Chapter 16 pt.3 – Meiosis and Comparison to Mitosis

• Source: Chapter 16, Part 3 (McGraw-Hill, 2020) provides comprehensive coverage of cell division.

• Material prepared for classroom use only; reproduction restricted to ensure proper educational context.

• Figures/tables referenced are pre-inserted in the separate slide deck to visually represent complex cellular processes like chromosome segregation and chiasmata formation.

• Learning objective: understand the fundamental differences between mitotic and meiotic cell division and the intricate chromosome dynamics involved in each.

Page 2 – Big-Picture Numbers

• Mitosis (in humans): 1 diploid parent cell (containing 46 chromosomes) accurately divides to produce 2 genetically identical diploid daughter cells, each with 46 chromosomes.

• Meiosis (in humans): 1 diploid parent germ-line cell undergoes two rounds of division to produce 4 genetically unique haploid gametes, each containing 23 chromosomes.

• Key vocabulary: diploid (2n) refers to cells containing two sets of chromosomes (one maternal, one paternal), while haploid (n) refers to cells containing a single set of chromosomes.

Page 3 – Definition of Meiosis

• Purpose: to convert diploid germ-line cells (found in the gonads) into haploid gametes (sperm or egg cells) for sexual reproduction.

• Human example: a cell with 46 chromosomes (diploid content) is reduced to cells with 23 chromosomes (haploid content).

• Two sequential divisions: Meiosis I, known as the reductional division (halves the chromosome number), and Meiosis II, known as the equational division (separates sister chromatids, similar to mitosis).

Pages 4–5 – Homologous Chromosome Pairs

• Each homologous pair shown with two colors: typically one chromosome of the pair is inherited from the maternal parent and the other from the paternal parent.

• Humans possess 23 such pairs (total 46 chromosomes) in diploid somatic cells, including 22 autosomal pairs and 1 pair of sex chromosomes.

• “Homologous” chromosomes are defined by having the same overall length, the same centromere position, and the same specific gene loci (physical locations of genes) along their length; however, they may carry different alleles (variant forms) for those genes.

Pages 6–7 – Chromosome Accounting Through Meiosis

Start:

• Diploid germ-line cell: contains 46 replicated chromosomes (each consisting of 2 sister chromatids, resulting in 92 chromatids) organized into 23 homologous pairs. At this stage, the chromosome count is determined by the number of centromeres.

After Meiosis I:

• The two resulting cells become haploid (23 chromosomes) because homologous pairs have separated. Crucially, each of these 23 chromosomes still consists of 2 sister chromatids (total 46 chromatids), meaning the DNA content is halved but each chromosome is still duplicated.

After Meiosis II:

• The cells remain haploid (23 chromosomes). During this division, the sister chromatids separate, resulting in each chromosome now consisting of a single chromatid (total 23 chromatids per cell). This leads to four haploid cells, each with half the original chromosome number and half the original DNA content.

• Asterisks in figure mark chromo-counts after each stage, illustrating the reductional nature of Meiosis I and the equational nature of Meiosis II.

Page 8 – Essential Event of Meiosis I

• Homologous pairs separate and move to opposite poles, which is the defining event that reduces the ploidy level (chromosome number) by half.

• Outcome: two haploid cells are formed by the end of meiosis I, each containing replicated chromosomes.

Page 9 – Essential Event of Meiosis II

• These haploid cells from Meiosis I undergo a second division without further DNA replication.

• Sister chromatids of each chromosome separate and move to opposite poles, similar to mitosis.

• Final product: four genetically unique haploid cells (gametes), each containing unreplicated chromosomes.

Page 10 – Mitosis vs. Meiosis: Shared Foundations & Key Differences

Shared prelim: Before either mitosis or meiosis, cells must pass through interphase, which includes G1 (cell growth and normal function), S (DNA synthesis, where chromosomes are replicated), and G2 (further growth and preparation for division).

Meiosis-specific distinctions:

1. Synapsis → formation of a bivalent/tetrad: Homologous chromosomes physically associate and align closely along their entire length.

2. Crossing over (genetic recombination): The exchange of genetic material between non-sister chromatids, leading to increased genetic diversity.

3. Separation of homologous pairs (not sister chromatids) during anaphase I: This is the crucial reductional step that halves the chromosome number.

• Nomenclature: Both processes utilize similar phase names (Prophase, Metaphase, Anaphase, Telophase), but Meiosis specifies I or II (e.g., Prophase I/II, Metaphase I/II) due to having two distinct divisions.

