Lewin Benjamin (ed.) Genes IX

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are bound to the telomere via

the same Dro-

tein, Cdcl3:

The Stnl

protein protects

against

de-

gradation (specifically,

against

,any

ex-

tension

of the degradation

of the

C-A-strand

that

generates

the G-tail).

A telomerase

enzyme extends the C-

A-rich strand. Its

activity is influenced

by two

proteins

that have

ancillary

roles,

such as controlling

the

lengttL

of the

extension.

The

telomerase uses the 3'-OH

of the G+T

telomeric strand

as a

primer

for synthesir; of tan-

dem TTGGGG repeats.

Only dGTP and dTTP

are

needed for the activity. The

telomerase irs a large

ribonucleoprotein

that consists of a templating

RNA

(coded

by TLCl) and

protein

with cat-

alytic activity

(EST2)

The short RNA

,rompo-

nent

(159

bases long in Tetrahymena,

and 192

bases long in Euplotes) includes

a sequence of 1 5

to 22 bases that is identical

to two repeats of the

C-rich repeating

sequence. This RNA

F,rovides

the template for synthesizing

the G-rich

repeat-

ing

sequence.

The

protein

component of the

telomerase

a catalytic subunit

that can act

only upon the RNA template

provideclby

the

nucleic acid component.

i'ri. r :

i::i:' .:.i

;i i

shows the action of telomerase.

The enzyme

progresses

discontinuou:;ly: The

template RNA

positioned

on the DNA

primer,

several nucleotides are added

to the

prin:rer,

and

then the enzyme translocates

to begin again.

The

telomerase

a specialized example of a

reverse transcriptase, an enzyme that

synthe-

sizes

a DNA sequence

using an RNA template

(see

Section 22.4, Yiral DNA Is

Generated

Reverse Transcription). We do not know how

the complementary

(C-A-rich)

stran<i of the

telomere is assembled,

but we may spreculate

that it could be synthesized by using th,.' 3'-OH

of a terminal G-T

hairpin

as a

primer

for DNA

synthesis.

Telomerase

synthesizes the inclividual

repeats

that are added to the chromosome ends,

but does

not itself

control the number of repeats.

Other

proteins

are involved in determining the

length

the telomere. They

can be

identified

by the

ESTI

and

ESTJ mutants

yeast

that have

altered telomere

lengths.

These

proteJLns

may

bind telomerase, and influence the le ngth of

the telomere by controlling the access of telo-

merase to its substrate. Proteins that bind telo-

meres in mammalian cells have

been

found,

but less is known about their functionrs.

Each organism has a characterisic range of

telomere lengths. They are long in mammals

i:Li:li-:iii.

iii .i i Tetomerase

positions

itsetf by base

pair-

ing between the

RNA template

and the

protruding

single-

stranded

DNA

primer.

adds

G and

T bases one

ata time

to the

primer,

as directed

by the

template.

The cycle starts

again when one

repeating unjt

has been

added.

(typically

5 to l5 kb

in human

beings) and

short

yeast

(typically

-300

bp in S.

cerevisiae).

The

basic control

mechanism

that the

probability

that a telomere will

a substrate

for telo-

merase

increases as

the

length of the

telomere

shortens; we

do not

know

if this

continu-

ous effect or

if it depends

on the

length

falling

below some critical

value.

When

telomerase

acts on

a telomere,

it may

add several

repeat-

ing units.

The enzyme's

intrinsic

mode

of action

is to dissociate

after adding

one

repeat;

addition

of several repeating

units

depends

on other

pro-

teins

that cause telomerase

to undertake

than one

round of extension.

The

number of

repeats that

added

is not

influenced

by the

length of the

telomere

itself,

but instead

is con-

trolled by ancillary

proteins that associate

with

telomerase.

Binding: RNA template

pairs

with DNA

primer

DNA

primer

3'W

3',

RNA templal

Polymerization:

RNA template direc

of nucleotides to 3'end

of DNA

Polymerizationcontinues

to end of template

region

3',

Translocalio;r:

Enzyme moves

5',..TTGGGGTTGGGGTTGC

3',

.,.AACCCC 5',

MCCC

3',

28.18

Tetomeres

Are Synthesized

by a Ribonucleoprotein

Enzyme 757

The

minimum features required

for exis-

tence as a

chromosome are:

Telomeres

to ensure

survival.

A centromere to

support segregation.

An origin to initiate

replication.

All of these

elements have been

put

together

to construct

yeast

artificial chromosome

(YAC).

