Purines and PyrimidinesPurines and Pyrimidines

Purine and Pyrimidine are the names of the Purine and Pyrimidine are the names of the parent compounds of two types of nitrogen-parent compounds of two types of nitrogen-containing heterocyclic aromatic compounds.containing heterocyclic aromatic compounds.

PyrimidinePyrimidine PurinePurine

NN

NN NNHH

NN

NNNN 11

22

3344

5577

88

99

11

22

33

44

55

6666

Important PyrimidinesImportant PyrimidinesPyrimidines that occur in Pyrimidines that occur in DNADNA::

Cytosine and Thymine. (Cytosine and Thymine. (CC & & TT))

Pyrimidines in Pyrimidines in RNARNA::

Cytosine and Uracil. (Cytosine and Uracil. (CC & & UU)) HHNN

NNHH

OO

OO

UracilUracil

HHNN

NNHH

OO

OO

CHCH33

ThymineThymine

HHNN

NNHH

NNHH22

OO

CytosineCytosine

Important PurinesImportant Purines

Adenine and Guanine are the principal purines Adenine and Guanine are the principal purines of both DNA and RNA.of both DNA and RNA.

AdenineAdenine

NN

NN

NNHH22

NN

NNHH

GuanineGuanine

OO

HHNN

NNHH

NN

NN

HH22NN

NucleosidesNucleosides

A A nucleosidenucleoside is a pyrimidine or purine N- is a pyrimidine or purine N-glycoside of D-ribofuranose or 2-deoxy-D-glycoside of D-ribofuranose or 2-deoxy-D-ribofuranose.ribofuranose.

The purine or pyrimidine part of a nucleoside is The purine or pyrimidine part of a nucleoside is referred to as a referred to as a purine or pyrimidine purine or pyrimidine BaseBase..

Pyrimidine nucleosidesPyrimidine nucleosides NNHH22

HOHO OHOH

OOHOCHHOCH22

OO

NN

NN

Cytidine occurs in RNA; Cytidine occurs in RNA; its 2-deoxy analog occurs in DNAits 2-deoxy analog occurs in DNA

CytidineCytidineCytidineCytidine

Pyrimidine nucleosidesPyrimidine nucleosides OO

NN

NNHH

OO

HOHO

OO

HH33CC

HOCHHOCH22

Thymidine occurs in DNAThymidine occurs in DNA

ThymidineThymidineThymidineThymidine

Pyrimidine nucleosidesPyrimidine nucleosides

HOCHHOCH22

OO

NN

NNHH

OO

OHOHHOHO

OO

Uridine occurs in RNAUridine occurs in RNA

UridineUridineUridineUridine

Purine nucleosidesPurine nucleosides

Adenosine occurs in RNA; Adenosine occurs in RNA; its 2-deoxy analog occurs in DNAits 2-deoxy analog occurs in DNA

HOCHHOCH22 OO

OHOHHOHO

NN

NN

NN

NHNH22

NN

AdenosineAdenosineAdenosineAdenosine

Purine nucleosidesPurine nucleosides

Guanosine occurs in RNA; Guanosine occurs in RNA; its 2-deoxy analog occurs in DNAits 2-deoxy analog occurs in DNA

HOCHHOCH22

NN

NHNH

OO

OO

HOHO

NN NHNH22

NN

OHOH

GuanosineGuanosineGuanosineGuanosine

NucleotidesNucleotides

Nucleotides are phosphoric acid Nucleotides are phosphoric acid esters of Nucleosides.esters of Nucleosides.

Adenosine 5'-Monophosphate (AMP)Adenosine 5'-Monophosphate (AMP)

Adenosine 5'-monophosphate (AMP) is also Adenosine 5'-monophosphate (AMP) is also called 5'-adenylic acid.called 5'-adenylic acid.

OCHOCH22PPHOHO

OO

HOHO

OO

OHOHHOHO

NN

NN

NN

NHNH22

NN

5'5'

1'1'

2'2'3'3'

4'4'

Adenosine Diphosphate (ADP)Adenosine Diphosphate (ADP)

OOPP

OO

HOHO

HOHO

OCHOCH22PP

OO

HOHO

OO

OHOHHOHO

NN

NN

NN

NHNH22

NN

Adenosine Triphosphate (ATP)Adenosine Triphosphate (ATP)

ATP is an important molecule in several ATP is an important molecule in several biochemical processes including:biochemical processes including:

energy storage.energy storage.

OOPP

OO

HOHO

OO

OCHOCH22PP

OO

HOHO

OO

OHOHHOHO

NN

NN

NN

NHNH22

NN

PP

OO

HOHO

HOHO

Nucleic AcidsNucleic Acids

Nucleic acids are polynucleotides.Nucleic acids are polynucleotides.

