Fig. 1. Effect of heat shock on chromosome morphology. (A) Phasecontrast image of one of the 18 bivalent chromosomes from an oocyte heat shocked for 4 hours at 31.5°C and allowed to recover for 3 hours at 18°C. The overall contraction of the chromosome and loss of lateral loops is typical of transcriptionally inactive chromosomes. (B) Immunofluorescent image of a chromosome from an oocyte heat shocked for 4 hours at 31.5°C and allowed to recover for 21 hours at 18°C. Pol II transcription has returned to normal levels as shown by overall lengthening of the chromosome and re-extension of the lateral loops. As in control preparations, chromosome loops are specifically stained with mAb H14, which recognizes the largest subunit of Pol II when the C-terminal domain is phosphorylated on serine 5.