The document discusses nucleic acids, including the structures and differences between DNA and RNA, highlighting their roles in storing and expressing genetic information. It explains nucleotide composition, the double helix structure of DNA, base pairing rules, and the organization of DNA within cells. Additionally, it covers ribosomal RNA and its significance in protein synthesis.

1. NUCLEIC ACIDS
Presented by
Salwa Soliman Salman

2. Nucleic Acid Structure
Nucleic acids are required for the storage
and expression of genetic information.
There are two chemically distinct types
of nucleic acids:
A. RNA (ribonucleic acid) is a polymer of
ribonucleotides
B. DNA (deoxyribonucleic acid) is a
polymer of deoxyribonucleotides

3. Nucleotides
• Monomers for nucleic acid polymers
• Each nucleotide consists of:
1. Pentose sugar
2. Nitrogenous base
3. Phosphate group

4. 1-Sugars
Pentoses (5-C sugars)
 Numbering of sugars is “primed”

5. 2
-
Nitrogenous Bases
Purines Pyrimidines

6. 3
-
Phosphate Groups
• Phosphates can be bonded to either C3 or C5
atoms of the sugar.
• The second and third phosphates are each
connected to the nucleotide by a "high energy
"bond. But first phosphate is connected to the
sugar by ester bond (low energy bond)
• The phosphate groups are responsible for the
negative charges associated with
nucleotides ,and cause DNA and RNA to be
referred to as "nucleic acids

7. 3
-
Phosphate Groups

8. Nucleosides
Result from linking one of the sugars with a
purine or Pyrimidine base through an N-
glycosidic linkage
Purines bond at their N9 atoms to the C1’
carbon of the sugar
Pyrimidines bond at their N1 atoms to the C1’
carbon of the sugar

9. Nucleotides
• Result from linking one or more
phosphates with a nucleoside onto the
5’ end of the molecule through
esterification

11. Differences between DNA & RNA
 They contain different sugars
 DNA contains deoxyribose
 RNA contains ribose
 Nitrogenous bases
 DNA contains A, G, T, & C
 RNA contains A, G, U, & C
Uracil (U) replaces thymine (T) in RNA, thus A pairs
with U when DNA is used as a template to make RNA
 Form
 DNA – most stable as double helix
 RNA most often exists as a single strand of nucleotides

12. Differences between DNA & RNA
• Size
􀁺 DNA molecules are larger
􀁺 RNAs are smaller
 Mobility
􀁺 DNAs are basically immobile
􀁺 RNAs are highly mobile
 Life span
􀁺 DNAs are long-lived
􀁺 RNAs are broken down soon after their job is
done

13. Structure of DNA
A- Phosphodiester bonds = 1ry structure of DNA
1- Phosphodiester bonds join the 5'-hydroxyl group of
the phosphate of one nucleotide to the 3'-hydroxyl
group of the deoxypentose of an adjacent nucleotide .
2- Polarity of DNA: with both a 5'-end (the end with
the free phosphate) and a 3'-end (the end with the free
hydroxyl) that are not attached to other nucleotides.
3- The bases located along the resulting
deoxyribosephosphate backbone are, written in
sequence from the 5'-end of the chain to the 3'-end.
For example " (5'-TACG-3').

15. Structure of DNA
B- The Double Helix Structure of DNA = 2ry
structure of DNA (B-form= Watson & Crick 1953)
 The double helix, the two chains are coiled
around acommon axis called the axis of symmetry.
 The chains are paired in an antiparallel manner,
that is, the 5'-end of one strand is paired with the 3'-
end of the other strand.
In the DNA helix, the hydrophilic
deoxyribosephosphate backbone of each chain is
on the outside of the molecule, whereas the
hydrophobic bases are stacked inside.

16. Structure of DNA
 The spatial relationship between the two strands
in the helix creates a major (wide) groove and a
minor
(narrow) groove.
 These grooves provide access for the binding of
regulatory proteins to their specific recognition
sequences along the DNA chain.
 Factors stabilize the structure of the double helix:
• A-Hydrogen bonds between bases.
• B- The hydrophobic interactions between the
stacked bases

18. Structure of DNA
• Chargaff's Rule:
DNA has equal numbers of adenine and
thymine residues (A=T) and equal
numbers of cytosine and guanine residues
(G=C)
the total amount of purines equals the total
amount of pyrimidines.
The base pairs are held together by
hydrogen bonds: two between A and T
(A=T) and three between G and C (G = C).

19. complementary base pairing
Each base will only bond with one other specific base.
 Adenine (A)
 Thymine (T)
 Cytosine (C)
 Guanine (G)
Because of this complementary base pairing, the
order of the bases in one strand determines the
order of the bases in the other strand.

20. DNA Organization
:
• DNA in a single human cell, if stretched
to its full length is about two meters.
• Eukaryotic DNA is associated with
tightly bound basic proteins, called
histones to form nucleosomes which
appear as beads on strings, which then
further organized to form chromosomes.

21. Nucleosomes
• There are 5 classes of
histones, designated H1,
H2A, H2B, H3, andH4.
• These small proteins
are at physiologic
positively charged pH
as a result of their high
content of lysine and
arginine.

22. Nucleosomes
• Two molecules each of H2A, H2B, H3, and H4 form
the structural core of the individual nucleosome
“beads.” = octamer = 8
 Around this core, a segment of the DNA double helix
is wound nearly twice, forming a negatively charged
super-twisted helix .
 Neighboring nucleosomes are joined by “linker” DNA
approximately 50 base pairs long.
 Histone H1 is not found in the nucleosome core, but
instead binds to the linker DNA chain between the
nucleosome beads

23. Higher levels of organization
1-Nucleosomes can be packed more tightly to
form a polynucleosome (6-7 Nucleosomes per
turn) to form 30nm fiber also called
nucleofilament .
2-The fiber is organized into loops that are
anchored by a nuclear scaffold containing
several proteins.
3- Additional levels of organization lead to the
final chromosomal structure .

30. Ribosomal RNA
• rRNAs are found in association with several proteins
as components of the ribosomes the sites for protein
synthesis.
• In prokaryotic cells there are three distinct size
species of rRNA (23S, 16S, and 5S).
• In the eukaryotic cytosol, there are four rRNA species
(28S, 18S, 5.8S, and 5S.
• rRNAs make up about 80% of the total RNA in the cell.

32. THANK YOU