a. Organisms are creatures of time; for example:

b. All animal activities are based on regular daily schedules; thus animals have circadian rhythms (circa =a bout; dia = day), which are entrained by light cycles.

c. Biological clocks, or rhythms of internal hormonal and nervous mechanisms, might be controlled by the DNA to RNA information transfer system and its feedback mechanisms.

a. In simple organisms that grow clonally (e.g. bacteria), evolution generally requires a series of mutations and rearrangements of the genome before new structures or functions are made.

b. Pseudosexual processes in bacteria are covered in Chapter 17.

c. Organisms that are diploid (have two copies of each gene) and undergo sexual reproduction (regularly mix copies of their genomes) have reproductive, genetic, and evolutionary advantages over clonal organisms.

d. A cell that has only one copy of its genome is haploid.

e. Sexual organisms have sexual cycles (Figure 15.5), whereby they alternate between haploid and diploid phases.

f. Diploid cells are formed when two gametes (haploid cells) fuse during fertilization or syngamy.

g. Haploid cells are formed after diploid cells undergo meiosis, a nuclear division that divides the cell's chromosomal complement in half.

h. To keep the chromosomal complement constant each generation, meiosis is essential in sexually reproducing organisms.

a. A typical human life cycle is illustrated in Figure 15.6.

b. Somatic cells are body cells, all of which are diploid and reproduce (for growth and repair) by mitosis.

c. All somatic cells of a given organism have essentially the same genetic complement.

d. Germ cells are the haploid gametes (e.g. sperm and eggs) that are made by meiosis, which occurs strictly in the gonads (sex organs) or sexually reproducing organisms.

e. In humans, testes are the male gonads, which produce sperm, and ovaries are the female gonads, which produce eggs (ova).

f. A zygote is the diploid cell formed after sperm and egg cells combine during fertilization, and which gives rise to a new individual.

g. Vegetative growth is growth of an organism via mitotic divisions; it is called the diplontic cycle when organisms in the diploid stage grow this way, the haplontic cycle, when haploid organisms grow this way, and the haplodiplontic cycle, when organisms grow this way in both haploid and diploid stages.

h. Sidebar 15.1 explores variations on sexual reproduction, including hermaphrodity, and parthenogenesis.

i. Figure 15.7 illustrates the haplontic growth cycle of Chlamydomonas, where gametes are called mating types (+ and -) and zygotes become spores.

j. Figure 15.8 illustrates the haplodiplontic cycle of the yeast Sacchraromyces, where both haploid and diploid cells grow vegetatively until conditions cause cells of opposing haploid mating types to fuse and make a diploid cell.

k. Land plants typically have haplodiplontic cycles; the gametophyte is the haploid phase that produces gametes via mitosis, and the sporophyte is the diploid phase (produced by the syngamy of haploid gametes) that produces spores via meiosis.

l. Figure 15.9 illustrates isogamy (gametes of the same size), anisogamy (gametes of differing sizes), and oogamy (motile sperm and nonmotile ovum), all distinctions of various sexual life cycles.

m. The evolutionary trend in organisms tends to be toward oogamy, whereby one cell can store nutrients and an elaborate development apparatus.

a. A karyotype (Figure 15.10) is a picture of an individual's set of chromosomes.

b. Each diploid species has a characteristic number of chromosomes, 2n, where n is the haploid number.

c. Chromosomes in diploid organisms are matched into n pairs of homologous chromosomes, or homologs, which are similar morphologically and also in the genes that they carry.

d. Section 19.1 discusses chromosome banding pattern morphologies.

e. Meiosis is a nuclear division that divides chromosome sets in half, producing haploid cells from diploid ones, for instance.

f. Meiosis must be precise in dividing a set of chromosomes into two sets of the same chromosome types, rather than randomly separating chromosomes in a set.

g. Euploid refers to a cell that has an integral number of chromosome sets (e.g. n, 2n, ...).

h. Aneuploid refers to a cell that has extra or missing chromosomes, rather than a full set.

i. Meiosis must begin with a cell that has had an S phase, during which the DNA in the chromosomes has been replicated.

j. Text micrographs illustrate meiosis stages for the Amphiuma, which has 2n=28 chromosomes.

k. Text drawings illustrate meiosis stages for an imaginary organism that has 2n=4 chromosomes, and where colored chromosomes are maternal and black chromosomes are paternal.

l. Meiosis stages are as follows:

m. Four haploid cells have been formed, though they differ genetically from the haploid cells that, through fertilization, formed the original diploid germ cell giving rise to them.

n. Due to independent assortment of maternal and paternal chromosomes during metaphase I and due to cross over exchanges during prophase I, the new haploid cells contain original combinations of genetic material.

a. Spermatogenesis (Figure 15.11) is the formation of sperm (haploid male gametes).

b. Oogenesis (Figure 15.11) is the formation of eggs (haploid female gametes, or ova).

c. In plants (Figure 15.13), the ovary produces eggs, and the anther produces sperm.

a. Normal growth cycles depend on genes turning on or off in proper sequences.

b. The cycle of genes switching on or off in vegetative cells that grow or divide can be monotonous, but other cells have more complex cycles.

c. In aging bacteria cultures, each cell walls off a nuclear body in a protective coating, making an endospore (Figure 15.14), which can endure environmental stresses and grow again during favorable conditions.

d. Anthrax and clostridial spores can lie dormant in soil or on the animal, until passed into another organism, where they again can grow and reproduce.

e. Figure 15.15 show Caulobacter, which varies its life cycle with differentiation, the production of different cell types, including stalked cells and flagellated swarmers.

f. These bacteria show simple patterns of gene regulation related to ecologically advantageous changes during a life cycle.

g. Chapters 20 and 21 cover the complex details of multicellular organism development, including the complicated gene regulation necessary for normal development.

a. Insects are one group of organisms that have distinct phases in the lives of individuals, including egg, larva, pupa, and adult stages. Each stage may be characterized by completely different forms and ways of life (Figure 15.16).

b. The wheat rust, Puccinia graminis (Figure 15.17), has been shown to evolve as fast as its host. It has a complex life cycle with distinct phases, including an intermediate phase that depends on the barberry plant as host, and a dikaryon phase.

c. The liver fluke (Figure 15.18) also has a complex life cycle with an intermediate host and morphologically distinct stages that were once thought to be different species.

d. Every organism goes through a distinctive cycle as it reproduces, all part of the mechanisms for producing generations of organisms with complete genomes, which in turn control the life and development cycles of the organisms.

e. These mechanisms have evolved over a very long period of time, and sexual reproduction has probably made this evolution more efficient, by taking advantage of increased variation.