The aim of this study was to analyse science teachers' language in the classroom within the context of explanation sequences, in order to determine the coherence of the explanations, and the opportunities presented for learners' exposure to the register of science. The study is framed by the concepts of semiotic mediation, register and coherent discourse drawn from Systemic Functional Linguistics (SFL). Data were collected through observation and video recording of teachers teaching physical science topics. The unit of analysis were teachers' utterances--their words, sentences and explanation sequences, as they taught a specific science topic and a particular causal relationship. The analysis identified significant differences in the use of coherence devices in the explanation sequences that enhanced or limited transmission of the register of science. Specific cohesive devices that contribute to coherent and incoherent discourse have been distinguished. On the whole, the lesson that was characterised by stronger cohesive devices provided greater opportunities for semiotic mediation necessary for understanding and learning the register of science.