Packaging of DNA and repetitive DNA

-

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. 


Comments

Post a Comment

Popular posts from this blog

Nucleic Acid : DNA and RNA (PART -1)

-
Image
 Nucleic acid is biopolymer,essensial for all living beings. They were first reported by Friedrich Miescher (1871) in nuclei of pus cell. He named them nucleic. They can be found in both nucleus and cytoplasm . Altman renamed nuclein and called them Nucleic acid . Nucleoprotein  Nucleic acid along with basic proteins make nucleoprotein which is sub unit of chromosome.   In nucleoprotein, nucleic acid is prosthetic group and protein part consist of protamine and histone (basic nature).  They are very complex and high molecular proteins.  Composition  Basic structure of nucleic acid consist of Sugar, nitrogenous base *(adenine,guanine,thymine,cytosine,uracil) and phosphorus. This combined structure of sugar, phosphorus and nitrogenous base is known as nucleotide.   DNA is made up of deoxyribonucleotides  which is further made up of Deoxyribose sugar, phosphorus, and nitrogenenous bases (adenine,thymine guanine, cytosine) RNA is made up of ribonucleotides which is further composed of Ri
Read more

Shell Coiling in snails 🐌

-
Image
 Maternal inheritance V/S Predetermination  Maternal inheritance : Inheritance of some traits of mother to the offspring through the cytoplasm of female gamete.  Predetermination : It is the effect of mother's genes on the characters of offspring through the cytoplasm of egg and embryo without involving plasmagenes of cytoplasm.  Shell coiling in snails 🐌  There are two types of coiling present in present in snails. 1.Dextral type 2. Sinistral type  The direction of coiling in shell is controlled by the orientation of mitotic spindle of the first cleavage of zygote. This all depends on the organization of egg before fertilisation. The spindle organisation is controlled by organisation of cytoplasm in the egg.  Dextral type : Shell is coiled clockwise (toward right when viewed from apex). It is shown by ( DD or Dd ) because D is dominant over d.  Sinistral type : Shell is coiled anticlockwise . It is shown by dd . 1.Cross between sinistral Female and Dextral male When eggs of Si
Read more

DNA :Structue and Functions

-
Image
  Deoxyribose nucleotide acid is polymer of deoxyribonucleotide . Each deoxyribonucleotide is made up of Ribose sugar , phosphate , and Nitrigenous base .  Sugar: DNA consist deoxyribose* sugar . Nitrigenous Base : It is of two types.  Purines : Adenine (A), Guanine (G) are purines,9_membraned double ring structure having N at 1,3,7,9 position  Add caption Pyrimidines : Thymime (T) ,Cytosine (C)  are Pyrimidines, 6_memberaned single ring structure having N at 1,3 position.  Bonding in DNA  There are two types of binding present in DNA i .e. Hydrogen bonding and covalent bonding.  Hydrogen bonding is present between nitrogenous bases. Adenine joined by  double bond with Thymime . Guanine joined with cytosine by Triple bond.  Covalent bonding :  Phosphodiester bond is formed between 5'phoshphate group of one  nucleotide with 3'OH of another nucleotide.  Bond between nitrogenous base and sugar is known as glycosidic bonds .  X-ray diffraction method    Maurice Wilkins,  R
Read more