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The human genome has many different regulatory sequences which are crucial to controlling gene expression. Conservative estimates indicate that these sequences make up 8% of the genome, [29] however extrapolations from the ENCODE project give that 20 [30] or more [31] of the genome is gene regulatory sequence.
In many organisms, a significant fraction of the genomic DNA is repetitive, with over two-thirds of the sequence consisting of repetitive elements in humans. [1] Some of these repeated sequences are necessary for maintaining important genome structures such as telomeres or centromeres. [2]
Distribution of CpG sites (left: in red) and GpC sites (right: in green) in the human APRT gene. CpG are more abundant in the upstream region of the gene, where they form a CpG island, whereas GpC are more evenly distributed. The 5 exons of the APRT gene are indicated (blue), and the start (ATG) and stop (TGA) codons are emphasized (bold blue).
The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes. [96] The information carried by DNA is held in the sequence of pieces of DNA called genes. Transmission of genetic information in genes is achieved via complementary base pairing. For example, in ...
Top: An ancestral gene duplicates to produce two paralogs (Genes A and B). A speciation event produces orthologs in the two daughter species. Bottom: in a separate species, an unrelated gene has a similar function (Gene C) but has a separate evolutionary origin and so is an analog.
The human genome is the total collection of genes in a human being contained in the human chromosome, composed of over three billion nucleotides. [2] In April 2003, the Human Genome Project was able to sequence all the DNA in the human genome, and to discover that the human genome was composed of around 20,000 protein coding genes.
Gene structure is the organisation of specialised sequence elements within a gene. Genes contain most of the information necessary for living cells to survive and reproduce. [ 1 ] [ 2 ] In most organisms, genes are made of DNA, where the particular DNA sequence determines the function of the gene.
The human genome is believed to contain around 20,000–25,000 genes. In addition to studying chromosomes to the level of individual genes, genetic testing in a broader sense includes biochemical tests for the possible presence of genetic diseases, or mutant forms of genes associated with increased risk of developing genetic disorders.