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Nucleotides and Nucleic acids

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Title: Nucleotides and Nucleic acids


1
Chemistry 20
Chapter 17 Nucleotides and Nucleic acids
2
Introduction
  • Each cell of our bodies contains thousands of
    different proteins.
  • How do cells know which proteins to synthesize
    out of the extremely large number of possible
    amino acid sequences?
  • the transmission of hereditary information took
    place in the nucleus, more specifically in
    structures called chromosomes.
  • The hereditary information was thought to reside
    in genes within the chromosomes.
  • Chemical analysis of nuclei showed chromosomes
    are made up largely of proteins called histones
    and nucleic acids.

3
Nucleic acids
Backbones of chromosomes
Ribonucleic acids (RNA) Deoxyribonucleic acids
(DNA)
Nucleic acids
RNA and DNA are polymers (monomers nucleotides).
4
Nucleotide
  • A nucleotide is composed of
  • Nitrogen-containing bases (amines)
  • Sugars (monosaccharides)
  • Phosphate

Phosphate
5
Bases
6
Sugars (monosaccharide)
  • RNA contains
  • Ribose sugar
  • DNA contains
  • 2-Deoxy-D-ribose sugar (without O on carbon 2)

7
Nucleoside
When a primidine or purine forms a glycosidic
bond to C1 of a sugar (ether ribose or
deoxyribose).
Base Sugar
Nucleoside
8
Nucleotide
A nucleotide forms with the -OH on C5 of a sugar
bonds to phosphoric acid.
Phosphate ester bond
5
5
1
A nucleotide
9
Primary structure of DNA and RNA
Carry all information for protein synthesis.
Sequence of nucleotides. Each phosphate is
linked to C3 and C5 of two sugars.
10
Primary structure of DNA and RNA
A nucleoside Base Sugar A nucleotide Base
Sugar Phosphate A nucleic acid A chain of
nucleotides
Like amino acids (C-terminal and N-terminal)
Base sequence is read from the C5 (free
phosphate) end to the C3 (free hydroxyl) end.
-ACGU-
11
Secondary structure of DNA
5
3
  • Two strands of polynucleotide form a double helix
    structure like a spiral.
  • Hydrogen bonds link paired bases
  • Adenine-Thymine (AT)
  • Guanine-Cytosine (G-C)
  • Sugar-Phosphate backbone is hydrophilic and stays
    on the outside (bases are hydrophobic).

3D structure
Sugar phosphate backbone
5
3
12
Secondary structure of DNA
13
Complementary base pairs
14
Higher structure of DNA
  • DNA is coiled around proteins called histones.
  • Histones are rich in the basic amino acids
  • Acidic DNA basic histones attract each other and
    form units
  • called nucleosomes.

Core of eight histones
15
Higher structure of DNA
Chromatin Condensed nucleosomes
16
Higher structure of DNA
Chromatin fibers are organized into loops, and
the loops into the bands that provide the
superstructure of chromosomes.
17
Difference between DNA RNA
  • DNA has four bases A, G, C, and T.
  • RNA has four bases A, G, C, and U.

2. In DNA Sugar is 2-deoxy-D-ribose. In RNA
Sugar is D-ribose.
3. DNA is almost always double-stranded (helical
structure). RNA is single strand.
4. RNA is much smaller than DNA.
18
RNA molecules
Transmits the genetic information needed to
operate the cell.
1. Ribosomal RNA (rRNA)
Most abundant RNA Contains ribosomes sites for
protein synthesis.
2. Messenger RNA (mRNA)
Carries genetic information from DNA (in nucleus)
to ribosomes (in cytoplasm) for protein
synthesis. They are produced in Transcription.
3. Transfer RNA (tRNA)
Smallest RNA. Translates the genetic information
in mRNA and brings specific Amino acids to the
ribosome for protein synthesis.
19
Genes
A section of a DNA molecule that contains a
specific sequence of the four bases (A, G, T,
and C)
1000 to 2000 nucleotides
Base sequence of the gene carries the information
to produce one protein molecule.
Change of sequence New protein
20
Functions of DNA
1. It reproduces itself (Replication)
2. It supplied the information to make up RNA,
proteins, and enzymes. (chapter 18)
21
Replication
Separation of the two original strands and
synthesis of two new daughter strands using the
original strands as templates.
By breaking H-bonds
22
Replication
Replication is bidirectional takes place at the
same speed in both directions.
Replication is semiconservative each daughter
molecule has one parental strand

and one newly synthesized one.
Origin of replication specific point of DNA
where replication begins.
Replication fork specific point of DNA where
replication is proceeding.
23
Replication
Leading strand is synthesized continuously in
the 3 ? 5 direction toward the replication
fork.
Lagging strand is synthesized discontinuously in
the 5 ? 3 direction away from the replication
fork.
24
Replication
Replisomes assemblies of enzyme factories.
25
Helicases
  • Unwinds the DNA double helix.
  • Replication of DNA starts with unwinding of the
    double helix.
  • Unwinding can occur at either end or in the
    middle.
  • Attach themselves to one DNA strand and cause
    separation of the double helix.

26
Primases
  • Catalyze the synthesis of primers.
  • Primers are short nucleotides (4 to 15).
  • They are required to start the synthesis of both
    daughter strands.
  • Primases are placed at about every 50 nucleotides
    in the lagging strand synthesis.

27
DNA Polymerase
Catalyze the formation of nucleotides. Joins
the nucleotides to produce a new strands
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