Which nitrogen-based substance is unique to RNA?

Animal cells are eukaryotic cells, characterized by the presence of a true nucleus enclosed within a membrane-bound nuclear envelope. Eukaryotic cells also contain other membrane-bound organelles, such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes, which perform specialized functions within the cell. These organelles compartmentalize cellular processes, allowing for greater efficiency and specialization of cellular functions. Eukaryotic cells are typically larger and more structurally complex than prokaryotic cells, which lack a true nucleus and membrane-bound organelles. Animal cells are found in organisms belonging to the kingdom Animalia and are essential building blocks of tissues and organs in multicellular animals. They play diverse roles in processes such as metabolism, growth, reproduction, and cellular communication.

Transcription is the process by which an RNA molecule is synthesized from a DNA template. It is a key step in gene expression and occurs in the nucleus of eukaryotic cells (and in the cytoplasm of prokaryotic cells). During transcription, RNA polymerase enzyme binds to a specific region of DNA called the promoter and unwinds the DNA double helix. One of the DNA strands, known as the template strand, serves as a template for RNA synthesis, while the other strand remains untranscribed. RNA polymerase catalyzes the formation of phosphodiester bonds between ribonucleotides, resulting in the synthesis of a complementary RNA molecule. The RNA transcript is synthesized in the 5' to 3' direction and is complementary to the template DNA strand, except that uracil (U) is used in place of thymine (T) in RNA. Transcription terminates when RNA polymerase reaches a sequence called the terminator, causing RNA polymerase to dissociate from the DNA template, and the newly synthesized RNA molecule is released. Transcription produces several types of RNA molecules, including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which play essential roles in protein synthesis and gene regulation.

Cellular respiration is the metabolic process by which cells convert biochemical energy stored in nutrients such as glucose into adenosine triphosphate (ATP), the universal energy currency of cells. Cellular respiration occurs primarily in the mitochondria, which are membrane-bound organelles found in the cytoplasm of eukaryotic cells. Mitochondria are often referred to as the "powerhouses" of the cell due to their role in ATP production through aerobic respiration. The process of cellular respiration involves several interconnected metabolic pathways, including glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain (oxidative phosphorylation). In glycolysis, glucose is partially oxidized to produce pyruvate and a small amount of ATP in the cytoplasm. Pyruvate is then transported into the mitochondria, where it enters the citric acid cycle, generating more ATP and reducing equivalents (NADH and FADH2). The reduced coenzymes NADH and FADH2 donate electrons to the electron transport chain located in the inner mitochondrial membrane, leading to the production of ATP through oxidative phosphorylation. Overall, cellular respiration is essential for providing energy for cellular processes such as biosynthesis, muscle contraction, active transport, and maintenance of cellular homeostasis.

Exocytosis is the cellular process by which cells actively transport materials from the intracellular space to the extracellular space by fusing secretory vesicles with the plasma membrane. It is a form of bulk transport used to release molecules such as proteins, lipids, hormones, neurotransmitters, and waste products from the cell into the extracellular environment. Exocytosis plays a crucial role in various cellular functions, including secretion, communication, signaling, and membrane remodeling. The process of exocytosis involves several steps: first, secretory vesicles containing the cargo molecules bud off from the Golgi apparatus or other membrane-bound organelles within the cell. Next, these vesicles move towards the plasma membrane along cytoskeletal elements such as microtubules and actin filaments. Upon reaching the plasma membrane, the vesicles dock and fuse with the membrane, releasing their contents into the extracellular space through a fusion pore. The membrane components of the vesicles are then incorporated into the plasma membrane, allowing the cell to maintain its surface area and membrane composition. Exocytosis is essential for cellular communication, neurotransmission, hormone release, immune responses, and the maintenance of tissue structure and function.