Molecular Definition of a Gene - Molecular Cell Biology - NCBI Bookshelf
The RNA polymerase that is responsible for transcription of the gene is same with cistron codes for a polypeptide chain: Typical Prokaryotic Gene structural it is a house keeping gene or special gene, either from prokaryote or eukaryote. Informational implications of gene function in eukaryotes include: the nuclear Benzer's term cistron can be retained as functional unit of hereditary material as base composition of different DNA's, an important numerical relationship was. Cistron, Recon, And Muton. The genome contains genetic information that is transmitted from parents to offspring, from one generation to the next. The genome.
Bio Cistrons, Genes, and Exons
Many other RNA molecules described in later chapters also are transcribed from non- protein -coding genes. Bacterial Operons Produce Polycistronic mRNAs As discussed in Chapter 4, genes encoding enzymes involved in related functions often are located next to each other in bacterial chromosomes. For example, the five genes encoding the enzymes required to synthesize the amino acid tryptophan from simple precursor molecules map in one contiguous stretch of the E.
This cluster of genes comprises a single transcription unit referred to as an operon. Ribosomes initiate translation at the beginning of each of the genes in this mRNA producing the five polypeptides required for tryptophan synthesis. Since a cistron is defined as a genetic unit that encodes a single polypeptidetrp mRNA, which encodes several polypeptides, is said to be polycistronic.
Figure Comparison of bacterial operons and simple eukaryotic transcription units. A control region located near more One consequence of the arrangement of bacterial genes into operons is that a single mutation can influence the expression of several proteins.
For example, if a single point mutation e. This distinction correlates with a fundamental difference in mRNA translation in prokaryotes and eukaryotes.
As explained in Chapter 4, a bacterial polycistronic mRNA contains multiple ribosome -binding sites located near the start sites for all the protein-coding regions in the mRNA. As a consequence, translation initiation can begin at internal sites in a polycistronic mRNA molecule, producing multiple proteins see Figure a.
The primary transcripts of eukaryotic protein-coding genes generally are processed into a single type of mRNA, which is translated to give a single type of polypeptide see Figure Such eukaryotic mRNAs and their corresponding genes are monocistronic. Although the primary transcripts of some protein-coding genes are processed into more than one type of mRNA, each mRNA is translated into a single polypeptide.
Unlike bacterial and yeast genes, which generally lack introns, most genes in higher animals and plants contain introns, which are removed during RNA processing. In many cases, the introns in a gene are considerably longer than the exons. Many large proteins in higher organisms have repeated domains and are encoded by genes consisting of repeats of similar exons separated by introns of variable length.
For instance, fibronectina component of the extracellular matrixis encoded by a gene containing multiple copies of three types of exons. Another example is the largest known human gene, the mutation -defined DMD gene, which is associated with Duchenne muscular dystrophy. The DMD gene, containing more than two million base pairs, is longer than the entire Haemophilus influenzae genome!
Mutations in exons, introns, and transcription-control regions all may influence the expression of proteins encoded by simple transcription units Figure b.
Unlike some mutations in bacterial operons, which can affect multiple proteins, mutations in simple eukaryotic transcription units can affect only one protein. Although many transcription units in eukaryotes are simple, complex transcription units are quite common in multicellular organisms.
The primary RNA transcript encoded by complex transcription units can be processed in more than one way by use of alternative poly A sites or splice sites, leading to formation of mRNAs containing different exons. Examples of both types of alternative RNA processing occur during sexual differentiation in Drosophila see Figure The DNA of eukaryotic cells is tightly bound to small basic protein histones that package the DNA in an orderly way in the cell nucleus.
The complexes between eukaryotic DNA and protein are called chromatin. The basic structural unit of chromatin is called nucleosome. Chromatin may be functional or non-functional. Heterochromatin is inactive chromatin, whereas euchromatin is actively involved in RNA transcription. Changing interpretations of gene function: The first meaningful insight into gene function was made by Garrod who concluded that persons suffering from inherited metabolic disease were missing specific enzymes.
