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HAZEL SIVE: Welcome back to
Getting Up to Speed in Biology.
This is lecture
three, class three,
because you're going to
participate in the class
with a number of exercises,
as you have been doing.
Today we're going to
talk about information
transfer in biology.
Information transfer
and molecular biology
are some of the most
key, crucial aspects
of modern biology, and you
need to know this material
very carefully in order
to understand anything
at a higher level in biology.
It is very cool stuff.
Understanding
information transfer
explains lots and
lots of things.
For example, why babies
look like their parents,
how you control the number
of fingers, how a bird gets
its colors, and how pathogens,
including viruses, make us ill.
This is just the
tip of the iceberg
when we think about
what information
transfer and molecular
biology can teach us.
Today I want to
cover four aspects.
We're going to talk about the
gene and some rules about DNA.
Then we're going to talk
about DNA replication, how
DNA makes more DNA.
We're going to talk about a
process called transcription,
and we're going to talk
about a process called .
These aspects of
the lecture will
draw on what you know
before, and you'll also
have an opportunity to
expand your practice
of the various concepts.
Let's get started, then,
with the notion of a gene
and some rules
associated with DNA.
We previously discussed that the
gene, the unit of hereditary,
of how things are passed on
from generation to generation,
is usually made of DNA.
If we pose the question,
what is a gene,
there is a number of ways
that we can answer it.
We can say that it is this unit
of inheritance, hereditary,
and we'll explore that
more in the next lecture
of this series.
But there is another
way that I want
to define the gene, which is
that it is a piece of DNA that
contains all the
instructions to make
the final product of the gene.
I'm going to be generic and talk
about nucleic acid and not DNA,
because genes can be RNA
sometimes, in a virus,
for example.
But let us say that a gene are
the nucleic acid instructions
in order to define a product,
and the product can be RNA
or it can be a protein.
The gene is usually
DNA, as I've mentioned,
and it is, as we discussed,
the unit of hereditary.
The notion of
information transfer
is part and parcel of
the field of biology
called molecular biology.
So molecular biology,
a term you should know,
includes biological
information transfer,
and in this notion of
information transfer,
the gene, usually
DNA, replicates.
It makes more of itself.
The gene is copied into another
related nucleic acid, RNA.
So the gene is copied into RNA,
and the process involved here
is called transcription.
And then the RNA is used in
a process called translation
to make a protein.
So the RNA is
translated into protein.
This is the
information transfer,
and it's often graphically
represented as DNA
replicating itself, with
the circular arrow leading
to production of RNA and leading
to production of protein.
Some of these arrows
can go backwards,
but these are really
the essential arrows
that you need to understand
information transfer.
We can depict this
graphically, DNA
making RNA and making protein.
So that's something about the
gene and information transfer.
Let's now talk about
some rules of DNA.
These rules of DNA are
incredibly important,
and with them you'll be able
to manipulate nucleic acid
and understand this information
transfer pretty well.
Here they go.
DNA rules.
It does.
Here they are.
We talked last time
about nucleotides.
We talked about A, G,
C, and T and U in RNA.
What I didn't tell you is that
those nucleotides can hydrogen
bond to one another,
and they do that
in a very defined,
stereotypical, always
the same way.
So the bases hydrogen bond such
that A makes two hydrogen bonds
with T, and G makes three
hydrogen bonds with C.
This is called base
pairing, and what it does
is to allow two strands
of DNA to hydrogen bond
to one another, if
the bases match,
in a way that is
called complementary.
So the base pairing
is associated
with complementary DNA strands.
For example, we
could write AATC,
and that would be
one strand of DNA,
and we would know what
the opposite strand of DNA
looked like because of
these rules of base pairing.
The other strand would
be T opposite the A, TA,
opposite the T, and G. Those
are our complementary strands.
But there's more.
Remember that five
prime and three prime
that we talked about, the five
prime phosphate, the three
prime hydroxyl?
We have to put
those somewhere in.
I told you you always
had to write them.
If we put in the five prime
and the three prime ends,
look what we see.
Here is five prime AATC, and on
the other end is three prime.
The complementary matching
strand goes the other way.
Three prime TTAG and five
prime on the other end.
This arrangement, whereby
the complementary or base
paired strands, go in
opposite directions
is called an
antiparallel arrangement,
and it is how
nucleic acids always
lie when they are base paired.
So these are called
antiparallel strands.
This rule-- these
rules of DNA really
make double stranded nucleic
acid, especially DNA,
very, very stable.
So we can think about the notion
of double stranded, or DS DNA,
as the kind of
currency of the gene.
And then the last thing
I want to remind you
in these DNA rules is something
we talked about previously.
When you add on nucleotides
to nucleic acid,
they always add to the three
prime end, to that free,
three prime .
So we will write this
over here as a reminder.
Three prime nucleotide
addition, and this
is a reminder from what
we spoke about last time.
Let's look at some
slides that have
to do with these really
important DNA rules.
Here are the base pairs.
Guanine base pairing with
cytosine, adenine base
pairing with thymine through
three or two double bonds.
This base pairing leads to
very stable double stranded DNA
structures that roll up, because
of the thermodynamics involved,
into this famous double helix
that you may have heard of.
So the double helix
is double stranded
DNA that takes on this
particular spiral,
or helical, structure
because it is
a chemically stable structure.
This is what genes are made of,
and you will see in a moment
why this is so important
for hereditary.
This reminds you--
I took it from one
of our previous slides.
This reminds you that the free
group on the last nucleotide
is the place where the
next nucleotide adds,
and the direction of
nucleic acid polymerization
goes from five prime
to three prime.
Good.
Now it's time for
you to practice
using some of these concepts
and practicing some DNA rules.
