MOLECULAR BASIS OF INHERITANCE ONE SHOT
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NUCLEIC ACIDS
* F. Meischer discovered nucleic acid in nucleus of pus cell and called it "nuclein". The term nucleic acid was
coined by "Altman."
* Nucleic acids are polymer of nucleotides.
Example : DNA and RNA.
Nucleotide = Nitrogen base + pentose sugar + phosphate
Nucleoside = Nitrogen base + pentose sugar.
A. Nitrogen base :
On the basis of structure nitrogen bases are broadly of two types :–
1. Pyrimidines – Consist of one pyrimidine ring. Skeleton of ring composed of two nitrogen and four Carbon
atoms. e.g. Cytosine, Thymine and Uracil.
DNA
* DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1869.
* DNA term was given by - Zacharis
* DNA is long polymer of deoxyribonucleotides.
* DNA is negatively charged.
* In DNA pentose sugar is deoxyribose sugar and four types of nitrogen bases A,T,G,C
* Wilkins and Franklin studied DNA molecule with the help of X-Ray crystallography
Purine= Pyrimidine
[A] + [G] = [T] + [C]
In a DNA A + T > G + C Þ A – T type DNA. Base ratio of A – T type of DNA is more than one.
eg. Eukaryotic DNA
* In a DNA G + C > A + T Þ G – C type DNA. Base ratio of G – C type of DNA is less than one.
eg. Prokaryotic DNA
* Melting point of DNA depends on G – C contents.
More G – C contents means higher melting point.
Tm = Temperature of melting.
Tm of prokaryotic DNA > Tm of Eukaryotic DNA
* DNA absorbs U.V. rays of 2600Ã… wavelength.
* Denaturation and renaturation of DNA - If a normal DNA molecule is placed at high temperature (80-
90°C) then both strands of DNA will separate from each other due to breaking of hydrogen bonds. It is called
DNA-denaturation.
When denatured DNA molecule is placed at normal temperature then both strand of DNA attached and
recoiled to each other. It is called renaturation of DNA.
Configuration of DNA Molecule :–
* Two strands of DNA are helically coiled like a revolving ladder. Back bone of this ladder (Reiling) is composed
of phosphates and sugars while steps (bars) are composed of pairs of nitrogen bases.
Two chains have anti-parallel polarity. It means, if one chain has the polarity 5'®3', the other has 3'®5'.
* Distance between two successive steps is 3.4 A0 . In one complete turn of DNA molecule there are such 10 steps
(10 pairs of nitrogen bases). So the length of one complete turn is 34 A0. This is called helix length.
* Diameter of DNA molecule i.e. distance between phosphates of two strands is 20A0.
* Each step of ascent is represented by a pair of bases. At each step of ascent, the strands turns 36°.
* In nucleus of eukaryotes the DNA is associated with histone protein to form nucleoprotein.
* Bond between DNA and Histone is salt linkage (Mg+2).
PACKAGING OF DNA HELIX
The average distance between the two adjacent base pairs of 0.34 nm (0.34 × 10–9 m or 3.4 Å). Length of DNA
for a human diploid cell is 6.6 × 109 bp × 0.34 × 10–9 m/bp = 2.2 m. The length is far greater than the
dimension of a typical nucleus (approximately 10–6 m).
The number of base pairs in Escherichia coli is 4.6 × 106. The total length is 1.36 mm. The long sized DNA
accommodated in small area (about 1 mm in E. coli) only through packing or compaction. DNA is acidic due to
presence of large number of phosphate group. Compaction occurs by folding acid attachment of DNA with
basic proteins, polyamine in prokaryotes and histone in eukaryotes.
DNA packaging in Prokaryotes : DNA is found in cytoplasm in supercoiled state. The coils are maintained
by non histone basic protein like polyamines. This compact structure of DNA is called nucleoid or genophore.
DNA packaging in Eukaryotes : It is carried out with the help of lysine and ariginine rich basic proteins called
histone. The unit of compaction is nucleosome. There are five types of histone proteins H1 H2A, H2B, H3 and
H4. Four of them occur in pairs to produce histone octamer (2 copies of each – H2A, H2B, H3 and H4), called
nubody or core of nucleosome.Their positively charged ends are directed outside. The negatively charged DNA
is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical
nucleosome contains 200 bp of DNA Helix. DNA present between two adjacent nucleosome is called linker
DNA. It is attached to H1 histone protein. Length of linker DNA varies from species to species. Nucleosome
chain gives a beads on string appearance under electron microscope. The nucleosomes furthers coils to form
solenoid. It has diameter of 30 nm as found in chromatin. The beads on string structure in chromatin is pack-
aged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to form
chromosomes. The packaging at higher level requires additional set of proteins (acidic) that collectively are
referred to as non-histone chromosomal (NHC) proteins.
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