DNA and Chromosome Structure Flashcards
(31 cards)
Describe the structure of nucleosomes.
Nucleosome is a repeating unit of DNA wrapped around an octamer of histone proteins.
8 histone proteins compose the histone core (histone octamer).
H2A and H2B associate to form a dimer through ‘the handshake interaction’, same goes for H3 and H4.
2 copies of each protein associate to form the histone octamer.
Describe the ‘beads on a string’ structure.
DNA (- charged) wraps itself around histone (+ charged) proteins (nucleosome) with sections of linker DNA in between.
Describe the structure of histones.
The tails of the histone protein is not inside the core, but actually exposed.
The N terminals are usually exposed, and therefore modified.
While the C terminals are inserted in the particles for function of DNA.
What are the different levels of compaction?
‘Beads on a string’, zig-zag, loops, compaction of loops to heterochromatin, chromosomes
What are radial loop domains?
In metaphase chromosomes, highly compact radial loops remain anchored to a scaffold formed from proteins in the nuclear matrix.
Describe euchromatin.
Less condensed and capable of gene transcription.
Occupies most chromosomal regions in non-dividing cells.
Describe heterochromatin.
Highly compact and transcriptionally inactive.
Occupies some localized regions in nondividing cells.
What happens to chromatin before mitosis?
Euchromatin will be converted to heterochromatin.
What is constitutive heterochromatin?
Regions that are always found in heterochromatic states.
What is facultative heterochromatin?
DNA regions that can switch between heterochromatin and euchromatin states.
What are chromodomain proteins?
Chromodomain proteins bind to DNA to keep the heterochromatin state.
What regions are always heterochromatic?
Centromeres
Telomeres
Describe the localization of chromatin.
Each chromosome has its specific position in distinct and non-overlapping chromosome territories.
Chromatin can move to specific sites within the nucleus to alter gene expression. This happens due to changes of the cell, which happen due to signals.
Describe the localization of chromatin.
Each chromosome has its specific position in distinct and non-overlapping chromosome territories.
Chromatin can move to specific sites within the nucleus to alter gene expression. This happens due to changes of the cell, which happen due to signals.
When a set of genes become ‘on’ together, they both go to a specific active location in the nucleus.
What are some important histone methylations?
Acetylation, methylation and phosphorylation.
How can histones be modified for transcription?
Eukaryotic transcription regulators help initiate transcription by recruiting chromatin modifying proteins.
This changes the state of chromatin to allow transcription, this state is determined by transcription regulators.
Histone acetyltransferase adds acetyl groups to N terminal tail residues. It’s a modifier of the state of chromatin, as well as a component of transcriptional activation complexes.
What are histone variants?
Proteins that substitute the core histones nucleosomes and usually grant special structural or functional features.
For example, transcription activation; centromere function and kinetochore assmebly… etc.
How do histones have a dynamic structure?
Their changes are catalyzed by ATP - dependant chromatin - remodelling complexes.
How do histones have a dynamic structure?
The movement of nucleosomes with DNA is possible due to specific chromatin remodelling complexes which use ATP to catalyze the sliding of nucleosomes around DNA.
What is meant by histone having a dynamic structure?
Nucleosomes have a dynamic structure, and are frequently subjected to changes.
The nucleosome is wrapped for a short amount of time before undergoing dynamic modification. Then it can be unwrapped before being rewrapped.
When DNA is less wrapped around histones, it can expose specific sequences. During unwrapping, these sequences can be bound to sequence-specific DNA-binding proteins.
After binding, the proteins become a complex with the nucleosomes and carries out its function.
How can histones be changed by proteins?
Negatively charged proteins serve as chaperones by using their state to detach histones from chromatin and replace them with other dimers, using up ATP in the process.
An entire nucleosome can be replaced, or its components by specific remodelling complexes which uses ATP to do its job.
What is meant by ‘the histone code’?
The covalent modifications and the histone variants act in concert to produce a ‘histone code’ that helps to determine biological function.
Describe chromatin reader complexes.
The histone code must be read by chromatin reader complexes, which reads and interprets this code to transform it into different states of the chromatin.
The complex is generally made by scaffold proteins and other specific proteins which recognize different types of modifications.
How do code readers and writer spread chromatin modifications?
A complex of code readers and code writer proteins can spread specific chromatin modifications for long distances along a chromosome.
When a regulatory protein is attempting to bind to DNA, it brings a writer protein, which starts ‘writing’ - adding a modification to a specific N-terminal tail of a histone.
This modification is then read by a reader complex. The complex behaves as a bridge with histone modifying enzymes to spread the modification to consecutive histones.
A wave of chromatin condensation is eventually spread.
