Question 3 The Nucleus Of A Cell Is Never Surrounded By A Membrane. True Or False

Name a cell that does not have a nucleus. What are they called?

You have them. Mammalian red blood cells are enucleated, that is, they lack nucleus.Similarly, in plants, seive tube cells are enucleated as they are associated with companion cells. These companion cells regulate the activities of seive tube cells through their nuclei.Also, prokaryotes, which are necessarily single celled organisms, lack a proper or well-defined nucleus. They have DNA but no nuclear membrane which leaves the DNA just like a thread like structure embedded in the cytoplasm. This lack of nuclear membrane makes us believe that prokaryotes are the most primitive organisms, the ones from which the other forms of life evolved.

How can a cell exist without a nucleus? What is it called?

A cell without nucleus lost it in an enucleation process. Which means that they still have a supply of ATP, proteins, enzyme, etc. They don't simply come into being from scratch. They are called anucleate cells. Example is red blood cell. (More on enucleation of red blood cell:[2], [3] and [4] )You can also perform enucleation of cells using cytochalasin B. Goldman, R. D., et. al. [1]  performed enucleation on fibroblasts and epithelial cells, and showed that they still exhibit locomotion and mitochondria still gets moved around.Nucleus is just the the storage of data for proteins. If cells have already produced ATP, glycolysis enzyme, kinase enzyme, etc,  it can still perform functions (glycolysis, phosphorylation, membrane tranport (based on kinase activity), intracellular signalling (which can depend on kinase actvity)). And GTP can still interact with cytoskeleton influencing the shape of the membrane. That is until the supply of ATP or GTP is depleted or the enzymes have degraded. (if only they can acquire these biochemicals (probably excreted by other cells) through endocytosis, they may just be able to sustain themselves... I am talking in an analogous manner to how VLDL just picks up membrane component that it needs from its surrounding)[1] Goldman, R. D., et. al., 1973, Preservation of Normal Behavior by Enucleated Cells in Culture, http://www.pnas.org/content/70/3... [2] Kathleen E. McGrath, 2014, Red cell island dances: switching hands, Inside Blood Commentary, Volume 123 Number 25 http://www.bloodjournal.org/cont... [3] Anna Rita Migliaccio ,2010, Erythroblast enucleation, Haematologica. 2010 Dec; 95(12): 1985–1988. http://www.ncbi.nlm.nih.gov/pmc/... [4] Ganesan Keerthivasan, et. al., 2011, Review Article Erythroblast Enucleation, Stem Cells International Volume 2011  Erythroblast Enucleation

Why does the cell nucleus need a double membrane?

This is a great question. First of all, why does a eukaryotic cell nucleus need a membrane in the first place? If we compare to a prokaryotic cell, which does not have a membrane-bound nucleus at all, we observe one major evolutionary benefit of a membrane-bound nucleus is that it allows for an additional level of regulating gene expression. Since membranes act as a barrier, eukaryotic cells can effectively control the rate at which mRNA is shipped out of the nucleus for translation in the cytoplasm (and then cotranslationally imported to the ER, if the right peptide signal is present). So whereas prokaryotes can only control the rate which DNA is transcribed by regulating activation of proteins needed for transcription/translation, etc., eukaryotic cells can regulate gene expression in multiple and complex ways owing to the compartmentalization of the nucleus with a membrane. Now, the real question is, why two membranes? We know that the outermost membrane is connected to the endoplasmic reticulum, so the two organelles effectively share the same membrane. This makes it easy for the nucleus to get intermembrane proteins made in the rough ER - all they have to do is slide over across the shared membrane. No endocytosis is necessary, which is efficient since all the steps in endocytosis - invagination of the ER, vesicle formation, and vesicle merging with the nuclear membrane - are all heavily energy expending activities. The additional inner membrane of the nucleus houses the DNA and the site of ribosome synthesis, called the nuclear organizing region, or NOR. Again, having another membrane compartmentalizes functions. With walls around it, the cell can maintain high concentrations of proteins and other molecules necessary for ribosome synthesis in one small section of the nucleus. Without walls, the molecules would diffuse across the larger nuclear area and more molecules would be required in order to maintain those high concentration levels. Additionally, eukaryotic cells gain another method of control over gene expression by regulating the rate of ribosome synthesis via membrane boundaries. So, long story short, eukaryotic cells are more complex and have more ways to regulate gene expression and maintain high concentrations of specific small molecules by compartmentalizing activities within membranes. More membranes means more control, more efficiency, and so on. Cells are the absolute coolest!

Why don't protons in a nucleus repel each other? Why doesn't the nucleus break apart?

Q: Why don't protons in a nucleus repel each other? Why doesn't the nucleus break apart?Because of the strong nuclear force, which is stronger than the electromagnetic force but only acts at extremely short distances. Even then, it’s close. If you put enough protons together, the mutual repulsion can overwhelm the strong force and allow the nucleus to split. This is why atomic fission can occur.All atoms are capable of having neutrons in their nucleus.As atoms grow larger, the presence of neutrons to space out the protons and keep the repulsive charge from rising too high in any one region becomes increasingly important. Dislodging a neutron (say, by showering atoms with neutrons from an external source) can make them unstable, causing them to cleave in two, or to have a higher probability of eventually breaking abort. Very large atoms are often unstable in this way inherently, because there is no way to have enough neutrons to “space out” the strong force enough to stop the repulsion from having at least a change of breaking the nucleus,Such unstable atoms are called “radioactive” because when taken en masse, a certain percentage of them will undergo fission at any given moment—releasing energy as various forms of radiation. Depending on how unstable they are, this can leave them quite benign (like the atoms of uranium in a kitchen counter top) or fairly dangerous (like those in a spent nuclear reactor core).Sometimes when atoms fission, the break down releases more neutrons, and under the right circumstances, using the right unstable elements are working material, this can create a chain reaction in which fissioning atoms release neutrons that cause their neighbours to fission, releasing energy and more neutrons, and so on.Such a chain reaction can then be used to boil water and turn a turbine, or blast a city back to the stone age.If you like science, you might like my free award-winning scifi sampler.