Histones and DNA packaging Flashcards
What size is one complete turn of DNA?
10 bases
3.4nm long
How much DNA is there in a human diploid cell?
~2m
What is the composition of chromatin?
DNA
RNA
Proteins
What is chromatin?
What chromosomes are made of
What are histones?
Proteins found in chromatin
They are the most common nuclear protein
Account for almost half of nuclear protein
Describe the basic features of histones
Small molecular weight
Positively charged
Name the core histones
H2A
H2B
H3
H4
Name the linker histone
H1
What is the width of double stranded DNA?
2nm
What is the basic unit of chromatin?
A nucleosome
How much DNA is in one nucleosome?
200bp
How many turns of DNA make one nucleosome?
1.8 (2)
List the components of a nucleosome
200bp of DNA 2x H2A 2x H2B 2x H3 2x H4 H1
How do H3 and H4 interact?
They form a heterodimer called the histone handshake motif
What is formed from the H3/H4 heterodimer?
A tetramer that is horse shoe like in structure
How do H2A and H2B interact?
They form a dimer
Where does the H2A/H2B dimer go?
One goes below and one goes above the H3/H4 tetramer
What would happen if a histone was altered?
The DNA packaging around the octamer would be altered
Name four variations of H2A
H2A.Z
macroH2A
H2A.Bbd
H2A.X
Compare H2A and H2A.Z
H2A.Z has 60% identity to H2A
It alters the stability between H2A and H2B
Alters interaction between the dimer and tetramer
Alters the canonical nucleosome
Associated with transcriptionally active chromatin
What is the role of histone variants?
Differentiates chromatin at centromeres, active genes and heterochromatin
What does H3.3 do?
It marks actively transcribed loci by replication independent nucleosome activity
What is level two of chromatin structure?
The 10nm fibre
“beads on a string”
Packing ratio: 6-7
What is level three of chromatin structure?
The 30nm solenoid
Further coiling of the 10nm fibre
Packing ratio: ~40
What is H1s role in the solenoid?
It forms the centre of the rings of the 10nm fibre
How many nucleosomes make a 30nm solenoid?
6
What is the fourth level of chromatin structure?
Loops of 60-100kb of DNA tethered by non histone scaffold proteins
Packing ratio: 680
What is level five of chromatin structure?
700nM fibre
Coiled coil
Condensed fibre
Packing ratio: 10^4
What is level six of chromatin structure?
A metaphase chromosome
What is the packing ratio?
How much smaller the folded version is than the extended length of DNA
Describe the metaphase chromosome
Telomeres at each end
A centromere
Sections of heterochromatin and euchromatin
Where is the centromere?
At the primary constriction
What is the purpose of the centromere?
Site that spindles attach to
Chromosome segregation occurs here
What are the two types of chromatin?
Heterochromatin
Euchromatin
Describe heterochromatin
Highly condensed
Transcriptionally inactive
Repetitive DNA sequences
What is constitutive heterochromatin?
All cells of a given species package the same regions of DNA into constitutive heterochromatin
Genes contained within constitutive heterochromatin will be poorly expressed
What is facultative heterochromatin?
DNA packaged into facultative heterochromatin is not consistent throughout the cells of a species
Facultative heterochromatin is regulated and associated with morphogenesis and differentiation
In different cells it may be packaged in euchromatin
When is heterochromatin replicated?
Late in the S phase
Describe euchromatin
More open configuration
Replicated early in S phase
Contains both transcriptionally active and inactive genes
