Biological Cycles, Reproduction, and Cell Communication
According to the biological cycles at which meiosis occurs, there are three biological cycles.
Haplonte Cycle
In species where adult individuals are haploid, the only diploid part of the life cycle is the zygote. Gametes are formed from the adult via mitosis. By uniting, gametes form a diploid zygote, which undergoes meiosis (zygotic meiosis) and gives rise to haploid cells that develop into haploid adults. This cycle occurs in some algae and fungi.
Diplonte Cycle
In species where adults are always diploid, meiosis occurs during gametogenesis (gametogenic meiosis). The union of gametes forms a diploid zygote, which also gives rise to diploid adult organisms. This cycle is present in many protozoa, some algae, and some fungi.
Diplohaplonte Cycle
This cycle is found in species with alternation of generations, where haploid and diploid generations alternate. Meiosis occurs during sporogenesis (sporogenic meiosis). Haploid spores are formed by meiosis. The diploid adult is called the sporophyte; the spore produces a haploid gametophyte. The gametophyte contains male (antheridia) and female (archegonia) sex organs. Fertilization of the oosphere in the archegonium by an antherozoid forms a diploid zygote, from which the sporophyte develops. This cycle appears in metaphytes, algae, and some fungi.
Asexual and Sexual Reproduction
In asexual reproduction, all divisions are by mitosis; in sexual reproduction, there is always a stage of division by meiosis. Asexual reproduction produces genetically identical offspring, while sexual reproduction produces genetically different offspring. This variability allows for survival in changing environments; almost all multicellular species reproduce sexually. The variability in offspring from sexual reproduction is due to random recombination of parental and recombinant chromatids during gamete formation. The combination of chromosomes in gametes is also random. If a new individual is formed by the union of two gametes, it is also influenced by chance. A drawback is that sexual reproduction is inefficient if the environment is stable and produces individuals similar to the parents. Sexual reproduction presents significant challenges: meeting, mating, fertilization, and embryonic development. Despite these difficulties, sexual reproduction has significant advantages. Initially, individuals may have been unicellular and haploid, reproducing asexually by mitosis. To cope with unfavorable seasons, spore formation may have been initiated. This could have led to the exchange of genetic material between different individuals, resulting in the formation of diploid individuals. Meiosis may have been initiated to resume the haploid phase, leading to the haplonte cycle. The diploid phase may have been extended, leading to the diplohaplonte cycle, and eventually the diplonte cycle.
Differences Between Mitosis and Meiosis
Mitosis consists of one division process, while meiosis consists of two division processes (meiosis I and meiosis II). In mitosis, two diploid daughter cells are obtained from one diploid cell; in meiosis, four haploid daughter cells are obtained from one diploid cell. In mitosis, DNA is duplicated during interphase; in meiosis, DNA is duplicated before meiosis I, but not between meiosis I and meiosis II. Mitosis applies to any cell type, while meiosis is only for reproductive cells. In mitosis, the two daughter cells have the same genetic information; in meiosis, the four daughter cells have different genetic information. Crossing over and genetic recombination occur in prophase I of meiosis, but not in mitosis. In metaphase of mitosis, sister chromatids are aligned; in metaphase I of meiosis, homologous chromosome pairs are aligned. In anaphase of mitosis, sister chromatids separate; in anaphase I of meiosis, homologous chromosomes separate.
Role of Cell Communication
All organisms exist in a changing environment. Their ability to sense and respond to these changes is crucial for survival. In unicellular organisms, a single cell must capture all stimuli and develop appropriate responses. In multicellular organisms, cells are specialized: some capture stimuli (sensory organs), others transmit signals (nerve and endocrine cells), and others carry out responses (muscle and glandular cells). Coordinated communication between cells is carried out by chemical agents.
Modes of Communication
In plant cells, communication depends on the cell wall and the transport of plant hormones via the sap. However, communication primarily occurs through plasmodesmata, which connect the cytoplasm of adjacent cells.
- Gap junctions: Direct cytoplasmic connections between adjacent cells allow the passage of ions and small molecules. This is crucial for embryonic development.
- Membrane-bound signaling molecules: A signaling cell with a signaling molecule interacts with a target cell with a receptor that captures the signal molecule. This is involved in the immune response.
- Long-distance communication via secreted molecules: A signaling cell releases signal molecules, which are captured by a target cell with a receptor.
- Paracrine signaling: Signal molecules act on nearby target cells.
- Endocrine signaling: Endocrine glands secrete hormones distributed throughout the body via the blood.
- Synaptic signaling: Nerve cells transmit impulses rapidly from cell to cell via synapses. A presynaptic neuron releases neurotransmitters into the synaptic cleft, causing changes in the postsynaptic neuron and transmitting the nerve impulse.
Receptors and Signal Transduction
The interaction between signal molecules and receptors provides a specific link, causing a change in the receptor’s structure and initiating a response. There are two types of receptors:
Intracellular receptors: Signal molecules are small, lipid-soluble, and water-insoluble. They can pass through membranes. Transported by blood proteins, they separate from the protein upon reaching the target cell and bind to an intracellular receptor. The hormone-receptor complex enters the nucleus and induces protein synthesis, leading to physiological changes in the cell.
Membrane receptors: Signal molecules are large, water-soluble, and lipid-insoluble; they cannot cross the membrane. Receptors are located in the membrane and act as signal transducers. Three types of membrane receptors exist:
- Receptor-associated ion channels: Located in nerve cells, neurotransmitters induce the opening or closing of ion channels, affecting synaptic transmission and muscle contraction.
- Enzyme-associated receptors: Act as enzymes; the best-studied are protein kinases, which cross the membrane, acting as extracellular receptors and intracellular catalysts (e.g., insulin receptors).
- G protein-coupled receptors: Binding of the signal molecule changes the receptor’s conformation, allowing it to bind to a G protein, changing its conformation and activating a second messenger that acts on other proteins, triggering a cascade of reactions. cAMP and calcium ions are examples of second messengers.