BIOLOGI SMA Kelas 12 - Pembelahan Sel | GIA Academy

Cell Division and Gametogenesis

Cell division supports growth, repair, and reproduction. Examples include increasing plant length, hair and nail growth, and wound healing, which involve cell multiplication or replacement.

Three Types of Cell Division

• Amitosis (binary fission): A direct division without the usual staged process of nuclear division. It commonly occurs in prokaryotes such as bacteria and cyanobacteria. DNA is copied, and the cell membrane constricts to separate the cell into two daughter cells.
• Mitosis: A staged division in body cells that produces two genetically identical daughter cells with the same chromosome number as the parent cell. It supports growth and the replacement of damaged cells.
• Meiosis: A two-stage division associated with the formation of sex cells. It produces four haploid daughter cells, each with half the parent cell’s chromosome number. It helps prevent chromosome numbers from doubling in each generation and contributes to species continuity.

Mitosis and the Cell Cycle

The cell cycle consists of interphase and the division phase. Interphase takes up most of the cycle and includes:

• G1: Cell growth and production of proteins and organelles.
• S: DNA and chromosome replication.
• G2: Further growth and preparation for division.

Mitosis includes karyokinesis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm.

1. Prophase: Chromatin condenses into visible chromosomes, and the nuclear membrane begins to disappear.
2. Prometaphase: The nuclear membrane breaks down, allowing spindle microtubules to attach to chromosomes at their kinetochores.
3. Metaphase: Chromosomes line up at the cell’s equator. Each chromosome has two sister chromatids.
4. Anaphase: Spindle fibers shorten and pull sister chromatids toward opposite poles.
5. Telophase: Chromosomes begin to return to chromatin form; nuclear membranes and nucleoli re-form.
6. Cytokinesis: The cytoplasm divides, producing two daughter cells.

Meiosis

Meiosis consists of Meiosis I and Meiosis II, following chromosome replication during interphase.

Meiosis I

• Prophase I: Homologous chromosomes pair. The stages named in the lesson are leptotene, zygotene, pachytene, diplotene, and diakinesis. Crossing over occurs during this phase, exchanging genetic material between homologous chromosomes.
• Metaphase I: Homologous chromosome pairs line up at the equator.
• Anaphase I: Homologous chromosomes separate and move to opposite poles; sister chromatids remain joined.
• Telophase I: The cell divides into two haploid cells whose chromosomes still consist of two connected chromatids.

Meiosis II

• Prophase II: Chromosomes condense, the nuclear membrane disappears, and spindle fibers form.
• Metaphase II: Chromosomes line up individually at the equator.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Nuclear structures re-form and the spindle disappears. The divisions ultimately yield four haploid cells.

Mitosis Compared with Meiosis

• Purpose: Mitosis increases or replaces body cells; meiosis produces cells for sexual reproduction and maintains chromosome number across generations.
• Where it occurs: Mitosis occurs in body cells; meiosis occurs in cells that give rise to gametes.
• Outcome: Mitosis produces two diploid daughter cells; meiosis produces four haploid cells.
• Number of divisions: Mitosis has one division; meiosis has two.
• Chromosome arrangement: Mitosis aligns individual chromosomes at the equator; Meiosis I aligns homologous pairs.
• Crossing over: Crossing over is not part of mitosis; it occurs during Prophase I of meiosis.

Animal Gametogenesis

Gametogenesis is the process of forming haploid gametes from diploid cells.

Spermatogenesis

Spermatogenesis takes place in the testes:

1. A diploid spermatogonium divides by mitosis to produce a primary spermatocyte.
2. Meiosis I produces two haploid secondary spermatocytes.
3. Meiosis II produces four haploid spermatids.
4. Spermiogenesis matures the spermatids into sperm cells.

The process produces four functional sperm cells from a primary spermatocyte. The lesson describes it as beginning at puberty and continuing through life.

Oogenesis

Oogenesis takes place in the ovaries:

1. A diploid oogonium divides by mitosis to produce a primary oocyte.
2. Meiosis I produces a large secondary oocyte and a smaller polar body.
3. Further division produces one ovum and polar bodies.
4. The polar bodies degenerate; the overall result described is one functional ovum and three polar bodies.

The lesson contrasts sperm production—which produces millions of small, functional cells—with oogenesis, which produces one large functional ovum per cycle. It describes oogenesis as beginning early in life and following a monthly cycle after puberty until menopause.

Plant Gametogenesis

Microsporogenesis

Microsporogenesis forms the male gametes in the anther:

1. A diploid microspore mother cell undergoes meiosis, forming four haploid microspores grouped as a tetrad.
2. Each microspore divides to form a vegetative nucleus and a generative nucleus.
3. The generative nucleus divides to form two sperm nuclei.

The resulting pollen structure is described as containing three nuclei: one vegetative nucleus and two sperm nuclei.

Megasporogenesis

Megasporogenesis forms the female gametophyte in the ovule:

1. A diploid megaspore mother cell undergoes meiosis, producing four haploid megaspores.
2. In flowering plants, one megaspore remains functional while the other three degenerate.
3. The functional megaspore undergoes three mitotic divisions, producing eight nuclei.
4. These are organized as three antipodal cells, two polar nuclei, one egg cell, and two synergids.

Practice-Question Takeaways

The lesson ends with three sample questions:

• A question identifying statements about meiosis gives 2, 3, and 5 as correct (option E).
• A diagram-labeling question on spermatogenesis identifies the sequence of cell types from spermatogonium through spermatozoa (option C).
• An embryo-sac diagram question tests recognition of the secondary embryo-sac nucleus, egg, synergids, and antipodal cells (option B).

Speakers and Sources

• One unnamed presenter or narrator from the GIA Academy YouTube channel.
• No guest speakers or other named sources are identified.