Gene structure The phenomenon of parent-of-origin gene expression
By Live Dr - Sun Jan 11, 12:16 pm
1. Gene structure
1.1 Exon and intron
The double-helical structure of DNA serves as the repository（storage room） for genetic information as well as the basis for DNA replication. These topics are addressed in detail in molecular biology texts and will not be reviewed exhaustively(entire) here of particular importance in considering genetic contributions to medicine is an appreciation(evaluation) of the structure of individual genes. Genes represent discrete(segregative) regions of DNA they may be quite short or may extend over hundreds of kilobases (kb, 1kb = 1000 individual base pairs[bp]). Individual regions of genes are defined by specific sequence features. One of the most prominent features of most human genes is the presence of distinct segments, some of them responsible for protein-coding information and others separating such coding sequences. The former-coding-sequences are referred to as exons. The latter-noncoding-regions between exons are referred to as introns.
1.2 Gene clusters
Genes sometimes occur in clusters, with genes of similar function located near each other. Many genes are related to one another. They make up so-called gene families. These are groups of genes of often similar structure and function. Some gene families are grouped as contiguous arrays. By far the best-studied gene cluster are those for the globin gene cluster.
These so-called pseudogenes, designated by Ψ, are DNA sequences that have some of the structures of expressed genes and were presumably once functional but have acquired one or more mutations during evolution that render(to result in) them incapable of producing a protein product.
2. Control of Gene Expression
With a few notable(remarkable) exceptions, all of the cells of the human body contain the complete genome. Yet(however), in any given tissue only a subset of these genes are being expressed. Therefore, the control of gene expression is fundamental to understanding virtually all aspects of human biology.
2.1 The promoter
The promoter is somewhat loosely defined as the sequence elements located immediately 5′ to the gene that interact with RNA polymerase and other components of the transcription machinery. These elements fix the site of transcription initiation and control mRNA quantity and sometimes tissue specificity. While in some situations the promoter may extend for several kilobases, the important promoter elements are generally located in the region 100-200 bp 5′ to the gene.
As noted previously, most eukaryotic genes have their coding regions interrupted by introns, which must be removed in a process called splicing to generate a mature mRNA that can be translated into a functional protein. While the function of introns remains unclear the mechanism of splicing is beginning to be understood. At the beginning and end of an intron, certain nucleotide sequences are found.
Most messenger RNAs that code for protein are characterized by the addition of a string of about 200 adenosine residues at their 3′ end (polyadenylatiun).
Enhancers are DNA sequences defined by the following properties: (1) they increase transcription from a nearby gene; (2) they can operate over considerable distances and are relatively unaffected by altering this distance; and (3) they are effective even if inverted.
Mutations represent differences in DNA organization or sequence in an individual with respect to some standard sequence. Many differences lead to observable amino acid changes in proteins, but some do not. Nevertheless, identifying different mutations has proved useful both for diagnostic studies and for determining the locations of genes and their alterations.
3.1 Base substitution
The simplest mutations represent local DNA base changes. These can include the substitution of one purine for another (A for G or G for A) or one pyrimidine for another (T for C or C for T) these are called transitions. Alternatively, mutations may exchange a pyrimidine for a purine or vice versa (C for A, T for G etc.) these are called transversions.
3.2 Frame shift mutation
Another possible DNA alteration is the loss of one or more bases. Deletions can cause serious problems. The loss of three contiguous’ bases can either lead to the loss of a single amino acid codon (as occurs in the most common mutation for cystic fibrosis [OMIM #219700] or affect two contiguous codons. Nevertheless, with the loss of three bases in a row (or any multiple of three), the reading frame of the gene remains intact. The loss of different numbers of bases destroys the triplet reading frame and leads to complete aberrancy in the protein produced. Deletions also can occur on a larger scale, such that entire DNA regions can be lost.
The movement of a small or large piece of DNA from one position to another, a process referred to as transposition, is another source of DNA variation. In bacteria and many less complicated organisms, transposition of DNA sequences from one position to another occurs relatively readily.
3.4 Dynamic mutation
The repeat copy numbers of nuclear sequence composed of DNA molecule were increased with the amplification in different degree.
4. Genomic Imprinting
The phenomenon of parent-of-origin gene expression. The expression of a gene depends upon the parent who passed on the gene. /The allele from one parent is expressed and the allele from the other parent is not.