Q:

Mark at multiple positions

I'm drawing a symbolic image of a DNA with tikz:

I did this using decorations.markings:
\documentclass[margin = 12mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}\begin{tikzpicture}
  \newcommand*{\adenin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = red]%
        \fill(0, -.1) -- (.35, -.1) -- (.45, 0) -- (.35, .1) -- (0, .1) -- cycle;%
  \end{scope}}%
  \newcommand*{\thymin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = blue]%
        \fill(0, -.1) -- (.35, -.1) -- (.25, 0) -- (.35, .1) -- (0, .1) -- cycle;
  \end{scope}}%
  \newcommand*{\guanin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = green]%
        \fill(0, -.1) -- (.35, -.1) arc(-90:90:.1) -- (.35, .1) -- (0, .1) -- cycle;% -- (0, .1) -- cycle;%
  \end{scope}}%
  \newcommand*{\cytosin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = yellow]%
        \fill(0, -.1) -- (.35, -.1) arc(270:90:.1) -- (.35, .1) -- (0, .1) -- cycle;%
  \end{scope}}%

  \draw[double distance = 1pt, line cap = rect, decoration = {
        markings, 
        mark = at position 5pt with { \adenin{(0, 0)}{270} },
        % 200 more lines with at position ...
        mark = at position 370pt with { \cytosin{(0, 0)}{270}{1.25} }
     }, preaction = {decorate}, shorten <= -3pt]
        (-.5, -1.2) .. controls (2, 1) and (4, -1) .. (6, 0)
        .. controls (9, 1.5) and (10, 0) .. (12, 0);
        % the other three paths with each 200 lines with at position...
\end{tikzpicture}\end{document}

According to the tikz manual, 31.5 markings can only be specified using either
/pgf/decoration/mark=at position <pos> with <code>
where <pos> is one number or
/pgf/decoration/mark=between positions <start pos> and <end pos> step <stepping> with <code>
this one being only for regularly repeating markings, which is not the case.
Because it looks like there is no way to specify a list of positions of markings I thought the easiest way would be to do a loop for each position, but I can't figure out how to put the loop inside the options definition.
Although it wouldn't change anything on the outcome, I'm interested in how to simplify my code. Sorry for the long example. Thanks for any help in advance

A:

I would do that slightly differently. I would store the information which protein should be at which position in a list, and instruct the markings to use this list. I added a command \somein, which takes one argument expands to \adenin{(0, 0)}{270}, \thymin{(0, 0)}{270}, \guanin{(0, 0)}{270}, or   \cytosin{(0, 0)}{270}, depending on whether the argument is 1, 2, 3, or 4. (One can also write a command that takes the name of the protein, or A, B and so on.) And then there is a list which indicates which protein should be at position 1, 2, etc.
\documentclass[margin = 12mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}\begin{tikzpicture}
  \newcommand*{\adenin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = red]%
        \fill(0, -.1) -- (.35, -.1) -- (.45, 0) -- (.35, .1) -- (0, .1) -- cycle;%
  \end{scope}}%
  \newcommand*{\thymin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = blue]%
        \fill(0, -.1) -- (.35, -.1) -- (.25, 0) -- (.35, .1) -- (0, .1) -- cycle;
  \end{scope}}%
  \newcommand*{\guanin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = green]%
        \fill(0, -.1) -- (.35, -.1) arc(-90:90:.1) -- (.35, .1) -- (0, .1) -- cycle;% -- (0, .1) -- cycle;%
  \end{scope}}%
  \newcommand*{\cytosin}[2]{\begin{scope}[shift = {#1}, rotate = #2, fill = yellow]%
        \fill(0, -.1) -- (.35, -.1) arc(270:90:.1) -- (.35, .1) -- (0, .1) -- cycle;%
  \end{scope}}%
  \newcommand*{\somein}[1]{\ifcase#1
  \or 
  \adenin{(0, 0)}{270}
  \or
  \thymin{(0, 0)}{270}
  \or
  \guanin{(0, 0)}{270}
  \or
  \cytosin{(0, 0)}{270}
  \fi
  } 
  \newcommand\myinlst{3, 1, 2, 2, 2, 2, 1, 3, 1, 4, 2, 2, 1, 1, 3, 4, 1, 
 2, 4, 4, 4, 4, 4, 4, 2, 2, 3, 1, 3, 4, 1, 4, 1, 4, 
 2, 3, 2, 4, 3, 4, 2, 3, 3, 2, 2, 3, 2, 1, 3, 1, 2, 
 3, 4, 3, 2, 4, 1, 2, 4, 4, 3, 3, 1, 1, 1, 4, 3, 4, 
 2, 2, 1, 2, 2, 2, 2, 2, 1, 4, 4, 1, 3, 4, 1, 1, 1, 
 3, 1, 2, 3, 3, 4, 2, 4, 3, 3, 3, 4, 2, 3, 2}
  \draw[double distance = 1pt, line cap = rect, decoration = {
        markings, 
        mark=between positions 5pt and 0.99 step 10pt
         with
        {\pgfmathtruncatemacro{\myin}{{\myinlst}[\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}-1]}
         \somein{\myin}},
        % 200 more lines with at position ...
     }, preaction = {decorate}, shorten <= -3pt]
        (-.5, -1.2) .. controls (2, 1) and (4, -1) .. (6, 0)
        .. controls (9, 1.5) and (10, 0) .. (12, 0);
        % the other three paths with each 200 lines with at position...
\end{tikzpicture}
\end{document}

A:

Thanks to AndréC's comment, I solved the problem using lists:
\begin{document}\begin{tikzpicture}[
        adenin/.style = {decoration = {markings,
        mark = at position #1 with { \adenin{(0, 0)}{270} }
  }}, thymin/.style = {decoration = {markings,
        mark = at position #1 with { \thymin{(0, 0)}{270} }
  }}, guanin/.style = {decoration = {markings,
        mark = at position #1 with { \guanin{(0, 0)}{270} }
  }}, cytosin/.style = {decoration = {markings,
        mark = at position #1 with { \cytosin{(0, 0)}{270} }
  }}]

for each nucleotide, and then called them:
     \draw[double distance = 1pt, line cap = rect,
        thymin/.list = {5pt, 15pt, 35pt},
        adenin/.list = {25pt, 45pt, 55pt},
        guanin/.list = {75pt, 85pt, 115pt},
        cytosin/.list = {65pt, 95pt, 105pt},
      preaction = {decorate}, shorten <= -3pt]
        (-.5, -1.2) .. controls (2, 1) and (4, -1) .. (6, 0)

However this violates the principle that markings have to be placed along the path:
In [the] case [of using multiple marks], however, it is necessary that the positions on the path are in increasing order. (p 384 of the pgf manual).
Because of this, the markings are placed very weirdly:

