LECTERE 6LECTERE 6

Nucleic acids: classification, structure and biological role.

Lecturer: Dmukhalska Yevheniya. B.

PLAN1. Nucleic acids. Types of nucleic acids. 2. Nucleic acids composition. 3. Levels of structural organization DNA. Biological

functions. 4. Levels of structural organization RNA. Biological

functions.

Nucleic acids A Nucleic acids are polymers of nucleotides joined by 3',5' -phosphodiester bonds; that is, a phosphate group links the 3' carbon of a sugar to the 5' carbon of the next sugar in the chain. Each strand has a distinct 5' end and 3' end, and thus has polarity. A phosphate group is often found at the 5' end, and a hydroxyl group is often found at the 3' end.TThe Swiss physiologist Friedrich Miescher (1844 – 1895) discovered nucleic acids in 1869 while studying the nuclei of white blood cells. The fact that they were initially found in cell nuclei and are acidic accounts for the name nucleic acid. Although are now know that nucleic acids are found throughout а cell, not just in the nucleus, the name is still used for such materials.

Types of nucleic acids.

Two types of nucleic acids are found within cells of higher organisms: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nearly all the DNA is found within the cell nucleus. Its primary function is the storage and transfer of genetic information. This information is used (indirectly) to control many functions of a living cell. In addition, DNA is passed from existing cell to new cells during cell division RNA occurs in all parts of a cell. It functions primarily in synthesis of proteins, the molecules that carry out essential cellular functions.

The monomers for nucleic acid polymers, nucleotides, have а more complex structure than polysaccharide monomers (monosaccharides) or protein monomers (amino acids). Within each nucleotide monomer are three subunits. А nucleotide is, а molecule composed of a pentose sugar bonded to both a group and a nitrogen-containing hetero-cyclic base.

• Pentose sugars. • The sugar unit of а nucleotide is either the

pentose ribose or the 2-deoxyribose.

Nitrogen-containing bases. • Five nitrogen-containing bases are nucleotide

components. Three of them are derivatives of pyrimidine, а monocyclic base with а six-membered ring, and two are derivatives of purine, а bicyclic base with fused five- and six-membered rings.

Nucleosides• Nucleosides are compounds formed when a base is linked to a

sugar via a glycosidic bond. • Glycosidic bonds by definition involve the carbonyl carbon

atom of the sugar, which in cyclic structures is joined to the ring O atom. Such carbon atoms are called anomeric. In nucleosides,the bond is an N-glycoside because it connects the anomeric C-1 to N-1 of a pyrimidine or to N-9 of a purine. Glycosidic bonds can be either or , depending on their orientation relative to the anomeric C atom. Glycosidic bonds in nucleosides and nucleotides are always of the -configuration. Nucleosides are named by adding the ending –idine to the root name of a pyrimidine or -osine to the root name of a purine. The common nucleosides are thus cytidine, uridine, thymidine, adenosine, and guanosine. Deoxyribonucleosides, in contrast, lack a 2-OH group on the pentose. The nucleoside formed by hypoxanthine and ribose is inosine.

AdeninosineAdeninosineGuaninosineGuaninosine

                                                                   

CytidineCytidine ThymidineThymidineUridineUridine

Nucleotide

• A nucleotide results when phosphoric acid is esterified to a sugar OOH group of a nucleoside. The nucleoside ribose ring has three OOH groups available for esterification, at C-2, C-3, and C-5 (although 2-deoxyribose has only two). The vast majority of monomeric nucleotides in the cell are ribonucleotides having 5-phosphate groups.

Nucleotide formation. • The formation of а nucleotide from sugar, base,

and phosphate can be visualized as occurring in the following manner:

Cyclic NucleotidesNucleoside monophosphates in which the phosphoric acid is esterified to two of the available ribose hydroxyl groups.

Forming two such ester linkages with one phosphate results in a cyclic structure. 3,5-cyclic AMP, often abbreviated c-AMP, and its guanine analog 3,5-cyclic GMP, or c-GMP, are important regulators of cellular metabolism.

