SeedRandom[13122345];
seq = BioSequence["DNA", "ATAAACGTACGTTTTTAGGCT"];
randompos = RandomInteger[seq["SequenceLength"]];
seq = StringReplacePart[seq, RandomChoice[{"A", "T", "C", "G"}], {randompos, randompos}];
randompos = RandomInteger[seq["SequenceLength"]];
seq = StringReplacePart[seq, "", {randompos, randompos}];
randompos = RandomInteger[seq["SequenceLength"]];
seq = StringInsert[seq, RandomChoice[{"A", "T", "C", "G"}], randompos];
seq = BioSequence["DNA", StringJoin@RandomChoice[{"A", "T", "C", "G"}, 250]];
size = 50;
parts = StringPartition[seq["SequenceString"], 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"]
Tally[Characters[seq["SequenceString"]]]
Do[
 type = RandomChoice[{1, 2, 3}];
 Switch[type, 1,
  randompos = RandomInteger[seq["SequenceLength"]];
  seq = StringReplacePart[seq, RandomChoice[{"A", "T", "C", "G"}], {randompos, randompos}];
  , 2,
  randompos = RandomInteger[seq["SequenceLength"]];
  seq = StringReplacePart[seq, "", {randompos, randompos}];
  , 3,
  randompos = RandomInteger[seq["SequenceLength"]];
  seq = StringInsert[seq, RandomChoice[{"A", "T", "C", "G"}], randompos];
  ]
 ,
 {10}
 ]
parts = StringPartition[seq["SequenceString"], 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"]
Tally[Characters[seq["SequenceString"]]]