# Chromosome --- **Chromosomes** are discrete bundles of [[DNA]] that are formed in all your cells when it is time to [[cell cycle|divide]]. They are often depicted in a characteristic X-like shape, but they only form that shape for a short time during [[DNA replication]]. Most of their existence is spent in an un-bundled mass of of spaghetti-like [[chromatin]]. Located on the... ## Structure Chromosomes are made up of [[DNA]] that is bundled up in the string-like [[chromatin]]. When it's time to [[DNA replication|replicate]] the chromatin scrunches up tighter and tighter into the cheeto-like shape that is commonly associated with chromosomes. When the chromosome has replicated but before it has split apart is the only time it exist in that classic X shape. Each cheeto-half of the X-shaped chromosome is called a [[chromatid|sister chromatid]], and they are attached at the center by a specialized region called a **centromere**. The two end pieces of a chromosome are the [[telomere|telomeres]]. An **autosome** is a chromosome that is *not* a sex chromosome. **[[Sex chromosomes]]** are the X and Y chromosomes in an organism; they distinguish an organisms sex. Humans only have one pair (the 23rd pair) of sex chromosomes, and all other chromosomes are autosomes. ## Ploidy [[Ploidy]] refers to the number of matching chromosomes an organism has. For example [[human|humans]] have 22 *pairs* of chromosomes (plus one potentially-not-matching pair) for a total of 44 chromosomes (plus 2, or 46). This makes humans *diploid*. ### Homologs Organisms that have more than one of each chromosome have what are called [[homologous pair|homologues]], or chromosomes of the same size, shape, and DNA make-up. ## Chromosomes in Humans [[human|Humans]] are [[ploidy#Diploid|diploid]] organisms, meaning they have chromosomes that come in pairs of two. In other words, we have 46 chromosomes: 22 pairs of homologous autosomal chromosomes and one pair of sex chromosomes, which may or may not be homologous, depending on the sex. (22 x 2 + 2) ![[human chromosomes.png]] ## Karyotype A picture of all the chromosomes in an organism is called a "karyotype". Here is the human karyotype. ![[karyotype.png]] ___