Chapter 10: Cell and Organelles
10.2. The Nucleus and its contents
Learning Objectives
By the end of this section, you will be able to:
- define the characteristics of living organisms.
- describe the structure of the cell nucleus, including the nuclear organization of the DNA and the function of the nucleolus.
- differentiate the cell nucleus from its surrounding cytoplasm.
Characteristics of living organisms
The cell is the smallest unit (the functional unit) in the body that demonstrates Life. Below are some of the characteristics that biologists use to characterize life.
- Order: Living organisms are highly organized with tissues, organs, systems performing specific function to maintain life.
- Sensitivity and Responsive to Stimuli: Our sensory and nervous systems allow us to receive information from and respond to the environment around us.
- Reproduction: The gonads (ovaries and testes) of the reproductive system produce the cells (egg and sperm) that can fuse and produce a new individual.
- Homeostasis: literally means a steady (stable) state, a concept that optimal function to maintain life. Our body systems function to keep us at optimal body temperature, pH, blood pressure, heart rate, etc. By extension, deviation from homeostasis (conditions outside the optimal range) is pathology (disease).
- Energy processing; the ability to use chemical energy (ATP) for metabolism (the chemical reactions of the cells).
Nucleus
The nucleus is the largest and most prominent of a cell’s organelles (Figure 10.2.1.). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Most cells contain one nucleus. The exception is some types of muscle cells which many have ore than one nucleus (are multinucleated) and mature red blood cell which have lost their nucleus (are anuclear).
Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of germ or reproductive cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.
Organization of nuclear DNA
Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.
Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.
The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 10.2.2.). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.
License and attributions:
- Anatomy and Physiology, Second edition, 2022, Betts, J.G. et al. License: CC BY 4.0. Located at https://openstax.org/books/anatomy-and-physiology-2e/pages/3-3-the-nucleus-and-dna-replication