Nucleic AcidsNucleic Acids

Nucleic acids first isolated in 1869 (Johann Nucleic acids first isolated in 1869 (Johann Miescher)Miescher)

Oswald Avery discovered (1945) that a Oswald Avery discovered (1945) that a substance which caused a change in the substance which caused a change in the genetically transmitted characteristics of a genetically transmitted characteristics of a bacterium was DNA.bacterium was DNA.

Scientists revised their opinion of the function of Scientists revised their opinion of the function of DNA and began to suspect it was the major DNA and began to suspect it was the major functional component of genes.functional component of genes.

Composition of DNAComposition of DNA

Erwin Chargaff (Columbia Univ.) studied DNAs Erwin Chargaff (Columbia Univ.) studied DNAs from various sources and analyzed the from various sources and analyzed the distribution of purines and pyrimidines in them.distribution of purines and pyrimidines in them.

The distribution of the bases adenine (A), The distribution of the bases adenine (A), guanine (G), thymine (T), and cytosine (C) guanine (G), thymine (T), and cytosine (C) varied among species.varied among species.

But the total purines (A and G) and the total But the total purines (A and G) and the total pyrimidines (T and C) were always equal.pyrimidines (T and C) were always equal.

Moreover: %A = %T, and %G = %CMoreover: %A = %T, and %G = %C

Composition of Human DNAComposition of Human DNA

Adenine (A) 30.3%Adenine (A) 30.3% Thymine (T) 30.3%Thymine (T) 30.3%

Guanine (G) 19.5%Guanine (G) 19.5% Cytosine (C) 19.9%Cytosine (C) 19.9%

Total purines: 49.8%Total purines: 49.8% Total pyrimidines: 50.1%Total pyrimidines: 50.1%

For example:For example:

PurinePurine PyrimidinePyrimidine

Base PairingBase Pairing

A and T are present in equal amounts in DNA A and T are present in equal amounts in DNA because of complementary hydrogen bonding.because of complementary hydrogen bonding.

2-deoxyribose2-deoxyribose

N

NN

N N

H

H

N

N

O CH3

O

H

2-deoxyribose2-deoxyriboseAA TT

Base PairingBase Pairing

Likewise, the amounts of G and C in DNA were Likewise, the amounts of G and C in DNA were equal because of complementary hydrogen equal because of complementary hydrogen bonding.bonding.

N

NN

N O

NH

H

H

N

N

N

O

HH

2-deoxyribose2-deoxyribose

2-deoxyribose2-deoxyribose

GG CC

The DNA DuplexThe DNA Duplex

A double-stranded structure for DNA in which a A double-stranded structure for DNA in which a purine or pyrimidine base in one chain is purine or pyrimidine base in one chain is hydrogen bonded to its complement in the hydrogen bonded to its complement in the other.other.

••Gives proper ratios (A=T and G=C)Gives proper ratios (A=T and G=C)

••Because each pair contains one purine and Because each pair contains one purine and one pyrimidine, the A---T and G---C distances one pyrimidine, the A---T and G---C distances between strands are approximately equal. between strands are approximately equal.

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

O

O

ĞO

ĞO

ĞO

O

O

O

O

OP

O

O

P O

OO

P O

OO

O

O

OĞ

OĞ

OĞ

O

O

O

O

O P

O

O

PO

OO

PO

OO

C G

AT

AT

CG

3'

5'

5'

5'

5' 5'

5'

5'

5'

3'

3'3'

3'3'

3'

3'

DNA is coiledDNA is coiled

A strand of DNA is too long (about 3 cm in length).A strand of DNA is too long (about 3 cm in length).

To fit inside a cell, it is coiled.To fit inside a cell, it is coiled.

Efficient coiling of DNA is accomplished with the Efficient coiling of DNA is accomplished with the aid of proteins called aid of proteins called histoneshistones..

DNA winds around histone proteins to form DNA winds around histone proteins to form nucleosomes.nucleosomes.

Ribonucleic Acids (RNA)Ribonucleic Acids (RNA)

Three main types of RNA :Three main types of RNA :

1- Messenger RNA1- Messenger RNA (mRNA): (mRNA):

Transfers DNA code to ribosomes for translation.Transfers DNA code to ribosomes for translation.

2- Transfer RNA2- Transfer RNA (tRNA): (tRNA):

Brings amino acids to ribosomes for protein Brings amino acids to ribosomes for protein synthesis.synthesis.

3- Ribosomal RNA3- Ribosomal RNA (rRNA): (rRNA):

Ribosomes are made of rRNA and protein.Ribosomes are made of rRNA and protein.

ABDULLATIF TAHA ABDULLAABDULLATIF TAHA ABDULLAMSc. BIOCHEMISTRYMSc. BIOCHEMISTRY