Gene Repetitive sequences

Examples of a polymorphism include single nucleotide substitutions, insertions and deletions of nucleotides, and repetitive sequences. While most polymorphisms are harmless and part of normal human genetic variations, studies have established links between certain gene polymorphisms and metabolic alterations or human diseases. 

The DNA in a eukaryotic genome can be divided into different sequence classes. These are unique-sequence, or nonrepetitive, DNA and repetitive-sequence DNA. In the haploid genome, unique-sequence DNA generally includes the single copy genes that code for proteins. The repetitive DNA in the haploid genome includes sequences that vary in copy number from two to as many as 10 copies per cell. 

This estimation (and the distribution of repetitive-sequence DNA) is based on a variety of DNA-RNA hybridization techniques and, more recently, on direct DNA sequencing. Similar techniques are used to estimate the number of active genes in a population of unique-sequence DNA. In brewers yeast Saccha-romyces cerevisiae, a lower eukaryote), about two thirds of its 6200 genes are expressed. In typical tissues in a higher eukaryote (eg, mammalian liver and kidney), between 10,000 and 15,000 genes are expressed. Different combinations of genes are expressed in each tissue,... 

Figure 27-10 Organization of the globin genes on human chromosomes 11 and 16. The composition of the various embryonic, fetal, and adult hemoglobins is also indicated. Closed boxes indicate active genes and open boxes pseudogenes. The triangles ( ) indicate Alu repetitive sequences and their orientation. The shaded boxes indicate Kpn repeat sequences and the half-arrows their respective orientation. The Kpn sequence between the e and 7° genes in fact consists of two tandemly linked Kpn repeats. From Karlsson and Nienhuis196 and Proudfoot.197...

Hull GA, Halford NG, Kreis M, Shewry PR. Isolation and characterisation of genes encoding rye prolamins containing a highly repetitive sequence motif. Plant Mol Biol 1991 17 1111-1115. 

Bacterial genomes are considerably less complex than human or other eukaryotic genomes. Common bacteria have only one chromosome, usually a circular DNA double helix of 4 million to 5 million base pairs, about 1000 times less than the amount of DNA in a human cell. About 90% of the DNA in bacteria codes for protein. There are no introns, but there are multiple small intergenic regions, dispersed throughout the genome, that carry repetitive sequences. The common, bacterium Escherichia co/f contains about 4300 genes. 

The common feature of these protein polymers is the presence of repetitive sequence motifs which form defined secondary structures. These repetitive amino acid sequences offer the possibihty to construct artificial genes by mul-timerization of small synthetic oligonucleotide sequences and thus the build up of high molecular weight proteins. The constructed artificial genes can be incorporated into an expression plasmid, which can subsequently be transferred to a bacterial host for production of the desired polypeptide (Fig. 19). The most commonly used host is E. coli. 

Some examples of moderately repetitive sequences are transposable elements including retroviruses, histone genes that are repeated 30-40 times in the human genome, and genes coding for ribosomal RNAs and transfer RNAs. Most of the moderately repetitive genes do not code for proteins but serve other functions in the cell. Histones are involved in condensation of DNA in chromosomes and both ribosomal RNAs and transfer RNA are involved in protein synthesis. 

Another class of highly repetitive sequences is the Alu family, which consist of about 300 bp that are repeated millions of times throughout the genome. Any Alu sequence is at least 85% homologous in base sequence to any other Alu sequence hence, the family of genes has been highly conserved. Each Alu sequence contains a restriction site that is recognized by the Alul restriction enzyme, from which the name of the family of sequences is derived. 

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