Page 11 – Bivalent (Tetrad) Formation

• During synapsis, homologous sister-chromatid pairs align precisely side-by-side, forming a structure called a bivalent (or tetrad), which consists of 4 chromatids.

• The Synaptonemal complex is a highly organized protein lattice that transiently glues the homologous chromosomes together, ensuring their accurate alignment for crossing over.

Page 12 – Crossing Over (Genetic Recombination)

• This involves the physical exchange of corresponding DNA segments between non-sister chromatids (i.e., between a maternal chromatid and a paternal chromatid) within a bivalent.

• This process produces recombinant chromosomes, which are mosaics of maternal and paternal genetic information, significantly increasing genetic variation in gametes.

• Crossing over specifically occurs during late Prophase I and is vital for genetic diversity.

Pages 13–14 – Chiasma & Visualization

• A crossover site remains physically connected as a chiasma (plural: chiasmata) after the synaptonemal complex disassembles, holding the homologous chromosomes together until Anaphase I.

• Chiasmata become visibly distinct structures when the homologous chromosome arms begin to pull apart in late prophase I, indicating where recombination has occurred.

• Figure 16.12 sequence illustrates this process:

  1. Homolog condensation: Chromosomes condense, becoming visible.

  2. Synapsis initiation: Homologous chromosomes begin to align.

  3. Bivalent completion: Full formation of the synaptonemal complex and bivalent.

  4. Crossing over: Reciprocal exchange of DNA segments occurs.

  5. Chiasma visible: Physical manifestation of crossover events.

Page 15 – Detailed Events of Prophase I

• Chromosome condensation continues, making the duplicated chromosomes compact and visible.

• Bivalents form via synapsis, where homologous chromosomes pair up precisely.

• Crossing over is executed, leading to the exchange of genetic material between non-sister chromatids.

• The nuclear envelope breaks down, allowing the spindle microtubules to gain access to the chromosomes.

Page 16 – Prometaphase I

• The spindle apparatus becomes fully assembled, extending microtubules from opposite spindle poles.

• A crucial difference from mitosis: Each pair of sister chromatids within a bivalent attaches via its kinetochore to microtubules originating from ONE spindle pole. This co-orientation is key for ensuring that homologous chromosomes (rather than sister chromatids) successfully separate during Anaphase I.

Page 17 – Metaphase I

• The bivalents, still structurally connected by chiasmata, line up as a DOUBLE row along the metaphase plate (equatorial plane of the cell).

• The orientation of each bivalent (i.e., which homologous chromosome faces which pole) is entirely random and independent of other bivalents. This

Page 17 – Metaphase I

• The bivalents, still structurally connected by chiasmata, line up as a DOUBLE row along the metaphase plate (equatorial plane of the cell).

• The orientation of each bivalent (i.e., which homologous chromosome faces which pole) is entirely random and independent of other bivalents. This independent assortment of homologous chromosomes leads to significant genetic variation in the resulting gametes. For n homologous pairs, there are 2^n possible combinations of chromosomes in the gametes.

Page 18 – Anaphase I

• Homologous chromosomes (each still composed of two sister chromatids) separate and move to opposite poles of the cell, pulled by the spindle microtubules.

• Sister chromatids remain attached at their centromeres, unlike in mitosis or Meiosis II.

• This separation is the physical manifestation of the reductional division.

Page 19 – Telophase I & Cytokinesis I

• Telophase I: The separated homologous chromosomes arrive at opposite poles. Each pole now has a haploid set of replicated chromosomes (n chromosomes, each with 2 chromatids). The nuclear envelope may reform around each set of chromosomes, and the chromosomes may decondense slightly.

• Cytokinesis I: The cytoplasm divides, typically concurrently with telophase I, forming two haploid daughter cells. These cells are now haploid with respect to chromosome number (e.g., 23 chromosomes in humans), but each chromosome still consists of two sister chromatids. There is no S phase (DNA replication) between Meiosis I and Meiosis II.

Page 20 – Meiosis II: Overview

• The two haploid cells produced in Meiosis I immediately enter Meiosis II.

• Meiosis II is often referred to as the equational division because it separates sister chromatids, much like mitosis, but starts with haploid cells. The chromosome number n remains haploid throughout Meiosis II, but the DNA content per chromosome is reduced (from replicated to unreplicated).