This is

a useful method for

perpetuating

foreign

sequences.

It turns

out that the synthetic

chro-

mosome

is stable

only

it is longer

than

50 kb.

We do not know

the basis for

this effect,

but the ability

to construct a

synthetic chromo-

some

allows us to investigate

the nature

of the

segregation

device in a controlled

environment.

Telomeres

Are Essential

for

SurvivaI

Telomerase

activity

is found in

all dividing cells

and is

generally

turned

off

terminally differ-

entiated

cells that do not

divide.

F{{;i.ffts

I*.3f

shows

that if telomerase is

mutated in a

divid-

ing

cell, the telomeres

become

gradually

shorter

with each

cell division. An

example

of the effects

of such a mutation

yeast

are shown in

lii;Lsgtt

IS.i3,

where the

telomere length

short-

ens

over

I 20

generations

from 400

bp to zero.

Loss

of telomeres

has very

bad effects. When

the

telomere length reaches

zero, it

becomes

difficult

for

the cells to

divide successfully.

Attempts

to divide typically

generate

chromo-

some

breaks and translocations.

This

causes an

increased

rate of mutation.

yeast

this is asso-

ciated

with a loss

of viability

and the culture

becomes

predominantly'occupied

by senescent

i:i$11*il

;.18.-iii

Mutation in

tetomerase

causes telomeres

shoden in

each cetl division. Eventual

loss

ofthe telom-

ere causes

chromosome

breaks and rearranqements.

CHAPTER

28 Chromosomes

f,ttwild-tvpe frtT-deficient

Divisions

40 120 40

80 120

ijil'tjEtil f

S.S.3

Tetomere

length

maintained

-350

wiLd-type

yeast,

but a

mutant

in the

frfl

gene

coding

for

the RNA component oftelomerase

rapidty

shortens its

telomeres

to zero Length. Reproduced

with

permission

from Nakamura,

T. M., et a|.1,997.

Science. 277:955-959.

1997

AAAS. Photo

courtesy of

Thomas

Cech and Toru

Nakamura, University

of Co[orado.

cells

(which

are elongated

and nondividing,

and

eventually die).

Some

cells

grow

out of the

senescing

cul-

ture. They

have acquired

the ability to

extend

their

telomeres by an alternative

to telomerase

activity. The survivors

fall into two

groups.

The

members

of one

group

have

circularized

their

chromosomes:

They now have

no telomeres,

and

as a

result

they have

become independent

of telomerase. The

other

group

uses

unequal

crossing-over

to extend

their telomeres

(ft**ftf

f *-34). The telomere

is a repeating

struc-

ture, so it is

possible

for two telomeres

to mis-

align when chromosomes

pair.

Recombination

between the mispaired

regions

generates

unequal crossing-over,

as shown

previously

Figure 6.1: When

the length

of one recombi-

nant chromosome

increases.

the length

of the

other decreases.

Cells usually

suppress unequal

crossing-

over because

its

potentially

deleterious

con-

sequences. Two

systems

are responsible

for

suppressing

crossing-over

between telomeres.

One is

provided

by telomere-binding proteins.

yeast,

the frequency

of recombination

between telomeres is

increased

by deletion

the

gene

tazl,

w};'ich codes for

protein

that

Telomere

752

regulates

telomerase activity. The

second is a

general

system that

undertakes mismatch repair.

addition to correcting mismatched

base

pairs

that may arise in DNA,

this system suppresses

recombination

between mispaired regions. As

shown in Figure 28.34, this includes

telomeres.

When

it is mutated,

greater

proportion

telomerase-deficient

yeast

survive the loss of

telomeres because recombination

between

telomeres

generates

some chromosomes with

longer telomeres.

When eukaryotic cells

are

placed

in cul-

ture, they usually divide for a fixed number

generations

and then

enter senescence. The rea-

son appears to be a decline in telomere length

because of the absence of telomerase

expres-

sion. Cells enter a crisis from

which some

emerge, but typically the cells that emerge have

chromosome

rearrangements

that have resulted

from

lack

protection

of chromosome ends.

These rearrangements may

cause

mutations

that contribute to the

tumorigenic state. The

absence of telomerase expression in this situa-

tion is due to failure to express

the

gene,

and

reactivation of telomerase is one of the mech-

anisms by

which these

cells then survive con-

tinued culture.

(This

of course was not an option

in the

yeast

experiments in which

the

gene

had

been deleted.)

