Nucleus

provide for structural support of the cell. In the nucleus, lamin and other proteins help support the shape of the nuclear envelope.

• Microtubules—these are the largest and thickest of the cytoskeleton fibers.

They are hollow and are made from alpha and beta tubulin. They are responsible for making flagella and cilia in animal cells, which are the appendages that protrude out of the cell and move in relation to the cells. They are made in the centrosome, which organizes microtubules. Centrosomes have multiple microtubules emanating from them. The microtubules will pull the chromatids away from each other in the process of cell division called mitosis. Transport of organelles inside the cell takes place because of microtubules.

NUCLEUS

The nucleus is probably the largest organelle in the cell. It is bound by a lipid bilayer membrane and contains the genetic information of the cell. While most animal cells will have only one nucleus, a few will have several nuclei and cells like red blood cells or RBCs have no nucleus (as it is extruded before the cells mature). Animal cells have DNA as their main source of genetic information. Within the nucleus in many cases is a nucleolus, which is contains protein, RNA, and DNA. It is responsible for making and modifying a type of RNA called ribosomal RNA or rRNA. This is RNA used to make ribosomes, which act to make proteins.

Within the nucleus is DNA, the main genetic information within the cell. These are held in different strands of nucleic acid called chromosomes. The numbers of chromosomes within the cell depends on the animal. In humans, there are 23 pairs of chromosomes for a total of 46 chromosomes. While the majority of the DNA in animal cells is noncoding and have no known function, chromosomes contain genes—segments of DNA that code for proteins. The way they do this will be discussed later in this chapter. Normal DNA in the cells is folded and not visible until the time of cell division.