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NUCLEIC ACIDS:

• Are biological molecules essential for known forms of life on earth

• They include DNA and RNA

• Discovered by Friedrich Miescher in 1869

• They are named so because of their initial discovery in nucleus

DNA stands for deoxyribo nucleic acid

This chemical substance is present in the nucleusof all cells in all living organisms

DNA controls all the chemical changes which take place in cells

The kind of cell which is formed, (muscle, blood,nerve etc) is controlled by DNA

The kind of organism which is produced (buttercup,giraffe, herring, human etc) is controlled by DNA

DNA 2

DNA is a very large molecule made up of a long chain of sub-units

The sub-units are called nucleotides

Each nucleotide is made up of

a sugar called deoxyribose

a phosphate group -PO4 and

an organic base

DNA molecule 3

The deoxyribose, the phosphate and one of the bases

adenine

deoxyribose

PO4

Combine to form a nucleotide

NUCLEOTIDES6

The most common organic bases are

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

THE BASES5

The bases always pair up in the same way

Adenine forms a bond with Thymine

and Cytosine bonds with Guanine

Bonding 1 10

Adenine Thymine

Cytosine Guanine

• Adenine and guanine are known as PURINES

• Thymine and cytosine are known as PYRIMIDINES, in RNA instead of thymine it is Uracil

• Adenine pairs with Thymine forming two hydrogen bonds

• Cytosine pairs with Guanine forming with three hydrogen bonds

Purines Pyrimidines

Purines are double ringed structures Pyrimidines are single ringed structures

Example of purine bases are Adenine and Guanine

Example of pyrimidine bases are Thymine and Cytosine( in DNA), Uracil and cytosine ( in RNA)

Ribose is a sugar, like glucose, but with only fivecarbon atoms in its molecule

Deoxyribose is almost the same but lacks one oxygen atom

Both molecules may be represented by the symbol

THE SUGARS:RIBOSE & DEOXYRIBOSE4

STRUCTURE OF DEOXYRIBONUCLEOTIDE AND RIBONUCLEOTIDE

NUCLEOSIDE NUCLEOTIDE

Nucleoside is a component formed by the union of a nitrogen base with a pentose sugar

Nucleotide is a component formed by the union of a nitrogen base, a pentose sugar and phosphate

It is a component of nucleotide It formed after phosphorylation of nucleoside

FUNCTIONS OF DNA:

• It is the genetic material, therefore responsible for carrying all the hereditary information.

• It has property of replication essential for passing genetic information from one cell to its daughters or from one generation to next.

• Crossing over produces recombination

• Changes in sequence and no. of nucleotides causes Mutation which is responsible for all variations and formation of new species.

• It controls all the metabolic reaction of cells through RNAs and RNA directed synthesis of proteins.

A molecule of DNA is formed by millions of nucleotides joined together in a long chain

PO4

PO4

PO4

PO4

sugar-phosphate backbone

+ bases

Joined nucleotides 7

In fact, the DNA usually consists of a double strand of nucleotides

The sugar-phosphate chains are on the outsideand the strands are held together by chemical bonds between the bases

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PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

2-stranded DNA 9

PO4

PO4

PO4

thymine

PO4

PO4

PO4

PO4

adenine

cytosine

PO4

guanine

Bonding 2 11

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

Pairing up 12

ROLE OF PHOSPHODIESTER LINKAGE

• Phosphodiester Bonds Link Successive Nucleotides in Nucleic Acids

• The successive nucleotides of both DNA and RNA are covalently linked through phosphate-group “bridges,” in which the 5’-phosphate group of one nucleotide unit is joined to the 3-hydroxyl group of the next nucleotide, creating a phosphodiester linkage .

The chain has a 5 end, which is usually attached to a phosphate, and a 3 end, which is usually a free hydroxyl group

The paired strands are coiled into a spiral called

A DOUBLE HELIX

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sugar-phosphatechain

bases

THE DOUBLE HELIX

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ANTIPARALLEL NATURE

• The two strands of DNA are also antiparallel(run in opposite directions) to one another.

• A strand of DNA can have the direction 5'-3' or 3'- 5'.

• One strand in the DNA molecule is 5'-3' and the other strand is 3'-5'.

DNA STRUCTURE

• The two strands run antiparallely and are complimentary to each other.

• The coiling is right handed.