Sum

mary

The

genetic

material

of all organisms and viruses

takes the form of tightly

packaged

nucleopro-

tein. Some virus

genomes

are inserted into

pre-

formed

virions, whereas

others assemble

protein

coat around the

nucleic

acid.

The

bac-

terial

genome

forms

a dense nucleoid, with

-20o/o

protein

by mass, but details of the inter-

action of

the

proteins

with DNA

are

not known.

The DNA is organized into

-100

domains that

maintain

independent

supercoiling, with a den-

sity of unrestrained supercoils corresponding

-llI00

to 200 bp. In eukaryotes, interphase

chromatin

and metaphase chromosomes both

appear to be organized

into large loops. Each

loop may be an independently supercoiled

domain.

The

bases of the

loops

are connected

to a metaphase scaffold or to the nuclear

matrix

by specific DNA sites.

Transcriptionally active sequences reside

within the euchromatin that comprises the

majority of interphase chromatin. The regions

of heterochromatin are

packaged

-5

to l0x more

compactly,

and are transcriptionally inert. AII

{- it-'

ii 11 .,i

:.;.r.

-'-l

Crossi

g-ove

r i n

telo

meric

regio ns is usu-

a[[y suppressed

by mismatch-repair

systems, but

can occur

when

they are

mutated.

An unequal

crossing-over

event

extends

the

tetomere of

one of the

products.

a[[owing

the

chromosome to survive

in the absence

telomerase.

chromatin becomes

densely

packaged

during

cell division, when

the individual

chromosomes

can be distinguished.

The existence

a repro-

ducible ultrastructure

in chromosomes

is indi-

cated by the

production

G-bands

by treatment

with Giemsa

stain.

The bands

are very

large

regions

(-10?

bp)

that can

be used

to map chro-

mosomal translocations

or other

large

changes

in structure.

Lampbrush chromosomes

amphibians

and

polytene

chromosomes

insects

have

unusually extended

structures,

with

packing

ratios < I 00. Polytene

chromosome

s of

D. melano

gaster

are divided

into

-5000

bands.

These bands

vary in size by an

order

of magnitude,

with

average

-25

kb. Transcriptionally

active

regions can be

visualized

in even

unfolded

("puffed")

structures,

in which

material

extruded from

the axis

of the

chromosome.

This

may resemble the

changes

that

occur

on a

smaller

scale when

a sequence

in euchromatin

is transcribed.

The centromeric

region

contains

the

kine-

tochore,

which

is responsible

for attaching

chromosome

to the

mitotic

spindle.

The cen-

tromere often

is surrounded

by heterochro-

matin. Centromeric

sequences

have

been

identified only

yeast

cerevisiae,

where

they

consist of short

conserved

elements.

These

ele-

lnCNTS, CDE-I ANd

CDE-III,

biNd

CBFI

ANd thc

CBF3 complex,

respectively,

and

a long

A-T-

rich region called

CDE-II

binds

Cse4

form a

specialized

structure

in chromatin.

Another

group

proteins that

binds

to this

assembly

provides

the

connection

to microtubules.

Telomeres

make

the ends

of chromosomes

stable.

Almost all

known

telomeres

consist

multiple repeats

in which

one strand

has

the

Crossing-over occurs

when

\,/

mismatch repair is absent

28.20

Summarv

753

general

sequence

C"(A,1T)*, where n>l

and

| to 4. The

other strand

Gn(T lA\*, has a

single

protruding

end that

provides

a template

for

addition

of individual

bases in defined

order. The

enzyme telomerase

is a ribonucle-

oprotein

whose RNA

component

provides

the

template

for synthesizing

the

G-rich strand.

This

overcomes

the

problem

of the inability

to replicate

at the

very end

of a duplex. The

telomere

stabilizes the

chromosome

end

because

the

overhanging single

strand

G"(TlA)m

displaces

its homolog

in earlier

repeating

units

in the telomere

to form a loop,

so there

are no free ends.

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16.

756 CHAPTER

Chromosomes

Nucleosome

CHAPTER OUTLINE

Introduction

The Nucleosome Is

the Subunit of

A[[

Chromatin

MicrococcaI nuclease

releases individuaI nucleosomes

from

chromatin as

115

oartictes.

nucteosome

contains

-200

bp of DNA, two copies of each

core

histone

(H2A,

HZB, H3, and H4).

DNA is wrapped around

the outside

surface of the

protein

0cramer.

DNA Is Coiled in Arrays

Nucleosomes

>95%

of the

DNA is recovered in nucleosomes

multimers

when micrococcal nuclease

cleaves

DNA

of chromatin.

