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ZOOLOGY: SEM- II, PAPER- C4T: CELL BIOLOGY, UNIT 6: NUCLEUS

N N u u c c l l e e o o t t i i d d e e

B B B Y Y Y

D D R R . . P P O O U U L L A A M M I I A A D D H H I I K K A A R R Y Y M M U U K K H H E E R R J J E E E E A A S S S S I I S S T T A A N N T T P P R R O O F F E E S S S S O O R R

D D E E P P A A R R T T M M E E N N T T O O F F Z Z O O O O L L O O G G Y Y

N N A A R R A A J J O O L L E E R R A A J J C C O O L L L L E E G G E E

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ZOOLOGY: SEM- II, PAPER- C4T: CELL BIOLOGY, UNIT 6: NUCLEUS

Nucleic Acids: Properties, Structure, and Functions

Nucleic acids are the biopolymers, or large biomolecules, essential to all known forms of life.

Nucleic acid is an important class of macromolecules found

in all cells and viruses.

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The term nucleic acid is the overall name for DNA and RNA.

There are two types of nucleic acids, namely deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Primarily, nucleic acids serve as repositories and

transmitters of genetic information. The functions of

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nucleic acids are concerned with the storage and expression of genetic information.

The genetic material in most organisms is DNA or Deoxyribonucleic acid; whereas in some viruses, it is RNA or Ribonucleic acid.

They are composed of nucleotides, which are the

monomers made of three components: a 5-carbon sugar, a

phosphate group and a nitrogenous base.

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A DNA molecule consists of two polynucleotide chains i.e.

chains with multiple nucleotides.

Structure of Polynucleotide Chain:

A nucleotide is made of the following components:

 Pentose sugar: A pentose sugar is a 5-carbon sugar. If

the sugar is a compound ribose, the polymer is RNA

(ribonucleic acid); if the sugar is derived from ribose as

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deoxyribose, the polymer is DNA (deoxyribonucleic

acid).

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 Phosphate group

 Nitrogenous base – These can be of two types –

Purines and Pyrimidines. Purines include Adenine and

Guanine whereas pyrimidines include Cytosine and

Thymine. In RNA, thymine is replaced by Uracil.

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ZOOLOGY: SEM- II, PAPER- C4T: CELL BIOLOGY, UNIT 6: NUCLEUS

Nitrogenous base + pentose sugar (via N-glycosidic linkage) = Nucleoside.

Nucleoside + phosphate group (via phosphodiester linkage) = Nucleotide.

Nucleotide + Nucleotide (via 3′-5′ phosphodiester linkage)

= Dinucleotide.

Many nucleotides linked together = Polynucleotide.

A polynucleotide has a free phosphate group at the 5′ end

of the sugar and this is called the 5′ end.

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Similarly, the sugar also has a free 3′-OH group at the other end of the polynucleotide which is called the 3′ end.

The backbone of a polynucleotide chain consists of pentose sugars and phosphate groups; whereas the nitrogenous bases project out of this backbone.

Nucleic acids are the most important of all biomolecules.

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These are found in abundance in all living things, where they function to create and encode and then store information of every living cell of every life-form organism on Earth.

In turn, they function to transmit and express that

information inside and outside the cell nucleus—to the

interior operations of the cell and ultimately to the next

generation of each living organism.

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The encoded information is contained and conveyed via the nucleic acid sequence, which provides the 'ladder-step' ordering of nucleotides within the molecules of RNA and DNA.

Strings of nucleotides are bonded to form helical

backbones—typically, one for RNA, two for DNA—and

assembled into chains of base-pairs selected from the five

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primary, or canonical, nucleobases, which are: adenine, cytosine, guanine, thymine, and uracil.

Thymine occurs only in DNA and uracil only in RNA. Using

amino acids and the process known as protein synthesis,

the specific sequencing in DNA of these nucleobase-pairs

enables storing and transmitting coded instructions as

genes.

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In RNA, base-pair sequencing provides for manufacturing new proteins that determine the frames and parts and most chemical processes of all life forms.

Definition:

A nucleic acid is a chain of nucleotides, which stores genetic information in biological systems. It creates DNA and RNA, which store the information needed by cells to create proteins.

This information is stored in multiple sets of three nucleotides,

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known as codons. Thus, a nucleic acid is a large molecule made

up of a string, or “polymer,” of units called “nucleotides.”

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Brief History:

Nuclein were discovered by Friedrich Miescher in 1869.

In the early 1880s Albrecht Kossel further purified the substance and discovered its highly acidic properties. He later also identified the nucleobases.

In 1889 Richard Altmann creates the term nucleic acid

In 1938 Astbury and Bell published the first X-ray

diffraction pattern of DNA.

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In 1953 Watson and Crick determined the structure of DNA.

DNA was discovered in 1869 by Johann Friedrich Miescher, a Swiss researcher. The demonstration that DNA contained genetic information was first made in 1944, by Avery, Macleod and MacCary.

