NLN NEX Online Access

Vestigial structures are anatomical features that are reduced in size and function compared to their ancestral form. In whales, vestigial leg bones are remnants of hind limbs that were fully functional in land-dwelling ancestors. These bones no longer serve a locomotor purpose but provide strong evolutionary evidence of descent from terrestrial mammals. The spinal column, rib cage, and jawbones are found in many vertebrates and do not specifically indicate a land-based ancestry. Vestigial leg bones, however, directly point to a time when whales had legs and lived on land before adapting to an aquatic environment. The NEX Science exam frequently tests evolutionary evidence, especially the significance of vestigial structures as proof of common ancestry and evolutionary change over time.

======

Crossing over is a key event that occurs during prophase I of meiosis. During this process, homologous chromosomes pair up and exchange corresponding segments of DNA. Specifically, non-sister chromatids exchange genetic material, which results in new combinations of alleles. The primary result of this exchange is increased genetic variation in gametes, which is essential for evolution and diversity within a species. While chiasmata are structures formed during crossing over, they describe the physical points of contact rather than the outcome of the process. Lining up at the metaphase plate and random assortment occur later during meiosis and are separate mechanisms of genetic variation. Therefore, the correct result of crossing over is the exchange of DNA segments between homologous chromosomes. This process ensures that offspring have unique genetic combinations that differ from both parents, which is a core concept tested on the NEX Science exam.

======

Copper is a chemical element, meaning it consists of only one type of atom. If a penny were made entirely of copper, it would be classified as a pure substance and specifically as an element. Compounds consist of two or more different elements chemically bonded together. Mixtures contain substances physically combined, not chemically bonded. Since the penny is composed of only copper atoms, it does not qualify as a mixture or a compound. The NEX Science exam frequently includes questions on classification of matter, requiring students to distinguish between elements, compounds, and mixtures. Therefore, option D is correct.

======

Electrolytes are charged ions that help regulate nerve function, muscle contraction, and fluid balance in the body. Calcium ions (Ca²⁺) are an important electrolyte found in the blood and play a crucial role in muscle contraction, blood clotting, and nerve signaling. Water is not an electrolyte because it does not carry a charge. Iodine is a trace element but not a primary blood electrolyte. Lactase is an enzyme involved in digestion, not an electrolyte. The NEX Science section often includes questions related to human physiology and emphasizes understanding ions and their roles in body function.

======