Nucleus
The nucleus is a membrane bound organelle within a cell that contains all of the cells genetic information. The nucleus is surrounded by a nuclear envelope that separates it from the cytoplasm. The cells chromosomes are contained within the nucleus, chromosomes house all of the cells DNA. The main function of the cell nucleus is to control gene expression and control the replication of DNA during the cell cycle. Thins means that if cells did not have nucleus that when cells went through mitosis that not all, if any, of the chromosomes would be copied incorrectly, which would cause many problems for the organism the cell was a part of. The nucleus of an animal cell was first discovered in 1831 by a man named Robert Brown.
Below Jenna and I have a video describing the functions of the nucleus and the nucleolus and how they operate within the cell and details about each of their specific functions.
Nucleolus
Function
The nucleolus is one of the major parts of the nucleus and was discovered by the Italian scientist, Felice Fontana, in 1774. The nucleolus is the most dense region of the nucleus and is where the synthesis of RNA takes place by making copies of DNA strands. With that RNA they can then make ribosomes, which are used to make proteins and are in high demand by cells; most mammalian cells contain from 5 to 10 millions ribosomes. Ribosomes need to be synthesized each time the cell divides; in essence, the nucleolus is a ribosome factory. I have a link below to a scientific paper discussing how DNA builds RNA and how the two are interrelatted.
Structure
The nucleolus is the most dense part of the nucleus. The nucleolus is like the blue prints of the city, the cell being the city. Without the blueprints you can't make any more buildings or know what you can build. The nucleolus contains all the DNA of a cell and is where ribosomes are created which in turn create proteins. When the nucleus is dyed, the nucleolus sticks out the most becoming the darkest part of the nucleus.