chromatin: DNA stored as chromatin, composed of DNA and proteins (histones).
• Chromosomes: condensed chromatin visible during cell division; each species has a characteristic number of chromosomes (e.g., humans: 46).

1.3 Cell Division

1.3.1 Cell Cycle

• The cell cycle includes interphase, mitosis, and cytokinesis.
• Interphase: includes G1, S, and G2 phases; DNA replication occurs during the S phase, resulting in chromosomes composed of two identical halves (sister chromatids).

1.3.2 Mitosis

• Prophase: Chromosomes condense, become visible, consist of sister chromatids held at the centromeres; the spindle forms from microtubules extending from centrosomes, which move to opposite poles.
• Prometaphase: Nucleolus disappears, nuclear envelope breaks down; spindle fibers attach to chromosomes, moving them to the cell's mid-region.
• Metaphase: Chromosomes align along the mid-region; sister chromatids connect to opposite poles via spindle fibers.
• Anaphase: Sister chromatids separate and move to opposite poles, now referred to as chromosomes; anaphase ends when chromosomes reach the poles.
• Telophase: Chromosomes decondense, new nuclear envelopes form around each set of chromosomes, and spindle microtubules disappear.
• Mitosis yields two genetically identical daughter nuclei.

1.3.3 Cytokinesis

• Cytokinesis: division of cytoplasm to form two cells.
• In animal cells, microfilaments constrict, dividing the cell.
• Mitosis & Cytokinesis: one division of the nucleus and cytoplasm produces two genetically identical cells, each with a complete set of chromosomes.

1.3.4 Cell Cycle Control System

• Checkpoints ensure proper cell division.
• G1 checkpoint: determines if the cell divides, enters G0 (non-dividing), or undergoes apoptosis if DNA damage cannot be repaired.
• G2 checkpoint: Checks for correct DNA replication; mitosis will not proceed if errors exist.
• M checkpoint: Between metaphase and anaphase, verifies chromosome attachment to the spindle for separation.

1.4 Cells and Reproduction - Meiosis

1.4.1 Reproduction

• Asexual Reproduction: a single parent produces genetically identical offspring via mitosis and cytokinesis.
• Sexual Reproduction: union of gametes (sex cells) to form a zygote; offspring are not identical to parents.

1.4.2 More About Chromosomes

• Homologous Chromosomes: partner chromosomes in a body cell, similar in shape, size, and centromere position; Carry information for same genetic traits.
• Karyotype: display of chromosomes in a cell.
• Diploid (2n): two sets of chromosomes (somatic cells).
• Haploid (n): single set of chromosomes (gametes).

1.4.3 Production of Gametes - Meiosis

• Meiosis: involves two nuclear divisions, with cytokinesis after each.
• Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I.
• Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II.
• Interkinesis: resting phase between meiosis I and II.

Meiosis I

• Prophase I: Chromatin condenses; homologous chromosomes pair up (synapsis) forming a tetrad; crossing over occurs.
• Metaphase I: Tetrads align on the equator.
• Anaphase I: Homologous chromosomes separate and move to opposite poles (random distribution).
• Telophase I: Chromosomes decondense; nuclear envelope may reorganize; cytokinesis occurs.

Meiosis II

• Prophase II: Chromosomes appear; nuclear envelope breaks down.
• Metaphase II: Chromosomes line up along the cell equator.
• Anaphase II: Sister chromatids separate and move to opposite poles (now chromosomes).
• Telophase II: Nuclear envelope reforms, chromosomes elongate; cytokinesis occurs.
• Meiosis results in four cells, each with different genetic combinations.
• Genetic recombination occurs due to crossing over in prophase I and random distribution in anaphase I.
• Meiosis involves two divisions but chromosomes are only duplicated once, resulting in four haploid cells with unique gene combinations.