CHEM 203                                        Biochemistry
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Unit  Nucleic acids PowerPoint PPT Presentation

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CHEM 203 Biochemistry. Unit  Nucleic acids. Nucleoproteins : are conjugated proteins formed of: a) basic protein ( histone or protamine ) and b) nucleic acid as prothetic group.

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Unit  Nucleic acids

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CHEM 203 Biochemistry

Unit 

Nucleic acids

Ola Fouad Talkhan

Nucleoproteins : are conjugated proteins formed of:

a) basic protein (histone or protamine) and

b) nucleic acid as prothetic group.

They are very complex high molecular weight proteins present in every cell.

Functions of nucleic acids

1-In cell nuclei they form the chromosomes which are responsible for cell division and carries of hereditary factors known as (genes).

2-In cytoplasm are associated with ribosome, the center of protein biosynthesis in every cell.

Ola Fouad Talkhan

Nucleic acid can be easily separated from nucleoprotein by addition of acids or alkalis .




Histone or protamin

(one or more molecules)

Nucleic acids

  • Nucleic acids

  • Nucleic acids is composed of large number of nucleotides,which considered as basal structural component of nucleic acids.

  • There are two types of nucleic acid

  • Deoxyribonucleic acid (DNA)

  • Ribonucleic acid (RNA)

Ola Fouad Talkhan



Histone or protamin

Nucleic acid


Large number of mononucleotides


Phosphoric acid

Nitrogenous base


Ribose Deoxyribose

Purin base Pyrimidine base

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• Nucleic acids are polynucleotides

• Their building blocks are nucleotides

  • Nucleic acids consist of nucleotides that have a nitrogen base, Pentose sugar , and phosphate

Ola Fouad Talkhan

Nitrogenous Bases

The nitrogen bases in nucleic acids are:

  • Pyrimidine bases:




  • purine bases:



Ola Fouad Talkhan

Pentose Sugars

The pentose (five-carbon) sugar:

  • In RNA is ribose.

  • In DNA is deoxyribose.

  • Has carbon atoms numbered with primes to distinguish them from the nitrogen bases.

Ola Fouad Talkhan


A nucleoside:

  • Has a nitrogen base linked by a glycosidic bond to C1’ of a ribose or deoxyribose.

  • Is named by changing the the nitrogen base ending to -osine for purines and –idine for pyrimidines


Ola Fouad Talkhan


A nucleotide:

  • Is a nucleoside that forms a phosphate ester with the C5’ OH group of ribose or deoxyribose.

  • Is named using the name of the nucleoside followed

    by 5’-monophosphate.

  • In a nucleoside ,the glycosidic C-1 atom of the pentose bonded to

    N-1 of the pyrimidine

    or N-9 of the purine base

Ola Fouad Talkhan

Nitrogenous bases + ribose = NucleosidesNucleoside + Phosphate group = Nuclotides

Ola Fouad Talkhan

Names of Nucleosides and Nucleotides

Ola Fouad Talkhan

Primary Structure of Nucleic Acids

In the primary structure of nucleic acids:

  • Nucleotides are joined by phosphodiester bonds.

  • The 3’-OH group of the sugar in one nucleotide forms an ester bond to the phosphate group on the 5’-carbon of the sugar of the next nucleotide.

Ola Fouad Talkhan

Primary Structure of Nucleic Acids

Ola Fouad Talkhan

Structure of Nucleic Acids

A nucleic acid polymer:

  • Has a free 5’-phosphate group at one end and a free 3’-OH group at the other end.

  • Is read from the free 5’-end using the letters of the bases.

  • This example reads


Ola Fouad Talkhan


  • In RNA, A, C, G, and U are linked by 3’-5’ ester bonds between ribose and phosphate.

Ola Fouad Talkhan

Types of RNA

Ola Fouad Talkhan


  • In DNA, A, C, G, and T are linked by 3’-5’ ester bonds between deoxyribose and phosphate.

Ola Fouad Talkhan

DNA Double Helix: A Secondary Structure


  • There are two strands of nucleotides that wind together in a double helix.

  • Two hydrogen bonds form between the complementary base pairs A-T.

  • Three hydrogen bonds form between the complementary base pairs G-C.

