This includes coding and non-coding gene content and contributes 25% of nuclear genome. As we have seen earlier in our comparison of genome fragment from different organisms one thing becomes clear that genes are not arranged indefinite pattern but rather arranged unevenly throughout the entire genome. There were two lines of evidence, one of which related to the banding patterns that are produced when chromosomes are stained. The dyes used in these procedures bind to DNA molecules, but in most cases with preferences for certain base pairs. Giemsa, for example, has a greater affinity for DNA regions that are rich in A and T nucleotides. The dark G-bands in the human karyogram are therefore thought to be AT-rich regions of the genome. The base composition of the genome as a whole is 59.7% A + T so the dark G-bands must have AT contents substantially greater than 60%. Cytogeneticists therefore predicted that there would be fewer genes in dark G-bands because genes generally have AT contents of 45-50%. This prediction was confirmed when the draft genome sequence was compared with the human karyogram. The second line of evidence pointing to uneven gene distribution derived from the isochoremodel of genome organization. According to this model, the genomes of vertebrates and plants (and possibly of other eukaryotes) are mosaics of segments of DNA, each at least 300kb in length, with each segment having a uniform base composition that differs from that of the adjacent segments. Support for the isochore model comes from experiments in which genomic DNA is broken into fragments of approximately 100 kb, treated with dyes that bind specifically to AT- or GC-rich regions, and the pieces separated by density gradient centrifugation. When this experiment is carried out with human DNA, five fractions are seen, each representing a different isochore type with a distinctive base composition: two AT-richisochores, called L1 and L2, and three GC-rich classes: H1, H2 and H3. The last of these, H3, is the least abundant in the human genome, making up only 3% of the total, but contains over25% of the genes. This is a clear indication that genes are not distributed evenly through the human genome.
The genes present in an organisms can be classified using two approaches first is based according to the function of genes and other is based on particular domain of the protein a gene codes for. The second approach is more informative and better because it shows that particular genome specifies a number of protein domains that are absent Genes and Genomic Organization from the genomes of other organisms, these domains including several involved in activities such as cell adhesion, electric couplings, and growth of nerve cells. These functions are interesting because they are ones that we look on as conferring the distinctive features of vertebrates compared with other types of eukaryote of the genome as a whole is 59.7% A + T so the dark G-bands must have AT contents substantially greater than 60%. Cytogeneticists therefore predicted that there would be fewer genes in dark G-bands because genes generally have AT contents of 45 50%. This prediction was confirmed when the draft genome sequence was compared with the human karyogram. The second line of evidence pointing to uneven gene distribution derived from the isochoremodel of genome organization. According to this model, the genomes of vertebrates and plants (and possibly of other eukaryotes) are mosaics of segments of DNA, each at least 300kb in length, with each segment having a uniform base composition that differs from that of the adjacent segments. Support for the isochore model comes from experiments in which genomic DNA is broken into fragments of approximately 100 kb, treated with dyes that bind specifically to AT- or GC-rich regions, and the pieces separated by density gradient centrifugation. When this experiment is carried out with human DNA, five fractions are seen, each representing a different isochore type with a distinctive base composition: two AT-richisochores, called L1 and L2, and three GC-rich classes: H1, H2 and H3. The last of these, H3,is the least abundant in the human genome, making up only 3% of the total, but contains over25% of the genes. This is a clear indication that genes are not distributed evenly through the human genome. The genes present in an organisms can be classified using two approaches first is based according to the function of genes and other is based on particular domain of the protein a gene codes for. The second approach is more informative and better because it shows that particular genome specifies a number of protein domains that are absent from the genomes of other organisms, these domains including several involved in activities such as cell adhesion, electric couplings, and growth of nerve cells. These functions are interesting because they are ones that we look on as conferring the distinctive features of vertebrates compared with other types of eukaryote. Since the earliest days of DNA sequencing it has been known that multigene families groups of genes of identical or similar sequence - are common features of many genomes. The rRNA genes are examples of ‘simple’ or ‘classical’ multigene families, in which all the members have identical or nearly identical sequences. These families are believed to have arisen by gene duplication, with the sequences of the individual members kept identical by an evolutionary process. Other multigene families, more common in higher eukaryotes than in lower eukaryotes, are called ‘complex’ because the individual members, although similar in sequence, are sufficiently different for the gene products to have distinctive properties. One of the best examples of this type of multigene family are the mammalian globin genes. The globins are the blood proteins that combine to make hemoglobin, each molecule of haemoglobin being made up of two α-type and two β-type globins. Why are the members of the globin gene families so different from one another? The answer was revealed when the expression patterns of the individual genes were studied. It was discovered that the genes are expressed at different stages in human development: for example, in the β-type cluster ε is expressed in the early embryo, G γ and A γ (whose protein products differ by just one aminoacid) in the fetus, and δ and β in the adult. The different biochemical properties of the resulting globin proteins are thought to reflect slight changes in the physiological role that hemoglobin plays during the course of human development.
In some multigene families, the individual members are clustered, as with the globingenes, but in others the genes are dispersed around the genome. An example of a dispersed family is the five human genes for aldolase, an enzyme involved in energy generation, which are located on chromosomes 3, 9, 10, 16 and 17. The important point is that, even though dispersed, the members of the multigene family have sequence similarities that point to a common evolutionary origin.