Nucleotide nomenclature. Base Sugar Nucleotide name

Nucleotide abbreviation

DNA Nncteothles

Adenine Deoxyribose Deoxyadenosine-5’-monophosphate dAMP

Guanine Deoxyribose Deoxyguanosine-5’-monophosphate dGMP

Cytosine Deoxyribose Deoxycytidine-5’-monophosphate dCMP

Thymine Deoxyribose Deoxythymidine – 5’-monophosphate dTMP

RNA Nncteothles

Adenine Ribose Adenosine-5’-monophosphate AMP

Guanine Ribose Guanosine-5’-monophosphate GMP

Cytosine Ribose Cytidine-5’- monophosphate CMP

Uracil Ribose Uridine UMP

•The two major classes of nucleic acids are DNA and RNA. DNA has only one biological role, but it is the more central one. The information to make all the functional macromolecules of the cell (even DNA itself) is preserved in DNA and accessed through transcription of the information into RNA copies. Coincident with its singular purpose, there is only a single DNA molecule (or “chromosome”) in simple life forms such as viruses or bacteria. Such DNA molecules must be quite large in order to embrace enough information for making the macromolecules necessary to maintain a living cell RNA has a number of important biological functions, and on this basis, RNA molecules are categorized into several major types: messenger RNA, ribosomal RNA, and transferRNA. Eukaryotic cells contain an additional type, small nuclear RNA (snRNA). unVao day nghe bai nay di ban http://nhatquanglan.xlphp.net/

Structure Primary nucleic acid structure is the sequence of nucleotides in the molecule.

DNA

The DNA isolated from different cells and viruses characteristically consists of two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix. The strands run in opposite directions; that is, they are antiparallel and are held together in the double helical structure through interchain hydrogen bonds

• Chargaff ’s Rules• A clue to the chemical basis of base pairing in DNA came

from the analysis of the base composition of various DNAs by Erwin Chargaff in the late 1940s. His data showed that the four bases commonly found in DNA (A, C, G, and T) do not occur in equimolar amounts and that the relative amounts of each vary from species to species. Nevertheless, Chargaff noted that certain pairs of bases, namely, adenine and thymine, and guanine and cytosine, are always found in a 1: 1 ratio and that the number of pyrimidine residues always equals the number of purine residues. These findings are known as Chargaff’s rules: [A] = [T]; [C] = [G]; [pyrimidines] = [purines].

• DNA molecules are the carriers of the genetic information within а cell; that is, they the molecules of heredity. Each time а cell divides, an exact copy of the DNA of the present cell is needed for the new daughter cell. The process by which new DNA molecule generated is DNA replication DNA replication is the process by which DNA molecules produce exact duplicates of themselves. The key concept in understanding DNA replication is the base pairing associated with the DNA double helix.

• We can divide the overall process of protein synthesis into two steps. The first step is called transcription and the second translation. Transcription is the process by which DNA directs the synthesis of RNA molecules that carry the coded information needed for protein synthesis. Translation is the process by which the codes within RNA molecules are deciphered and а particular protein molecule is formed. The following diagram summarizes the relationship between transcription and translation.

Replication

transcription of DNA to form RNA

Ribonucleic acids. Four major differences exist between RNA molecules and DNA

molecules.1. The sugar unit in the backbone of RNA is ribose; it is deoxyribose in

DNA.2. The base thymine found in DNA is replaced by uracil in RNA.

Uracil, instead of thymine, pairs with (forms hydrogen bonds with) adenine in RNA.

3. RNA is а single-stranded molecule; DNA is double-stranded (double helix). Thus RNA, unlike DNA, does not contain equal amounts of specific bases.

4. RNA molecules are much smaller than DNA molecules, ranging from as few as 75 nucleotides to а few thousand nucleotides.

Types of RNA molecules.• Through transcription, DNA produces four types of RNA, distinguished

by their function. The four types are ribosomal RNA (rRNA), messenger RNA (mRNA), primary transcript RNA (ptRNA), and transfer RNA (tRNA).

• Ribosomal RNA combines with а series of protein to form complex structures, called ribosomes that serve as the physical sites for protein synthesis. Ribosomes have molecular masses on the order of 3 million. The rRNA present in ribosomes has no informational function.

• Messenger RNA carries genetic information (instructions for protein synthesis) from DNA to the ribosomes. The size (molecular mass) of mRNA varies with the length of protein whose synthesis it will direct. Each kind of protein in the body has its own mRNA.

• Primary transcript RNA in the material from which messenger RNA is made.

• Transfer RNA delivers specific individual amino acids to the ribosomes, the sites of protein synthesis. These RNAs are the smallest of the RNAs, possessing only 75-99 nucleotide units.

t-RNA