suprachiasmatic nucleus

Examples of suprachiasmatic nucleus in the following topics:

• Visual Processing

  • Finally, there is the pathway from the retina to the suprachiasmatic nucleus (SCN) of the hypothalamus.

• The Nucleus and Ribosomes

  • Found within eukaryotic cells, the nucleus contains the genetic material that determines the entire structure and function of that cell.
  • One of the main differences between prokaryotic and eukaryotic cells is the nucleus.
  • The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm .
  • Lastly, the boundary of the nucleus is called the nuclear envelope.
  • The nucleus is the control center of the cell.

• Characteristics of Eukaryotic Cells

  • Because a eukaryotic cell's nucleus is surrounded by a membrane, it is often said to have a "true nucleus. " Organelles (meaning "little organ") have specialized cellular roles, just as the organs of your body have specialized roles.
  • Typically, the nucleus is the most prominent organelle in a cell.
  • Eukaryotic cells have a true nucleus, which means the cell's DNA is surrounded by a membrane.
  • The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus.
  • The nucleoplasm is the semi-solid fluid inside the nucleus where we find the chromatin and the nucleolus.

• Prokaryotic versus Eukaryotic Gene Expression

  • Prokaryotic organisms are single-celled organisms that lack a defined nucleus; therefore, their DNA floats freely within the cell cytoplasm.
  • In eukaryotic cells, the DNA is contained inside the cell's nucleus where it is transcribed into RNA.
  • The newly-synthesized RNA is then transported out of the nucleus into the cytoplasm where ribosomes translate the RNA into protein.
  • The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus within the cytoplasm.
  • Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm.

• Fertilization

  • In fertilization, the sperm binds to the egg, allowing their membranes to fuse and the sperm to transfer its nucleus into the egg.
  • The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum.
  • Fusion between the oocyte plasma membrane and sperm follows and allows the sperm nucleus, centriole, and flagellum, but not the mitochondria, to enter the oocyte.
  • (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus.

• Reproductive Cloning

  • The zygote nucleus contains the genetic information to produce a new individual.
  • If the haploid nucleus of an egg cell is replaced with a diploid nucleus from the cell of any individual of the same species (called a donor), it will become a zygote that is genetically identical to the donor.
  • Somatic cell nuclear transfer is the technique of transferring a diploid nucleus into an enucleated egg.
  • To create Dolly, the nucleus was removed from a donor egg cell.
  • The nucleus from a second sheep was then introduced into the cell, which was allowed to divide to the blastocyst stage before being implanted in a surrogate mother.

• Endosymbiotic Theory and the Evolution of Eukaryotes

  • However, the role of endosymbiosis in the development of the nucleus is more controversial.
  • There are several other competing hypotheses as to the origin of eukaryotes and the nucleus.
  • One idea about how the eukaryotic nucleus evolved is that prokaryotic cells produced an additional membrane which surrounded the bacterial chromosome.
  • If the eukaryotic nucleus evolved this way, we would expect one of the two types of prokaryotes to be more closely-related to eukaryotes.
  • Another hypothesis, the nucleus-first hypothesis, proposes the nucleus evolved in prokaryotes first, followed by a later fusion of the new eukaryote with bacteria that became mitochondria.

• Intracellular Hormone Receptors

  • They then adhere to intracellular receptors residing in the cytoplasm or in the nucleus.
  • Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors located in the nucleus.
  • The hormones diffuse across both the plasma membrane and the nuclear envelope, then bind to receptors in the nucleus.
  • Upon hormone binding, the receptor dissociates from the heat shock protein and translocates to the nucleus.
  • In the nucleus, the hormone-receptor complex binds to a DNA sequence called a hormone response element (HRE), which triggers gene transcription and translation.

• Characteristics of Eukaryotic DNA

  • The single characteristic that is both necessary and sufficient to define an organism as a eukaryote is a nucleus surrounded by a nuclear envelope with nuclear pores.
  • All extant eukaryotes have cells with nuclei; most of a eukaryotic cell's genetic material is contained within the nucleus.
  • In contrast, prokaryotic DNA is not contained within a nucleus, but rather is attached to the plasma membrane and contained in the form of a nucleoid, an irregularly-shaped region that is not surrounded by a nuclear membrane .
  • Because eukaryotes have mitochondria and prokaryotes do not, eukaryotic cells contain mitochondrial DNA in addition to DNA contained in the nucleus and ribosomes.
  • Eukaryotic DNA is stored in a nucleus, whereas prokaryotic DNA is in the cytoplasm in the form of a nucleoid.

• Electron Shells and the Bohr Model

  • Niels Bohr proposed an early model of the atom as a central nucleus containing protons and neutrons being orbited by electrons in shells.
  • The Bohr model shows the atom as a central nucleus containing protons and neutrons with the electrons in circular orbitals at specific distances from the nucleus .
  • For example, 1n represents the first energy level located closest to the nucleus.
  • Under standard conditions, atoms fill the inner shells (closer to the nucleus) first, often resulting in a variable number of electrons in the outermost shell.
  • An electron normally exists in the lowest energy shell available, which is the one closest to the nucleus.