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So let's start by talking about mRNA.

So each mRNA has the potential of having

three different reading frames in any particular region.

So what do I mean by a reading frame?

So the amino acids are encoded by 3-base so-called codons.

They form a non-overlapping code.

So you could have imagined that you read AUG here

as the start codon.

And then the next code on would not be AAA, as it actually is.

It could have been that it's UGA.

That you shifted by one base at a time

and then read the next three bases.

Can anybody think why that's not a great idea?

Yeah.

It's more constrained in terms of what amino acids

you can put there.

Right.

Each amino acid can only have probably only a few amino acids

that could follow it.

So you'd be very restricted in the types

of proteins you could make if you had that kind of code.

So instead the code doesn't shift by one,

it shifts by three.

So that means it's a completely non-overlapping code.

And each region of mRNA can have three different ways

to read that non-overlapping code.

So in this case, the RNA encodes a protein--

a short protein in this case.

In fact, generally, if people sequenced a genome

and found an ORF this big, they would say it's nonsense

and not relevant.

So many times when cells are making short proteins,

they make long proteins and process them

into shorter proteins.

You don't typically find ORFs this short.

But this is for illustrative purposes.

So you can see you start here with an AUG.

You end with a stop codon.

There are three different stop codons.

UGA, UAG, and UAA.

And so everything in between that-- as long as there's not

a stop codon-- will be part of the polypeptide that started

with this initiating AUG.

And the start codon is almost always AUG.

In bacteria, occasionally it can be GUG.

And there are a few other organisms that occasionally

use an alternate start codon.

Although the starting amino acid,

regardless of what that codon is, is always methionine.

OK?

So AUG codes from methionine.

And even in bacteria when it's GUG,

it's still methionine that it's starting with.

And what you can see here is in these two additional potential

reading frames-- first of all, neither of them

has a start codon, so there's no AUG.

And in at least the middle case, there's

actually two stop codons that are represented in there.

So in this case, while this could

be part of a larger open reading frame,

if this weren't the end of the RNA,

it doesn't have a start codon, so we can't really

call that an open reading frame.

It also doesn't have a stop codon.

So if this were the entirety of the RNA,

there would only be one possible open reading frame here.

It's the only one with a start codon.