L2: DNA packaging Flashcards
How long is the DNA of one cell in human and what is the diameter of the nucleus?
1.8 Meter - 6 Micrometer
What is DNA packaging?
- It is packing all the DNA in the Nucleus and still be functional.
What does Eukaryotic DNA look like?
- It is linear helix which is associated with proteins to form the chromosome.
When does DNA packaging take place?
In prophase in mitosis prior to being separated into two daughter cellsz
What do chromosomes form and what is chromatin?
-Chromosomes form the chromatin network and chromatin is Double helix DNA associated with proteins
What are chromosomes?
-They are DNA helices associated with histones and other structural proteins
What are the types of chromatin and when does it appear like that?
-It appears in interphase as Euchromatin and heterochromatin.Euchromatin is less densely packed so it shows gene expression while heterochromatin is densely packed thats why it shows no gene expression as it is not accessible to enzymes and factors
What are the proteins associated with DNA?
- Histone proteins:- They are proteins that contain high content of basic amino acids like lysine,arginine and histidine.
- They bind with the Phosphate group of the DNA as they are negatively charged.
- There are five types of histones which are H1,H2A,H2B,H3 and H4.
- Non histone proteins:- Structural proteins which help in packaging the DNA.
- Enzymes that help in packaging, Replication and transcription.
- Regulatory proteins which help in transcription and replication.
What are the stages of packaging DNA?
1- Nucleosome
2- 11nm-chromatin fibril
3- 30nm-chromatin fiber
4- 700nm-chromatid
5- 1400nm-chromosome
“1-2-3 are Euchromatin”
Nucleosome stage
-The DNA is wounded 1.75 turns (146 bp) around octamer proteins which consists of 2 molecules of each of (H2A - H2B - H3 - H4), These structures are connected together by linker DNA (50bp) associated with H1, This Structure protects The DNA from digestion by nucleases and it is 11-nm in diameter and 5 nm in lenght.
11nm - chromatin fibril
-The Nucleosomes are connected together by Linker DNA forming a string of nucleosomes “beads-on-a-string”-packaging ratio is 10 folds.
-The Linker DNA and histone protein H1 make it more compact.
30-nm chromatin fiber
-The supercoiling of the Nucleosomes so that each turn contains 6-7 Nucleosomes-Stabilized by H1 histone-packaging ratio is 50 folds
700 nm chromatid
-The 30 nm fiber is supercoiled into twisted looped structure where each 6 loops are wrapped around scaffold protein (rosettes), The packaging ratio is 8000 folds, Each 30 rosettes form a loop of the coil that form the chromatid
1400 nm chromosome
-It is identified in prophase
-They are arranged according to their length and the site of centromere.
-packaging ration is 10000.
-Each chromosome consists of two sister chromatid.
The chromosme
-The centromere is rich in AT bp (106) which is bonded to certain proteins forming the kinetochore which forms a mitotic spindle.
-The centromere forms the arms where the short ones are called p and long are Q.
-At the ends of the DNA there are tandems 5’ TTAGGG 3’ which are called telomeres (many kilos bp long)
