DNA and RNA[Introduction, Components,

and Structure]

M A Y A N N R O G O N

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DNA[Introduction, Components,

and Structure]

DNA [Deoxyribonucleic acid]

a polymer of deoxyribo-nucleotides.

Usually double stranded.

And have double-helix structure.

found in chromosomes, mitochondria and

chloroplasts.

It acts as the genetic material in most of the

organisms.

Carries the genetic information

A Few Key Events Led to the Discovery of the Structure of DNA

Friedrich Meischer

DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1868.

He named it as ‘Nuclein’.

James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

In 1953 , James Watson and Francis Crick, described a very simple but famous DoubleHelix model for the structure of DNA.

X-Ray EvidenceRosalind Franklin

British Scientist

Used a technique called X-Ray diffraction

Provided important clues about the structure of DNA

X-Ray Evidence

There were 2 strands

Strands were twisted around each other (helix)

The nitrogen bases are in the middle

The Double Helix• Francis Crick & James Watson

• Trying to understand the structure of DNA by building models.

• Unsuccessful until early 1953, Watson was shown a copy of Franklin’s X-ray pattern.

• “The instant I saw the picture my mouth fell open and my pulse began to race.”– James Watson

• Within weeks Watson and Crick had figured out the structure of DNA

• Published their results in a historic one page paper in April of 1953

• Watson and Crick later discovered what held the two strands together.

• Hydrogen bonds could form between certain nitrogen bases and provide enough force to hold the two strands together.

• Hydrogen bonds could only form between certain base pairs adenine and thymine and guanine and cytosine.

• This principal is called Base pairing.

• This explains Chargaff’s Rule.

Components of DNA

Deoxyribose[a pentose

sugar]

Nitrogen Base(there are four different ones)

Phosphate

H

OH

OCH2

Base

Phosphate

Ribose

OHH

5′

4′

1′

3′2′

H

A, G, C or U

HH

H

OCH2

Base

Phosphate

Deoxyribose

5′

OHH

4′ 1′

3′ 2′

OO

O

P

O–HH

DNA Nucleotide RNA Nucleotide

A, G, C or T

CCytosine

GGuanine

AAdenosine

TThymine

4 Kinds

of

Nitrogen Bases

Purines

Pyrimidines

Nitrogenous Bases of DNA & RNA

Nucleotide Structure

Nucleotides are formed by the condensation of a sugar, phosphate and one of the 4 bases

The following illustration represents one nucleotide

Phosphate

Deoxyribose

NitrogenousBases

HH

H

OCH2

Base

DNA nucleotide

Phosphate

Deoxyribose

5′

OH

4′ 1′

3′ 2′

OO

O

P

O–HH

Base + sugar nucleoside

Example

Adenine + ribose = Adenosine

Adenine + deoxyribose = Deoxyadenosine

Base + sugar + phosphate(s) nucleotide

Example

Deoxyadenosine monophosphate (dAMP)

Deoxyadenosine diphosphate (dADP)

Deoxyadenosine triphosphate (dATP)

Nomenclature of Nucleic Acid Components

Base Nucleoside Nucleotide Nucleic

acid

Purines

Adenine Adenosine Adenylate RNA

Deoxyadenosine Deoxyadenylate DNA

Guanine Guanosine Guanylate RNA

Deoxy guanosine Deoxyguanylate DNA

Pyrimidines

Cytosine Cytidine Cytidylate RNA

Deoxycytidine Deoxycytidylate DNA

Thymine Thymidine Thymidylate DNA

(deoxythymidine) (deoxythymidylate)

Uracil Uridine Uridylate RNA

SugarBase

P

SugarBase

P

Nucleotides are linked together by covalent bondscalled phosphodiester linkage.

1

23

4

5

1

23

4

5

A chemical bond that involves sharing a pair of electrons between atoms in a molecule.

Antiparallel strands

The strands run opposite of each other.

The 5’ end always has the phosphate attached.

It is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside.

The two chains have anti- parallel polarity. It means, if one chainhas the polarity 5’-3’, the other has 3’-5’.

5’ 3’

3’ 5’

G C

T A

C G

A T

DNA Double Helix & Hydrogen bondingSalient features of the Double-helix structure of DNA:

DNA Double Helix & Hydrogen bonding

3 Hydrogen bonds2 Hydrogen bonds

The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thyminefrom opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds.

Based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenineand Thymine; and Guanine and Cytosine are constant and equals one.

Hydrogen bond

a chemical bond inwhich a hydrogen atomof one molecule isattracted to anelectronegative atom,especially a nitrogen,oxygen, or fluorineatom, usually ofanother molecule.

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DNA Double Helix

There are two asymmetrical grooves on the outside of the helix:a)Major grooveb)Minor groove

Groove any furrow(slight depression in the smoothness of a surface)or channel on a bodily structure or part.

Certain proteins can bind within these grooveThey can thus interact with a particular sequence of bases.

(b) Space-filling model of DNA(a) Ball-and-stick model of DNA

Minorgroove

Majorgroove

Minorgroove

Majorgroove

Structure of Double-helix

B- DNA

A- DNA

Z- DNA

Biologically THE MOST COMMONIt is a -helix meaning that it has

a Right handed, or clockwise, spiral.

Complementary base pairing• A-T• G-C

Minor Groove is Narrow, Shallow.Major Groove is Wide, Deep.

B- DNA

This structure exists when plenty of water surrounds molecule and there is no unusual base sequence in DNA-Condition that are likely tobe present in the cells.

B-DNA structure is most stable configuration for a random sequence of nucleotides under physiological condition.

A- DNA

Right-handed helixWider and flatter than B-DNAIts bases are tilted away from

main axis of moleculeNarrow Deep major Groove and

Broad, Shallow minor Groove.Observed when less water is

present. i.e.Dehydratingcondition.

