"A"

"ACGT"

st := "ACGT"

len(st)

""

len("")

import "math/rand"
st = "ACGT"
string(st[rand.Intn(len(st))]) // generating a random nucleotide

string(st[rand.Intn(len(st))]) // repeated invocations might yield different nucleotides

string(st[rand.Intn(len(st))]) // repeated invocations might yield different nucleotides

string(st[rand.Intn(len(st))]) // repeated invocations might yield different nucleotides

string(st[rand.Intn(len(st))]) // repeated invocations might yield different nucleotides

// make random nucleotide string by concatenating random nucleotides
import "bytes"
var b bytes.Buffer
for i := 0; i < 40; i++ {
    b.WriteByte(st[rand.Intn(len(st))])
}
st = b.String()
st

st[1:3] // substring, starting at position 1 and extending up to but not including position 3
// note that the first position is numbered 0

st[0:3] // prefix of length 3

st[:3] // another way of getting the prefix of length 3

st[len(st)-3:len(st)] // suffix of length 3

st[-3:] // another way of getting the suffix of length 3

st1, st2 := "CAT", "ATAC"

st1

st2

st1 + st2 // concatenation of 2 strings