There are three major structural forms of double helical DNA. The ‘B’-form, described by Watson and Crick, the ‘A’-form and the Z-form
The B-form is a right-handed helix with ten residences per 360° turn and with planes of bases perpendicular to the helix axis. The chromosomal DNA primarily consists of B-DNA.
If B-DNA is moderately dehydrated the A-DNA is produced. A-form is also a right-handed helix, but with eleven base pairs per 360° turn and the planes of the bases are tilted 20° away from the perpendicular to helical axis. The DNA regions found in DNA- RNA hybrid or RNA-RNA double stranded regions are very close to A-form.
The Z-DNA has ‘Zigzag’ backbone and hence has the name. It is a left-handed helix containing twelve base pairs per turn. The Z-DNA stretches occurring naturally in DNA have a sequence of alternating purines and pyrimidines, for example Poly GC regions. The transitions among the three forms of DNA may Fig-8.4.Differcnt forms of dank play an important role in regulating gene expression.
ADVERTISEMENTS:
RNA
RNA is a polynucleotide made of ribonucleotide units having ribose sugar, phosphoric acid and nitrogen bases (Adenine or Guanine or Cytosine or Uracil). It is single stranded. Cellular RNAs are non-genetic and are of three types. Messenger RNA (mRNA)
It is the RNA formed during the protein synthesis. Five to ten percent of cellular RNA is of this type. The molecular weight of m-RNA varies from 30000-1000000. It is short lived. DNA transfers the genetic information to ribosome through this type of RNA during the protein synthesis. Ribosomal RNA (r RNA)
The most stable form of RNA in the cell is the r-RNA. About 80% of cellular RNA is of this type. The molecular weight of r-RNA ranges from 40000-1000000. It may have some folds to have a complex structure. R-RNA units along with protein constitute the protein synthesizing factory or the ribosome. Transfer RNA (tRNA)
ADVERTISEMENTS:
It is smallest form of RNA made of only 75 to 100 nucleotides. It is also known as the soluble RNA. It forms about 10-15% of total cellular RNA. The molecular weight of t RNA varies from 25000-30000. It transfers the amino acids from the cytoplasm to the ribosome.
In 1964 Ilolley gave the detailed structure of t RNA through the ‘Clover leaf model’. In that model it was proposed that t RNA has three loops and a lump. The anticodon loop has the complementary base sequence with respect to a codon of mRNA facilitating the attachment of t-RNA with the later. Other two loops are rPFC loop or ribosomal binding loop and DIIU loop or amino acyl synthetase binding loop. The 3′ end of t RNA ends with CCA-OH, which acts as the amino acid attachment site. The other end ends with G.
In eukaryotes a variety of other RNAs are found. These are small nuclear RNA or Sn RNA, concearned with mRNA processing and small nucleolar RNA (Sno RNA) concearned with ribosomal RNA processing in nucleolus. DNA Replication
In a multicellular organism all the cells posses the same quantity and quality of DNA as all the cells result from the successive divisions of a single cell. It is a fact that whether a cell has only one chromosome (as in prokaryote) or many chromosomes (as in eukaryote) the entire genome must replicate precisely once per every cell division. The basic principles of DNA replication in relations to cell cycle are:
- Once the DNA replication is initiated the cell is committed to a division.
- The cell division cannot occur until the replication of entire genomic DNA is complete.