Download 06. Nucleic acids

LECTERE 6
Nucleic acids: classification, structure and
biological role.
Lecturer: Dmukhalska Yevheniya. B.
PLAN
1. Nucleic acids. Types of nucleic acids.
2. Nucleic acids composition.
3. Levels of structural organization
functions.
4. Levels of structural organization
functions.
DNA.
Biological
RNA.
Biological
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 nitrogencontaining 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.
Guaninosine
Adeninosine
Cytidine
Uridine
Thymidine
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 Nucleotides
Nucleoside 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
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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