Later, Beadle and Tatum were able to produce mutations in the genes of Neurospora and identifying the specific metabolic reactions that were affected. According to Beadle and Tatum a biosynthetic pathway might have four steps, where 1 is the starting material and 5 is the final product. Each step is catalyzed by an enzyme: In turn, each enzyme is specified by a particular gene. This would imply that gene a specifies enzyme A, gene b specifies enzyme B, and so on. Therefore, if we inactivate the gene responsible for an enzyme, we initiate one required step and the pathway is interrupted.
Following diagram shows that enzyme B is eliminated due to mutation in gene b. Since, the cell cannot carry out the reaction that converts compound 2 to compound 3, it is blocked at compound 2, and can not go further. The model one-gene-one enzyme hypothesis provided the first exciting insight into the function of genes. Genes some how were responsible for the function of enzymes and each gene apparently controlled one specific enzyme.
The hypothesis can be summarized as under: I biochemical reactions in vivo occur as a series of discrete stepwise reactions ii each reaction is specifically catalyzed by a single enzyme iii each enzyme is specified by a single gene.
The gene is a unit of function which is equivalent to the cistron and that can be defined experimentally by a cis-trans complementation test. The cistron is a region of the genetic material that codes for one polypeptide chain.
Therefore, one-gene-one-enzyme hypothesis be referred more precisely as the one-cistron-one-polypeptide hypothesis, thereby emphasizing that cistron can code for proteins other than enzyme. Byit become clear that the DNA associated with the nucleus contains genes which direct the synthesis of ribosomal RNA Perry This was conclusively proved in case of mammalian cells by McConkey and Hopkins The one-cistron-one-polypeptide hypothesis is strongly supported by the studies of Schwartz dealing with esterase mutations in maize.
His evidence that isozymes characteristic of an allelic series arise as a result of charge differences serves to clarify this and similar situations which initially appeared to be in potential conflict with this hypothesis in its simplest form. Full-length cDNA sequencing allows unique sequences to be selected that will distinguish between otherwise closely related genes.
This will facilitate selection of oligos and larger regions for expression array. For example, complex protein mixtures can be subjected to peptide analysis by mass spectrometry, and a unique matches identified in the genome sequence. This also allows simple comparative analysis to identify protein-coding regions that can then be readily built into gene models. Comparative analysis is far more powerful than compositional analysis for gene finding. Exon-intron structure is typically conserved between species, which should make gene modeling for more accurate.
If the right genome is selected, it should be possible to identify conserved regulatory elements in promoters. The availability of complete annotated genomic sequence allows each insertion to be annotated automatically by simply searching insertion sites against the genomic sequence for sequence matches at the nucleotide level.
Genebank annotation from the nearest gene can then be automatically downloaded and parsed into the Genetrap database on a periodic basis Tissier et al.
Similarly, three structural proteins VP6, VP7 and VP4 were sequenced at the DNA level in several rotavirus strains in terms of their recognition and reactively with panels of antibodies Hoshino and Kapikian, and their role in virus assembly and infectivity Patton, VP6, which makes up inner capsid of rotavirus is the major structural protein of this virus characterized in detail by using mass spectrometry and two-dimensional gel-electrophoresis Emslie et al.Difference between Prokaryotic and Eukaryotic Gene Expression
In the characterization of VP6, SA11 had the predicted C-terminus with no modification, and peptide mass fingerprinting data was used to clarify the amino acid sequence at sites where there were conflicts in the DNA sequence Smith-Huerta et al. Feed forward loop FFL system: The expression of many genes encoding transcriptional activators in prokaryotes and eukaryotes is up regulated through positive feedback activation. During positive feedback activation, a transcriptional activator binds to its own promoter and thus increases its own expression as well as expression of its target genes.
