Physical maps of all length variants of cloned ribosomal RNA (rRNA) genes were constructed using restriction endonucleases EcoRI, BamHI, BglII, HindIII, and SalI. Length variation in repeat units was attributed to differences in spacer regions. Comparison of HaeIII and HpaII digestion of cereal rDNA and cloned repeats suggested that most methylated cytosines in natural rDNA are in -CpG- sequences. Incomplete methylation occurs at specific BamHI sites in barley DNA. Ribosomal spacer sequences are not present elsewhere in the genome except in rDNA repeats. The study reports the isolation of DNA enriched for rDNA from wheat and barley, and its use to clone full-length, stable rDNA repeats in plasmid pAC184. Restriction maps of wheat and barley repeats were determined using cloned rDNA, and the organization of rDNA and spacer sequences were characterized. DNA was isolated from barley and wheat, enriched for rDNA using actinomycin D-CsCl gradients. The DNA was digested with EcoRI and ligated to pAC184. Cloned rDNA was used to transform E. coli, and transformants were tested for inserts. The results showed that wheat and barley rDNA could be cloned efficiently in pAC184. The sizes of inserts were determined, with a mean of 2.9 Kb for wheat DNA. Cloning of wheat and barley rDNA repeats was successful, with five wheat and thirteen barley clones obtained. The wheat rDNA inserts were 9 Kb, matching the repeat unit size. Eleven of thirteen barley inserts were full-length repeats, with two size classes present. Plasmids pHV309, pHV256, and pHV279 contained full-length, partial, and deleted repeats, respectively. BamHI digestion further defined length variation in rDNA repeats. Wheat rDNA had three size classes, while barley had two. The length heterogeneity in wheat was likely due to differences in the non-transcribed spacer. In barley, the longer repeat produced 3.1 and 1.8 Kb fragments, while the shorter produced 2.2 and 1.8 Kb fragments. Physical maps of rDNA repeats were constructed using restriction endonucleases. The major wheat repeat was 8.8 Kb, while the smaller was 9.0 Kb. The barley repeats were 9.9 Kb and 9.0 Kb. The rDNA repeats from wheat and barley were closely related. Methylation of cytosine residues in rDNA was studied. Wheat DNA had 25% methylated cytosines, while barley had 23%. HaeIII and HpaII digestion of rDNA showed that most methylated cytosines were in CpG sequences. Methylation of BamHI sites in barley rDNA was observed, with some sites showing incomplete methylationPhysical maps of all length variants of cloned ribosomal RNA (rRNA) genes were constructed using restriction endonucleases EcoRI, BamHI, BglII, HindIII, and SalI. Length variation in repeat units was attributed to differences in spacer regions. Comparison of HaeIII and HpaII digestion of cereal rDNA and cloned repeats suggested that most methylated cytosines in natural rDNA are in -CpG- sequences. Incomplete methylation occurs at specific BamHI sites in barley DNA. Ribosomal spacer sequences are not present elsewhere in the genome except in rDNA repeats. The study reports the isolation of DNA enriched for rDNA from wheat and barley, and its use to clone full-length, stable rDNA repeats in plasmid pAC184. Restriction maps of wheat and barley repeats were determined using cloned rDNA, and the organization of rDNA and spacer sequences were characterized. DNA was isolated from barley and wheat, enriched for rDNA using actinomycin D-CsCl gradients. The DNA was digested with EcoRI and ligated to pAC184. Cloned rDNA was used to transform E. coli, and transformants were tested for inserts. The results showed that wheat and barley rDNA could be cloned efficiently in pAC184. The sizes of inserts were determined, with a mean of 2.9 Kb for wheat DNA. Cloning of wheat and barley rDNA repeats was successful, with five wheat and thirteen barley clones obtained. The wheat rDNA inserts were 9 Kb, matching the repeat unit size. Eleven of thirteen barley inserts were full-length repeats, with two size classes present. Plasmids pHV309, pHV256, and pHV279 contained full-length, partial, and deleted repeats, respectively. BamHI digestion further defined length variation in rDNA repeats. Wheat rDNA had three size classes, while barley had two. The length heterogeneity in wheat was likely due to differences in the non-transcribed spacer. In barley, the longer repeat produced 3.1 and 1.8 Kb fragments, while the shorter produced 2.2 and 1.8 Kb fragments. Physical maps of rDNA repeats were constructed using restriction endonucleases. The major wheat repeat was 8.8 Kb, while the smaller was 9.0 Kb. The barley repeats were 9.9 Kb and 9.0 Kb. The rDNA repeats from wheat and barley were closely related. Methylation of cytosine residues in rDNA was studied. Wheat DNA had 25% methylated cytosines, while barley had 23%. HaeIII and HpaII digestion of rDNA showed that most methylated cytosines were in CpG sequences. Methylation of BamHI sites in barley rDNA was observed, with some sites showing incomplete methylation