(a)+Chromosomes

Vocabulary: Chromosome: A long composite of genes that carries DNA and forms chromatin. Chromatin: DNA and proteins that, together, make up the nucleus of a cell. Centro-mere: A central area that joins two sister chromatids that form a chromosome.

Chromosome is an organized structure of DNA, protien found in cells.Chromosome also has a DNA - bound protiens which serve to package the DNA and control its functions
Chromosomes are located within the nucleus of our cells. They are not visible in the cell's nucleus -not even under a microscope- unless the cell is dividing. The DNA that makes up chromosomes becomes tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division. This is the structure of chromosomes.


 * Diagram of a chromosome containing genetic information:**

[[image:http://www.chromosome18.org/Portals/c18/images/genetics/karyotype.gif width="300" height="260" align="left" caption="karyotype.gif"]]
====A karyotype is simplified picture of a person's chromosomes. In order to get this picture, the chromosomes are isolated, stained and examined under a microscope. Mostly, this is done using the chromosomes in the white blood cells. A picture of the chromosomes is taken through the microscope. Then, the picture of the chromosome is cut up and rearranged by the chromosome's size. The chromosomes are lined up from largest to smallest. A trained cytogeneticist can look for missing or extra pieces of chromosomes. ====

Often there are variations between species of chromosome numbers and other detailed informations. Some of this differences are 1. variation between genders 2. variation between germ line and soma 3. variation between members of the population 4. geographical variation between races 5. mosaics or other abnormal individuals

Humans have 22 pairs of chromosomes called autosomes. There is an extra pair of chromosomes, X and Y that determine an individual's gender. Males have both a X and a Y chromosme, whereas females have 2 X chromosmes. In total, humans have 23 pairs of chromosomes. Variations in this can cause both hereditary and non-herditary mutations. The most common of these is cancer or other types of tumor.

Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 10,000 to 1,000,000,000 nucleotides in a long chain. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes. In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, whereas duplicated chromosomes (copied during synthesis phase) contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure. Chromosomal recombination plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may aberrantly evade apoptosis leading to the progression of cancer. In practice "chromosome" is a rather loosely defined term. In prokaryotes and virus the term genophore is more appropriate when no chromatin is present. However, a large body of work uses the term chromosome regardless of chromatin content. In prokaryotes DNA is usually arranged as a circle, which is tightly coiled in on itself, sometimes accompanied by one or more smaller, circular DNA molecules called plasmids. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest genophores are found in viruses: these DNA or RNA molecules are short linear or circular genophores that often lack structural proteins.


 * __Work cited:__**


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 * "Chromosome." //Wikipedia, the Free Encyclopedia//. Web. 09 Feb. 2012. .
 * //Structure of Chromosomes//. //YouTube.com//. Web. <[]>.
 * "What Is a Chromosome?" //THE MEDICAL NEWS//. Web. 03 Mar. 2012. .

