cryptic satellite DNA

cryptic satellite DNA their effect on plant aand animal

1. PRESENTATION ON CRYPTIC SATELLITE DNA Submitted To :- Submitted By :- Dr. D.K. Garg Sanjay Kumar Sanadya M.Sc (Ag) 1 st year

2. INTRODUCTION DNA is a polymers of polynucleotide sequences. Its found in living organism, either may be double helical or may be circular form. Double helical DNA Circular DNA

3. DNA subdivided on the basis of sequences of nucleotide in two type :- 1.Non repetitive DNA (unique DNA) :- single copy per genome in living organism. 2. repetitive DNA :- few to million copy per genome. Also subdivided on the basis of no. of copies of nucleotide sequences :- A. Highly repetitive DNA :- million copies per genome. B. Moderately repetitive DNA :- few to thousand copies per genome. Information on repetitive DNA has been gathered through Density Gradient Centrifugation Method.

4. SATELLITE DNA Some very short highly repetitive sequences (repeated many time in tandem) differ in density from the remaining chromosomal DNA and are separable as minor band in C sCl 2 Density Gradient Centrifugation , Called as SATELLITE DNA. An important feature of satellite is that they represent very long stretches of DNA of very low sequence complexity, within constancy of sequences can be maintained.

5. Where are the blocks of highly repetitive DNA located in the genome? It is located in heterochromatin regions of chromosomes, Heterochromatin is commonly found at centromeres. Why? (the regions where the kinetochores are formed at mitosis and meiosis for controlling chromosome movement). The centromeric location of satellite DNA suggests that it has some structural function in the chromosome. This function could be connected with the process of chromosome segregation. 

6. CRYPTIC SATELLITE DNA Generally single satellite sequences found , but some time multiple satellite sequences can be there, which may anomalous behavior in CsCl2 Density Gradient Centrifugation due to base methylation so that cannot be separated from the main-band DNA so it is called CRYPTIC SATELLITE DNA . cryptic and apparent satellites usually account for all the large tandemly repeated blocks of highly repetitive DNA. When a genome has more than one type of highly repetitive DNA, each exists in its own satellite block although sometimes different blocks are adjacent.

7. Minisatellite :- Some sequences that resemble satellite DNA consists of tandem repeats of a short sequences of 2 to 8 bp , the repeats being identical. Variation in this type of DNA arises due mainly to differences in the number of repeat units, which is highly variable. These DNAs are called minisatellite or VNTRs (variable number of tandem repeats) and are used for fingerprinting and genome mapping.

8. Examples:- 1. Drosophila virilis , three major satellites, viz., ACAAACT, ATAAACT, ACAAATT, and a cryptic satellite DNA (AATATAG) make up 40% of the genome . but satellites in other species may have unrelated sequences. Each satellite has arisen by a lateral amplification of a very short sequences, which may be a variant of a preexisting satellite or may have Some other origin.

9. 2.Mus musculus Satellite DNA consists mainly of a repeating unit of 234 bp. This fragment is composed of two smaller sub units of 117 bp each. Each 117 bp subunit contains two smaller sequences of 58 and 59 (one T added to the second subunit) bp. Each 58/59 bp subunit, in turn, has two subunits called α & β ; the α sequences have an insertion of one C ,while β sequences have an insertion of a 3 bp sequences. The analysis of 58/59 bp reveals consensus of 9 bases, which is the basic repeating unit. This repeating sequences is supposed to have become amplified by a process called saltatory replication to yield 54 bp subunit, which became 58/59 bp by addition. The α & β subunits of the 58/59 bp subunit diverged due to the accumulation of mutation. Subsequently, the 58/59 bp subunit underwent saltatory replication as a unit to yield the much larger 234 bp repeating unit. Mutation accumulation would have resulted in further divergence among the repeating units.

10. Cryptic Satellites in Inverted Repeats Comprise 30% of the Genome of a HERMIT CRAB :- One major very highly repeated (VHR) DNA (repeat unit = 156 bp ), a family of three minor VHR DNAs ( repeat units = 71-74 bp ), and a number of trace components account for almost 30% of the genome of a hermit crab . The repeat units of the three minor variants are defined by identical 14-bp G + C rich inverted repeats that might form cruciform . Two copies of the repeat unit (CCTA) of one of two patent satellites of this crab occur at the center of one in seven of the G + C- rich inverted repeats ; copies of the other patent satellite are found in main component DNA .

11. The sequences of both the major and minor VHR DNAs are characterized by short tracts of An and/or Tn (n = 4-7) residues whose presence would permit the formation of perfectly matched stems separated by loops of 8-16 bp . The An and/or Tn tracts are interspersed with segments of G + C-rich DNA and are arranged differently in the major and minor VHR DNAs. Although the repeat units of the major and the three minor VHR DNAs are arranged in tandem, the composition and sequence of their bases are such that they do not form distinct bands in CsCl2 gradients. They are called Cryptic Satellites . Other example of cryptic satellite DNA :- Complex organization of a cryptic satellite DNA in the genome of the marine invertebrate Rapana thomasiana ( Gastropoda ).

12. Thank you