Purines and Pyrimidines 4

Purine and Pyrimidine are the names of the parent compounds of two types of nitrogen-containing heterocyclic aromatic compounds. N H 1 2 3 4 5 7 8 9 6 Pyrimidine Purine

Important Pyrimidines Pyrimidines that occur in DNA: Cytosine and Thymine. (C & T) Pyrimidines in RNA: Cytosine and Uracil. (C & U) NH2 HN N H O HN N H O CH3 HN O N H Uracil Thymine Cytosine

Important Purines Adenine and Guanine are the principal purines of both DNA and RNA. N NH2 N H O HN N H N H2N Adenine Guanine

Nucleosides A nucleoside is a pyrimidine or purine N-glycoside of D-ribofuranose or 2-deoxy-D-ribofuranose. The purine or pyrimidine part of a nucleoside is referred to as a purine or pyrimidine Base.

Cytidine occurs in RNA; its 2-deoxy analog occurs in DNA Pyrimidine nucleosides NH2 HO OH O HOCH2 N Cytidine occurs in RNA; its 2-deoxy analog occurs in DNA

Thymidine occurs in DNA Pyrimidine nucleosides O N NH HO H3C HOCH2 Thymidine occurs in DNA

Uridine Pyrimidine nucleosides HOCH2 O N NH OH HO Uridine occurs in RNA

Adenosine occurs in RNA; its 2-deoxy analog occurs in DNA Purine nucleosides HOCH2 O OH HO N NH2 Adenosine occurs in RNA; its 2-deoxy analog occurs in DNA

Guanosine occurs in RNA; its 2-deoxy analog occurs in DNA Purine nucleosides HOCH2 N NH O HO NH2 OH Guanosine occurs in RNA; its 2-deoxy analog occurs in DNA

Nucleotides Nucleotides are phosphoric acid esters of Nucleosides. 4

Adenosine 5'-Monophosphate (AMP) Adenosine 5'-monophosphate (AMP) is also called 5'-adenylic acid. OCH2 P HO O OH N NH2 5' 1' 2' 3' 4'

Adenosine Diphosphate (ADP) OCH2 OH N NH2

Adenosine Triphosphate (ATP) ATP is an important molecule in several biochemical processes including: energy storage. O P HO OCH2 OH N NH2

Nucleic acids are polynucleotides. 4

Nucleic Acids Nucleic acids first isolated in 1869 (Johann Miescher) Oswald Avery discovered (1945) that a substance which caused a change in the genetically transmitted characteristics of a bacterium was DNA. Scientists revised their opinion of the function of DNA and began to suspect it was the major functional component of genes.

Composition of DNA Erwin Chargaff (Columbia Univ.) studied DNAs from various sources and analyzed the distribution of purines and pyrimidines in them. The distribution of the bases adenine (A), guanine (G), thymine (T), and cytosine (C) varied among species. But the total purines (A and G) and the total pyrimidines (T and C) were always equal. Moreover: %A = %T, and %G = %C

Composition of Human DNA For example: Purine Pyrimidine Adenine (A) 30.3% Thymine (T) 30.3% Guanine (G) 19.5% Cytosine (C) 19.9% Total purines: 49.8% Total pyrimidines: 50.1%

Base Pairing A and T are present in equal amounts in DNA because of complementary hydrogen bonding. 2-deoxyribose A T

Base Pairing Likewise, the amounts of G and C in DNA were equal because of complementary hydrogen bonding. 2-deoxyribose G C

The DNA Duplex A double-stranded structure for DNA in which a purine or pyrimidine base in one chain is hydrogen bonded to its complement in the other. •Gives proper ratios (A=T and G=C) •Because each pair contains one purine and one pyrimidine, the A---T and G---C distances between strands are approximately equal.

Two antiparallel strands of DNA are paired by hydrogen bonds between purine and pyrimidine bases.

DNA is coiled A strand of DNA is too long (about 3 cm in length). To fit inside a cell, it is coiled. Efficient coiling of DNA is accomplished with the aid of proteins called histones. DNA winds around histone proteins to form nucleosomes.

Ribonucleic Acids (RNA) 4

Three main types of RNA : 1- Messenger RNA (mRNA): Transfers DNA code to ribosomes for translation. 2- Transfer RNA (tRNA): Brings amino acids to ribosomes for protein synthesis. 3- Ribosomal RNA (rRNA): Ribosomes are made of rRNA and protein.

ABDULLATIF TAHA ABDULLA MSc. BIOCHEMISTRY سبحانك لا علم لنا إلا ما علمتنا * إنك أنت العليم الحكيم 0