Chromatin and DNA Packaging

what configuration must chromosomes be in to go through mitosis and meiosis?

the x-configuration

About how much DNA is in a single cell?

~6 billion base pairs

~2 meters of DNA

When is the chromosome in x-configuration?

metaphase

What is a chromatin?

DNA+ Protein (histones)

euchromatin

lightly packed chromatin rich in gene concentration and is most often under active transcription

CODING REGIONS OF DNA

heterochromatin

tightly packed chromatin consisting mainly of genetically inactive sequences

how can you tell that a part of chromosome on a karyotope is heterochromatin?

the darkest areas are the tightly packed regions

constitutive heterochromatin

VERY gene poor

cannot access DNA at ANY time

centromeres and telomeres

facultative heterochromatin

gene silencing

Barr bodies

sometimes accessible; can go back and forth between euchromatin and heterochromatin

gene silencing

controlling genes by changing their chromatic state

Folded Fiber Model

whole mounts of human white blood cells

found few or no free fiber ends and concluded chromatid must consist of a single fiber

extensive folding of Type B forms a chromatid

replication from each end toward centromere

Nucleosome Model

most commonly accepted model for DNA packing

Nucleosome=DNA wrapped around core histone

nucelosome

simplest packaging structure of all eukaryotic chromatin

localized areas of transcription; better fit for protein biosynthesis

histones

constituently expressed proteins (we make them all the time)

core histones

consists of 120 amino acids each

Highly conserved during evolution

in combination form the core particle

very basic in charge

how many proteins are in a core histone?

8 proteins total (2 in each subunit)

what does it mean when we say that core histones are very basic in charge?

the are approx. 25% Lysine and Arginine

what are the four core histones?

H2A, H2B, H3, H4

Linker histone

not part of core

consist of approx. 200 amino acids

tissue specific expression and not highly conserved

loosely associated with core particle

what does it mean that linker histones have tissue specific expression?

H1 is different in our lungs than in our hearts

how many histones are in a nucleosome?

9 total (8 core histones, and 1 linker)

10nm fiber

primary packaging of the chromatin

linear arrangement

200bp DNA- linker DNA

how much does 10nm fiber reduce DNA length by?

about 7 times

what do the tails in histones do?

when they are clamped down DNA cannot move freely but when they arent clamped down DNA can move

allow DNA to move from euchromatin to heterochromatin and vice versa

what are nucleosomes sometimes referred to as?

beads on a string

What fiber causes super coiling of DNA (Formation of the Solenoid)?

30 nm fiber

how much does the 30nm fiber reduce DNA by?

about 7 times

formation of the solenoid

H1 Histone responsible for packaging

30 nm fiber

solenoid

helical coiling of 10 nm fibers consisting of 6 nucleosomes

what is H1 histones role in supercoiling?

its responsible for packaging

How does Histone H1 work?

binds two distinct regions of DNA:

1. Linker DNA

2. Portion of the 146bp core

binds to both linker DNA and central core DNA

induces tighter DNA wrapping around histones

how much compaction does histone H1 work?

40x

formation of the Zig Zag Model

Allows for greater amount of compaction

DNA backbone not flexible enough to bend between nucleosomes→ straight linker DNA connects opposite DNA

this model doesnt have as sharp as a bend as the solenoid model

zig zag or solenoid model?

experimental results show that both solenoid and zig zag topologies may both simultaneously be present in chromatin fiber,

what fiber causes higher order coiling?

300 nm fiber

300 nm fiber

chromatin loops

built around a scaffold of topoisomerase II

compaction level of euchromatin (even though the DNA is wrapped it can still move)

what fiber causes final condensation?

700 nm

what is the 1400 nm fiber known as?

the metaphase chromosome

700nm fiber

spiral scaffold composed of topoisomerase II and about 15 non-histone proteins

compaction level of heterochromatin

when do chromosomes condense in the cell cycle?

early prophase

when does most of the making of our DNA occur?

interphase when DNA is replicated during S phase

what chromatins does G2 phase contain?

30 nm fiber and 300 nm fiber

what fiber does prophase contain?

700 nm fiber

what fiber does metaphase contain?

x-config chromosome

what fibers does g1 phase contain?

2nm - 10nm fiber

karyotype

picture of metaphase chromosomes in the nucleus of eukaryotic cells

• chromosome size

• centromere positon

• heterochromatin vs. euchromatin