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Abstract. The lampbrush chromosomes of amphibian oocytes are highly active in RS~A transcription. The extent of transcription and features of the transcriptional product have been studied both cytologically and by molecular hybridization. Each lampbrush loop is considered to be a unit of transcription which generates many primary transcript molecules. In Triturus the primary transcripts can be recovered as nuclear RNP particles which can be dispersed in the presence of formamide to give linear forms of up to 20 ~m or more in length. Only part of the RNA extracted from nuclear RNP codes for protein, the remainder containing non-informationM repetitive sequences. Although about 4% of the chromosomal DNA of Triturus is transcribed during oogenesis, only 0.05-0.1% of the DNA is expressed as coding sequences. This informational sequence complexity is equivalent to approximately 10 ~ different mRNA species. A model for the organization of genetic sequences in the lampbrush chromosomes is suggested.

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The meiotic lampbrush chromosomes of amphibian oocytes display readily distinguishable regions of transcription (lateral loops) which extend from axial condensates of chromatin (chromomeres). The chromomeres contain most of the chromosomal ENA which, along with histone, is tightly compacted as regular arrays of DNP. Many RNA transcripts are generated on the lateral loops, and heterogeneous non-histone proteins associate with these transcripts, forming periodic condensates of 20--30 nm ribonucleoprotein (RNP) particles. These unit particles aggregate in various ways and to varying degrees and thereby confer distinctive gross morphologies to particular loops. There are about 10(4) lateral loops per haploid complement of newt chromosomes and this figure is similar to the experimentally derived number of different messenger RNA sequences found in oocytes. From cytological and biochemical studies it is now possible to consider individual lateral loops from various aspects: as morphologically distinct units; as units of inheritance; as units of functional activity; as units of transcription; as units of transcribed repetitive sequences; and as units containing one coding sequence. The difficulties in arriving at a simple explanation of the organization of transcription in lampbrush chromosomes are discussed.