The

length of DNA

per

nucteosome varies for individuat tis-

sues in a range from 154 to 260 bp.

Nucleosomes Have

a Common Structure

NucteosomaI

DNA is divided into the core

DNA

and [inker

DNA

depending on

its

susceptibitity to

micrococca[

nuctease.

The

core

DNA is

the length of 146 bp that

is found

the

core

particles produced

prolonged

digestion with

micro-

coccaI nuctease.

Linker DNA is

the

region

of 8 to

114

bp that

susceptibte

to early cleavage

by the enzyme.

Changes

the length of [inker

DNA

account

for the varia-

tion

in total

length of

nucleosomal DNA.

associated with linker DNA and

may

lie at the

point

where DNA enters and [eaves the nucleosome.

DNA Structure Varies

the Nucleosomal Surface

DNA

is wrapped

1.65

times around the

histone octamer.

The structure of the DNA is altered so that

it has

increased

number of base

pairs/turn

the

middl.e, but a

decreased

number

at the ends.

The Periodicity

DNA

Changes on

the Nucleosome

-0.6

negative turns of DNA are absorbed by the

change in

bp/turn from 10.5 in solution to an average of

10.2 on the

nucleosomaI

surface.

which

explains

the [inking-number

parao0x.

Organization

of the Histone 0ctamer

The

histone

octamer

has

kernel of an H3z-H4z tetramer

associated

with

two

H2A-H2B dimers.

Each

histone is

extensively

interdigitated with

its

partner.

A[[ core

histones have

the structural

motif of the

histone

fold. N-terminaI taits extend out of the

nucteosome.

The

Path of

Nucleosomes

in the Chromatin

Fiber

L0 nm chromat'in

fibers are

unfolded

from 30

nm fibers and

consjst

of a

string of

nucleosomes.

nm fibers

have s'ix

nucleosomes/turn,

which are

orga-

nized into a

sotenoid.

Histone H1

is required

for formation ofthe

nm fiber.

Reproduction

Chromatin

Requires

Assemb[y

Nucteosomes

Histone octamers

are

not conserved

during

replication, but

H2A-H2B

dimers and

H32-H42

tetramers are

conserved'

There are different

pathways for the assembty

of nucteo-

somes

during

replication

and

independentty

of reptication.

Accessory

proteins

are

required to assist

the assembly

nucte050mes.

CAF-1

is an assembty

protein

that

is Linked

to the PCNA sub-

unit

of the

rept'isome;

it is required

for deposition

z-H4z

tetramers

foltowi

ng reptication.

A different assemb[y

protein

and

a variant

of histone

may be used

for

reptication-independent

assembty.

Do Nucleosomes

Lie

Specific

Positions?

Nucteosomes

may form at specific

positions

as the

resutt

either

of the [oca[

structure

of DNA

or of

proteins

that

inter-

act

with specific

sequences.

The

most common

cause

of nucteosome

positioning

is when

proteins

binding

DNA establish

a boundary.

Positioning

may affect

which regions

of DNA are

in the

finker

and

which

face of DNA

is exposed

on the

nucleosome

rfa ce.

Are Transcribed

Genes

0rganized

in Nucleosomes?

Nucleosomes

are found

at the same

frequency

when

tran-

scribed

genes

nontranscribed

genes

are digested

with

micrococcaI

nuclease.

Some

heavily

transcribed

genes

appear

to be except'ionaI

cases that

are devoid

of nucteosomes.

Histone

0ctamers

Are

Disptaced

Transcription

RNA

potymerase disptaces

histone

octamers

during tran-

scription

in a

model system.

but

octamers

reassociate

with

DNA as

soon as the

potymerase has

passed.

Nucleosomes

are

reorganized

when

transcription

passes

through

gene.

Continued on

page

757

?DM

Nucteosome Displacement

and

Reassembly Require

SpeciaI Factors

Ancitlary

factors

are

required

both

for

RNA

potymerase

to disptace

octamers during

transcription

and for the histones

reassemb[e

into

nucleosomes

after

transcri

ption.

Insulators

Btock

the Actions

Enhancers

and Heterochromatin

Insutators

Can

Vary in

Strength

Insulators

can differ

how

effectively

they bl"ock

passage

of an activating

signat.

DNAase

Hypersensitive

Sites Reftect

Changes in

Chromatin Structure

Hypersensitive

sites are found

at the

pro-

moters

of expressed

genes.

