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so rna has many functions and some of
these functions include the following
obviously we've discussed in other
presentations the synthesis of proteins
and this includes the various most
common types of rna the transfer rna
the ribosomal rna and the messenger rna
rna as i noted is the genetic material
for certain rna-containing viruses
rna has been found to catalyze reactions
in some cases and this type of rna has
the name of a ribozyme
rna is also more recently described as
being able to control
gene expression and these little tiny
rnas called
mirna and sirna participate
in a process we describe as rna
interference
and last rnas participate in the
processing of yet even other rnas
and these are called small nuclear rnas
or snow rnas as we shall see
now rna of course uh has a structure
made
that comes from the nucleotides
comprising it and the nucleotides
are ribonucleotides a u c and g
the the rna strands are a little bit
different from dna strands in that rnas
are usually present in a single stranded
form
although some viruses have a
double-stranded nature
the single-stranded form of rna can pair
with itself
in many cases and one of the reasons
this is more commonly happening with rna
than it is with for example a single
strand of dna is that in rna
a base pair between g and u is
somewhat stable a gt base pair in dna
is not stable watson and crick were
credited with discovering the b
form of dna which you can see on the
right side of this slide
rosalind franklin whose data they
borrowed discovered the a form of dna
that you see on the left
now the reason i'm showing you these two
slides is that rna
also has some specific configurations
that it makes
the a form of dna and the b form of dna
though they look very similar
are not the same the b form of dna you
see in the base is aligned in what looks
like a stair-step fashion
on the right but in the a form those
bases are not flat
now it turns out that the a form of dna
is also the form that
dna rna duplexes form and it's also the
form that rna rna duplexes form so for
our purposes
the a form will be the more relevant
structure to be thinking about
with respect to the structure of rna
when it's in a duplex
now i want to illustrate one of the
common types of
self-pairing things that rna molecules
can do
and this depends on the sequences of
course so we see on the the screen a
sequence of a single strand of rna
now the base bases in this rna actually
are able to
form complementary pairs with each other
and you can see this in the red
nucleotides that i've marked here what
happens with this
rna is that these nucleotides can find
each other very readily
and when they do they form what's called
a stem loop structure
this stem loop structure arises from the
fact that there is
an inverted repeat of those nucleotides
and that inverted repeat of those
nucleotides allows for a pairing
structure such as we
see we call this structure a stem loop
and it's sometimes also called a hair
pin
a stem loop has obviously a loop at the
top
and a stem at the bottom
now we see this self-pairing nature of
rna
in many different forms of rna here is
actually a ribozyme
a catalytic rna and you can see that it
has extensive sets of structures
or base pairings within itself these
types of base pairings are referred to
as secondary structures
so the stem loops are examples that we
can see very simply in this figure
and we can also see that there are stems
that have mismatches within there
so it's not like everything is perfectly
paired and not like everything
has to be perfectly paired in order to
give this structure
there are also unpaired regions within
the molecule as you can see here
and last there are stems that have
bulges and bulges arise where there are
portions that pair
and then inserts as you can see on the
bottom
sequence that don't fit into the overall
structure
now we've seen before in another
presentation that the various
ribosomal rnas have secondary structure
as well
and these of course arise from the self
pairings that you see here
it's thought that these secondary
structures of these small ribosomal rnas
are important for the binding of
proteins in the ribosome and to give
those
proteins and the structure that the
ribosome ultimately has
now we've also seen in another
presentation the structure of transfer
rnas
and the transfer rnas have this internal
base pairing
sequence that we've seen here and last
we've also seen the larger
ribosomal rnas whose sequence is so
large we can't show you
the individual nucleotides very clearly
but in the figure on the right you can
get an idea of all the stems and stem
loops and mismatches and so forth
that are appearing in that molecule
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you