The chemical structure of DNA III - Genes - the units of inheritance (5/10)

Uploaded by OUlearn on 25.07.2008

If you're going to build anything complicated,
you need some sort of plan, a blueprint, a set of instructions.
Now that's just as true of living cells as anything else.
But they contain the instructions within themselves.
They're inherited.
They're copied and passed on
from generation of cell to generation of cell.
Now what we're going to do is look at what the instructions are
and how they work.
Before we do that, just stop for a moment
and think what sorts of characteristics
any set of inherited instructions is likely to have.
I think they are probably three.
First of all, the instructions have to be stable.
They have to last long enough to be copied
and passed on to the next generation of cells.
And there's the second point.
They need to be capable of being copied
and copied accurately.
Finally, and most obviously,
as instructions they need to contain information.
Well, that's all very well but it's just speculation, theory.
What about the reality?
The reality is a molecule called DNA.
Now you might think that the structure of a molecule
that contains the instructions for making a complete cell
must itself be very complicated.
But, in fact, the basic structure of DNA
is remarkably simple.
Here's a model of DNA.
Unwind the double helix and it consists of two long strands
that form a ladder-like structure.
The strands of DNA are strings of chemically repeating units,
which act as basic building blocks.
Each unit contains a sugar - deoxyribose...
...a phosphate group and a base.
Together these form a nucleotide.
There are actually four types of base.
And guanine.
Within a strand of DNA, the bases can come in any order.
Just how important this sequence is will become clear later.
What's more, in double-stranded DNA,
the bases match up in a particular fashion.
Adenine always pairs up with thymine.
And guanine with cytosine.
This precise base pairing means that the base sequence in one strand
is complimentary to the sequence in the other.
The base pairs are held together by relatively weak hydrogen bonds.
But when summed up over the whole DNA double helix,
these hydrogen bonds impart great stability.