The increased levels of the transcriptional activator can be directly correlated with increased expression of its target genes.
In transiently and stably transformed plant cells, the FFL system yields expression levels of a luciferase reporter gene exceeding the levels observed with the potent CaMV35S promoter. Schwechheimer and his colleagues were able to generate transgenic plants which ubiquitously express high levels of a luciferase reporter gene. The activity of transcription factors is regulated by a variety of mechanisms, e.
Schwechheimer and Bevan, The numerous transcriptional activators vary in their strength and possibly in some cases also in their tissue-specific activity Schwechheimer et al. However, it can be hypothesized that the presence of high concentrations of transcription factor in the nucleus may be deleterious for eukaryotic cells and that the cells possess mechanisms which allow us to stop expression of these genes.
Hence, a regulatory mechanism which excludes the transcription factor from the nucleus, e. Extensive information through cDNA libraries: Targeting expressed genes, through the construction, characterization and the analysis of extensive cDNA libraries, provides information that is potentially of direct relevance to a particular phenotype Clarke et al.
Due to worldwide efforts in sequencing expressed genes and chromosomal DNA in Arabidopsis and rice, it is now possible to apply the developing knowledge to more complex genomes present in the wheat and barley. This is possible because there is a significant level of conserved gene order and content between the genomes, particularly between wheat and rice Larik and Soomro, ; Larik et al. An estimate of the total number of genes in rice is approximately 30, Yamamoto and Sasaki, The concept of homoeology between the genomes and chromosomes of wheat and its relatives implies that similar homoeologies should exist between the individual genes and the polypeptides coded by them Siddiqui et al.
The genes for the formation of various iso-enzymes may ultimately be correlated with genes for visible and physiological effects, thus elucidating the biological function of many enzymes Barber et al. Al-saheal and Larik determined the changes in the gene activity between large L and small S genotrophs crossed to the PL plastic genotypes when grown either at the lower temperature T2, outside all times or at the higher temperature T1, the first 5 weeks in greenhouse. Results demonstrate that I the DNA show increased reversion in the crosses, compared with parents at T1 and less reversion in the crosses, than in the parent, at T2; ii more reversion occurs in the PL maternal crosses than in the PL paternal crosses; and iii the PL genotroph possess a nuclear and a cytoplasmic factor which plays a direct part in determining nuclear DNA changes in the chromosomes of the stable genotypes.
Feedback from nucleus to cytoplasm: Information on the genetic control of environmentally induced DNA variation in flax genotrophs Al-Saheal and Larik, brevealed that changes occurred in the amount of nuclear DNA of R when the PL nuclear and cytoplasmic factors were introduced by crossing.
The results suggest that there may be feedback from nucleus to cytoplasm in Lo, which inactivates the cytoplasmic factor. Possibly a paramutation like mechanism adjusts the different amount of DNA, or heterochromatin between the homologous chromosomes in heterozygotes. This reversion in DNA appears to be accompanied by an increase in stability rather than in any gain in plasticity.
Plasticity, viewed here as a character determined by nuclear and cytoplasmic factor Al-Saheal and Larik, bcan be assumed as a regulatory system whose elements, insofar as they are identifiable at the level of the genetic analysis, interact in a manner suggestive of the lactose operon in Escherichia coli Jacob and Monod, Changes in the number of repeated sequences seems to be a more likely source of the changes in the amount of nuclear DNA than polyteny. Since a general inducing environment promoting rapid healthy growth Durrant, is essential for induction in flax, cell division may be essential for changes in the number of gene sequences to occur.
In a specific environment; by increasing the amount of DNA, the number of sequences increases at each cell division and decreases at each cell division in an environment that induces less DNA untill a level is attained where no further changes can occur and the system becomes relatively stable.
Gene activity in coupling and repulsion phases: Changes appear to be paramutation-like where one allele alters its homologue on the other chromosome, as a directly or via associated elements. In repulsion, in the large L genotroph, H phenotype had changed to the h phenotype at the time of induction by a heterochromatic region extending over this locus. In the small S genotroph, which has less DNA, it is assumed that the heterochromatin does not cover the h locus, and so it is a fully active H.
