Chromosomes are highly condensed rods of Deoxyribonucleic Acid (DNA), the genetic material which contains the building blocks of life. DNA stores important information about the structure of an animal or plant, and it helps direct the organism as it grows and manages daily tasks. Chromosomes serve as the storage for this important material, periodically dividing along with cells and replicating to make copies of the DNA they contain. Chromosomes are also very important in sexual reproduction, as they allow an organism to pass genetic material on to descendants.
In organisms with cell nuclei, known as eukaryotes, chromosomes are found inside the nucleus. Most of these organisms have a set of chromosomes which come in pairs. In structural cells, each cell retains a complete set of chromosomes, in what is known as diploid form, referring to the fact that the chromosome set is complete. In cells for sexual reproduction like eggs or sperm, each cell only has half of the parent organism's genetic material, stored in haploid form, ensuring that the parent passes down half of its genes.
Each end of a chromosome is capped by a telomere, a string of repetitive DNA which protects the chromosome from damage. Scientists sometimes look at telomeres to gain important information about an organism, as they appear to change over time and they may be associated with aging. When chromosomes divide to make copies, the telomere also ensures that all of the important genetic material is copied.
Every time a cell divides, the chromosomes inside are replicated. In mitosis, normal cell division, the chromosomes make copies of themselves which later pair up, so that at the end of the cell division process there are two cells with a set of diploid chromosomes each. When cells divide to create reproductive material, a process called meiosis, each division yields four cells, each with a haploid set of chromosomes. These cells are known as gametes, and when they meet, they contain enough genetic material to create an entirely new organism.
In humans, the normal number of chromosomes is 46, appearing in 23 pairs. Each pair of chromosomes stores distinct information, and any damage to a chromosome can cause serious problems for the parent organism. Errors usually occur during cell division, creating gaps in the genetic material in the chromosome. In some cases, an abnormal number of chromosomes appears, in a condition known as aneuploidy. Aneuploidy in reproductive cells can be a major issue, as it will cause birth defects.