They

are

generated

by the binding

of tran-

scription factors

that displace histone

octamers.

Domains Define

Regions That

Contain

Active

Genes

A domain

containing a transcribed

gene

defined by increased

sensitivity to

degra-

dation by DNAase I.

An LCR May

ControI

Domain

LCR is located

at the 5'end

ofthe domain

and consists of several hypersensitive

sites.

What

Constitutes

a Regutatory Domain?

A domain

may have an insulator.

an LCR.

matrix

attachment site.

and transcription

unit(s).

Summarv

?EfB

JEETI

Insutators

are

able to btock

passage

of any

activating

or inactivating

effects from

enhancers,

silencers,

and LCRs.

Insulators

may

provide

barriers

against the

spread of heterochromatin.

Insulators

Can

Define

Domain

Insulators

are speciatized

chromatin

struc-

tures

that

have

hypersensitive

sites. Two

insulators

can

protect

the

region

between

them from a[[

external effects.

Insutators

May Act in

One Direction

Some insutators

have

directionatity.

and

may

stop

passage

of effects in

one direc-

tion but not

the other.

Introduction

Chromatin

has a compact

organization

in which

most DNA

sequences

are

structurally inacces-

sible

and functionally

inactive.

Within

this mass

is the

minority

of active

sequences.

What is the

general

structure

of chromatin,

and what is

the difference

between active

and inactive

sequences?

The

high overall

packing

ratio

the

genetic

material

immediately

suggests

that

DNA

cannot

be directly

packaged

into

the final

structure

of chromatin. There

must

be hierarchies

of organization.

The

fundamental

subunit

of chromatin

has the

sqme type

of design in

all eukaryofes.

The nucleo-

some

contains

-200

DNA,

organized

octamer

of small,

basic

proteins

into

a bead-

structure.

The

protein

components

are his-

tones.

They

form an interior

core; the DNA

lies

on the

surface

of the

particle.

Nucleosomes

are

an invariant

component

of euchromatin

and

heterochromatin

the interphase

nucleus

and

of mitotic

chromosomes.

The

nucleosome pro-

vides

the first

level

of organization, giving

packing

ratio

-6.

Its

components

and

struc-

ture are

well

characterized.

The

second

level

of organization

is the

coil-

ing

of the series

of nucleosomes

into a helical

il:l1JffiTi"[1':,',f

:i:ffi,tlil.'"'ff

mitotic

chromosomes

lsee

Figure

28.11).In

chromatin

this brings

the

packing

ratio

of DNA

CHAPTER

29 Nucteosomes

-40.

The

structure

of this fiber requires

addi-

tional

proteins,

but is not

well defined.

The

final

packing

ratio

is determined

by the

third level of

organization, the

packaging

of the

30 nm fiber itself.

This

gives

an overall

packing

ratio of

-I000

in euchromatin,

cyclically

inter-

changeable

with

packing

into

mitotic

chromo-

somes to

achieve an overall

ratio

-10,000.

Heterochromatin generally

has a

packing

ratio

-10,000

both interphase

and

mitosis.

We need

work through

these levels

organization to

characterize

the events

involved

in cyclical

packaging,

replication,

and transcrip-

tion. We

assume that

association

with additional

proteins.

or modifications

of existing

chromo-

somal

proteins,

are involved

changing

the

structure

of chromatin.

We do

not know

the

individual

targets for

controlling

cyclical

pack-

aging. Both

replication

and transcription

require

unwinding

DNA,

and thus

must involve

unfolding

of the

structure

that allows

the rele-

vant

enzymes

to manipulate

the DNA.

This

likely

to involve

changes

all levels

organization.

When

chromatin is

replicated,

the

nucleo-

somes must

be reproduced

on both

daughter

duplex

molecules.

In addition

asking

how

the nucleosome

itself

is assembled,

we must

inquire

what happens

other

proteins

pres-

ent in

chromatin. Replication

disrupts

the struc-

ture

of chromatin,

which indicates

that it

both

poses

problem

for

maintaining

regions

with

758

specific structure and

offers an opportunity to

change the structure.

The mass

chromatin

contains up to twice

as much

protein

as DNA. Approximately half

of the

protein

mass

is accounted for by the nucle-

osomes. The mass of RNA is < l0%

the mass

of DNA. Much of the RNA consists

nascent

transcripts still associated with the template

DNA.

The nonhistones include all

the

proteins

of chromatin except the histones. They are more

variable between tissues and

species,

and they

comprise

a smaller

proportion

of the

mass

than

the histones. They also comprise a much larger

number of

proteins,

so that any individual

pro-

tein is

present

amounts much smaller than

any

histone.

