٣٢٥ Molecular Biology Sabah Linjawi ١

Lecture 3

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What is nucleic acid?

A nucleic acid is a macromolecule composed of chains of monomeric nucleotidesThese molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).Nucleic acids are universal in living things, as they are found in all cells and viruses. Nucleic acids were first discovered by Friedrich Miescher in 1871.

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What is nucleic acid?

The term "nucleic acid" is the generic name for a family of biopolymers, named for their role in the cell nucleus.The monomers from which nucleic acids are constructed are called nucleotides.Each nucleotide consists of three components: a nitrogenous heterocyclicbase, which is either a purine or a pyrimidine; a pentose sugar; and a phosphate group.

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Building Blocks - Nucleotides

A nucleotide is composed of three parts: pentose, baseand phosphate group.In DNA or RNA, a pentose is associated with only one phosphate groupbut a cellular free nucleotide (such as ATP) may contain more than one phosphate group.If all phosphate groups are removed, a nucleotide becomes a nucleoside

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Pentose

The chemical structure of pentose which contains five carbon atoms, labeled as C1' to C5'.The pentose is called ribose in RNA and deoxyribose in DNAbecause the DNA's pentose lacks an oxygen atom at C2'.Recalling that RNA stands for "ribonucleic acid", and DNA for "deoxyribonucleic acid".

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Bases

There are five different bases, each is denoted by a single letter as given in the parenthesis:Adenine (A), Cytosine (C), Guanine (G), Thymine (T), and Uracil (U).Among them,A, C, G and T exist in DNA;A, C, G and U exist in RNA.Their chemical structures are shown in the following figure. A and G contain a pair of fused rings, classified as purines. C, T, and U contain only one ring, classified as pyrimidines.

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Cellular Nucleotides and Nucleosides

In cells, a free nucleotide may contain one, two or three phosphate groups.The energy carrier ATP (adenosine triphosphate) has three phosphate groupsADP (adenosine diphosphate) has twoAMP (adenosine monophosphate) has one.Their structures are shown in the following figure.

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The Nucleic Acid Chain

In a nucleic acid chain, two nucleotides are linked by a phosphodiester bond, which may be formed by the condensation reaction similar to the formation of the peptide bond.In cells, such process has been found in the ligation between two nucleic acid fragments.However, the whole

nucleic acid chain is usually synthesized by RNA polymerase or DNA polymerase.

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DNA Structure

In a normal DNA molecule, adenine (A) is paired with thymine (T)guanine (G) is paired with cytosine

(C).The uracil (U) of RNA can also pair with adenine (A)U differs from T by only a methyl

group located on the other side of hydrogen bonding. A DNA molecule has two strands, held together by the hydrogen bonding between their bases adenine can form two hydrogen bonds with thymine cytosine can form three hydrogen bonds with guanine.

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DNA's B Form, A Form and Z Form

In a DNA molecule, the two strands are not parallel, but intertwined with each other. Each strand looks like a helix.The two strands form a "double

helix" structure, which was first discovered by James D. Watson and Francis Crick in 1953.This structure, also known as the B

form, the helix makes a turn every 3.4 nmThe distance between two

neighboring base pairs is 0.34 nm.There are about 10 pairs per turn.

The intertwined strands make two grooves of different widthsThe major groove and the minor

groove, which may facilitate binding with specific proteins.

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DNA's B Form, A Form and Z Form

In a solution with higher salt concentrations or with alcohol added, the DNA structure may change to an A form, which is still right-handed, but every 2.3 nm makes a turn and there are 11 base pairs per turn. Another DNA structure is called the Z form, because its bases seem to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising 12 base pairs.

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RNA Structure and Function

Most cellular RNA molecules are single stranded.They may form secondary structures such as stem-loop and hairpin.

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RNA Structure and Function

The major role of RNA is to participate in protein synthesis, which requires three classes of RNA :messenger RNA (mRNA)transfer RNA (tRNA)ribosomal RNA (rRNA) Other classes of RNA includeRibozymesThe RNA molecules with catalytic activity .Small RNA moleculesRNA interference and other functions .

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The mRNA-ribosome-tRNA complex formed during protein synthesis

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References

Wolfram Saenger, Principles of Nucleic Acid Structure, 1984, Springer-Verlag New York Inc. Keith Roberts, Martin Raff, Bruce Alberts, Peter Walter, Julian Lewis and Alexander Johnson, Molecular Biology of the Cell 4th Edition, Routledge, March, 2002, hardcover, 1616 pages, 7.6 pounds, ISBN 0-8153-3218-1The Biochemistry of the Nucleic Acids - by Roger Lionel Poulter Adams, John T Knowler -708 pagesNucleic Acids in Chemistry and Biology - by G Michael Blackburn, ... - 503 pagesThe Double Helix: A Personal Account of the ... - by James D Watson, Gunther SiegmundStent - 264 pagesLinksInterview with Aaron Klug, Nobel Laureate for structural elucidation of biologically important nucleic-acid protein complexes provided by the Vega Science Trust. Nucleic Acid Research JournalUC Berkeley video lecture on nucleic acids www.visionlearning.com/library/module_viewer.php?midwww.answers.com/topic/nucleic-acid