Sexual vs Asexual Reproduction: Genetic Diversity

Posted on Jan 11, 2025 in Biology

There are two types of reproduction: sexual and asexual. In sexual reproduction, descendants are genetically different. For example, humans reproduce through the union of gametes. Individual differences are the norm in sexual reproduction because special haploid cells, the sexual reproductive cells (meiospores and gametes), are formed by meiosis. Meiosis is the division of cells. In asexual reproduction, descendants are genetically identical. For example, roses reproduce through cuttings, where a new individual is formed from a cell by mitosis, originating from the progenitor cell.

Meiospores vs Gametes

Meiospores differ from gametes in that gametes of different sexes unite to give rise to a cell that can be divided, whereas meiospores cannot.

Haploid and Diploid Biological Cycles

  • Haploid cycle: Typical of species where individuals are haploid, meaning they have a single set of chromosomes. The only diploid structure is the zygote.
  • Diploid cycle: Found in plants and fungi, where adult individuals are diploid, meaning they have two sets of chromosomes. Meiosis occurs during gametogenesis. After fertilization, the gametes (haploid) form a diploid zygote, which develops into an adult.
  • Diplohaploid cycle: Characterized by alternating generations of two types of adult individuals: some are diploid, and others are haploid. Meiosis takes place during sporogenesis.

Advantages of Sexual Reproduction

In asexual reproduction, all divisions are by mitosis, while in sexual reproduction, meiosis occurs. Asexual and sexual reproduction generate individuals that are different. This can be an advantage for the species.

Importance of Offspring Variability

Genetic variability in offspring is crucial for adaptation and evolution. This variability arises from:

  1. Recombination: The exchange of genes between two chromosomes, specifically between one chromatid of each chromosome.
  2. Chromosome combinations: Each cell receives a different combination of chromosomes.
  3. Fertilization: If the individual forms two separate gametes, fertilization further increases variability.

Stages of Meiosis

Meiosis is the process that generates genetic variability, resulting in cells with half the number of chromosomes. It involves two divisions:

Meiosis I

  1. Prophase I:
    • Leptotene: DNA filaments condense into chromosomes, each with two chromatids (four arms).
    • Zygotene: Chromosomes recognize their counterparts and join together in a process called synapsis, facilitated by the synaptonemal complex.
    • Pachytene: Synapsis is complete, forming bivalents. Recombination occurs between two of the four chromatids.
    • Diplotene: Chromosomes begin to separate, remaining joined at points called chiasmata.
    • Diakinesis: Chromosomes condense further, and the two sister chromatids become distinct. The nuclear envelope and nucleolus disappear, and bivalents move to the equatorial plane.
  2. Metaphase I: Bivalents align at the equatorial plane.
  3. Anaphase I: Homologous chromosomes separate and move towards opposite poles.
  4. Telophase I: Chromosomes reach the poles. In some species, chromosomes decondense, and a nuclear envelope forms; in others, meiosis II begins directly.

Meiosis II

Meiosis II is preceded by a brief interphase (interkinesis) without DNA replication. It includes:

  • Prophase II: The nuclear envelope breaks down, and chromosomes condense.
  • Metaphase II: Chromosomes align at the equatorial region.
  • Anaphase II: Sister chromatids separate and migrate to opposite poles.
  • Telophase II: Chromosomes decondense, and a nuclear envelope forms around each set, resulting in four haploid nuclei. Cytokinesis (cytoplasm division) follows.

Asexual and Sexual Reproduction in Multicellular Organisms

Multicellular organisms can reproduce both asexually and sexually. In sexual reproduction, descendants are different because each zygote is formed by two different gametes. In asexual reproduction, each descendant is born from a single haploid cell.

Reproduction is the generation of new individuals, while sexuality is the exchange and reception of genetic material. Asexual reproduction occurs without sexuality.

Types of Asexual Reproduction

  • Budding: New individuals form from multicellular buds that detach from the parent organism.
  • Excision: Longitudinal or transverse division of the body into two or more fragments.
  • Fragmentation: Generation of new individuals from plant parts, such as rhizomes, tubers, bulbs, and stolons.
  • Sporulation: Formation of single cells (spores) by mitosis.
  • Polyembryony: The first cell divisions of the zygote separate.

Types of Sexual Reproduction

  • Somatogamy: Union of two somatic cells, first plasmogamy (cytoplasm fusion) and then karyogamy (nucleus fusion).
  • Gametangiogamy: Fusion of multinucleated gametangia.
  • Gametogamy: Formation of new individuals from gametes, with two varieties:
    • Amphigony: Fusion of two different gametes.
    • Parthenogenesis: Development of an individual from an unfertilized egg.