nucleac acids Flashcards
nucleic acids
Nucleic acids are among the most important biological macromolecules (others being amino acids-proteins, sugars-carbohydrates, and lipids-fats).
They are found in abundance in all living things Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides.
chemistry of nucleotide
Each nucleotide has three components: a 5-carbon sugar, a phosphate group, and a nitrogenous base.
If the sugar is deoxyribose, the polymer is DNA. If the sugar is ribose, the polymer is RNA. When all three components are combined, they form a nucleic acid. Nucleotides are also known as phosphate nucleotides.
sugar
D-ribose and D-2-deoxyribose are the only sugars so far found in the nucleic acids from which the sugars have been isolated and identified, and they are assumed to be the sugars universally present in nucleic acids.
Both sugars are present in nucleic acids as the β-Furanoside ring structures as depicted below.
nitrogenous bases
It has 2 types
Pyrimidine and purine
Pyrimidine bases found in nucleic acids are mainly three:
* Cytosine is found both in DNA and RNA
* Thymine is found in DNA only
* Uracil is found in RNA only.
purine basis
The Purine ring is more complex than the Pyrimidine ring.
It can be considered the product of fusion of a pyrimidine ring with an imidazole ring
purine found in DNA
Adenine and guanine are the two principal purines found in both DNA and RNA.
1. Adenine
Chemically it is 6-aminopurine.
- Guanine
Chemically it is 2-amino-6-oxypurine.
nucleosides
The nucleosides are composed of purine or pyrimidine base linked to either D-ribose (in RNA) or D-2-deoxyribose (in DNA).
They are joined by β-N-glycosidic linkage.
This linkage in purine nucleosides is at position –9 of the purine base and carbon 1’ of sugar or deoxy sugar.
Example: Adenosine (adenine-9-riboside)
They are joined by β-N-glycosidic linkage.
This linkage in purine nucleosides is at position –9 of the purine base and carbon 1’ of sugar or deoxy sugar.
Example: Adenosine (adenine-9-riboside)
nucleotides
A nucleotide is a nucleoside to which a phosphoric acid group has been attached to the sugar molecule by ‘esterification’ at a definite – OH group and thus has the general composition base – sugar – PO4.
In ribose nucleosides, there are three possible positions for phosphate esterification namely 2’, 3’ and 5’.
In deoxynucleosides, there are free –OH groups only at the 3’ and 5’ positions in the deoxyribose nucleosides. PO4 can be attached only at these positions. The name of each nucleotide may be derived from that of its constituent nitrogenous base.
fuctions of cyclic AMP
Mediator of hormone action—acts as “Second messenger” in the cell.
* Regulates glycogen metabolism—increased cyclic AMP produces breakdown of glycogen (glycogenolysis).
* Regulates TG metabolism—increased cyclic AMP produces lipolysis (breakdown of TG).
* Cholesterol biosynthesis is inhibited by cyclic AMP.
* Cyclic AMP stimulates protein kinases so that inactive protein kinase is converted to active protein kinase.
* Cyclic AMP modulates both transcription and translation in protein biosynthesis.
* Cyclic AMP activates different steps of steroid biosynthesis.
* Cyclic AMP also regulates permeability of cell membranes to water, sodium, potassium and calcium.
* Plays an important role in regulation of insulin secretion catecholamine biosynthesis, and melatonin synthesis.
* Histamine increases cyclic AMP production in parietal cells which in turn increases gastric secretion.
* Decrease in cyclic AMP level is involved in the excitation of bitter taste receptors in tongue.
* Cyclic AMP plays an important role in cell differentiation. Addition of cyclic AMP to malignant cell lines in vitro reduces growth rates and restores their morphology to normal
deoxyribosenucleic acid (DNA)
DNA is a polymer of deoxyribonucleotides and is found in chromosomes, mitochondria an chloroplasts.
The nuclear DNA is found bound to basic proteins called histones. DNA is present in every nucleated cell and carries the genetic information. It is conveniently isolated from viruses, thymus gland, spleen, leucocytes, etc.
chemistry of DNA
Histones are a small family of closely related basic proteins in chromatin.
four types of histones.
They are—
H2A, H2B, H3 and H4, called core histones.
The structure of all four types of histones are highly conserved between species. This extreme conservation implies that the function of histones is identical in all eukaryotes.
Structure: The –COOH terminal 2/3rd of the molecules have a typical random amino acid composition, but 1/3rd of the –NH2 terminal are rich in basic amino acids.
Interaction of histones: The four types of histones interact with each other in very specific ways as follows:
H3 and H4 form a tetramer containing 2 molecules of each (H3H4)2
H2A and H2B form dimers (H2A–H2B).
In the nucleosome, the DNA is supercolied in a left hand helix over the surface of the disc-shaped histone octamer.
Function: This association of histone octamer with DNA protects the DNA from digestion by a nuclease.
primary structure of DNA
Chromosomal DNA consists of very long DNA molecules.
Each DNA is a polymer of about 1010 deoxyribonucleotides.
Normally there are only four different types of deoxyribonucleotides that are found in DNA molecule, namely,
1. adenine deoxyribonucleotide(dA),
2. thymine deoxyribonucleotide (dT),
3. guanine deoxyribonucleotide (dG), and
4. cytosine deoxyribonucleotide (dC
Nucleotides of each of the two helical strands are bound to each other by covalent 3’-5’ phosphodiester linkage.
Each such bond is formed by the ester linkages of a single phosphate residue with the 3’ –OH (i.e. C – 3’ –OH group of the ribose sugar) of one nucleotide with the C – 5’ –OH group of ribose of the next nucleotide.
This kind of bonding gives rise to a linear polydeoxyribonucleotide strand with 2 free ends on both sides.
what is primary structure
The primary structure is the number and sequence of different deoxyribonucleotides in its strands joined together by phosphodiester linkages.
The backbone of the primary structure is the linear strand of interconnected sugar phosphate residues while the purine or pyrimidine connected with the sugar residue projects laterally from the backbone.
secondary structure of DNA
This consists of a double stranded helix formed by the two polydeoxyribonucleotide strands around a central axis. This type of model was first proposed by Watson and Crick
DNA is a double helix. Each of its two strands is coiled about a central axis, usually a right handed helix.
The two sugar phosphate backbones wind around the outside of the bases like the banisters of a spiral staircase and are exposed to the aqueous solution.
The phosphodiester bonds in the two interwoven strands run in opposite directions. Therefore the strands are called antiparallel.
hus the polarity of the two strands will be 3’ – 5’ and 5’ – 3’. The 3’ – 5’ strand is called coding or “template strand” and 5’ – 3’ strand is called noncoding‘strand
denaturation of DNA
Two strands of DNA double helix can separate or unwind during processes such as DNA replication, RNA transcription and genetic recombination.
Complete unwinding of DNA can take place in vitro and is called denaturation of DNA or it is also known as a helix to coil transition.
The melting temperature of DNA is determined by its base composition. Since there are two hydrogen bonds between A and T while three between G and C, increasing G-C base pairs raises Tm.
Tm is strongly influenced by the base composition of the DNA.
DNA rich in G-C pairs has a higher Tm than DNA with high proportion of A-T pairs.
Mammalian DNA, which has about 40 per cent G-C pairs, has a Tm of about 87°C.
