There are three main parts of the chromosome: the telomere, the centromere and the chromatid. The telomere is the end part of the chromosome, and its primary purpose is to protect the chromosome. The centromere is the primary part of the chromosome; it is where the arms (short arm and long arm) of the chromosome cross over. Finally, there is chromatid, which make up the strands of chromosome and is an individual DNA molecule.
Chromosomes are classified based on where the centromere divides the structure. The three main types of chromosomes are metacentric (which has the centromere in the middle), submetacentric (which divides the chromosome into two arms, one 1/3 length and the other 2/3 length) and acrocentric (which has the centromere near the bottom).
Chromosomes are found in all living organisms, though the chromosomes found in prokaryotes is different than eukaryotes. A prokaryote is an organism that does not have its chromosomes in a specialized organelle; bacteria are an example of a prokaryotic organism. As a result, the chromosomes in prokaryotes is circular in shape. In contrast, the chromosomes in an eukaryotic cell is coiled. All multicellular organisms (and the single-cell organisms called protists) are eukaryotic.
A cell reproduces through the process of mitosis, which involves the chromosomes uncoiling and dividing into two new cells (called daughter cells). The first step of mitosis is prophase, which is when the nucleus dissolves (prokaryotes do not have a prophase step). The second step is metaphase, which is when the chromosomes line up in the middle of the cell and then, in the third step, anaphase, they are pulled to separate ends of the cell. The final step of mitosis is when the cell finally divides to create two daughter cells, each with an identical number of chromosomes as the parent cell. In meiosis, the daughter cells divide again in order to create four new chromosomes, each with half the number of cells as the original parent cell; meiosis is how sex cells in multicellular organisms are created.