the

The Nuclear Envelope

okay here's where we're gonna start

going through the nucleus of the cell

and the nucleus is made up of a whole

bunch of parts okay it's not just one

sort of thing the nucleus is we call a

nuclear envelope so the nuclear envelope

itself at the core of the cell is a

double membrane that means there are

there are two phospholipid bilayers that

created now the outer phospholipid

bilayer is folded into these sac-like

compartments referred to as the ER or

also called the endoplasmic reticulum

now for now we're not going to really

talk about that but the endoplasmic

reticulum is important for part of the

job of you know of what the nucleus is

so that's again another piece of what we

want to discuss here and this is just

the beginning of this particular mini

lecture is sort of an introduction to

both the nucleus of the cell and an

introduction to sort of all the topics

that follow so the nucleus at the core

of a eukaryotic cell contains which we

know the DNA of the cell the DNA is held

within structures within the cell called

chromosomes so if we have this is a cell

and that's the cell's nucleus inside the

nucleus we have these structures that

are called chromosomes

the chromosomes are made up of DNA which

most of you all know but the chromosomes

are also made up of protein and it's

really about a 50/50 ratio in terms of

mass in terms of about how much of each

there is making up a chromosome so about

50% DNA 50% protein people originally

didn't know which was maybe more

important or which carried are passed

along genetic traits people actually

thought the proteins were those

molecules because the proteins are more

complex the proteins have at least 20

specific different amino acids that can

be put together in different orders

so like language with letters we can

have an almost infinite possibility of

combinations whereas DNA was a lot more

simplistic DNA is four four nucleotides

so essentially four letters it would be

like as if we had a language in which

every word we can speak in all meanings

were limited to those four letters in

each word can only be three letters long

so it would seem that would be very

limiting yet the way it is is that the

DNA is the code for the information the

protein is what helps organize the

chromosomes but together they make up

the chromosomes now we're gonna get into

the detail of the chromosomes and a

process called gene expression a little

bit later and so gene expression is kind

of an where toward the end of it where

the endoplasmic reticulum comes back

into place so the purpose of the DNA is

to code for the production of proteins

not directly for proteins I mean they do

but they essentially DNA codes for RNA

so DNA codes for RNA lock times people

say DNA codes for proteins and that's

sort of true but it's it's sort of

simplistic not all DNA directly codes

for a protein there are going to be

different types of RNA that we'll get

into there's transfer RNA messenger RNA

ribosomal RNA there's other types

heterogenous nuclear heterogeneous

nuclear RNA and there's old and there's

others as well and so they're not they

don't they're not protein

for proteins it's only the messenger RNA

that does so so DNA codes for RNA the

job of RNA all the different types of

RNA ultimately work together to make

polypeptides so polypeptide and protein

is sometimes used as a synonymous terms

the one for the other but the thing is

that structurally as they're built we

call them a polypeptide once they gain a

function then truly they would be a

protein sometimes you require several

polypeptides to make just one functional

protein so it can be a little more

complicated so we're using just the

proper work terms here and not kind of

overstepping or over over simplifying it

too much so DNA RNA pol pot's now when

the polypeptides are made there that's

gonna happen outside the nucleus in the

cytoplasm it's gonna have happen in

association with structures called

ribosomes which are made up of the RNA

many of those ribosomes are actually

going to attach to the surface of the ER

so along the ER the endoplasmic

reticulum you'll have the ribosomes so

essentially well the DNA is held in the

nucleus the RNA is going to be working

kind of doing what it does much of the

time attached to the surface of the

endoplasmic reticulum and that

endoplasmic reticulum is really a

continuation in a way of a nuclear

envelope of the double membrane it's the

outer membrane so the endoplasmic

reticulum is the outer membrane of the

nuclear envelope and then it just is

folded into a whole bunch of

compartments inside the endoplasmic

reticulum the polypeptides get processed

in proteins that get modified other

amino acids that aren't coded for in the

DNA could be added or or or amino acids

could be removed

there's modifications to them chemical

modifications all sorts of other things

that happen to really make the

polypeptides functional proteins and so

the ER that's its only job that's one of

its jobs so we're starting off it we're

looking at the nucleus alright the

nucleus is a structure within the

eukaryotic cells

that is made up of a double membrane now

to get out of that you know so we said

the DNA is inside the DNA never comes

out

it's the RNA that's gonna leave the

nucleus so DNA is in the nucleus RNA is

not RNA well it made there but then it

