Nucleus
Function of a nucleus
The nucleus regulates all cell activity. It does this by controlling the enzymes present. The chromatin is composed of DNA. DNA contains the information for the production of proteins. This information is encoded in the 4 DNA bases. Adenine, thymine, cytocine, and guanine. The specific sequence of these bases tells the cell what order to put the amino acids.
There are three processes that enable the cell to manufacture protein:
Replication allows the nucleus to make exact copies of its DNA
Transcription allows the cell to make RNA working copies of its DNA
In translation the Messenger RNA is used to line up amino acids into a protein molecule
There are three processes that enable the cell to manufacture protein:
Replication allows the nucleus to make exact copies of its DNA
Transcription allows the cell to make RNA working copies of its DNA
In translation the Messenger RNA is used to line up amino acids into a protein molecule
Nucleolus
Structure and Function
The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cells. The nucleolar organiser regions of chromosomes, which contain the genes for pre-ribosomal ribonucleic acid (rRNA), serve as the foundation for nucleolar structure. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre-rRNA. During transcription ribosomal and nonribosomal proteins attach to the RNA. Subsequently, there is modification and cleavage of pre-rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre-ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified. These include assembly of signal recognition particles, sensing cellular stress and transport of human immunodeficiency virus 1 (HIV-1) messenger RNA.
Nuclear Envelope
Structure and function
The nuclear envelope has two membranes, each with the typical unit membrane structure. They enclose a flattened sac and are connected at the nuclear pore sites. The outermost membrane is continuous with the rough endoplasmic reticulum (ER) and has ribosomes attached (see figure to the left). The space between the outer and inner membranes is also continuous with rough endoplasmic reticulum space. It can fill with newly synthesized proteins just as the rough endoplasmic reticulum does. The nuclear envelope is enmeshed in a network of filaments for stability.The nuclear envelope is shown in an electron micrograph in the figure to the right. The filaments outside the envelope are not visualized with these protocols. Also, the nuclear lamina just inside the nuclear envelope is not shown well (see paragraph below for description). However, one can see ribosomes on the outer membrane and the sac enclosed by the two membranes. Dense patches of Heterochromatin are seen just inside the inner membrane.