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Chromosome. A chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes: 22 pairs of numbered chromosomes, called autosomes, and 1 pair of sex chromosomes, X and Y. Each parent contributes 1 chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
Base pair. A base pair is 2 chemical bases bonded to one another, forming a “rung of the DNA ladder.” The DNA molecule consists of 2 strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is 1 of 4 bases—adenine (A), cytosine (C), guanine (G), or thymine (T). The 2 strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine and cytosine forming a base pair with guanine. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
Exon. An exon is the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons because they are expressed, whereas the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between—or interfere with—the exons. [Exon sequences are specified in a particular order to form messenger RNA (mRNA), which determines the amino acids that make up a protein. The 5′UTR (untranslated region) and 3′UTR sequences do not code for protein but do mark the first and last exon coding mRNA. Introns are noncoding sequences of DNA separating the exons of a gene; DNA sequences associated with introns are not specified in mRNA.] Author's note: Bracketed text added to figure caption from the genome.gov website. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
Allele. An allele is 1 of 2 or more versions of a gene. An individual inherits 2 alleles for each gene, 1 from each parent. If the 2 alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. Though the term “allele” was originally used to describe variation among genes, it now also refers to variation among noncoding DNA sequences. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
Genome. The genome is the entire set of genetic instructions found in a cell. In humans, the genome consists of 23 pairs of chromosomes, found in the nucleus, as well as a small chromosome found in the cells' mitochondria. These chromosomes, taken together, contain approximately 3.1 billion bases of DNA sequence. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
Polymorphism. Polymorphism involves 1 of 2 or more variants of a particular DNA sequence. The most common type of polymorphism involves variation at a single base pair. Polymorphisms also can be much larger and involve long stretches of DNA. Called a single-nucleotide polymorphism (SNP, pronounced “snip”), scientists are studying how SNPs in the human genome correlate with disease, drug response, and other phenotypes. Courtesy of Darryl Leja of the National Human Genome Research Institute, National Institutes of Health, http://www.genome.gov/. Accessed August 30, 2015.
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