A simple DNA simulator
Ruby, Rev B 171 161 bytes

Fixing the output for z=1 cost 10 bytes. It's a special case: the helix is really 3 characters wide if you looked at it at 90 degrees, but as we look at it at 0 degrees it looks only 1 character wide. zero leading spaces on z=1 no longer required

Some savings by eliminating brackets and by multiplying y.abs by 2 before truncation when calculating the number of - characters needed.

Finally, I avoided the include Math (required for sin and PI) by using complex number arithmetic with powers of the number i. The imaginary part of the complex number is equivalent to sin x, except that it repeats with period 4 instead of period 2*PI. Saving for this change was either 1 or 0 bytes.

-&gt;z,i{x=0
loop{y=z*("i".to_c**x).imag
s=(?-*(y.abs*2)).center z*2+1
s[z-y+0.5]='TGAC'[r=rand(4)]
x!=0&amp;&amp;s[z+y+0.5]='actg'[r]
puts s
sleep i
x+=2.0/z
x&gt;3.99&amp;&amp;x=0}}


Ruby, Rev A 165 bytes

This is way longer than expected. There are a few potential golfing opportunities to be explored.

include Math
-&gt;z,i{x=0
loop{y=z*sin(x)
s=('--'*(y.abs+h=0.5)).center(z*2+1)
s[z+h-y]='TGAC'[r=rand(4)]
x!=0&amp;&amp;s[z+h+y]='actg'[r]
puts s
sleep(i)
x+=PI/z
x&gt;6.28&amp;&amp;x=0}}


Commented in test program

include Math
f=-&gt;z,i{x=0
  loop{y=z*sin(x)
    s=('--'*(y.abs+h=0.5)).center(z*2+1)  #make a space-padded string of z*2+1 characters, containing enough - signs
    s[z+h-y]='TGAC'[r=rand(4)]            #insert random capital letter, saving index in r
    x!=0&amp;&amp;s[z+h+y]='actg'[r]              #insert small letter. This will normally go on top of the capital as it is done second, but supress for x=0 to make helix
    puts s
    sleep(i)
    x+=PI/z                               #increment x
    x&gt;6.28&amp;&amp;x=0                           #reset x if equal to 2*PI (this proofs against loss of floating point precision, making correct output truly infinite.)
  }
}

Z=gets.to_i
I=gets.to_f
f[Z,I]