Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 30 of 367 The nucleus is usually the largest organelle in a cell. The nucleus serves as the control center for cellular operations. A single nucleus stores all the information needed to direct the synthesis of the more than 100,000 different proteins in the human body. It determines the cell structure and functions by controlling which proteins are synthesized, under what circumstances, and in what amounts. A cell without a nucleus cannot direct these activities and repair itself, and it will disintegrate within three or four months. The fluid contents of the nucleus are called the nucleoplasm. The nucleoplasm contains the nuclear matrix. This matrix is a network of fine filaments that provide structural support and may be involved in the regulation of genetic activity. The nucleoplasm also contains ions, enzymes, RNA and DNA nucleotides, small amounts of RNA, and DNA. It is the DNA in the nucleus that stores the instructions for protein synthesis within packages called nucleosomes. DNA is bound to proteins called histones. In cells that are not dividing, the nucleosomes are loosely coiled within the nucleus and form a tangle of fine filaments known as chromatin. Just before cell division begins, the coiling becomes tighter, forming distinct structures called chromosomes . In humans, the nuclei of somatic cells contain 23 pairs of chromosomes. One member of each pair is derived from the mother and the other from the father. A gene is the functional unit of heredity. It contains all the DNA and divisions called “triplets” are used to produce specific proteins. The number of triplets in a gene depends on the size of the polypeptide needed. A relatively short polypeptide chain might require fewer than 100 triplets, whereas the instructions for building a large protein might involve 1000 or more triplets. Not all segments of the DNA carry instructions for proteins. Some segments contain instructions for the synthesis of transfer RNA or ribosomal RNA, some have a regulatory function, and others have no apparent function. DNA controls protein synthesis, cell structure, and cell function. Transcription is the production of RNA from a DNA template. After transcription, a strand of messenger RNA (mRNA) is created and carries information from the nucleus to the cytoplasm. Once it contacts the ribosomes, it allows for the translation of proteins. DNA is transcribed into RNA by nucleotide sequences matching to their Lysosomes Lysosomes are special vesicles that provide an isolated environment for potentially dangerous chemical reactions. These vesicles, produced at the Golgi apparatus, contain digestive enzymes and assist in cleanup. Lysosomes are small, often spherical bodies with contents that look dense and dark in electron micrographs. Lysosomes also perform recycling functions inside the cell. Mitochondria Cells, like all living things, require energy to carry out its functions. The organelles responsible for energy production are the mitochondria. The number of mitochondria in a cell varies with its energy demands. Most of the chemical reactions that release energy occur in the mitochondria, but most of the cellular activities that require energy occur in the surrounding cytoplasm. Cells must, therefore, store energy in a form that can be moved from place to place. Nucleus