6. MOLECULAR BASIS OF INHERITANCE
·
Nucleic acids (DNA &
RNA) are the building blocks of genetic material.
·
DNA is the genetic
material in most of the organisms.
· RNA is the genetic material in some viruses. RNA mostly functions as messengers.
THE DNA
STRUCTURE OF POLYNUCLEOTIDE CHAIN
Polynucleotides
are the polymer of nucleotides.
DNA & RNA are polynucleotides. A
nucleotide has 3 components:
1.
A nitrogenous base.
2. A pentose sugar (ribose in RNA & deoxyribose in
DNA).
3.
A phosphate group.
Nitrogen
bases are 2 types:
}
Purines: It includes Adenine (A) and Guanine (G).
} Pyrimidines: It includes Cytosine (C),
Thymine (T) & Uracil (U). Thymine (5-methyl Uracil) present only in DNA and Uracil only in RNA.
A nitrogenous
base is linked to the OH of 1' C pentose sugar through an N-glycosidic
linkage to form nucleoside.
|
Nucleosides in RNA |
Nucleosides in DNA |
|
Adenosine |
Deoxyadenosine |
|
Guanosine |
Deoxyguanosine |
|
Cytidine |
Deoxycytidine |
|
Uridine |
Deoxythymidine |
A
phosphate group is linked to OH of 5' C of a nucleoside through phosphoester
linkage to form nucleotide (or deoxynucleotide).
In RNA, each nucleotide
has an additional –OH group at 2' C of the ribose (2’- OH).
2
nucleotides are linked through 3’-5’ phosphodiester bond to
form dinucleotide.
When more nucleotides are
linked, it forms polynucleotide.
STRUCTURE OF THE DNA
}
Friedrich Meischer
(1869): Identified DNA
and named it as ‘Nuclein’.
} James
Watson & Francis
Crick (1953) proposed double helix model of DNA. It
was based on X-ray diffraction data produced
by Maurice Wilkins & Rosalind Franklin.
}
DNA
is made of 2 polynucleotide chains coiled in a right-handed fashion. Its
backbone is formed of sugar & phosphates. The bases project inside.
}
The
2 chains have anti-parallel polarity, i.e. one chain has the polarity 5’→3’
and the other has 3’→5’.
}
The
bases in 2 strands are paired through H-bonds forming base pairs
(bp).
A=T (2 hydrogen bonds) C≡G (3 hydrogen bonds)
}
Purine
comes opposite to a pyrimidine. This generates uniform distance between the 2
strands.
} Erwin
Chargaff’s rule: In
DNA, the proportion of A is equal to T and the proportion of G is equal to C.
v
Ф 174 (a bacteriophage) has 5386 nucleotides.
v
Bacteriophage lambda has 48502 base pairs (bp).
v
E. coli has 4.6x106 bp.
v
Haploid
content of human DNA is 3.3x109 bp.
Length of DNA = number of base pairs X
distance between two adjacent base pairs.
Number
of base pairs in human = 6.6 x 109
Hence, the length of DNA = 6.6 x109 x 0.34x 10-9
=
2.2 m
In E. coli,
length of DNA =1.36 mm (1.36 x 10-3 m)
PACKAGING OF DNA HELIX
§ In prokaryotes (E.g. E. coli), the DNA is not scattered
throughout the cell. DNA is negatively charged. So it is held with some
positively charged proteins to form nucleoid.
§
§ Histones are rich in positively charged basic amino
acid residues lysines and arginines.
§ 8
histones form histone
octamer.
§ Negatively charged DNA is wrapped around histone
octamer to give nucleosome.
§ A typical nucleosome contains 200 bp.
Therefore, total number of nucleosomes in human =
§ Nucleosomes constitute the repeating unit to form chromatin.
Chromatin is the thread-like stained bodies.
§ Nucleosomes in chromatin = ‘beads-on-string’.
§ Chromatin is packaged → chromatin fibres → coiled
and condensed at metaphase stage → chromosomes.
§ Higher level packaging of chromatin requires non-histone
chromosomal (NHC) proteins.
§ Chromatin has 2 forms:
· Euchromatin: Loosely packed and transcriptionally active region
of chromatin. It stains light.
· Heterochromatin: Densely packed and inactive region of chromatin. It stains dark.
- Topic 1: The DNA
- Topic 2: The Search for Genetic Material
- Topic 3: Properties of Genetic Material, RNA World
- Topic 4: DNA Replication
- Topic 5: Transcription
- Topic 6: Genetic Code, Types of RNA
- Topic 7: Translation (Protein Synthesis)
- Topic 8: Regulation of Gene Expression, Operon Concept
- Topic 9: Human Genome Project (HGP)
- Topic 10: DNA Fingerprinting