nucleus

Examples of nucleus in the following topics:

• 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.

• Nuclear Binding Energy and Mass Defect

  • A nucleus weighs less than its sum of nucleons, a quantity known as the mass defect, caused by release of energy when the nucleus formed.
  • This mass, known as the mass defect, is missing in the resulting nucleus and represents the energy released when the nucleus is formed.
  • As the size of the nucleus increases, the strong nuclear force is only felt between nucleons that are close together, while the coulombic repulsion continues to be felt throughout the nucleus; this leads to instability and hence the radioactivity and fissile nature of the heavier elements.
  • This graph shows the nuclear binding energy (in MeV) per nucleon as a function of the number of nucleons in the nucleus.
  • Notice that iron-56 has the most binding energy per nucleon, making it the most stable nucleus.

• Photon Interactions and Pair Production

  • Pair production refers to the creation of an elementary particle and its antiparticle, usually when a photon interacts with a nucleus.
  • Below is an illustration of pair production, which refers to the creation of an elementary particle and its antiparticle, usually when a photon interacts with a nucleus.
  • In nuclear physics, this reaction occurs when a high-energy photon (gamma rays) interacts with a nucleus.
  • The nucleus in the process carries away (or provides) access momentum.
  • Describe process of pair production as the result of photon interaction with nucleus

• 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.

• Clusters of Neuronal Cell Bodies

  • A nucleus can either by a relatively compact collection of neurons or a distinctly identifiable group of neurons spread over a large area.
  • In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons.
  • In anatomical sections, a nucleus shows up as a region of gray matter, often bordered by white matter.
  • In addition, nucleus can refer to an identifiable distinct group of neurons that can spread over an extended area.
  • For example, the reticular nucleus of the thalamus is a thin layer of inhibitory neurons that surround the thalamus.

• Multielectron Atoms

  • In hydrogen-like atoms (those with only one electron), the net force on the electron is just as large as the electric attraction from the nucleus.
  • However, when more electrons are involved, each electron (in the $n$-shell) feels not only the electromagnetic attraction from the positive nucleus, but also repulsion forces from other electrons in shells from '1' to '$n$'.
  • Therefore, these electrons are not as strongly bonded to the nucleus as electrons closer to the nucleus.
  • As an approximation, the effective nuclear charge on each electron can be estimated by: Zeff=Z−σZ_\text{eff} = Z - \sigma, where $Z$ is the number of protons in the nucleus and σ\sigma is the average number of electrons between the nucleus and the electron in question. σ\sigma can be found by using quantum chemistry and the Schrodinger equation or by using Slater's empirical formula.
  • A multielectron atom with inner electrons shielding outside electrons from the positively charged nucleus

• Nuclear Size and Density

  • However, the nucleus can be modelled as a sphere of positive charge for the interpretation of electron scattering experiments: because there is no definite boundary to the nucleus, the electrons "see" a range of cross-sections, for which a mean can be taken.
  • Rutherford was able to put an upper limit on the radius of the gold nucleus of 34 femtometers (fm).
  • This gives a charge radius for the gold nucleus ($A=197$) of about 7.5 fm.
  • The nuclear density for a typical nucleus can be approximately calculated from the size of the nucleus:
  • Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell.

• Nuclear Fission

  • Within the nucleus, there are different forces that act between the particles.
  • This force binds protons and neutrons together inside the nucleus, and it is most powerful when the nucleus is small and the nucleons are close together.
  • These two forces produce opposite effects in the nucleus.
  • In these nuclei, it's possible for particles and energy to be ejected from the nucleus.
  • In order to initiate fission, a high-energy neutron is directed towards a nucleus, such as 235U.

• Pons

  • Within the pons is the pneumotaxic center, a nucleus in the pons that regulates the change from inspiration to expiration.
  • The alar plate produces sensory neuroblasts, which will give rise to the solitary nucleus and its special visceral afferent column, the cochlear and vestibular nuclei (which form the special somatic afferent fibers of the vestibulocochlear nerve), the spinal and principal trigeminal nerve nuclei (which form the general somatic afferent column of the trigeminal nerve), and the pontine nuclei, which is involved in motor activity.
  • Basal plate neuroblasts give rise to the abducent nucleus (forms the general somatic efferent fibers), the facial and motor trigeminal nuclei (form the special visceral efferent column), and the superior salivatory nucleus, which forms the general visceral efferent fibers of the facial nerve.
  • the chief or pontine nucleus of the trigeminal nerve sensory nucleus (V)- mid-pons

• 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.