· 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.
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
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.
} 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.
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)
§ 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