Molecular Genetics

Moderately repeated DNA includes some gene families for abundant products.

Facts

Nucleoli are the sites of ribosomal RNA (rRNA) synthesis. The large and small rRNAs are derived from a single transcript by processing. Oscar Miller observed nucleolar chromatin spread on grids by electron microscopy. The above diagrams the appearance of this chromatin. The lateral branches were interpreted as nascent RNA transcripts. The genes (rDNA) coding for rRNA are moderately repeated (several hundred copies) in most eukaryotes.

The E. coli genome contains 7 genes for rRNAs. As in eukaryotes, one gene codes for large and small rRNAs. The genes are distributed at 7 loci on the E. coli genetic map. The loci do not seem to result from duplications of larger chromosomal segments. Interpretations

• Genes for the family of histone proteins are clustered near one another in Drosophila melanogaster (above) and other invertebrates.
• The genes for individual histones are separated from one another by dissimilar sequences (indicated by lower case letters).
• There are about 200 copies of histone genes in sea urchins.
• The D. melanogaster 4.8 kbp unit containing all 5 genes is tandemly repeated.

Interpretations

• Kinetically, rDNAs are repetitive DNAs. The multiple copies are maintained probably because the rDNA gene needs voluminous transcription.
• rDNA genes are tandemly repeated in eukaryotes. They differ from satellite DNAs in that the length of the repeat is longer and the DNA is not highly repeated.
• The eukaryotic transcribed rDNA unit is flanked by not transcribed or lightly transcribed segments.
• rDNA genes are dispersed at different locations on the E. coli DNA.
• Like rDNA genes, the D. melanogaster histone gene unit is moderately repetitive DNA, is arranged in tandem repeats, and has products that are required in large amounts by all cells.
• Unlike rDNA genes, the histone unit consists of transcribed regions separated by non-identical spacer regions.
• Gene families are groups of genes related, but not necessarily identical, in structure. The members of the family can have functions that are identical or similar. Examples of further gene families with members with related functions are the growth hormone and the immunoglobulin gene families.
• Whether gene families correspond to repeated sequence families depends on the degree of relatedness of the members of the gene family and the stringency of hybridization used to define the sequence family.
• It is difficult to analyze genetically the function of multi-copy genes since mutating one copy or adding one extra copy should have only a small effect on the level of expression from the multi-copy family.
• Note that single locus and multi-locus VNTR loci are tandem moderately repeated sequences used in forensic and legal investigations. Multi-locus VNTRs are dispersed sets of tandem repeats.

Further information

• Further experiments established that the rDNA repeat unit is truly tandem, consisting of both transcribed and non-transcribed segments.
• Unlike sea urchin and fruit fly histone genes, mammalian histone genes are dispersed (like E.coli rRNA genes are) at various chromosomal locations, as are other genes whose products are needed in most cells (actin and tubulin, for example). Genes for histones are clustered in Drosophila.
• Genes for ribosomal RNA (rDNA) are moderately repeated sequences organized in tandem repeats. The large intergenic spacer of the human rDNA unit has many Alu repeats.
• Retrotransposons can be a significant fraction of middle repetitive DNA.
• Short tandemly repeated sequences can occur in regions of genomes that are transcribed and translated.
  • The expansion of trinucleotide repeats is associated with disease.
  • Translation of 23 nucleotide repeats of the K12 gene of the Karposi Sarcoma herpesvirus is reading frame independent.

This is page 112285 of Molecular Genetics by Ulrich Melcher, © 1998-2000