The functions

nonhistone

proteins

include control of

gene

expression and

higher-

order structure. Thus RNA

polymerase

may be

considered

to be a

prominent

nonhistone. The

HMG

(high-mobility group) proteins

comprise

a discrete and well-defined subclass of

nonhis-

tones

(at

least some of which are transcription

factors). A major

problem

working

with other

nonhistones

that they tend to be contami-

nated with other nuclear

proteins,

and so far it

has

proved

difficult to obtain those nonhistone

proteins

responsible for higher-order structures.

The Nucleosome Is

the

Subunit of

Atl.

Chromatin

MicrococcaI nuclease releases individual.

nucteosomes from chromatin as 11S

particles.

nucteosome

contains

-200

bp of

DNA, two

copies of each core

histone

(H24,

H2B. H3.

and H4).

DNA is wrapped around the outside surface of the

protein

octamer.

When interphase

nuclei are

suspended

in a solu-

tion of

low ionic strength, they swell and

rup-

ture to release

fibers

of chromatin.

$'i{;i"J{":*:

:li;.1,

shows

a lysed nucleus in which

fibers are

streaming

out. In some regions, the

fibers con-

sist of tightly

packed

material, but in

regions

that have become stretched,

they can be seen

to consist

of discrete

particles.

These

are the

nucleosomes.

In especially extended regions,

individual nucleosomes are connected

by a fine

thread, which

is a free duplex of DNA. A contin-

uous duplex thread of

DNA runs

through

the series

particles.

i:iri

iii'lL

iill.

Chromatin

spitting

out of tysed

nuctei consists

of a compactly organized

series

particles.

The bar

100 nm. Reproduced

fron Cell,

vot.

4, Oudet.

P., etal.,

ELec-

tron microscopic

. . .

pp.

281-300.

1975,

wlth

permission

from Elsevier.

Photo courtesy

Pierre Chambon.

Fiili,jiii:

j':;r,.,

IndividuaI

nuc[eosomes

are

reteased by

digestion of chromatin

wjth micrococcal

nuclease.

The bar

100 nm. Reproduced

from CelL,

vo[. 4,

Oudet.

P., et at.,

Electron

microscopic .

. .

pp.

281-300.

1975,

with

permission from Etsevier.

Photo courtesy

Pierre

Chambon.

Individual

nucleosomes

can

be obtained

treating

chromatin

with

the endonuclease

micrococcal

nuclease,

which

cuts

the

DNA

thread at

the

junction

between

nucleosomes.

First it releases

groups

particles, and

then,

releases single

nucleosomes.

Individual

nucle-

osomes

can be seen

i,l{,i.iSil

;1F"f

as compact

particles. They sediment

-I

lS.

The nucleosome

contains

-200

bp of

DNA asso-

ciated with

a histone

octamer

that

consists

of two

copies

each of H2A,

H2B,

H3, and

H4.

These

are

known

as the

core

histones.

Their

association

29.2 Ihe

Nucteosome

the

Subunit

Att Chromatin

759

200

bp DNA= 130 kD

Length

67 nm

H2A

2=z8kD

@@Hzex2=28kD

(lfi)ttsx2

=3okD

@ @

=22kD

Total

protein =

108

6 nm

H1=z4kD

-___-------____-

11 nm

iliuRI

tF.3 The

nucteosome

consists

of approximately

equaI

masses

of DNA and histones (inctuding

H1). The

predicted

mass

of the nucleosome

is 262

kD.

DNA "leaves"

DNA

"enters"

FiStiRg

*9.4 The nucteosome

may

be a

cylinder with DNA

organized

into

two turns

around

the surface.

illustrated

diagrammatically

in f.gGiJRf

*S"i. This

model

explains

the

stoichiometry

of the core

histones

chromatirL;

H2A,

H2B,

H3, and H4

are

present

equimolar

amounts,

with

two

molecules

of each

per

-200

DNA.

Histones

and H4

are among

the most

conserved proteins

known.

This

suggests

that

their functions

are identical

all eukaryotes.

The types

of H2A and

H2B can

be recognized

all eukaryotes,

but

show appreciable

species-

specific

variation

sequence.

Histone

comprises

set of closely

proteins

that

show appreciable

variation

between

tissues

and

between

species.

The role

of Hl

is different

from

that

of the

core histones.

It is

present

half

the amount

of a core his-

tone

and can

be extracted