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                        GC-Depth, GC-Content 
                        Distribution 
                    
                        In molecular biology and genetics, GC-content (or guanine-cytosine content) 
                        is the percentage of nitrogenous baseson 
                        a DNA or RNA molecule 
                        that are either guanine or cytosine (from 
                        a possibility of four different ones, also including adenine and thymine in 
                        DNA and adenine and uracil in 
                        RNA).[1] This 
                        may refer to a certain fragment of DNA or RNA, or that of the whole genome. 
                        When it refers to a fragment of the genetic material, it may denote the 
                        GC-content of section of a gene (domain), single gene, group of genes (or gene 
                        clusters), or even a non-coding region. 
                        Genomic content Within-genome variation
                         The GC ratio within a genome is found to be markedly variable. These variations 
                        in GC ratio within the genomes of more complex organisms result in a mosaic-like 
                        formation with islet regions called isochores.[11] This 
                        results in the variations in staining intensity in the chromosomes.[12] GC-rich 
                        isochores include in them many protein coding genes, and thus determination of 
                        ratio of these specific regions contributes in mapping gene-rich regions of the 
                        genome.[13][14] 
                         
                        
                        Coding sequences 
                        
                        
                        
                         
                     
                    
                        Within a long region of genomic sequence, genes are often 
                        characterised by having a higher GC-content in contrast to the background 
                        GC-content for the entire genome. Evidence of GC ratio with that of length of 
                        the coding region of a gene has shown that the length of the coding 
                        sequence is directly proportional to higher G+C content.[15] This has been 
                        pointed to the fact that the stop codon has a bias towards A and T 
                        nucleotides, and, thus, the shorter the sequence the higher the AT bias.[16] 
                    
                        Comparison of more than 1,000 orthologous genes in mammals showed 
                        marked within-genome variations of the third-codon position GC content, with a range 
                        from less than 30% to more than 80%.[17] 
                    
                        
                         
                        Among-genome variation
                         
                     
                    
                        GC content is found to be variable with different 
                        organisms, the process of which is envisaged to be contributed to by variation 
                        in selection, mutational bias, and 
                        biased recombination-associated DNA repair.[18] 
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                    Nucleotide bonds showing AT and GC pairs. Arrows point to the hydrogen bonds. 
                     
                     
                     Influence of GC Content on Mean Read Depth in WGS | 
            
            
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                        The average GC-content in human genomes ranges from 35% 
                        to 60% across 100-Kb fragments, with a mean of 46.1%.[17] The GC-content 
                        of Yeast (Saccharomyces cerevisiae) is 38%,[19] and that of 
                        another common model organism, thale cress (Arabidopsis thaliana), is 36%.[20] Because of the 
                        nature of the genetic code, it is virtually impossible for an 
                        organism to have a genome with a GC-content approaching either 0% or 100%. 
                        However, a species with an extremely low GC-content is Plasmodium falciparum (GC% 
                        = ~20%),[21] and it is 
                        usually common to refer to such examples as being AT-rich instead of GC-poor.[22] 
                    
                        Several mammalian species (e.g., shrew, microbat, tenrec, rabbit) have independently undergone a marked increase in 
                        the GC-content of their genes. These GC-content changes are correlated with 
                        species life-history traits (e.g., body 
                        mass or longevity) and genome size,[17] and might be 
                        linked to a molecular phenomenon called the GC-biased gene conversion.[23] 
                    
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