A-DNA has been observed in two context:• Active site of DNA polymerase

(~3bp)• Gram (+) bacteria undergoing

sporulation

Z- DNA

A left-handed helixSeen in Condition of High salt

concentration.In this form sugar-phosphate

backbones zigzag back andforth, giving rise to the nameZ-DNA(for zigzag).

12 base pairs per turn.

A deep Minor Groove.No Discernible Major Groove.Part of some active genes form

Z-DNA, suggesting that Z-DNA may play a role in regulating gene transcription.

RNA[Introduction, Components,

and Structure]

RNA [Ribonucleic acid]

RNA is a polymer of ribonucleotides linked

together by phosphodiester linkage.

RNA was first genetic material.

In 1967 Carl Woese found the catalytic

properties of RNA and speculated that the

earliest forms of life relied on RNA both

to carry genetic information and to catalyse

biochemical reactions.

Their theories were not validated until the work of

Nobel Prize laureate Thomas R. Cech. In the

1970s, Cech was studying the splicing of RNA in a

single-celled organism, Tetrahymena thermophila,

when he discovered that an unprocessed RNA

molecule could splice itself. He announced his

discovery in 1982 and became the first to show

that RNA has catalytic functions.

RNA [Ribonucleic acid]

RNA exists in several different single-stranded

structures, most of which are directly or indirectly

involved in protein synthesis or its regulation.

It also acts as the genetic material in some

viruses.

It function as messenger(mRNA), adapter(tRNA),

structural(rRNA) and in some cases as a catalytic

molecule(Ribozyme).

RNA strands are typically several hundred to

several thousand nucleotides in length.

Components of DNA

Ribose[a pentose

sugar]

Nitrogen Base(there are four different ones)

Phosphate

CCytosine

GGuanine

AAdenosine

UUracil

4 Kinds

of

Nitrogen Bases

Purines

Pyrimidines

Nucleotide Structure

Nucleotides are formed by the condensation of a sugar, phosphate and one of the 4 bases

The following illustration represents one nucleotide

Phosphate

Ribose

NitrogenousBases

HH

OH

OCH2

Base

RNA nucleotide

Phosphate

Ribose

5′

OH

4′ 1′

3′ 2′

OO

O

P

O–HH

Base + sugar nucleoside Example

Adenine + ribose = Adenosine

Base + sugar + phosphate(s) nucleotide

Example

Adenosine monophosphate (AMP)

Adenosine diphosphate (ADP)

Adenosine triphosphate (ATP)

SugarBase

P

SugarBase

P

Nucleotides are linked together by covalent bondscalled phosphodiester linkage.

1

23

4

5

1

23

4

5

A chemical bond that involves sharing a pair of electrons between atoms in a molecule.

Adenine (A)

Guanine(G)

Uracil (U)

BasesBackbone

Cytosine (C)

O

HHHH

OOO

O –

P CH2

O –

HHHH

OOO

O

P CH2

O –

NH2

H

N

HHHH

OOO

O

P CH2

O –

H

H

HH

OH

HH

OOO

O

P CH2

O –

Sugar (ribose)

Phosphate

5′

4′ 1′

2′3′

5′

4′ 1′

2′3′

5′

4′ 1′

2′3′

5′

4′ 1′

2′3′OH

OH

OH

OH

RNAnucleotide

Phosphodiesterlinkage

3′

5′

NH2

OH

H

H

O

NH O

N

NN

N

N

N

N

N N

N

NH2

H

H

Structure of RNA

Types of RNA

In all prokaryotic and eukaryotic organisms, three main classes of RNA molecules exist

1) Messenger RNA(m RNA)

2) Transfer RNA (t RNA)

3) Ribosomal RNA (r RNA)

Messenger RNA (mRNA) 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.

In eukaryotic cells, once

precursor mRNA (pre-mRNA)

has been transcribed from DNA,

it is processed to mature mRNA

This removes its introns—non-

coding sections of the pre-

mRNA

The mRNA is then exported

from the nucleus to the

cytoplasm, where it is bound to

ribosomes and translated into its

corresponding protein form with

the help of tRNA

Transfer RNA (tRNA)

Transfer RNA (tRNA) is a small

RNA chain of about

80 nucleotides

It 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

Ribosomal RNA (rRNA)

Ribosomal RNA (rRNA) is the

catalytic component of the

ribosomes

Three of the rRNA molecules

are synthesized in

the nucleolus, and one is

synthesized elsewher

In the cytoplasm, ribosomal

RNA and protein combine to

form a nucleoprotein called a

ribosome

The ribosome binds mRNA

and carries out protein

synthesis

Several ribosomes may be

attached to a single mRNA at

any time.

Nearly all the RNA found in a

typical eukaryotic cell is rRNA.

RNA vs. DNA

Structurally ,DNA and RNA are nearly identical .However there are three fundametaldifferences that account for the very different functions of two molecules.

Differences between RNA and DNA

REFERENCES:• J. D. Watson and F. H. C. Crick. Molecular structure of nucleic acids: a

structure for deoxyribose nucleic acids. Nature 171:737–738 (1953).

• J. D. Watson and F. H. C. Crick. Genetical implications of the structure of deoxyribonucleic acid.Nature 171:964–967 (1953).

• U.satyanarayana. Structure of DNA and RNA. Biochemistry. Retrieved from: http://www. Slideshare.com./ph

• Lehninger, Micheal M. Cox and David l. Nelson .Principle of biochemistry. Retrieved from: www. Slideshare.com.ph

• Tazeen Anwaar and Uzma Imtiyaz. Presentation on DNA and RNA Structure. Retrieved from: http://www. Slideshare.com./ph

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