leaves the nucleus and it goes out into

the cytoplasm and then out in the

cytoplasm it may stay there or it may be

then come sort of back in a way and

connect to the endoplasmic reticulum

which is kind of part of that the

nuclear envelope all right pi peptides

are made out in the cytoplasm now when

they're done when they're made and

actually from functional proteins are

manufactured some of those proteins

could be the ones that they could come

back into the nucleus and attach to the

DNA they could stay out in the cytoplasm

and be enzymes in metabolism they could

be transported to the cell's membrane

and be actively moving molecules across

it they could be released from the cell

itself so where they end up is a totally

different kind of story that we'll go

into so nucleus nuclear membrane

chromosomes so what is a the chromosome

right the chromosome we said is DNA and

protein the DNA is we'll get into its

structure in a different lecture cut the

DNA is really as simple as these long

strings okay and the thing is if you can

imagine having a very long string say a

piece of you know love yarn or rope and

yet it's 100 feet long or maybe it's 300

feet long the size of a football field

and you need to put it in your pocket

and then you need to take it back out

again often and then maybe there's

sections of it that have information and

even on it and then you need to get

certain specific pieces at only certain

specific times not the whole thing you

don't want to take the whole thing out

but the thing is there's all kind of

problems with that just imagine things

that you actually do deal with like that

like a garden hose or an extension cord

or Christmas lights things that are not

even this long but that's fairly long

objects that end up being coiled up and

stored in some way and often when

they're taken back out of storage they

end up being tangled they end up having

knots in them

and sometimes we're kind of amazed how

that happened because you thought if you

put it away properly and it didn't so

this is what happens to the DNA if this

is the diameter of a cell cells diameter

many chromosomes are about ten times the

diameter of a cell so the individual

chromosome is really long okay now the

chromosomes remember they're packaged up

into the nucleus though into a smaller

compartment within the cell and we don't

just have a single chromosome prokaryote

to do back to bacteria but we have many

so that we have 20s humans we have 23

different chromosomes and then there we

have two of each so we have 46 and

they're all shoved in this little space

and they're all fairly large so it has

to be some really good way of organizing

them and that's where the proteins come

in so the DNA isn't just in some random

sort of long string the DNA is coiled

but if we just made it into one coil

then if we wonder to access little

segments somewhere in the middle you

have to undo the whole thing to get

there so instead the DNA is coiled like

this this is a DNA structure printer

this is structure of the chromosome

so now what we have here are these

little circles that kind of drew in here

they're called histone proteins histone

proteins organize the DNA so this would

be the DNA and the the DNA wraps around

each histone protein so there's sort of

two loops per histone protein now the

way in which these histone proteins bind

to the DNA and then organize it has a

whole number of functions which are

really going to be beyond the scope of

what we're really getting into in this

class but to give you some idea this is

an organizing function okay first off so

it helps the DNA become organized in one

way so it doesn't all get tangled up the

histone proteins can then attach to

other histone proteins histones

attaching making DNA more and more and

more compact but then if we want to get

at the DNA if your cell wants to read

some of it it can then detach some of

these histones from each other and then

unravel the DNA the thing is it can also

unravel sort of a section or a piece not

necessarily the whole thing if it wants

to get in between the histone proteins

you can see there are some regions where

there's there's nothing attached these

are usually binding sites for specific

other proteins or molecules that are

going to read the DNA so these are

binding sites for maybe things called

regulators and things called

transcription factors which we can we'll

get into in future future lecture so if

you want to know about them there will

be a future lecture to look at look at

transcription they'll talk about

transcription factors but basically if

there's a specific part of the DNA that

a protein has to attach to the thing is

it has to be available so it has to be

in between the histones all of your DNA

all of your cells have DNA all that DNA

is the same

in all of your cells so cells in your

ear or your eye or your toe or your

stomach or your heart there have the

same DNA but those cells are not the

same and they're full of different sorts

of proteins what controls what decides

which proteins are going to be made in a

particular type of cell well the

decision is somewhat made at the level

of this organization if the beginning of

a gene the beginning of the information

is available because it's between

histone proteins then that section could

potentially if the right regulators and

transcription factors were there be read