The heterochromatic transfer frequencies were higher in coupling than in repulsion. In the heterozygote, stable equilibria of the homozygotes are destroyed and transfer of heterochromatin, or number of reiterated sequences, or a decrease in one homologue and an increase in the other, occur in this region between homologous chromosomes. The amount and direction of the association is dependent upon the frequency of heterochromatic transfer: This mechanism of heterochromatic transfer preserves the Mendelian ratio of 3: A gene encoding wheat endosperm SBE-1 was characterized by Rahman et al.
There are two types of branching enzymes in plants, starch branching enzyme-1 SBE-1 and starch branching enzyme-II SBEand both are about kda in mass. While SBE-1 and SBE catalyse identical reactions, evidence from mutational and gene expression experiments demonstrate that the enzymes differ in their roles, and biochemical evidence suggests that they differ in their patterns of action Gaun et al.
In maize Boyer and Preiss,rice Mizuno et al. Mutants lacking SBE activity are not known. Several possible reasons for this can be advanced: I SBE-1 may be encoded by multiple genes, ii the null mutation does not lead to a phenotype identified in coarse screens for seed morphology or starch structure, or iii an SBE-1 mutant is lethal for reasons which are not yet evident.
These proteins were compared with respect to their effects on flour-processing properties such as dough mixing, extensibility and maximum resistance which are the important features in the enduse of wheat food products.
Ectopic expression of genes and aberrant transcripts: Girard and Freeling have found Lg3-O, a sami dominant neomorphic mutation that transforms regions of leaf blade, auricle and ligule into sheath.
The liguleless3 gene is a member of knox class 1 family of homeobox genes and the dominant alleles which define it are due to ectopic expression of the gene in the leaf. They found that the transcripts produced by these alleles are significantly shorter than those of wild type as well as it progenitor, Lg3-O.
Similarly, green-fluorescent compounds induced by ectopic expression of P gene in maize was studied by Lin et al. The maize P gene, R2R3 myb transcription factor, controls 3-deoxy flavonoid and phlobaphene biosynthesis.
In the pericarp, P regulates the accumulation of a subset of flavonoid biosynthetic genes C2, Chi1 and A1. The ectopic expression of P in cultured BMS cells induces the accumulation of distinct classes of flavonoid and phenylpropanoid compounds, as well as orange-fluorescent bodies. The P1 gene encodes a transcriptional regulator of red phlobaphene pigment biosynthesis.
A P1-rr allele conditions uniform red pigmentation of the pericarp, cob glume, husks, silks and tassel glumes, whereas plants carrying a P1-wr allele lack pericarp pigmentation, but have uniform, darkred cob glumes and pigmented margins on the husks and tassel glumes.
When maize plants were transformed with constructs containing the P1-rr promoter deriving either P1-rr or P1-wr cDNA sequences, the transgenes promoted pigmentation in the floral organs as well as in the vegetative organs of plants. This pattern of trans gene expression is comparable to that caused by Ufo1 unstable factor for orangea dominant allele that induces phlobaphene production in vegetative organs when combined with a P1-wr allele Cocciolone et al.
In present review we have critically examined and discussed the gene structure and function in prokaryotes and eukaryotes with particular reference to DNA information technology. Methods of experimentation used to obtain information for bioinformatic analysis include electrophoresis, chromatography and a relatively new area biochips, which are miniaturized devices that can make biological experimentation more efficient.
Functionally, biochips include DNA chips, protein chips and lab chips. Biochips contain either immobilized DNA strands DNA chipsimmobilized protein strands protein chips are interconnected channels with fluid propulsion and control systems etched into glass, silicon, quartz or plastic lab chips that respectively permit gene, protein and expression system analysis on a single chip.