Neurospora crassa is a red mold that scientists use to study genetics. N. crassa commonly grows on bread as shown in the top left corner of this figure. To culture the mold in lab, researchers grow it in glassware such as test tubes, Erlenmeyer flasks, and petri dishes, as shown in the top right corner of the figure. In the glassware, researchers place a gel, called a medium, of agar, sucrose, salts, and vitamins. The mold grows on the medium, and cotton stoppers prevent anything from contaminating the mold. Under a microscope, researchers can see the structure of the mold's ascospores, which are haploid and oval-shaped structures and function in the mold's life cycle as seeds function in a plant's life cycle.

Mitochondria are organelles found in the cytoplasm of eukaryotic cells. They are composed of an outer membrane and an inner membrane. The outer membrane faces the cellular cytoplasm, while the inner membrane folds back on itself multiple times, forming inner folds, called cristae. The space between the two membrane layers is called the intermembrane space, and the space within the inner membrane is called the matrix.

The figure depicts three different molecular structures of estrogen found in mammals’ that differ by the arrangement of bonds and side groups. The molecular structures of the three estrogen molecules differ by the arrangement of chemical bonds and side groups attached to the core steroid structure, cholesterol, which contains three cyclohexane rings and one cyclopentane ring. Compared to the molecular structure of estriol, the molecular structure of estradiol is missing one oxygen-hydrogen or OH group, and estrone lacks the OH group, and one hydrogen molecule that results in a double bonded oxygen atom. These steroid hormones bind to specific cell receptor molecules and induce transcriptional changes in cells. The production of estriol increases during pregnancy, estradiol production increases during stages of the menstrual cycle, and estrone levels increase during menopause. The differing bonds and chemical arrangements enable scientists to determine the different concentrations of the molecules.

“Test-tube baby” is a term used to refer to a baby produced through artificial insemination or in vitro fertilization, also called IVF. During artificial insemination, a physician injects carefully selected sperm into a women’s uterus to fertilize her eggs. During IVF, a trained professional harvests eggs from a female donor. Those eggs are fertilized with carefully selected sperm in a petri dish. Those fertilized eggs are allowed to divide and grow in the dish for four days, at which point the trained professional inserts those eggs into the uterus of a female so she can carry the pregnancy.

Y-chromosomes exist in the body cells of many kinds of male animals. Found in the nucleus of most living animal cells, the X and Y-chromosomes are condensed structures made of DNA wrapped around proteins called histones. The individual histones bunch into groups that the coiled DNA wraps around called a nucleosome, which are roughly 10 nano-meters (nm) across. The histones bunch together to form a helical fiber (30 nm) that spins into a supercoil (200 nm). During much of a cell's life, DNA exists in the 200 nm supercoil phase. But when DNA replicates itself, supercoils condense further into visible chromosomes with diameters of about 1400 nm. The X- and Y-chromosomes carry the genetic information that determines the sex of many types of animals. The Y-chromosome contains a gene called the sex-determining region Y, or the SRY gene in humans. If a fertilized egg, called a zygote, has the SRY gene, the zygote develops normally into an adult organism with male sex traits. Zygotes without the SRY gene develop to have female traits. Zygotes with Y-chromosomes but mutated SRY genes can develop into adult organisms that have female traits.