if it's not if it's stuck in a histone

protein and it's sort of locked away so

there's an example say in your your eyes

to to build an eye during embryonic

development your eye produces a lens the

lens has a protein called crystal and

the crystal and protein you have the

gene for that protein and your cells in

your skin and cells all throughout your

body

but those other cells never produce that

protein only the cells that made the

lens of your eye so in those cells in

the eye the beginning of that gene the

readable part is available and all the

other cells in your body though the

beginning of that gene is unavailable or

it's locked up

all right locked up in these histone

proteins and then typically there may

not be the regulators or transcription

factors to even look for them but if

there were it would they be sort of

locked away and not not able to be found

that's just a little bit of how that how

that happens okay so do you nee is the

main molecule carrying genetic

information it's found inside the

nucleus it's organized into these little

loops and loops themselves have names we

call the nucleus ohm all right so one

histone protein with the two DNA loops

is a nucleus ohm if DNA was cut up into

little pieces if we were to chop the

entire chromosome up into sections and

then just have say a piece of DNA that

little piece of DNA would be called by

another word called cro chromatin

so chromosome is a whole strand of DNA

end-to-end double-stranded but it's a

whole piece of DNA double stranded with

all of its proteins that's the

chromosome if it were chopped up because

we're studying it or something happened

to sound a little piece of the DNA

section would be called chromatin and

that chromatin would be made up of these

nucleosomes which are doing a double

wrapped around a histone protein and so

that's what we find in the nucleus of

the cell I mean other things too but

that's how the DNA is organized right

into the chromosomes in general I mean

there's a lot more to it but this is a

introductory level course and this is

kind of an introduction to that topic so

next thing we want to get into is the

DNA itself what is the structure of DNA

all right we're not gonna get into the

histone proteins in their structure and

all really just the DNA itself the

structure of DNA is gonna lead directly

to another topic the topic of DNA

replication or how DNA is copied to make

more chromosomes and this drawing here

so before I end this we're gonna point

something out in this little drawing so

here I drew kind of little squiggly

zigzags as the chromosomes but here they

look like something you maybe have

recognized a little more so so like this

sort of X shaped structure a lot of

times that's what you see or think of as

a chromosome when you look at the cell

but the thing is well that is sort of a

chromosome it's sort of isn't as well

and that's really because it's not just

a chromosome it's actually two but it's

two of the exact same chromosome and not

just two versions of the same chromosome

it's exact copies okay so these are

actually single chromosomes joined

together with a protein and they're

called sister chromatids

so technically that the chromosome up

the chromatin but since you when they're

joined together we just call them by a

different name okay now they chromosomes

in your cell typically don't exist in

this form almost never very rarely the

reason that though you always see

pictures if you look at a picture of a

chromosome what's it look like someone

shows you this kind of picture the

reason is because most the time the DNA

is in these long stringy structures that

are coiled up around the histone

proteins and they're kind of bound

together to organize them but that's it

they're not they're not that dense and

there would be technically invisible

under light microscope so the type of

microscope that you would have access to

in a normal sort of teaching laboratory

you would not be able to see DNA and you

would not be able to see chromosomes but

during the process of cell division the

DNA doubles you first we make up more

DNA the doubled DNA actually sticks

together and forms the sister chromatids

and condenses it's really really tightly

so it's easy to separate again if you

have this really long 100 foot long

string and you have a bunch of them and

you're trying to pull them apart it

would be hard but if they were coiled

and wrapped up like yarn balls well then

it would be really easy to separate them

even though they're very long strands

they would be coiled up or essentially

condensed into something really tight

and that's allows us to see them under a

light microscope so we see pictures of

chromosomes most often depicted in a

state that they're usually not in

they're usually only in that state right

around the time during cell division

right and that's what we're seeing so to

kind of little squiggly X's kind of eye

we draw here they represent the

chromosomes in the way that we would see

them in a laboratory when you look at

microscope slides of cells and

chromosomes and then mitosis but most

the time they would be invisible DNA

with protein all right so the next step

is we're going to get into DNA structure

we're going to start with the most

simple unit of the DNA which is going to

nucleotide and that's the next step

where we begin