STRUCTURS OF DNA AND RNA

SUBMITTED BY S.GAYATHRI MSFBI1506 BMEM

DNA Deoxyribonucleic acid, a self replicating material

which is present in nearly all living organisms as the main constituent of chromosomes.

It is the carrier of genetic information.

Most DNA is located in the cell nucleus where is it called nuclear DNA, but a small amount of DNA can also be found in the mitochondrial it is called mtDNA.

STRUCTURE OF DNA

• DNA has three main components.

i. Deoxyribose (Pentose sugar)ii. Base (Nitrogen base pairs)iii. Phosphate group

Deoxyribose

• A sugar derived from ribose by replacement of a hydroxyl group by hydrogen.

• Role of deoxyribose: The nucleic acid DNA is built of nucleotides

with a deoxyribose sugar.

BASES

• A nitrogenous base is simply a nitrogen containing molecules that has the same chemical properties as a base.

• They are the particularly important since they make up the building blocks of DNA.

• Four bases are present in DNA(A,G,T,C).• They are classified into two divisionI. Pyrimidine II. Purine

• Pyrimidine made of one 6 members ring.• Purine made of 6member ring fused to a 5

member ring.

PHOSPHATE• DNA are made of nucleotides.• When it gets joined to the growing strand of

DNA , two of its phosphates are lost, and the remaining one attaches to another nucleotides sugar.

• 5’ end always has the phosphate attached.

<--Phosphodiester Bond

DOUBLE HELIX DNA

• The discovery in 1953 of the double helix, the twisted ladder structure of deoxyribonucleic acid by James watson and Crick.

How double helix structure maintained?• Sugar – phosphate backbone is hydrophilic so

is position on the outside.• Nitrogenous bases are very reactive so are

protected on inside.• Polynucleotide's are antiparallel, they run in

opposite directions.

DOUBLE HELIX AND HYDROGEN BONDING

• Made of two strands of nucleotides that are joined together by hydrogen bonding.

• Adenine and thymine are pair up and cytosine and guanine are pair up.

• Each pair is connected through hydrogen bonding.

• Hydrogen bonding always occurs between one pyrimiding and one purine.

TYPES AND CONFORMATIONS OF DNA B-DNA:• DNA duplex model proposed by watson and

crick is right handed spiral and is called B-DNA (Balanced DNA).

• It is more hydrated and most frequently found DNA in living cells.

• Single turn of helix has 10 base pairs.• It is physiologically and biologically active

forms. And it can changed into other forms.

A-DNA:• Another right handed duplex model is A-DNA

(Alternate DNA).• Here a single turn of helix has 11 base pairs.• Right handed DNA is known to change

temporarily into the left handed form at least for a short distance.

• Such changes may cause in gene expressions .

Z-DNA:• Z-DNA(zigzag DNA) is left handed double helix

with zigzag backbone alternate purine and pyrimidine bases.

• Single turn of 45Å length with 12 base pairs and single groove.

RNA

• Ribonucleic acid, a nucleic acid present in all living cells.

• It is principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins , although n some viruses RNA rather than DNA carries the genetic information.

• RNA is a polymer of ribonucleotieds linked together by 3’-5’ phosphodiester linkage.

PRIMARY STRUCTURE OF RNA

• Like DNA, each RNA strand has the same basic structure, composed of nitrogenous bases covalently bound to a sugar-phosphate backbone.

• However, unlike DNA, RNA is usually a single-stranded molecule.

TYPES OF RNA

• In all prokaryotic and eukaryotic organisms, three main classes of RNA molecules exit.

i. Messenger RNA (mRNA)ii. Transfer RNA (tRNA) iii. Ribosomal RNA (rRNA)

STRUCTURAL CHARACTERISTICS OF m-RNA• The 3’ end of most m-RNA have a polymer of

Adenylate residues (20-250)• The tail prevents the attack by 3’ exonucleases• Histones and interferons do not contain poly A

tails.• The m-RNA molecules are formed with the help of

DNA template during the process of transcription.

• The sequence of nucleotides in mRNA is complementary to the sequence of nucleotides on template DNA.

• The sequence carried on m-RNA is read in the form of codons.

• A codon is made up of 3 nucleotides.• The m-RNA is formed after processing of

heterogeneous nuclear RNA.

RIBOSOMAL RNA(r-RNA)• The eukaryotes ribosome contain two major

nucleoprotein subunits.• A large one with a molecular weight(60s) and

smaller subunit with a molecular weight(40s).• The 60s subunits contain a 5s ribosomal RNA,

5.8s rRNA, and a 28s rRNA; there are also probably more than 50 specific polypeptides.

• The 40s subunits smaller and contains a single 18s rRNA and approximately 30 distinct polypeptide chains.

TRANSFER RNA(tRNA)• Transfer RNA are the smallest of three major

species of RNA molecules.• They have 74-95 nucleotide residues.• They are synthesized by the nuclear

processing of a precursor molecules.• They transfer the amino acid from cytoplasm

to the protein synthesizing machinery, hence the name tRNA.

• They are easily soluble, hence called “Soluble RNA or sRNA.

• They are also called adaptor molecules.• They are at least 20 species of tRNA one

corresponding to each of the 20 amino acid required for protein synthesis.

STRUCTURAL CHARECTERSTICS OF tRNA

• tRNA classified into 3 divisions.i. Primary structure ii. Secondary structure iii. Tertiary structure

PRIMARY STRUCTURE OF tRNA

• Linear sequence of nucleotides is 60-90 in nucleotides long but most commonly 76.

• Many modified bases, sometimes accounting for 20% of the total bases in any one tRNA molecules.

• All of them are created post transcriptioally.

SECONDARY STRUCTURE OF tRNA

• Each single tRNA shows extensive internal base pairing and acquires a clover leaf like structure.

• The structure is stabilized by hydrogen bonding between the bases and in a consistent feature.

• All tRNA contain 5 main arms or loops which areas follows:

i. Acceptor armii. Anticodan armiii. D HU armiv. T Ψ C armv. Extra arm

TERTIARY STRUCTURE OF t-RNA

• The L-shaped tertiary structure is formed by further folding of the clover leaf due to hydrogen bonds between T and D arms.