Histones Flashcards
(25 cards)
How many bases are there in one complete turn of double helical DNA
There are 10 bases in one complete helical turn
How is DNA organised and packaged in the cell
It is packaged into chromatin
What is chromatin made of
Chromatin consists of proteins and RNA and is made of the nucleosome
The DNA and histones are in a 1:1 ratio
- proteins are - histones which are the most common nuclear protein
- RNA is non protein coding RNA instead is for transcriptional control
What is the nucleosome
It is formed of histones
Characteristics of histones
Histones are very small and positively charged to allow there function of packaging DNA as DNA is negatively charged
The core histones are highly conserved
What are the core histones and which ones are the most highly conserved
H2A H2B H3- highly conserved H4- highly conserved H1 linker- least conserved
How many levels of packaging does the nucleosome cause
6 levels of packaging
Most basic level of chromatin
Nucleosome consisting of DNA and histones which
How is the nucleosome formed
H3 and H4 form diners and bond tightly in a dimer handshake to form a tetramer which forms a horseshoe structure
H2A and H2B form a dimer and one binds above and one binds bellow the H3H4 tetramer this handshake is less conserved as the interaction is less strong
This forms an optometric core
DNA in a right handed double helix is wound round in a left handed super helix the DNA has major grooves and minor grooves
There is a flexible end terminal tail that comes out of the core of the nucleosome and through the DNA so the DNA can be remodelled and changed
What would happen if a histones was altered in the nucleosome
This would cause a change in the path of the DNA around the Octomer and this may result in a change in the packaging of DNA
Variations of H2A
H2A.Z- changes transcriptional activity
MarcoH2A- silencing chromosomes
H2A.X- signals DNA breaks
H2A.Bbd- keeps silenced genomes in condensed state
Difference between H2A and H2A.Z
60% identity to H2A
H2A.Z alters the interactional stability between H2A and H2B dimer with the H3 and H4 tetramer
Therefore it results in a looser nucleosome which results in a more transcriptionally active chromatin
H3 variations
There is very little difference between each
H3.3- used when we want to change the transcriptional activity without waiting for DNA transcription, usually the DNA is repackaged when DNA is replicated due to it being moved however we cant always wait for DNA replication so H3.3 is used when a gene needs to be turned on without replication happening
H3.1
H3.2
H3.lt
CenpA- enriched at your centromeres and telomeres it signifies that these regions do not have genes within them
Levels 1 of DNA packaging
146 base pairs rap around the nucleosome with two ends that stick out giving 200 base pairs the adjacent nucleosome join together
2 turns around the core forming a left handing super helix
There are major and minor grooves that allow for the docking of transcriptional machinery
Level 2 of DNA packaging
10nm fibre
Beads on a string structure
There is linker DNA between nucleosome and this is the most transcriptionally active DNA as it is free least transcriptionally active DNA is the DNA touching the core
Packing ratio 6-7
Level 3 of DNA packaging
30nm solenoid
Run of 6 nucleosome that form a circle and inside the circle is histones H1 and is essential as it stops the DNA from moving fixing it in place
Packing ratio of 40
Level 4 of DNA packaging
300 nm solenoid
Protein scaffold that has non histones chromatin proteins that will tether the loops of the 30nm solenoid to the protein scaffold
There are non histones chromatin proteins that keep the loops apart
Level 5 of DNA packaging
700nm solenoid
Protein scaffold coils again, called the coiled coil
Packing ration 10 to the power of 4
Why is a protein scaffold so important in DNA packaging
It means that the DNA can be unwound in a controlled way all the way back down to the nucleosome
Level 6 of DNA packaging
Metaphase chromosome, most highly compacted form of DNA that can be viewed in a light microscope
This is the form that is taken during metaphase- splitting of the DNA so transcription is not favourable so is tightly wound
No transcription can occur
The metaphase chromosome
Genes must be packaged so they can be unruffled once split even though transcription is not happening during metaphase
Telomeres at the end of the chromosome and centromeres in the center of the chromosome
Telomeres are long as a child and get shorter as you age these are protective regions that do not contain genes
Centromere is where the mitotic spindles attach during the splitting of chromatin during mitosis
Telomeres and centromeres are heterochromatin
Euchromatic is where the mRNA coding regions lie and are on the P and Q arms of the chromosome
Heterochromatin
Remains highly conserved throughout the cell cycle and contains no genes or genes that have been silenced forever transcriptionally inactive
it is darker on a micrograph
2 types of heterochromatin
-Constitutive
It is the telomeres and centromeres
All cells of a given species will package the same region of DNA into heterochromatin
It is a large part of the Y chromosome this is because the Y chromosome has to line up with the X chromosome so don’t want it to be damaged at all, there are very few genes on the Y chromosome so only a small number that is transcriptionally active on the p arm
-Facultative
Not consistent within the cell types of species
It is regulated and is associated with differentiation
For instance in females one of the X chromosomes is randomly silenced in every cell this is random so therefore genetic information that is packaged as heterochromatin in one cell will be euchromatin in another cell
Maternal and paternal X chromosome differs between all cells
When is heterochromatin replicated
It is replicated very late in S phase as it takes time to unwind the highly condensed DNA to transcriptionally active DNA
