During sexual reproduction, gametes are made that contain 1/2 the amount of genetic material (54).
The gametes fuse in fertilization. The process by which they are form is called gametogenesis (54).
Gametes are typically haploid, meaning that they contain 1/2 the number of chromosomes that diploid cells contain . Compared to a diploid, the haploid contains a single set of chromosomes, giving them 23 chromosomes exactly (54).
During meiosis, haploid cells are produced from a cell that was originally diploid (54).

Meiosis Producing Haploid Cells

The process begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle. There are then 2 successive divisions (54).
- Prior to meiosis, the chromosomes are replicated in S phase to produce pairs of sister chromatids. Then meiosis I and II occurs in a sequence just like mitosis.
The prophase of meiosis I has its own 5 phases: lepotene (replication), zygotene (homologues recognize each other and align) pachytene (syanpsis is complete), diplotena (synapses dissociate), and diakinesis (End of prophase)
- Leptotene: Replicated chromosomes begin to condense and become visible with a light microscope. Each structure is actually a pair of chromatids (54).
- Zygotene: Recognition process called synapsis, in which the homologous chromosomes recognize each other and then align themselves along their entire lengths (54).
  - In most eukaryotes, a synaptonemal complex is formed between homologues. This complex is composed of parallel lateral elements bound to the chromosomal DNA and a central element that promotes binding of the lateral elements to each other via transverse filaments, This is not a required complex. It's unknown what its use is, but it may help maintain homologus pairing in situations where the normal process has failed. Either that or it may play a role in meiotic chromosome structure, or it may serve to regulate the process of crossing over (55-56)
- Diplotene: Synaptonemal complex has largely disappeared. Bivalent pulls apart slightly (56).
  - A bivalent is also called a tetrad because it's composed of 4 chromatids.
- Diakinesis: Synaptonemal complex completely disappears (56).
Before pachytene occurs crossing over happens. Crossing over is critical for proper segregation of chromosmes (56).
- The connection that results from crossing over is called a chiasma.
After prophase, the rest of meiosis I mostly continues as.... Normal. Here are some differences : (56).
- In metaphase, the bivalents are aligned along the metaphase plate in a double row, and the arrangement of the sister chromatids are random.
- In metaphase, kinetochores are attached to the sister chromatids; one pair is of sister chromatids is linked to one of the poles and the homologous pair is linked to the opposite pole.
- In anaphase, the 2 pairs of sister chromatids within a bivalent separate from each other. The connectionn holding these chromatids, however, is not broken. Instead, each joined pair of chromatids migrates to 1 pole, and the homologous pair of chromatids move to the opposite.
Meiosis can yield many different results acctoss different cells. This is consistent with Mendel's Law of Independent Assorrtment (56).
The sister chromatids can be randomly aligned along the metaphase plate in many possible ways (56).
- Because the homologues are very similar, but not idential, the random alignment allows for a vast amount of genetic diversity.
The end result of meiosis I is 2 cells, each with 3 pairs of sister chromatids. This is different from the diploid cell that made it; the original had its chromosomes in homologous pairs. (56).

Ch 3: Reproduction and Chromosome Transmission