2.3 Meiosis

2026-03-04 23:32

Status: #new

Cells are divided into primarily two types:

Gamete Production

Many organisms contain pairs of chromosomes. A complete set originates from each parent. Such organisms are known as diploid.
Pairs of Chromosomes are identical to each other through their bands and shape
- They are paired together when they share the same layout of genes along the DNA. Such chromosomes are known as homologous chromosomes.
  - Also known as homologous pairs

The process of gamete production. The goal is to create haploid gametes. A gamete (1n) will create a single set of chromosomes for their respective organism type

Goals of Meiosis:

Before Meiosis begins, the cells which will undergo gametogenesis are in Interphase
Gametogenesis - the creation process of gametes (Genesis)
Right before Meiosis, cells have 2 sets of chromosomes, hence they are 2n.
Comparison with Mitosis:

Meiosis

First stage of Meiosis (creates the genetic diversity), also many steps of Mitosis apply here

Tetrads line up along the metaphase plate (instead of replicated chromosomes)
This is made possible by the meiotic spindle microtubules pushing and pulling
A key difference to mitosis
- 23 Tetrads vs 46 Replicated Chromosomes
- Meiotic Checkpoints are less strict. They prioritize ensuring pairs (Tetrads) of Homologous chromosomes exist.
- Meanwhile, Mitotic Checkpoints (like the M checkpoint) focus on near perfect alignment, which could lead to prolonged arrest. This means that their checkpoints prevent the kind of random orientation of homologous pairs (Tetrads) which are needed in Meiosis

Spindle Fibres retract to pull the homologous chromosomes towards opposite poles as the bond between the pair of homologous chromosomes in the Tetrad is broken.
- This is a result of seperase breaking down the cohesin proteins
Genetic content between the two resulting cells will be dramatically different
- A tetrad may be identical in extreme edge cases, but given there is likely more than one tetrad, a perfectly identical clone is for all intents and purposes impossible.
Difference with mitosis:
- While they do pull apart genetic material, the genetic material is different for each cell. In addition, the centromere cohesin proteins are not targeted by the enzyme seperase.
This is also the point where the diploid cell becomes a haploid gamete, as the ploidy is reduce by 1. This is because we are splitting the tetrads, leading to non-identical sister chromatid pairs. This is because unlike mitosis, we separate our homologous pairs, leading to single sets of chromosomes (say no 1-1 arrangement but a 2(1) arrangement)

Spindle Fibres are retracted
Cleavage furrow is developed (or cell plate for plants) to divide the cytoplasm
A nuclear membrane MAY reappear
Condensed chromosomes MAY begin to decondense into chromatin
Cytokinesis then occurs, but one KEY difference:
- We have distinct parent pairs/homologous chromosomes for each cell. What this means is that we have individual sister chromatids/ replicated chromosomes, instead of a half/half split that decondense into chromatin upon the breaking of the cohesin structures found in Mitosis

Meiotic spindle microtubules are formed and centrioles move to the opposite ends of the cell
In some cases:
- The nuclear membraned may disappear and chromosomes condense (if during Telophase the nuclear membrane and decondensing happened)
In animal cells, we can see the formation of centrosomes and asters along with the return of
- Kinetochore microtubules, Polar microtubules and Astral microtubules

Looks and feels like mitosis
Astral microtubules attach to the cell membrane
Individual replicated chromosomes (sister chromatids) line up on the metaphase plate
- Key differences:
  - Haploid Metaphase Line up, instead of Diploid Metaphase Line up found in Metaphase I
If applicable
- Aster microtubules will attach to the cell membrane
There is potential for further variety upon separation,
- Because sister chromatids aren't identical, this means that there is no strong mitosis like M-check. Because of this, we can have strong random orientations.

Meiotic Spindle Microtubules retract after the centromeres cohesin is cleaved by the protein seperase. If applicable, it is the kinetochore mictrotubules that pull on these centromeres, acting on motor proteins
As the split sister chromatids approach the poles, the Meiotic Spindle Microtubules begin to disassemble. In addition, the cell begins to elongate during this process.
Comparison to Anaphase 1:
- Anaphase 1 - Splits Homologous Chromosomes and reduces Ploidy (the # of complete sets of a chromosome) by 1 (2n - n).
- Anaphase 2 - Splits non-identical sister chromatids into 2, creating two distinct 1n cells.
- Mitotic Anaphase - In this stage, it splits identical sister chromatids, which keeps the ploidy at 2

Meiotic Spindle Microtubules (if applicable, kinetochore microtubules) are retracted
- Nuclear membrane begins to reform around genetic material
- Cleavage furrow forms
- The chromosomes decondense into chromatin

Cleavage furrow at metaphase plate forms
- If applicable - Caused by actin microfilaments that pull the cell inwards
- Eventually pinches the cell in two
Cytokinesis is a physical manifestation of a process, not a categorizable description

While Meiosis divides genetic material, additional cellular development is required for true gametes to form.
Mendel's law of Separation
- During gamete formation, the allele for each gene segregate from each other such that each gamete carries only allele for each gene.
Ova/Ovum Formation
- The cytoplasm is not evenly split in meiosis (I) and (II). This results in a larger cell and smaller cell.
- Smaller cell is known as a polar bodies
  - They are reabsorbed into the body (they die)
- The survivor is known as the egg/ova.
- They are a lot larger than sperm as they need lot of material to split upon fertilization.
Spermatogenesis
- Meiosis results in 4 sperm
Sperm are mature when
- Most of the cytoplasm inside the sperm is lost and the nucleus becomes the head (to squirm into a egg)
- A flagellum is developed
Ova mature when
- A single cell collects more cytoplasm than the other polar bodies during meiosis (aka killing everyone else)
- They fertilize to create a blastocyst (early fertilized cell)