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Cell Nucleus: Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (Cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the Endoplasmic reticulum. A cell may contain more than one nucleus. (From Singleton & Sainsbury, Dictionary of Microbiology and Molecular Biology, 2d ed)

The Nucleus

JoVE 10691

The nucleus is a membrane-bound organelle that contains a eukaryotic organism’s genetic instructions in the form of chromosomal DNA. This is distinct from the DNA in mitochondria or chloroplasts that carry out functions specific to those organelles. While some cells—such as red blood cells—do not have a nucleus, and others—such as skeletal muscle cells—have multiple nuclei, most eukaryotic cells have a single nucleus. The DNA in the nucleus is wrapped around proteins such as histones, creating a DNA-protein complex called chromatin. When cells are not dividing—that is, when they are in the interphase part of their cell cycle—the chromatin is organized diffusely. This allows easy access to the DNA during the transcription process when messenger RNA (mRNA) is synthesized based on the DNA code. When a eukaryotic cell is about to divide, the chromatin condenses tightly into distinct, linear chromosomes. Humans have 46 chromosomes in total. Chromatin is particularly concentrated in a region of the nucleus called the nucleolus. The nucleolus is important for the production of ribosomes, which translate mRNA into protein. In the nucleolus, ribosomal RNA is synthesized and combined with proteins to create ribosomal subunits, which later form functioning ribosomes in the cytoplasm of the cell. The interior of t

 Core: Biology

Cell Structure- Concept

JoVE 10587

Background

Cells represent the most basic biological units of all organisms, whether it be simple, single-celled organisms like bacteria, or large, multicellular organisms like elephants and giant redwood trees. In the mid 19th century, the Cell Theory was proposed to define a cell, which states:



Every living organism is made up of one or more cells.
The cells…

 Lab Bio

Cell Division- Concept

JoVE 10571

Cell division is fundamental to all living organisms and required for growth and development. As an essential means of reproduction for all living things, cell division allows organisms to transfer their genetic material to their offspring. For a unicellular organism, cellular division generates a completely new organism. For multicellular organisms, cellular division produces new cells for…

 Lab Bio

Reproductive Cloning

JoVE 10816

Reproductive cloning is the process of producing a genetically identical copy—a clone—of an entire organism. While clones can be produced by splitting an early embryo—similar to what happens naturally with identical twins—cloning of adult animals is usually done by a process called somatic cell nuclear transfer (SCNT).

In SCNT, an egg cell is taken from an animal and its nucleus is removed, creating an enucleated egg. Then a somatic cell—any cell that is not a sex cell—is taken from the animal to be cloned. The nucleus of the somatic cell is then transferred into the enucleated egg—either by direct injection or by fusion of the somatic cell to the egg using an electrical current. The egg now contains the nucleus, with the chromosomal DNA, of the animal to be cloned. It is stimulated to divide, forming an embryo, which is then implanted into the uterus of a surrogate mother. If all goes well, it develops normally and the clone is born. Although this process has been used to successfully clone many different types of animals—including sheep, cows, mules, rabbits, and dogs—its success rate is low, with only a small percentage of embryos surviving to birth. Cloned animals that survive to birth also appear to age and die prematurely. This is because their DNA comes from adult cells that have unde

 Core: Biology

Internal Receptors

JoVE 11011

Many cellular signals are hydrophilic and therefore cannot pass through the plasma membrane. However, small or hydrophobic signaling molecules can cross the hydrophobic core of the plasma membrane and bind to internal, or intracellular, receptors that reside within the cell. Many mammalian steroid hormones use this mechanism of cell signaling, as does nitric oxide (NO) gas.

