Packaging of DNA and repetitive DNA

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Satellite DNA (Repetitive DNA)

 




It is observed that after Denaturation(for more details click here https://singhsardarni.blogspot.com/2020/09/dna-structue-and-functions.html), DNA fails to come back into original form easily. This may be due to two reasons :
  1. They are degraded molecules from heterogeneous populations.
  2. They are of high molecular weight. 
However, Britten noticed that DNA can broken into small fragments. It gave conclusion that certain short sequences of bases are repeated hundreds of times in DNA .Such type of DNAs are known as repetitive DNA. 
Such repetitive form of DNA observed in chromosome near to centromere. 
Sometimes, repeating sequences of DNA may be similar to that of rest of DNA.  This type of DNA can be easily separated out by process of ultracentrifugation. 
  • Prokaryotic DNA doesn't bear repetitive DNA sequences.
  • Except yeast repetitive DNA has been observed in all eukaryotes.
  • In human beings 30% of DNA is repetitive which is repeated more than 20 times.
  • In Mouse 10% of repetitive DNA is of highly repetitive type.
  • In drosophila about 25% DNA is of repetitive nature.
In drosophila three types of Satellite DNA name has been identified. Each  Satellite DNA has the repeating sequence of 7 nucleotide pairs e. g. 
1. Satellite DNA I with repeat sequence                      5'-ACAAACT-3'
          3'-TGCTTTGA-5'
2. Satellite DNA II with repeat sequence 
           5'-ATAAACT-3'
           3'-TATTTGA-3'
3. Satellite DNA III with repeat sequence 
            5'-ACAAATT-3'
             3'-TGTTTAA-5'
  • Repetitive DNA has the replication power  but does not take part in protein synthesis. Thus, this type of DNA is of inert type and partially dispensable. 

Packaging of DNA/DNA-protein interaction 

  In humans,Total length of DNA is = distance between two consecutive base pair *bp present in diploid cell 
0.34*10raise to power-9m*6.6*10raise to power -9 bp = 2.2 m
Size of nucleus is 10 raise to power -6m
 How did such a long DNA get into such a small size nucleus?😲😲🤔🤔

Packaging in prokaryotes

Prokaryote don't have a well defined nucleus. But the DNA is not scattered throughout the cell . DNA is found in super coiled form in cytoplasm .The coils are maintained by non -histone basic proteins, polyamines which have positive charge. The packaged structure of DNA is known as genophore or nucleiod. 

Packaging in eukaryotes

The negatively charged DNA is wrapped around histone octomer *and form nucleosomes. 
P. Oudet (1975)gave the term nucleosome.
The nucleosomes are further coiled to form solenoids. 


Wooddock has shown that chromatin* consists of repeating pattern of bodies (nucleosomes). 
Most of chromatin is seen as a fibre with diameter of 30nm. The basic form of chromatin packaging is commonly known as 30nm chromatin fibre. 
The nucleosomes give chromatin it's beads on a string apperence. 


The nucleosomes beads appear as Disc shaped particles with a diameter of about 11nm. 
The histone form a protein protein core around which double stranded DNA fragments (146 loose long pair) is wounded. Histone are riched in basic amino acids residues lysins and arginin with charged side chains. 
DNA is wrapped over octomer in super helical state forming 1.75 turns 
Linker DNA : each nucleosomes beads is separated by linker DNA (60bp approx). 
The linker DNA plus the nucleosomes beads constitutes the entire nucleosomes which contain about 200 bp of DNA. 



*histone octomer : five type of histone (H2A, H2B, H3, H4, H1)  out of which 2copies of  four type except h1 forms together form histone octomer or nu(v8)body.


Nucleosomes (12.5nm)=200bp+2molecules of each of H2A, H2B, H3, H4.
*chromatin : Thread like stained bodies seen in nucleus. 


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