Repetitive DNA Sequences

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,... 

Repetitive-sequence DNA can be broadly classified as moderately repetitive or as highly repetitive. The highly repetitive sequences consist of 5-500 base pair lengths repeated many times in tandem. These sequences are usually clustered in centromeres and telomeres of the chromosome and are present in about 1-10 milHon copies per haploid genome. These sequences are transcriptionally inactive and may play a strucmral role in the chromosome (see Chapter 40). 

Primers can be custom synthesized commercially at a fairly reasonable rate. Primer sequences are selected so that they are typically 21 to 24 nucleotides in length with an average G + C content of 40 to 60%. The primers should not be part of repetitive-sequence DNA, nor should the DNA have palindromic sequences. There are computer programs to help select oligonucleotide primers for PCR, such as PRIMER (from White Head Institute www-genome. wi. mit. echi/ftp //distribution/software/primer. 0.5/manual. asc). In addition, commercial sources exist for software (National Biosciences) that includes the OLIGO program. 

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See also in sourсe #XX -- [ , ]

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