What Are Chromosomes

In the nucleus of each eukaryotic cell, the DNA is packaged together into chromosomes. Each chromosome is made up of DNA that is tightly coiled around proteins that give it the structure. Chromosomes usually occur in pairs, except for gametes (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. Chromosomes are made from long, coiled molecules of DNA known as a double helix. Within 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 2 sections, or arms. The location of the centromere on each chromosome is what gives it it’s characteristic shape. The p arm are the short arms, and the q arms are the long arm structure. Chromosomes are made up of DNA and proteins - with the DNA being coiled around the histone proteins that support the 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 condense into short structures which can then be stained and observed under a microscope. The centromere holds the duplicated chromosomes together. Duplicated chromosomes are commonly called sister chromatids. Note that one chromosome is just one half, but the X-shape is that duplicated chromosome joined at the centromere.