seq = "CGTAAAAAATTACAACGTCCTTTGGCTATCTCTTAAACTCCTGCTAAATGCTCGTGCTTTCCA\
ATTATGTAAGCGTTCCGAGACGGGGTGGTCGATTCTGAGGACAAAGGTCAAGATGGAGCGCATCGAACGC\
AATAAGGATCATTTGATGGGACGTTTCGTCGACAAAGTCTTGTTTCGAGAGTAACGGCTACCGTCTTCGA\
TTCTGCTTATAACACTATGTTCTTATGAAATGGATGTTCTGAGTTGGTCAGTCCCAATGTGCGGGGTTTC\
TTTTAGTACGTCGGGAGTGGTATTATATTTAATTTTTCTATATAGCGATCTGTATTTAAGCAATTCATTT\
AGGTTATCGCCGCGATGCTCGGTTCGGACCGCCAAGCATCTGGCTCCACTGCTAGTGTCCTAAATTTGAA\
TGGCAAACACAAATAAGATTTAGCAATTCGTGTAGACGACCGGGGACTTGCATGATGGGAGCAGCTTTGT\
TAAACTACGAACGTAAT";
size = 70;
parts = StringPartition[seq, UpTo[size]];
begins = Most[Accumulate[Prepend[StringLength /@ parts, 1]]];
ends = Rest[Accumulate[Prepend[StringLength /@ parts, 0]]];
StringRiffle[MapThread[ToString[#1] <> "-" <> ToString[#2] <> ": " <> #3 &, {begins, ends, parts}], "\n"]
StringRiffle[#1 <> ": " <> ToString[#2] & @@@ Tally[Characters[seq]], "\n"]