Ola Fouad Talkhan

The negatively charged phosphate group and the sugar units expose themselves to the outside of the chain.

Ola Fouad Talkhan

DNA Double Helix Structure

Ola Fouad Talkhan

The antiparallel nature of the DNA double helix.

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The DNA Double Helix

Watson and Crick were Proposed a structure of DNA double helix

The double helix is stabilized by hydrogen bonds and hydrophobic interactions

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1.Two helical polynucleotide chains are coiled around a common axis. The chains run in opposite directions, (anti parallel).2. The two antiparallel polynucleotide chains are not identical, but they are complimentary.3.The purine, pyrimidine bases are on the inside of the helix, the phosphate and deoxyribose groups are on the outside.4. The two chains are held together by hydrogen bonds between pairs of bases. Adenine is two hydrogen bonds (A= T), Guanine is bonded to cytosine by three hydrogen bonds (G=C)

Ola Fouad Talkhan

Comparison between DNA and RNA

  • DNA - one type, one purpose .

  • RNA - Several types, several purposes:

    • ribosomal RNA - the basis of structure and function of ribosomes (largest amount).

    • messenger RNA - carries the message for protein synthesis (fewest and unique).

    • transfer RNA - carries the amino acids for protein synthesis (smallest molecules).

Ola Fouad Talkhan

DNA Replication

The duplication of DNA to give two DNA molecules identical to the original one.

DNA in the chromosomes replicates itself every cell division

•Maintains correct genetic information

DNA replication involves:

  • Unwinding the DNA

  • Pairing the bases in each strand with new bases to form new complementary strands.

  • Producing two new DNA strands that exactly duplicate the original DNA.

Ola Fouad Talkhan

  • Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides.

  • Primase is the enzyme that synthesizes the RNA Primer.

  • DNA polymerase can then add the new nucleotides

DNA polymerase can only add nucleotides to the 3’ end of the DNA.

•This causes the NEW strand to be built in a 5’ to 3’ direction

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DNA Replication - General considerations

Replicating DNA

A. Function of replication.

  • Proteins must have the correct shape.

  • The shape is determined by the primary structure (amino acid sequence.

Base Pairing & Double Helix

  • The amino acid sequence is determined by the gene (the sequence of bases in the DNA).

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Information Transfer in Cells

Figure 10.1

The fundamental process of information transfer in cells.

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  • Process of copying DNA to mRNA

  • Differs from DNA synthesis in that only one strand of DNA, the template strand, is used to make mRNA

  • Does not need a primer to start as RNA polymerases have ability to initiate synthesis de novo

  • Can involve multiple RNA polymerases

  • Divided into 3 stages

  • Initiation

  • Elongation

  • Termination

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Transcription: Synthesis of mRNA


  • A section of DNA containing the gene unwinds.

  • One strand of DNA is copied starting at the initiation point, which has the sequence TATAAA.

  • An mRNA is synthesized using complementary base pairing with uracil (U) replacing thymine (T).

  • The newly formed mRNA moves out of the nucleus to ribosomes in the cytoplasm.

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RNA Polymerase

  • During transcription, RNA polymerase moves along the DNA template in the 3’-5’direction to synthesize the corresponding mRNA.

  • The mRNA is released at the termination point.

Ola Fouad Talkhan


  • Interpreting the information coded in the mRNA into proteins

  • The nucleotides are read in triplets (set of three) called codons

  • Each triplet code for a specific amino acid, and sometimes more than one codon exist for an amino acid

  • mRNA are read by the translational machinery including ribosomes, tRNAs and rRNAs

  • Like transcription, it also includes initiation, elongation and termination

Ola Fouad Talkhan


Ola Fouad Talkhan

DNA Replication - General considerations

The flow of genetic information in the cell.

A. Function of replication.

  • Proteins must have the correct shape.

  • The shape is determined by the primary structure (amino acid sequence.

  • The amino acid sequence is determined by the gene (the sequence of bases in the DNA).

DNA  RNA  protein

Ola Fouad Talkhan


  • A change or alteration that occurs in the DNA. Mutations can be caused by the environment (sun, radiation, or chemicals), aging, or chance. Some mutations do not affect the information contained in the DNA. Other mutations have serious consequences on how that gene functions.

Ola Fouad Talkhan

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