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5.1 Meiosis

Keywords

English Term 中文翻译 Definition & Explanation
Meiosis 减数分裂 A specialized type of cell division that reduces the chromosome number by half, creating four haploid cells.
Diploid (\(2n\)) 二倍体 A cell containing two complete sets of chromosomes, one from each parent.
Haploid (\(n\)) 单倍体 A cell containing only one set of chromosomes (gametes).
Homologous Chromosomes 同源染色体 A pair of chromosomes (one maternal, one paternal) that are similar in shape, size, and gene content.
Synapsis & Chiasmata 联会与交叉 The pairing of homologous chromosomes and the physical site where crossing over occurs.

1. The Purpose of Meiosis

Meiosis is a process that ensures the formation of haploid (\(n\)) gamete cells, sometimes referred to as daughter cells, in sexually reproducing diploid (\(2n\)) organisms.

Without this reduction, the chromosome number would double every generation upon fertilization. Meiosis maintains the species-specific chromosome count while introducing genetic variation.

2. Meiosis I: Reductional Division

Meiosis I is the first round of division, where the goal is to separate homologous chromosomes.

  1. Prophase I: Homologous chromosomes pair up and condense. Synapsis occurs, and chiasmata may form (where genetic material is exchanged). The meiotic spindle begins to form, centrosomes move to opposite poles, and the nuclear envelope breaks down.
  2. Metaphase I: Meiotic spindle fibers align homologous pairs along the equator of the cell at the metaphase plate.
  3. Anaphase I: Homologous chromosomes separate and are pulled toward opposite poles. Crucially, sister chromatids remain attached at their centromeres.
  4. Telophase I: The spindle breaks down, new nuclear envelopes may develop, and cytokinesis occurs.

Result: Two haploid daughter cells are formed.

Etymology: Meiosis

The word Meiosis comes from the Greek word meioun, meaning "to lessen." This perfectly describes the "lessening" of the chromosome number from \(2n\) to \(n\).

3. Meiosis II: Equational Division

Meiosis II is very similar to mitosis, as it involves the separation of sister chromatids.

  1. Prophase II: The meiotic spindle forms again; sister chromatids (still connected at the centromere) attach to the spindle.
  2. Metaphase II: Chromosomes align individually along the metaphase plate.
  3. Anaphase II: Proteins at the centromeres break down. Sister chromatids are pulled apart toward opposite poles.
  4. Telophase II: Nuclear envelopes reform, chromosomes decondense, and cytokinesis occurs.

Final Result: Four haploid daughter cells, each containing unduplicated chromatids.

(Placeholder: A diagram illustrating the stages of Meiosis I and II, highlighting the difference between homologous separation and sister chromatid separation.)

4. Comparison: Mitosis vs. Meiosis

While both processes use a similar spindle apparatus to move chromosomes, they serve different biological roles.

Feature Mitosis Meiosis
Number of Divisions One Two
Number of Daughter Cells Two (genetically identical) Four (genetically distinct)
Ploidy \(2n \rightarrow 2n\) (remains same) \(2n \rightarrow n\) (reduced)
Homologous Pairing No Yes (in Prophase I)
Function Growth, tissue repair, asexual reproduction Production of gametes for sexual reproduction

Quiz

Campbell Biology Chapter 13 Practice Test: Meiosis and Sexual Life Cycles

Click the link above to practice related multiple-choice questions (opens in a new tab).