• The major groove occurs where the backbones are far apart, the minor groove occurs where they are close together.

• Two helical polynucleotide chains are coiled around a common axis. The chains run in opposite directions.

• The sugar-phosphate backbones are on the outside and, therefore, the purine and pyrimidine bases lie on the inside of the helix.Adenine always pairs with Thymine, Guanine binds with Cytosine

• The bases are nearly perpendicular to the helix axis, and adjacent bases are separated by 3.4 Å.

• The helical structure repeats every 34 Å, so there are 10 bases (= 34 Å per repeat/3.4 Å per base) per turn of helix.

• There is a rotation of 36 degrees per base (360 degrees per full turn/10 bases per turn).

• The diameter of the helix is 20 Å.

CHARGAFFS RULE:

• “It states that DNA from any cell of all organisms should have a 1:1 ratio of pyrimidine and purine bases and more specifically that the amount of guanine is equal to amount of cytosine and amount of adenine is equal to amount of thymine”

• %A=%T

• %G=%C

DENATURATION OR MELTING

• The hydrogen bonds between the nitrogenous bases of complementray DNA can break due to high temperature, low or high pH known as Melting or Denaturation of DNA.

• A-T base pairs can go easy denaturation due to presence of only two hydrogen bonds, whereas G-C base pairs are comparitively more stable (high melting point) due to presence of three hydrogen bonds.

RNA

• Like DNA, RNA is also made of a long chain of

nucleotides.

• Some RNA molecules play an active role in cells by

catalyzing biological reactions, controlling gene

expression, or sensing and communicating responses to

cellular signals.

• In RNA molecules instead of the base T, base U is found.

• Generally three types of RNA molecules are found in the

cell – mRNA, rRNA, and tRNA.

MESSENGER RNA:mRNA• Comprise only 5% of total cellular RNA.

• Messenger RNA (mRNA) carries information about a protein

sequence to the ribosomes, the protein synthesis factories in the

cell.

• It is coded so that every three nucleotides (a codon) correspond

to one amino acid.

• The synthesis of mRNA takes place inside the nucleus and after

its synthesis, it is exported out where it attaches to ribosomes

for protein synthesis.

• After certain amount of time the message in mRNA is degraded

into its component nucleotides.

TRANSFER RNA:tRNA• tRNA represents 15% of total RNA in the cell.

• Transfer RNA (tRNA) is a small RNA chain of about 80

nucleotides that transfers a specific amino acid to a growing

polypeptide chain at the ribosomal site of protein synthesis

during translation.

• It has sites for amino acid attachment and an anticodon region

for codon recognition that binds to a specific sequence on the

messenger RNA chain through hydrogen bonding.

STRUCTURE OF tRNA• Anticodon arm :

• Made up of three nitrogen bases for recognising and attaching to the codon of mRNA

• A-A Binding Site:

It lies at the3’ end and has CCA-OH group

• TΨC Loop:

Contains pseudouridine, site for attaching the ribosomes

• DHU Loop:

Contains dihydrouridine, binding site for aminoacyl synthetase enzyme.

• Variable loop:

Lies between the anticodon and TΨC loop.

RIBOSOMAL RNA (rRNA):

• 80% of total RNA in the cells are rRNA

• Ribosomal RNA is a constituent of Ribosomes

• rRNA are found in combination with several proteins (about 82 proteins) as component of the ribosome which is the site of protein synthesis.

CENTRAL DOGMA• The set of ideas that describes how the cell uses the information

stored in DNA is called the Central dogma

• The first step of central dogma is Transcription

• Transcription uses DNA as a template to copy genetic information into form of RNA

• The second step of central dogma is Translation

• Translation synthesizes the protein using RNA as a template

Character RNA DNA

Nitrogenous bases RNA contains adenine, uracil, cytosine, and guanine.

DNA contain adenine, thymine, cytosine, guanine;

Strandedness RNA is single-stranded. DNA is double-stranded

Location RNA can be found in both the nucleus and cytoplasm.

DNA is only found in the nucleus

Functions RNA carries out protein synthesis. RNA transfers the information to other places in the cell

DNA contains the information for protein synthesis.DNA stores genetic information

Pentose sugar Hydroxyl group present at 2 carbon of the pentose sugar

Absence of hydroxyl group at 2 C of the pentose sugar