1. Polymers have a direction
The first thing to notice is that almost all of the chains
that can be made from these beads have two ends. The right end has a
unit with a free knob; that is, a knob unattached to a hole. The left
end is a bead with a hole that isn't connected to a knob. The result
of all of this is that the chain has two distinguishable ends. You
could call them a head and a tail end -- although which is the head
and which the tail is arbitrary. Notice that since the polymer has two
different kinds of ends, it -- by definition -- has a direction. In other words, you
can now speak of a hole-to-knob direction (left to right on the
picture), and a knob-to-hole direction (from right to left).
We'll be coming back to the concept of polymer direction many
times. It's a simple, but very important idea, and it's critical that you
understand it. If you find it difficult to picture the orientation of a polymer
using beads, you might find it helpful to think of circus elephants
instead. Imagine that polymers consist of individual elephants linked
together trunk to tail in long lines. It's clear that these lines
have a direction, and that the lead elephant has a trunk
that is not grabbing onto a tail, and the trailing elephant has a
tail that is not held by a trunk. In this elephant analogy, it's much
easier to see which end of the polymer is the head and which is the
tail.
2. Most biological polymers are unbranched
The second property to notice is that the pictured chain has no
branches. That is, starting at the end with a free hole, there's only
one path from there to the end with a free knob. This then is a
linear chain, and, as we'll see, most of the biological polymers that we'll be
discussing are built this way.
Can chains with branches be constructed with the kinds of beads or elephants pictured?What kind of beads or elephants would be necessary to make a branched chain?
3. Polymers can form closed loops
Third, it may have occurred to you that the head and tail ends of
a given chain can be connected (it's what happens if the first elephant grabs the tail of the last elephant). If you do connect them, you get, as
illustrated, a closed chain -- a chain without ends. Some people call
chains like this circles, although. strictly speaking, you can make
them look like squares, rectangles or any other closed figure. As
we'll see in later sessions, some useful polymers in living things
are closed.
Does the closed chain still have a directionality in spite of the fact that there is no beginning and no end, no head and no tail? (You might think of the elephants to answer this question).
4. Not all the units in a polymer have to be the same
Fourth, not all the units used in the building of the chain have
to be identical. For example, it is possible to construct chains from
a series of different colored units, or from units with slightly
different shapes. The only requirement to form a chain is that the
beads have to have a hole at one end into which all knobs fit, and a
knob that fits all holes. So, in the elephant analogy, we might have male and female elephants, pink and blue elephants, large and small elephants, and indian and african elephants. The only requirements are that all participating elephants have a trunk and a tail, and that the trunks of all elephants are capable of grabbing the tails of all elephants, and the tails are capable of being grabbed by all the other trunks. 
Assuming a random group of elephants that could form lines by grabbing any other elephant, what would happen to the line if you had a few elephants that had trunks but didn't have tails? Or tails without trunks?
If, in fact, different kinds of elehpants are used to construct a chain, then there are many different arrangements that could be formed. For instance, let's assume that you began construction with a group of red and blue elephants. They might get together to form a line of alternating colors, with red elehants always following blue. Or they might make a chain consisting of five reds followed by five blues, or nine blues followed by 1 red.
If you were restricted to chains with eight elephants and two different colors, how many different chains could you make?How many chains of length 12 could you make if you began with four different colored elephants?
What about 10 different colors?
Chains with different varieties of elephants hint of things to come later. It turns out that many biological polymers are composed of more than one type of basic unit -- like the different colored elephants pictured above. What is more, the arrangement of the different units -- their sequence -- is critical to the functioning of the chain. In fact, it is the sequence of their polymers that makes humans different from hummingbirds; pumas distinct from pine trees; and flies unlike finches.
5. Polymers could be synthesized in many different ways -- but
biological polymers are made only one way
Fifth, you can construct chains in many different ways. Let's go back to beads again. Starting
with a single bead, you can add additional beads to, say, the hole
end. Subsequent additions could always be made to the unoccupied hole
end of the growing chain. But other modes of synthesis are equally
plausible. For example, you could add single beads to both ends, or
assemble beads in groups of two and add them two at a time to one or
both ends of a chain. Many other possibilities could be envisioned.
As we will see, the biological polymers that we will be considering are almost always assembled in only one way: by adding a single bead at a time to only one end.
6. Chains can be flexible
Finally, you'll notice that the chains that you can assemble from
the plastic beads are quite flexible. If you jumble them together they form a ball-like shape with many indentations and protrusions. This behavior too, mirrors how
some biological polymers act. We'll come back to this important point soon.