• Its primary function is to finally separate sister chromatids, leading to four unique haploid gametes.

Page 21 – Prophase II & Prometaphase II

• Prophase II: Chromosomes in each of the two haploid daughter cells (from Meiosis I) condense again. The nuclear envelope (if reformed) breaks down, and the spindle apparatus begins to form again.

• Prometaphase II: Spindle microtubules attach to the kinetochores of individual sister chromatids. Unlike Metaphase I, where homologous chromosomes paired and attached to one pole, in Meiosis II, each sister chromatid attaches to microtubules from opposite poles, preparing for their separation.

Page 22 – Metaphase II

• The chromosomes (each still composed of two sister chromatids) align individually along the metaphase plate in each of the two cells.

• This alignment is similar to Metaphase in mitosis, but with a haploid number of chromosomes.

Page 23 – Anaphase II

• Sister chromatids finally separate at their centromeres and move as individual (unreplicated) chromosomes to opposite poles of the cell.

• This results in chromosomes composed of single chromatids.

Page 24 – Telophase II & Cytokinesis II

• Telophase II: Chromosomes arrive at the poles of the cell. Nuclear envelopes reform around the sets of unreplicated chromosomes, and the chromosomes decondense.

• Cytokinesis II: The cytoplasm divides, resulting in a total of four genetically unique haploid daughter cells (gametes). Each of these cells contains n chromosomes, and each chromosome consists of a single chromatid (e.g., 23 chromosomes, 23 chromatids in human gametes).

• The bivalents, still structurally connected by chiasmata, line up as a DOUBLE row along the metaphase plate (equatorial plane of the cell).

• The orientation of each bivalent (i.e., which homologous chromosome faces which pole) is entirely random and independent of other bivalents. This independent assortment of homologous chromosomes leads to significant genetic variation in the resulting gametes. For n homologous pairs, there are 2^n possible combinations of chromosomes in the gametes.

• Homologous chromosomes (each still composed of two sister chromatids) separate and move to opposite poles of the cell, pulled by the spindle microtubules.

• Sister chromatids remain attached at their centromeres, unlike in mitosis or Meiosis II.

• This separation is the physical manifestation of the reductional division.

• Telophase I: The separated homologous chromosomes arrive at opposite poles. Each pole now has a haploid set of replicated chromosomes (n chromosomes, each with 2 chromatids). The nuclear envelope may reform around each set of chromosomes, and the chromosomes may decondense slightly.

• Cytokinesis I: The cytoplasm divides, typically concurrently with telophase I, forming two haploid daughter cells. These cells are now haploid with respect to chromosome number (e.g., 23 chromosomes in humans), but each chromosome still consists of two sister chromatids. There is no S phase (DNA replication) between Meiosis I and Meiosis II.

• The two haploid cells produced in Meiosis I immediately enter Meiosis II.

• Meiosis II is often referred to as the equational division because it separates sister chromatids, much like mitosis, but starts with haploid cells. The chromosome number n remains haploid throughout Meiosis II, but the DNA content per chromosome is reduced (from replicated to unreplicated).

• Its primary function is to finally separate sister chromatids, leading to four unique haploid gametes.

• Prophase II: Chromosomes in each of the two haploid daughter cells (from Meiosis I) condense again. The nuclear envelope (if reformed) breaks down, and the spindle apparatus begins to form again.

• Prometaphase II: Spindle microtubules attach to the kinetochores of individual sister chromatids. Unlike Metaphase I, where homologous chromosomes paired and attached to one pole, in Meiosis II, each sister chromatid attaches to microtubules from opposite poles, preparing for their separation.

• The chromosomes (each still composed of two sister chromatids) align individually along the metaphase plate in each of the two cells.

• This alignment is similar to Metaphase in mitosis, but with a haploid number of chromosomes.

• Sister chromatids finally separate at their centromeres and move as individual (unreplicated) chromosomes to opposite poles of the cell.

• This results in chromosomes composed of single chromatids.

• Telophase II: Chromosomes arrive at the poles of the cell. Nuclear envelopes reform around the sets of unreplicated chromosomes, and the chromosomes decondense.

• Cytokinesis II: The cytoplasm divides, resulting in a total of four genetically unique haploid daughter cells (gametes). Each of these cells contains n chromosomes, and each chromosome consists of a single chromatid (e.g., 23 chromosomes, 23 chromatids in human gametes).