Experimental studies of nucleic acids constitute a major

part of modern biological and medical research, and form a

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foundation for genome and forensic science, and the biotechnology and pharmaceutical industries.

Functions of Nucleic Acids:

 DNA is the chemical basis of heredity and may be regarded as the reserve bank of genetic information.

 DNA is exclusively responsible for maintaining the identity

of different species of organisms over millions of years.

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 Further, every aspect of cellular function is under the control of DNA. The DNA is organized into genes, the fundamental units of genetic information.

 The genes control the protein synthesis through the

mediation of RNA, as shown below:

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This interrelationship of these three classes of biomolecules

(DNA, RNA and proteins) constitutes the central dogma of

molecular biology or more commonly the central dogma of life.

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Components of Nucleic Acids:

Nucleic acids are the polymers of nucleotides (polynucleotides) held by 3′ and 5′ phosphate bridges. In other words, nucleic acids are built up by the monomeric units—

nucleotides.

Nucleotides:

Nucleotides are composed of a nitrogenous base, a pentose

sugar and a phosphate. Nucleotides perform a wide variety of

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functions in the living cells, besides being the building blocks

or monomeric units in the nucleic acid (DNA and RNA)

structure. These include their role as structural components of

some coenzymes of B-complex vitamins (e.g. FAD, NAD+), in

the energy reactions of cells (ATP is the energy currency), and

in the control of metabolic reactions.

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Structure of Nucleotides:

The nucleotide essentially consists of base, sugar and phosphate. The term nucleoside refers to base + sugar. Thus, nucleotide is nucleoside + phosphate.

Purines and pyrimidine’s:

The nitrogenous bases found in nucleotides (and, therefore,

nucleic acids) are aromatic heterocyclic compounds. The bases

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are of two types—purines and pyrimidine’s. Their general

structure are given below.

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ZOOLOGY: SEM- II, PAPER- C4T: CELL BIOLOGY, UNIT 6: NUCLEUS

Tautomeric forms of purines and pyrimidine’s:

The existence of a molecule in a keto (lactam) and enol

(lactim) form is known as tautomerism. The heterocyclic rings

of purines and pyrimidine’s with oxo functional groups

exhibit tautomerism as simplified below:

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The purine—guanine and pyrimidine’s-cytosine, thymine and

uracil exhibit tautomerism. The lactam and lactim forms of

cytosine are represented below:

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At physiological pH, the lactam (keto) tautomeric forms are predominantly present.

Minor bases found in nucleic acids:

Besides the bases described above, several minor and unusual

bases are often found in DNA and RNA. These include 5-

methylcytosine, N4-acetylcytosine, N6– methyladenine, N6,

N6-dimethyladenine, pseudouracil etc. It is believed that the

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unusual bases in nucleic acids will help in the recognition of specific enzymes.

Nomenclature of Nucleotides:

The addition of a pentose sugar to base produces a nucleoside.

If the sugar is ribose, ribonucleosides are formed. Adenosine,

guanosine, cytidine and uridine are the ribonucleosides of A, G,

C and U respectively. If the sugar is a deoxyribose, deoxyribo-

nucleosides are produced.

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The term mononucleotide is used when a single phosphate moiety is added to a nucleoside. Thus, adenosine monophosphate (AMP) contains adenine + ribose + phosphate.

The principal bases, their respective nucleosides and

nucleotides found in the structure of nucleic acids are given in

Table below. Note that the prefix ‘d’ is used to indicate if the

sugar is deoxyribose (e.g. dAMP).

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ZOOLOGY: SEM- II, PAPER- C4T: CELL BIOLOGY, UNIT 6: NUCLEUS

The Binding of Nucleotide Components:

The atoms in the purine ring are numbered as 1 to 9 and for

pyrimidine as 1 to 6. The carbons of sugars are represented

with an associated prime (‘) for differentiation. Thus the

pentose carbons are 1′ to 5’. The pentose’s are bound to

nitrogenous bases by β-N-glycosidic bonds. The N9 of a purine

ring binds with C1(1’) of a pentose sugar to form a covalent

bond in the purine nucleoside. In case of pyrimidine

nucleosides, the glycosidic linkage is between N1 of a

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pyrimidine and C’1 of a pentose. The hydroxyl groups of adenosine are esterified with phosphates to produce 5′- or 3′- monophosphates. 5′-Hydroxyl is the most commonly esterified, hence 5′ is usually omitted while writing nucleotide names. Thus AMP represents adenosine 5′-monophosphate.

However, for adenosine 3′-monophosphate, the abbreviation 3′-AMP is used. The structures of two selected nucleotides namely AMP and TMP.

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Nucleoside Di- and Triphosphates:

Nucleoside monophosphates possess only one phosphate

moiety (AMP, TMP). The addition of second or third

phosphates to the nucleoside results in nucleoside

diphosphate (e.g. ADP) or triphosphate (e.g. ATP),

respectively. The anionic properties of nucleotides and nucleic

acids are due to the negative charges contributed by

phosphate groups.

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