The Molecular Basis
of Heredity:
Structures and
Functions
of Nucleic Acids
by Raye L. Alford, PhD
Baylor College of Medicine
DNA Model by J. Watson and F. Crick
© Cold Spring Harbor Laboratory Archives
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The Atomic Components of DNA and RNA
The Periodic Table of Elements

Carbon (C)

Hydrogen (H)

Nitrogen (N)

Oxygen (O)

Phosphorus (P)
Thomas Jefferson National Accelerator Facility, Office of Science Education
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The Molecular Components of DNA and RNA (I)

Bases

DNA and RNA




Adenine (A)
Cytosine (C)
Guanine (G)
Uracil
DNA Only


Cytosine
Adenine
Thymine (T)
RNA Only

Thymine
Guanine
Uracil (U)
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The Molecular Components of DNA and RNA (II)

The Sugar – Phosphate Backbone
 DNA – Deoxyribose (Pentose) Sugar



RNA – Ribose (Pentose) Sugar
Phosphate Group
Base + Sugar + Phosphate = Nucleotide
BASE
5’
1’
3’
2’
Deoxyribose + Triphosphate Group
BASE
5’
1’
3’
2’
Ribose + Triphosphate Group
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The DNA and RNA Strand

Nucleotide + Nucleotide(n)
= DNA (or RNA) strand
5'
Base
5'
1'

Deoxyribonucleotides = DNA

Ribonucleotides = RNA
3'
Base
Base
3'
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DNA is Double-Stranded

Hydrogen bonds
enable the DNA ladder.
5'
3'
3'
5'
Adenosine
Thymidine
7
8


A always pairs with T.
C always pairs with G.
The sequence of one
strand predicts the
sequence of the other.
5 4
6
6 5
1
3
1
2
2
9
4
3
Two Hydrogen
Bonds
Guanosine
7
Cytidine
5
6
4
3
1
2
Three Hydrogen
Bonds
6 5
1
2
8
9
4
3
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The DNA Double Helix

The DNA ladder spirals in a
double helix.




There are 10 base pairs per turn.
Each turn is 34 Angstroms high.
The helix is 24 Angstroms wide.
The major groove is the
wider of the two grooves.
 The other groove is called
the minor groove
Major groove
Minor groove
Major groove
Minor groove
Redrawn from National Institute of Medical Sciences, NIH
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DNA Replication



Replication relies on
complementary base
pairing.
5'
5'
3'
3'
Replication occurs before
Mitosis and Meiosis I.
DNA replication is
semi-conservative.


DNA is synthesized
by DNA polymerase.
DNA is synthesized in
a 5’ to 3’ direction.
3'
5‘
5'5'
3'
Redrawn from National Center for Biotechnology Information
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Transcription

Like replication, transcription relies on base pairing.

Instead of DNA, RNA is made.

RNA is synthesized by RNA polymerase II.
RNA Polymerase
Template
Strand
Elongation
Site
Coding
Strand
Rewinding
3'
5'
3'
5'
3'
Unwinding
5'
Nascent
RNA
Movement of polymerase
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RNA Processing in Eukaryotic Cells
RNA
3'
5'
Exon
Intron
Exon
Intron
Exon
Splicing out of introns,
capping with 5' methyl cap,
polyadenylation
AAAAAAAn
mRNA
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Translation

Ribosomes make proteins from the messages encoded in
mRNA.
 Three nucleotide groups called codons encode one
amino acid.

This is the genetic code.
Nearly completed
protein
Beginning of
protein
Ribosome
3'
5'
mRNA
Adapted from: L. Stryer. Biochemistry, 3rd ed..
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The Genetic Code



Each 3-nucleotide mRNA
codon specifies a particular
amino acid.
Each tRNA transfers a
particular amino acid.
To use the chart, read from
the mRNA.
METtRNA
National Center for Biotechnology Information,
National Library of Medicine, NIH
anticodon loop
mRNA
codon
codon
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codon
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Summary of DNA

DNA:
 is a complex macromolecule made of
several basic atomic components.
 encodes genes, the instructions needed
to make RNA and proteins.
 is a double helix. The complementary
nature of the DNA helix permits its
replication and transcription.
 is transcribed into RNA.
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Summary of RNA



mRNA (messenger RNA) is the template for the
production of proteins, the molecules that build and
comprise structures and perform enzymatic reactions.
tRNA (transfer RNA) enables the transfer of amino acids
into a growing polypeptide chain.
rRNA (ribosomal RNA) is a component of proteinproducing ribosomes.
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