Similar to membrane-bound receptors, binding of a ligand to a receptor located in the cytoplasm or nucleus of a cell causes a conformational change in the receptor. Like transcription factors, the active receptor can bind to receptor-specific DNA binding sites to increase or decrease the transcription of target genes. In the case of an intracellular receptor located in the cytoplasm, the receptor-ligand complex must first cross the nuclear membrane. Many steroid hormones, including estrogen and testosterone, use intracellular receptors to induce specific effects. As an example, estrogen can diffuse across the membrane; binding of estrogen to its receptor results in dimerization of the receptors and transport of the ligand-receptor complex to the nucleus. Once in the nucleus, the complex can bind to DNA sequences called Estrogen-Response Elements (EREs). Depending on the other transcription factors and co-activators, binding of activated estrogen receptors (ERs) to EREs may cause an incre

 Core: Biology

Eukaryotic Compartmentalization

JoVE 10689

One of the distinguishing features of eukaryotic cells is that they contain membrane-bound organelles—such as the nucleus and mitochondria—that carry out particular functions. Since biological membranes are only permeable to a small number of substances, the membrane around an organelle creates a compartment with controlled conditions inside. These microenvironments are often distinct from the environment of the surrounding cytosol and are tailored to the specific functions of the organelle. For example, lysosomes—organelles in animal cells that digest molecules and cellular debris—maintain an environment that is more acidic than the surrounding cytosol, because its enzymes require a lower pH to catalyze reactions. Similarly, pH is regulated within mitochondria, which helps them carry out their function of producing energy. Additionally, some proteins require an oxidative environment for proper folding and processing, but the cytosol is generally reductive. Therefore, these proteins are produced by ribosomes in the endoplasmic reticulum (ER), which maintains the necessary environment. Proteins are often then transported within the cell through membrane-bound vesicles. The genetic material of eukaryotic cells is compartmentalized within the nucleus, which is surrounded by a double membrane called the nuclear envelope. Sma

 Core: Biology

The Central Dogma

JoVE 10798

The central dogma of biology states that information encoded in the DNA is transferred to messenger RNA (mRNA), which then directs the synthesis of protein. The set of instructions that enable the mRNA nucleotide sequence to be decoded into amino acids is called the genetic code. The universal nature of this genetic code has spurred advances in scientific research, agriculture, and medicine. In the early 1900s, scientists discovered that DNA stores all the information needed for cellular functions and that proteins perform most of these functions. However, the mechanisms of converting genetic information into functional proteins remained unknown for many years. Initially, it was believed that a single gene is directly converted into its encoded protein. Two crucial discoveries in eukaryotic cells challenged this theory: First, protein production does not take place in the nucleus. Second, DNA is not present outside the nucleus. These findings sparked the search for an intermediary molecule that connects DNA with protein production. This intermediary molecule, found in both the nucleus and the cytoplasm, and associated with protein production, is RNA. During transcription, RNA is synthesized in the nucleus, using DNA as a template. The newly-synthesized RNA is similar in sequence to the DNA strand, except thymidine in DNA is replaced by uracil i

 Core: Biology

What are Cells?

JoVE 10687

Cells are the foundational level of organization of life. An organism may be unicellular, as with prokaryotes and most eukaryotic protists, or multicellular where the functions of an organism are divided into different collections of specialized cells. In multicellular eukaryotes, cells are the building blocks of complex structures and can have various forms and functions.

Cells are the building blocks of all living organisms, whether it is a single cell that forms the entire organism (e.g., a bacterium) or trillions of them (e.g., humans). No matter what organism a cell is a part of, they share specific characteristics. A living cell has a plasma membrane, a bilayer of lipids, which separates the watery solution inside the cell, also called cytoplasm, from the outside of the cell. Furthermore, a living cell can replicate itself, which requires that it possess genetic information encoded in DNA. DNA can be localized to a particular area of the cell, as in the nucleoid of a prokaryotic cell, or it can be contained inside another membrane, such as the nucleus of eukaryotes. Eukaryote means "true nucleus." The word prokaryote, hence, implies that the cell is from a group which arose before membrane-bound nuclei appeared in the history of life. Prokaryotic cells lack internal membranes. In contrast, eukaryotes have internal membran

 Core: Biology
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