Changes

==History==

==History==

The word "chromosome" means "colored body" and comes from the Greek χρώμα or "chroma" for "color" and σομε or "some" for "body". By 1900 scientists knew that [[cell|cells]] were the building blocks of living things. All living things start as a single fertilized cell which keeps dividing. Scientists identified tiny threads in the nucleus of the cells. Because they could stain the threads with colored dyes to study them under a microscope, they called the threads chromosomes, from the [[Greek]] words for colored bodies. They could see that chromosomes came in pairs, and that human cells all contained 23 matching pairs. American biologist [[Walter]] Sutton knew [[Mendel’s|Mendel's principles of genetics]] work on peas, and suggested that chromosomes held the secret of inheritance.

The word "chromosome" means "colored body" and comes from the Greek χρώμα or "chroma" for "color" and σομε or "some" for "body". By 1900 scientists knew that [[cell]]s were the building blocks of living things. All living things start as a single fertilized cell which keeps dividing. Scientists identified tiny threads in the nucleus of the cells. Because they could stain the threads with colored dyes to study them under a microscope, they called the threads chromosomes, from the [[Greek]] words for colored bodies. They could see that chromosomes came in pairs, and that human cells all contained 23 matching pairs. American biologist [[Walter]] Sutton knew [[Mendel’s|Mendel's principles of genetics]] work on peas, and suggested that chromosomes held the secret of inheritance.

Another American biologist, [[Thomas Hunt Morgan]], developed the idea that chromosomes were made up of linked groups of factors called [[gene|genes]]. He experimented with red-eyed [[fruit fly|fruit flies]] and found that sometimes a white-eyed fly appeared. When he mated them, he found that as he expected there were three red-eyed flies to every white-eyed fly, but that all the white-eyed flies were male. He concluded that the gene for white eyes must be on a chromosome that was related to being male. Later workers found that this is why some hereditary diseases such as [[hemophilia]] and [[muscular dystrophy]] only show in males, though women can carry the gene for the disease without showing it.

Another American biologist, [[Thomas Hunt Morgan]], developed the idea that chromosomes were made up of linked groups of factors called [[gene]]s. He experimented with red-eyed [[fruit fly|fruit flies]] and found that sometimes a white-eyed fly appeared. When he mated them, he found that as he expected there were three red-eyed flies to every white-eyed fly, but that all the white-eyed flies were male. He concluded that the gene for white eyes must be on a chromosome that was related to being male. Later workers found that this is why some hereditary diseases such as [[hemophilia]] and [[muscular dystrophy]] only show in males, though women can carry the gene for the disease without showing it.

In 1991 a project called the [[Human Genome Project]] began to use computers to map the three billion base pairs which make up the 46 human chromosomes.

In 1991 a project called the [[Human Genome Project]] began to use computers to map the three billion base pairs which make up the 46 human chromosomes.

Most chromosomes have the rough shape of an I when they condense prior to replication and are as bits of long invisible string when unwound. The familiar X shape actually refers to 2 identical chromosomes referred to as sister chromatids. Each chromosome has a constriction point called the [[centromere]], which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm” for "petite". The long arm of the chromosome is labeled the “q arm” for "not petite". The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.<ref>http://www.rothamsted.ac.uk/notebook/courses/guide/chromo.htm</ref>

Most chromosomes have the rough shape of an I when they condense prior to replication and are as bits of long invisible string when unwound. The familiar X shape actually refers to 2 identical chromosomes referred to as sister chromatids. Each chromosome has a constriction point called the [[centromere]], which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm” for "petite". The long arm of the chromosome is labeled the “q arm” for "not petite". The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.<ref>http://www.rothamsted.ac.uk/notebook/courses/guide/chromo.htm</ref>

The chromosomal DNA molecule contains three specific [[nucleotide]] sequences which are required for replication: a DNA replication origin; the [[centromere]] to attach the DNA to the mitotic spindle.; a [[telomere]] located at each end of the linear chromosome. <ref>http://ghr.nlm.nih.gov/ghr/chromosomes</ref> There is also a telomere region within the human chromosome two, as well as a non-functional second centromere. <ref>http://www.pnas.org/content/88/20/9051.abstract</ref> This indicates a historical chromosome fusion event.

The chromosomal DNA molecule contains three specific [[nucleotide]] sequences which are required for replication: a DNA replication origin; the [[centromere]] to attach the DNA to the mitotic spindle.; a [[telomere]] located at each end of the linear chromosome.<ref>http://ghr.nlm.nih.gov/ghr/chromosomes</ref> There is also a telomere region within the human chromosome two, as well as a non-functional second centromere.<ref>http://www.pnas.org/content/88/20/9051.abstract</ref> This indicates a historical chromosome fusion event.

==Number of Chromosomes==

==Number of Chromosomes==