A nucleus is the center of something. In science, the nucleus can refer to the center of an atom or a cell.
The nucleus of an atom is made of protons and neutrons. Protons are positively charged subatomic particles while neutrons are neutrally charged. It is in the nucleus where all the atomic mass resides (electrons orbit the nucleus of an atom, but the mass of an electron is insignificant to the mass of the entire atom).
The nucleus of a cell is where genetic material is stored (DNA, which stands for deoxyribose nucleic acid). The DNA is what is responsible for passing on traits from one generation. It also codes for the proteins that are made within an organism. The proteins are responsible for the shape, and therefore the function, of most biomolecular structures.
In animal cells, the nucleus is surrounded by nuclear membrane. The nucleus is the largest organelle in animal cells and occupy about 10% of the cell volume. It consists of nucleolus and other smaller sub-nuclear bodies. The nuclear membrane has pores spread through its surface (also called nuclear pores or nucleopores) that control the entry of molecules into the nucleus.
The nucleus contains all the genetic material of the cell (as DNA in the chromosomes) and directs the activities of the cell by regulating the gene expression. The nucleolus is the site for RNA synthesis. Nucleus acts as the site for gene transcription and it is here that mRNA is formed and undergoes post-transcriptional modification before being sent to the cytoplasm for translation (for making the proteins).
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The animal cell nucleus is a spherical structure within the cell’s plasma membrane. The nucleus is surrounded by its own porous membrane. The nucleus is the most noticeable structure within the cell taking up approximately 10% of its total volume. It is considered to be a specialized organelle. Other organelles and the nucleolus are found inside the nucleus. The nucleolus is the structure in the nucleus that aids in the manufacture of ribosomes and is usually seen as a dark spot within the nucleus. Ribosomes are the structures that are responsible for protein synthesis.
Many functions of the cell are controlled by the nucleus. These include protein synthesis, cell growth, and cell division or reproduction. In animal cells the DNA, or hereditary material, is located in the nucleus. In addition the RNA is copied and pushed out of the nucleus through pores in the nuclear cell membrane.