Chapter 10 Flashcards
is the complex of DNA and DNA-binding proteins such as histones.
chromatin
Histones are by far the most abundant proteins in chromatin and are organized in repetitive units, known as
composed mainly of a dual tetramer
nucleosomes
Is formed by each of the core histones (H2A, H2B, H3 and H4)
tetramer
The DNA connecting two consecutive nucleosomes is called
linker DNA
formed with 2 copies of all the major histones
can only be formed in the presence of DNA
octamer
a structural motif composed of three
alpha helices interconnected by two loops
Histones fold motif
head to tail between H3 and H4 or H2A and H2B is at the base of the tight DNA binding to the
histones
dimer of histone folds
forms a V-shaped structure including three DNA-binding sites.
each dimer
placed in the center of the nucleosome
H1 and linker DNA
are arranged with helical pitch of 11 nm, the size of a nucleosome
nucleosome units
nucleosome arranged in a spiral shape, with the flat sides of disks are facing each other; linker bent inside the spiral
solenoid model
nucleosomes from
two DNA portions twist around a central axis.
suggested by electron microscopy observation
zigzag model
Another empirical evidence supporting the
zigzag model is the crystal structure of the
tetranucleosome
Similarly, histone-depleted chromosomes* are composed of a
chromosomal scaffold
A protein complex with ATPase activity that translocates (i.e., slides) nucleosomes along the DNA.
chromatin remodeling complex
A class of enzymes
that covalently modify the N-terminal tails of
histones
histone modifying enzymes
The inheritance of
characteristics acquired by means that do not
involve the nucleotide sequence of the parental
chromosomes
epigenetic inheritance
is the position of nucleosomes in the genome fully random?
no
They all hydrolyze ATP to break some bonds between DNA and nucleosome so that the latter can be either
ejected or repositioned
Chromatin remodeling complexes
plays
a major role in altering chromatin structure.
Modification of the N-terminal tails of histones (H3 variants)
add an acetyl
group to lysine residues
Histone acetyltransferases
recognition of
acetylated residues.
Bromodomains
recognition of
methylated residues.
Chromodomain
a hypothetical code in which successive covalent modifications of histone tails and DNA trigger chromatin remodeling and transcriptional activation
events.
histone code
1st PTM to 2nd PTM to function (activation or repression of transcription)
PTM cross-talk.
there will be 4 nucleosome types
- Parental H3-H4 and parental H2A-H2B
- Parental H3-H4 and new H2A-H2B
- new H3-H4 and parental H2A-H2B
- New H3-H4 and new H2A-H2B
can recognize specific
PTMs and then act on nearby nucleosomes, expanding a given PTM pattern
Histone modifying complexes
are used to recruit specific histone
modifying complexes.
Epigenetic marks
can help maintain a closed state–repressing gene expression
HP1 complex
