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DNA
The Molecule of Life
What is DNA?

DeoxyriboNucleic Acid
 Chargaff’s Law


R. Franklin and M. Wilkins


A=T, G=C
Crystal X-ray
J Watson and F Crick

Model of DNA


Double stranded structure
Bases inside
What is DNA Made of?

Deoxyribose sugar
 Phosphate
 Base

Purine


A, G
Pyrimidine

T, C
What are the Structures of the
Bases?

Purines

Adenine

Guanine
What are the structures of the
bases?

Pyrimidines

Thymine

Cytosine
Assembly of the parts



Purines and pyrimidines form chemical bonds with
deoxy-ribose (5-carbon) sugar. The carbon atoms on
the sugar are designated 1', 2', 3', 4' and 5'.
It is the 1' carbon of the sugar that becomes bonded to
the nitrogen atom at position N1 of a pyrimidine or
N9 of a purine. RNA contains ribose.
The resulting molecules are called nucleosides and
can serve as elementary precursors for DNA (and
RNA synthesis)
Nucleosides (examples)
Nucleosides become Nucleotides
Nucleosides form bonds with phosphate
groups.
 Phosphate groups bind to the 5’ C of the
deoxyribose sugar.
 Nucleosides + phosphate group =
NUCLEOTIDE

A Nucleotide
A, G, C or T
Forms sugar
Phosphate
Backbone
What makes DNA
Different from
RNA?
A Single Strand of Nucleotides
The nucleotides connect by a series of 5'
to 3' phosphate-deoxyribose bonds.
 Note the sequence of the bases in the
next diagram.
 Polynucleotide sequences are
referenced in the 5' to 3' direction

Polynucleotide
Polynucleotide pairs are
Complementary:
One strand of DNA is arranged 5’ to 3’
 The partner strand is arranged exactly 3’
to 5’.
 Chargaff’s law states A = T and C=G
 The strands are held together by Hbonds between the bases

Base pairing: how it works

Hydrogen Bonding between bases:

A-T Bonding


2 hydrogen bonds
G-C Bonding

3 hydrogen bonds
H-Bond Orientation
H-bonds between
O```H and N```H
 Orientation in
space
 PAIRING of A with
T,
 PAIRING of G with
C

Double Stranded DNA:
Complementary strands (cont.)

The bases pair COMPLEMENTARY to
one another.
Use

This complementarity allows for DNA
replication and transcription
DNA:

Two
complementary
strands of
polynucleotides
 Like a zipper
but held
together by Hbonds (which
really are not
bonds, but
forces)
DNA: The Double Helix

Like a
ladder
twisted
about its
axis
 Each cell
in our
body
contains 2
m worth of
DNA.
The CODE

The specific base pairing and the
sequence of the bases are significant.
 We call the specific arrangement of bases
the CODE


The sequences of code form the GENE for a
specific trait. Genes are special sequences of
hundreds to thousands of nucleotide base pairs
that form templates for protein making
It codes for specific RNA bases for the making
of specific proteins for the trait.
GENE


Exon:
regions that
form the
code for
the trait
Intron:
regions that
are part of
the gene
but are
excised
Genes
Total number of genes is unknown, it
estimated to be 30 000 to 120 000
 Genes comprise only 3% of the
chromosome—the rest is called junk
DNA—its code is meaningless “junk”

What is important about base pairs?
Can predict sequence of one strand
based on the sequence of the other
because it is complementary
 Replication and Transcription: a single
strand of DNA acts as a TEMPLATE for
a new strand, or for making RNA.
 Repair of damaged DNA—the template
DNA allows for repairs.

DNA: From Chromatin to Chromosome
DNA supercoils
around tiny
proteins called
HISTONES.
 The resulting
strand with
histones
supercoils on
itself.

Size
CHROMOSOMES

The supercoiled DNA further coils until it
further supercoils as chromatin.
 This is how 2 m of DNA can be packed into the
nucleus of a single body cell.
 At interphase of MITOSIS or MEIOSIS I, the
DNA replicates itself. The chromatin become
visible as double stranded DNA (DNA that has
replicated).
 Chromosomal Wrapping
Replication:

Why?


Where?



S phase of cell cycle
What?


Nucleus in Eukaryotes.
Cytosol in Prokaryotes
When?


When cells replicate, each new cell needs it’s own copy of
DNA.
Many proteins: major is DNA Polymerase
How?
Replication

How?
5’3’ directionality
 Starts with RNA primer
 Leading Strand
 Lagging Strand



Okasaki Fragments
Sequence determined by basepairing
Nova-Cracking the Code of Life
 The Structure of DNA

Transcription
DNA RNA
 What is the difference between DNA and
RNA?



Ribose Sugar
Uracil not thymine
Transcription

Where?



What?


When RNA is needed
Why?


RNA Polymerase plus some minor proteins
When?


Nucleus in Eukaryotes
Cytosol in Prokaryotes
RNA’s serve many important functions in cells
How?
Transcription

How?
5’3’ directionality
 Usually only one strand
 Uses Base-pairing
 Same idea as with DNA replication


RNA Synthesis Animation
Translation

What?


Where?


When proteins are need, after RNA is made
Why?


Cytosol
When?


RNA Protein
Proteins are vital for cells
How?
Translation

How?

Ribosomal Subunits
Small subunit
 Large subunit


Codon



Triplet code used
tRNA, rRNA, mRNA
Translation Animation
The Genetic Code
Why is this important?

Genetic Engineering

Gene Splicing
Mutations
 Cloning

In Summary
1.
A nucleotide is made of three parts:
a)
b)
c)
A phosphate group
A five carbon sugar (deoxyribose)
And a nitrogen containing