Question

why only a segment of dna and only one of strands is copied into rna

Answer 1

Answer:

the RNA molecules produced by transcription are released from the DNA template as single strands. In addition, because they are copied from only a limited region of the DNA, RNA molecules are much shorter than DNA molecules

Answer 2

Answer:

It seems reasonable that only one strand is used, because transcription of RNA from both strands would produce two complementary RNA strands from the same stretch of DNA, and these strands presumably would produce two different kinds of protein (with different amino acid sequences).