PURINES, PYRIMIDINES, NUCLEOSIDE, & NUCLEOTIDE

 PURINES, PYRIMIDINES, NUCLEOSIDE

&

NUCLEOTIDE

• DNA Stores Genetic Information.

• The transformation was inherited permanently by subsequent generations.
• The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. 

• The observation that DNA was double-stranded was of crucial significance and provided one of the major clues that led to the Watson-Crick structure of DNA. 

 Purines and pyrimidines

• Purines and pyrimidines are nitrogen-containing heterocycles.
• cyclic compounds whose rings contain both carbon and other elements.
• They form Nucleosides and Nucleotides .

Nucleosides: 

• these Are derivatives of purines and pyrimidines that have a sugar linked to a ring nitrogen.The sugar in ribonucleosides is D-ribose, and in deoxyribonucleosides it is 2-deoxy-D-ribose.

•  ribonucleoside 

• deoxyribonucleoside  

• ribonucleosides 

• deoxyribonucleoside

Structure of purine and pyrimidine nucleotides

• nucleotide = ester of phosphoric acid and a nucleoside
• nucleoside = N-containing base + monosaccharide

b-N-glycosidic bond between base and saccharide

almost always to N-1 of a pyrimidine or to N-9 of a purine

• Nucleotides are the building blocks of nucleic acids.
•  Numerals with a prime ( 5′ or 3′) distinguish atoms of the sugar from those of the heterocyclic base.
• Mononucleotides are nucleosides with a phosphoryl group esterified to a hydroxyl group of the sugar.

•  Since most nucleotides are 5′-, the prefix “5′-” is usually omitted when naming them.

Functions of Nucleotides

1. monomeric units of RNA and DNA  substrates: nucleoside triphosphate
2. energetic metabolism    
   ATP = principal form of chemical energy                                        available to cells.
3. physiological mediators ; cAMP, cGMP   („second messengers“)
4. components of coenzyme ; NAD⁺, NADP⁺, FAD
5. allosteric efectors : regulation of key enzymes of metabolic pathways

SYNTHETIC NUCLEOTIDE ANALOGS ARE USED IN CHEMOTHERAPY

        Nucleic Acid Structure & Function

Structure of DNA

• DNA Contains Four Deoxynucleotides
• Nucleotides have three characteristic components:

1. nitrogenous (nitrogen-containing) base, pyrimidine and purine.
2.  pentose,
3. phosphate. 

• The molecule without the phosphate group is called a nucleoside. Both DNA and RNA contain two major purine bases,

1. adenine (A) and
2. guanine (G) Two major pyrimidines. In both DNA and RNA one of the pyrimidines is
3. cytosine (C), but The second major pyrimidine is not the same in both: it is 
4. thymine (T) in DNA and 
5. uracil(U) in RNA.

• I.e; (A, G, C, T) in DNA , (A, G, C, U) in RNA
• These monomeric units of DNA are held in polymeric form by 3′,5′-phosphodiester bridges constituting a single strand
• The informational content of DNA (the genetic code) resides in the sequence in which these monomers are ordered.
• Together with x-ray data from the DNA molecule and the observation of Chargaff: That in DNA molecules the concentration of deoxyadenosine (A) nucleotides equals that of  (T) nucleotides (A = T), while the concentration of (G) nucleotides equals that of (C) nucleotides  (G = C),  led Watson, Crick, and Wilkins to propose a model of a double stranded DNA molecule.
• The two strands of this double-stranded helix are held together by hydrogen bonds between the purine and pyrimidine bases.
•  The pairings between the purine and pyrimidine nucleotides on the opposite strands are very specific; A with T and G with C .
• The two strands of the double-helical molecule, each of which possesses a polarity, are antiparallel; ie, one strand runs in the 5′ to 3′ direction and The other in the 3′ to 5′ direction.
• DNA PROVIDES A TEMPLATE FOR REPLICATION & TRANSCRIPTION

DNA functions :

1. Store genetic information & transferring it to daughter cells during mitosis by replication 
2. Transfer of genetic information from DNA to RNA to be expressed as proteins

• THE CHEMICAL NATURE OF RNA DIFFERS FROM THAT OF DNA

• In RNA, the sugar moiety is ribose rather than the 2′-deoxyribose of DNA.
• The pyrimidine components of RNA differ from those of DNA. thymine (T) in DNA and uracil (U) in RNA.
• RNA exists as a single strand, whereas DNA exists as a double-stranded helical molecule.
•  : RNA  

A. MESSENGER RNA (mRNA)

B. TRANSFER RNA (tRNA)

C. RIBOSOMAL RNA (rRNA)