Taxonomy, Reproduction, and Cell Division: An Overview
Taxonomic Groups
Taxonomic ranks include: Domain, kingdom, phylum (or division), class, order, family, genus, and species.
- Archaea: Includes archaebacteria.
- Bacteria: Includes eubacteria.
- Eukarya: Includes all eukaryotes, organisms with cells whose nuclei are surrounded by a nuclear membrane.
Classified into six kingdoms to understand the origins of living beings:
Kingdoms: Archaea, Bacteria, Protista, Fungi, Plantae (Vegetal), Animalia (Animal).
Sexual and Asexual Reproduction
Asexual Reproduction
Asexual reproduction occurs when an individual produces offspring without the participation of sexual cells.
There are four main types:
- Bipartition or binary fission
- Sporulation
- Germination
- Fragmentation
Sexual Reproduction
Sexual reproduction is an important mode of reproduction among living beings, characterized by the production of sexual cells or gametes.
Sexual reproduction is composed of three stages:
- Gamete Production: Gametes are formed in specialized organs. In animals, the organs that produce gametes are gonads.
- Fertilization: Fertilization is the union of different sex gametes. When this union occurs, a unique cell called a zygote or egg is formed. The fusion of gametes (mixing of hereditary characters) activates the ovule, initiating development.
- Development: The zygote divides and forms an embryo, which is the new being in the first stages of its development.
Cellular Reproduction
Interphase
The cell spends most of its life in interphase, preparing for cellular division. It is divided into three sub-stages:
- G1 (Growth): Characterized by being genetically active.
- S (Synthesis) Phase: The hereditary material is duplicated.
- G2: Final preparation of all components for cellular division takes place.
Mitosis
Mitosis ensures that the same amount of genetic material (2n) from the progenitor cell is inherited by daughter cells.
- Prophase: The nucleolus and the nuclear membrane disappear. The duplicated genetic material appears as double-armed chromosomes.
- Metaphase: The centriole forms the achromatic spindle. The double chromatid chromosomes are fixed to the spindle by the centromere and are located in the middle of the cell.
- Anaphase: The spindle retracts to the poles. The chromosomes are divided by the centromere and separated into their chromatids. The chromatids are taken to the cell poles.
- Telophase: The spindle disappears, and the nuclear membrane and nucleolus reappear.
Meiosis
Meiosis is a mechanism that allows the reduction of the chromosome number by half and the formation of female and male gametes.
Meiosis I fulfills the cellular division phases: prophase I, metaphase I, anaphase I, and telophase I. As a result, all daughter cells obtained from a cell are characterized by having received half of the homologous chromosomes with gene exchange. Daughter cells produced in meiosis I enter a second process of cellular division known as meiosis II. Cells that begin meiosis II distribute their complete chromosomes into chromatids.
Spermatogenesis
Spermatogenesis is the process of sperm formation that happens in the testicles from diploid cells called spermatogonia. When these enter into meiosis I, they are known as primary spermatocytes, and when they enter meiosis II, they are called secondary spermatocytes. The four final cells produced in meiosis are called spermatids, and they undergo a series of changes that lead them to sperm formation.
Oogenesis
Oogenesis is the process of ovule formation in the ovaries. The ovules come from diploid cells located in the ovaries called oogonia. Those that begin with meiosis I are called primary oocytes, and those that begin with meiosis II are called secondary oocytes, which ultimately constitute an ovule and polar bodies.