Biological life cycle
The biological life cycle refers to the series of stages that living organisms, including plants and humans, undergo from conception to reproduction. Each stage marks significant changes, ultimately leading back to the starting point. In humans, this cycle begins with conception, progresses through prenatal development, and continues through infancy, childhood, adolescence, and adulthood, where individuals can become parents themselves. Similarly, flowering plants experience a cyclical life cycle that includes stages such as fertilization, seed development, and flowering, which facilitates the production of new gametes.
Key concepts in the study of biological life cycles include developmental stages and reproduction mechanisms. In plants, the alternation of generations describes the transition between haploid and diploid phases, affecting species within the Archaeplastida group, which includes both algae and land plants. This process involves the creation of gametes, zygotes, and the subsequent formation of multicellular structures like sporophytes and gametophytes.
In animals, including humans, sexual reproduction involves the combination of genetic material from two parents, leading to a genetically unique individual. The life cycle of a human continues from conception through various growth stages, ultimately allowing for the continuation of the life cycle through reproduction, where traits are inherited from both parents. Understanding these biological processes highlights the interconnectedness of life across different species and the importance of each stage in sustaining life.
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Biological life cycle
Living organisms, including plants and people, go through a set of stages. Each stage represents a change from the previous stage. Those changes, over the course of the organism’s life, lead back to the starting point. For example, each human being’s life cycle begins at conception, followed by prenatal development until birth, and then a maturation through infancy, toddlerhood, childhood, adolescence, and then finally to the stage of adulthood. Adult humans can then become parents. That is what makes it a cycle of life, the biological stages going from one beginning to the next.
Background
The study of biological life cycles includes developmental stages and how organisms reproduce. Many scholars have contributed to the wealth of available information about both; one such scholar was Wilhelm Hofmeister (1824–77), a German botanist and biologist who first advanced the idea that plants go through an alternation of generations.
The alternation of generations was one of the early key understandings established in the study of biological life cycles. It means that plants go through an alternation of phases, known as “metagenesis,” from one generation to the next. There are two primary phases, haploid and diploid.
Where n is a set number of chromosomes, cells of an organism in the haploid phase each have n chromosomes. Cells of an organism in the diploid phase have 2n chromosomes. In other words, diploid cells have double the chromosomes of haploid cells, and haploid cells have half the chromosomes of diploid cells.
This facet of the biological life cycle affects organisms in the Archaeplastida group of eukaryotes. Eukaryotes are organisms with cells that have a nucleus within a membrane that contains their key genetic material. That sets them apart from arguably more primitive prokaryotic cells, which lack nuclei. Archaeplastida includes both red algae and green algae, as well as land plants.
Their life cycle begins with haploid gametes. Each is a single cell with n chromosomes. When two gametes fuse, they become a new diploid cell with 2n chromosomes. The newly made diploid cell is called a “zygote.”
The diploid cell then goes through the process of mitosis, in which the cell makes identical copies of its own set of chromosomes. Mitosis is part of a sequence of processes, usually followed by cytokinesis and karyokinesis. Together, the processes result in duplicate diploid cells that are as complete as the first one and that each have 2n chromosomes. The new diploid cells are called “sporophytes.”
The sporophytes then produce sporocytes, which is the organism’s way of getting ready to make haploid spores. They are produced through the process of meiosis, which makes new cells that have only half the chromosomes of their parent cells. The new haploid cells are called “spores.”
The haploid spore germinates and divides by mitosis, just as its diploid predecessor did earlier. Since mitosis makes identical copies of the parent cells, each new cell made in this stage is haploid like the spore that made it. The multicellular organism that results from this stage is composed of haploid cells, and is called a “gametophyte.”
The gametophyte matures and produces gametes, which are the source of its name. Gametes are haploid and have mechanisms to connect to other haploid gametes, so together they can make diploid zygotes. This is a return to the first stage of the biological life cycle.
Impact
Studies of biological life cycles in plant life, as described above, have also affected studies of life cycles in the animal kingdom, including those of human beings. With humans, as with mammals in general, the life cycle is continued from one generation to the next through sexual reproduction.
Sexual reproduction involves two parents, one female and one male, each contributing half the chromosomes needed for the child they create together. The parents each contribute twenty-three chromosomes. The child, the next generation in that pair of human beings’ life cycle, then has twenty-three pairs of chromosomes, with half of each pair inherited from each parent. That is why sons and daughters are equally likely to inherit traits from either parent. While the mother gestates the child from conception to birth, each cell in that child reflects a genetically equal contribution from the father and from the mother.
The one exception to that rule concerns the chromosome pair determining the child’s sex. If the pair is XX, the child’s sex is female. If the pair is XY, the child’s sex is male. Mothers contribute one X, while fathers contribute an X or a Y, meaning that the father’s genetic contribution makes the final determination as to whether the child’s chromosomal sex will be male or female.
Starting with conception, each fetus goes through a series of stages during the ensuing nine months, following its internal instructions for growth and differentiation. The genetic code its parents gave it at conception directs the development process, leading to ever greater degrees of differentiation, with organs and limbs developing across the period of gestation.
Upon birth, the child enters the next stage of his or her life cycle. The child remains the same genetically unique individual from conception throughout the rest of the life cycle, but develops and changes physically while progressing through further stages. These include the very dependent stages of infancy and toddlerhood, in which the child’s basic needs have to be met by the parent or other caregiver. They continue through the more independent stages of late childhood and adolescence and culminate in adulthood, when the person is able to procreate with another adult of the opposite sex and continue the biological life cycle for another generation.
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