 Chromosomes In the nucleus of each eukaryotic cell, the DNA is packed together into chromosomes. Each chromosome is made up of DNA that is tightly coiled around proteins that give it the structure. This tight packaging system means that two meters worth of human DNA can fit into a tiny cell that is only two micrometers wide. In this video we are going to look at what chromosomes are and what their role is. Chromosomes usually occur in pairs except for gametes, which are eggs and sperm cells, but the number of chromosomes changes depending upon species. Humans have 23 pairs, elephants have 28 pairs and carrot plants have 9 pairs. If you want to learn more about DNA, watch this video. So chromosomes are made from long, coiled molecules of DNA known as double helix. In this long chain, there are shorter regions that carry the genetic code for particular proteins that are known as genes. Chromosomes carry all of the information that help a cell grow, survive and reproduce. Each chromosome has a centromere, which divides the chromosome into two sections, or arms. The location of the centromere on each chromosome is what gives it its characteristic shape. The P-arm are the short arms and the Q-arms are the long arm structure. Chromosomes are passed from parents to offspring and play an important role that ensures DNA is copied and distributed accurately in the process of cell division. Chromosomes are not usually visible in the cell's nucleus and can only be seen when the cell is dividing. During cell division, the chromosomes are duplicated. We will look at this in more detail in future videos, such as What is Mitosis? When the chromosomes are duplicated, they are condensed into short structures which can then be strained and observed under a microscope. The centromere holds the duplicated chromosomes together. Duplicated chromosomes are commonly called sister chromatids. So, from this video, you should know that chromosomes hold the DNA inside the nucleus. They have a centromere which holds the duplicated chromosomes together, and these duplicates are known as chromatids. Humans have 23 pairs of chromosomes, but other species have different amounts.