All language subtitles for 47031x_MH_Overview_04_Central_Dogma_v2-en

MICHAEL HEMANN: We have a pretty diverse group of people.

So we're just going to start with a little bit

of a refresher on some basics before we

get into the genetics in subsequent lectures.

So essentially the central dogma of biology

is that we have DNA that goes to RNA that goes to protein.

This is just the basic tenets of modern molecular biology.

So DNA is copied in a process called DNA replication.

DNA replication is essentially the polymerization

of new DNA molecules templated by existing molecules.

So if we just look at a short stretch of DNA here,

a four-base sequence, We have a 5 prime phosphate,

we have a 3 prime hydroxyl, we have two copies of this DNA.

So we have the complementary strand here.

And this is replicated into two daughter molecules of DNA

based on a process called semi-conservative replication,

which means that each of these strands

actually templates the addition of nucleotides

to the other strand.

So we have these two molecules that come apart,

and you have the synthesis of new strands

from 5 prime to 3 prime in both of the daughter strands.

So from the big-picture perspective,

we have a long sequence of DNA.

And we have the synthesis from 5 prime to 3 prime

on both strands.

One strand, you basically have continual synthesis

from lots of origins or replication,

and the other you have Okazaki fragments.

So again, a semi-conservative strategy

that leads to the production of two progeny molecules of DNA,

half of which are the old DNA and half of which

are newly synthesized from nucleotides that

are sort of swimming around.

So the second process is this process of DNA to RNA.

So transcription occurs off of a DNA template.

where we have the binding of an RNA polymerase

to a transcriptional start site, and you

have the new synthesis of an RNA that is essentially

a copy of this top strand--

this synth strand-- from a migrating transcriptional

bubble.

So this is early in transcription, and this is late

in transcription.

where we have the production of a long RNA molecule that,

again, is templated off this lower strand.

It's identical to the top strand, of course.

It's RNA and not DNA.

So it's a different molecule--

less stable molecule-- that essentially is

the active copy of our genome.

So it's actively producing information

that's used to encode proteins.

And so that synthesis of proteins--

so going from RNA to protein is called translation.

So not all RNAs are translated.

So you have lots of classes of RNAs

like tRNAs and ribosomal RNAs that aren't translated.

You have small RNAs that serve to regulate transcription,

like microRNAs.

But all proteins are encoded through the translation

of mRNAs.

And so you have a ribosome, you have an mRNA,

and then you have addition of amino acids

to a growing polypeptide through the introduction

of tRNA-loaded individual amino acids.

So the step wide addition of amino acids

to a polypeptide chain leads to the production

of a large polypeptide.

This large polypeptide is folded into a diverse class

of proteins in the cell--

proteins including enzymes and receptors

and structural proteins and hormones and lots

of different classes of proteins,

essentially, all made during this process.

And so again, the central dogma of molecular biology

and genetics is that you have a gene which

is a double stranded piece of DNA that yields an RNA that

is a smaller sequence from a transcriptional start site down

to a polyadenylation site.

And this encodes for the translation of a protein

from the start codon like AUG--

and there are some degenerate start codons--

but mostly AUG.

So this is the basic principle.

This is the gene as a structural entity.

So a typical gene in our cells--

average gene is around one to two kilobases,

so 1,000 to 2,000 bases.

That corresponds with a triplet code to 300 to 600,

or 333 to 666 amino acids.

There are genes that are much larger.

So dystrophin, for example, is a gene

that is two million base pairs.

This is the gene that is altered in the condition Duchenne's

muscular dystrophy.

It's mostly introns, which we'll talk about.

But in total physical length, it is two million bases.

Genes can be very small.

They can be 30 or 40 amino acids.

And sometimes it's actually very difficult actually to predict

whether something is a protein-coding gene or not--

and we'll talk about that--

how do we know what a gene is, how do we assess

whether it's protein coding.

When we say that we actually have

a certain number of genes in the genome,

how and why are we saying that?

How can we possibly predict that?

E. coli has around 4,000 genes.

We are at the other end of the spectrum.

So humans have 20,000 to 25,000 genes.

Actually, this number has evolved over time.

Initially, but prior to the human genome sequence,

it was thought that there were about 100,000 genes,

but we're simpler than we thought.

But there are much more simple organisms.

For example, an organism with the perhaps unfortunate name

Mycoplasma genitalium has the fewest genes of any organism.

It's 467 genes, so that's basically the bare bones.

That's what you need to get by in the world, around 467 genes.

Anybody know what organism has the most genes?

Any guesses?

So plants have the biggest genomes

in terms of overall sequence length.

There is a kind of flea that used to be the big winner.

So there was a flea that had 31,000.

But right now, it's the octopus, 33,000 genes.

They have all these weird N-cadherins

that are involved in their distributive neural network.

It's a really cool kind of thing.

But they have more than we do.