What is the difference between chromatids and chromosomes?

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When a single strand of DNA duplicates itself, it forms the shape of an X. It's almost as if one side of the X is a "mirror image" of the other side of the X. These two identical strands of DNA join together at the centromere. A duplicated strand of DNA is referred to as the chromosome. However, a chromosome can also be a linear shape, as a result of the DNA NOT duplicating itself. When looking at a chromosome in the shape of an X, ONE side of the chromosome is called a chromatid. Two sister chromatids come together to make a chromosome.

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A chromosome is the thread - like strand  that is housed in the nucleus of a cell and carries genetic information. It is composed partially of DNA (deoxyribose nucleic acid). This DNA looks like a double helix. It is made of a sugar-phosphate backbone (sides of the ladder) and nucleotide base pairs as its rungs (adenine and thymine, or cytosine and guanine). The DNA within a chromosome is tightly wound around histone proteins. Each species has a unique number of chromosomes. For example, humans have 46 chromosomes. 

A chromatid, on the other hand, is a replicated copy of a chromosome. This occurs at the beginning of cell division. Thus, a chromosome is made of two sister chromatids. The two copies are joined in the center by what is called a centromere. 

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The difference between chromatids and chromosomes is subtle and can be confusing. We know that DNA is a double-stranded nucleic acid that holds the blueprint for cell growth, division, and function. DNA is arranged into chromosomes. In eukaryotic cells, chromosomes consist of linear strands, whereas in prokaryotic cells chromosomes are circle-shaped. Human beings have 46 chromosomes in each cell (23 from each parent). In order to fit this much DNA into each cell, it is packaged around histone proteins. A chromatin or DNA protein complex is the result. When cell division takes place, two identical copies of DNA are created by DNA replication. These two copies are connected by a centromere and the unit is termed a chromatid pair. A chromatid is one of the two copies or strands. Upon completion of cell division, each daughter cell ends up with one of the chromatids.

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