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DNA
Life’s Instruction Manual
or
What Genes are Made Of
Discovery of DNA
• 1952 Rosalind Franklin-studies DNA
molecule using x-ray diffraction
• 1953 Watson and Crick- develop double
helix model
The Shape of the DNA Molecule
• DNA is a complex organic molecule with a
double helix shape. (a twisted ladder)
• It consists of 2 phosphate-sugar
backbones connected by a series of
nucleotide bases. (colored letters)
Labeling the Components
DNA Nucleotide Bases
•
•
•
•
•
Adenine A
Thymine T
Guanine G
Cytosine C
These bases are joined together in a
specific way…
• A with T
G with C
• We call this base pairing.
How does this work?
• One purine
always pairs with
one pyrimidine.
• Bonding between
A and T forms
two hydrogen
bonds
• Bonding between
C and G forms
three hydrogen
bonds
DNA Structure
DNA Replication
• The bonds between the base pairs are
weak hydrogen bonds and can be broken
easily. This means that the molecule can
unwind and unzip itself.
• Each side of the DNA molecule has all the
information necessary to make a
complementary (second) side.
• Each piece of “old” DNA will act as a
template for a “new” piece of DNA.
DNA Copies Itself!!
• Because of base pairing,
each strand of DNA can
replicate itself making
two new strands of DNA.
• It uses extra nucleotide
bases (in cell) to create
this copy.
• All of the work of DNA
replication is done by
enzymes!!
Enzymes!
• Enzymes are proteins that speed up
(catalyze) chemical reactions.
• Some important enzymes in the process of
DNA replication are
– Helicase: unwinds the DNA structure
– Polymerase: makes DNA for base pairing
(new strand)
– Ligase: “glues” the new DNA strand together
to fit to the old strand
Semi-Conservative Replication
• The result is that each
new copy of DNA
produced by this
process is made up of
½ original DNA
molecule and ½ new
DNA molecule. This
makes the process
semi-conservative.
RNA
• RNA is a nucleic acid like DNA
• RNA is single stranded.
• RNA does not contain thymine but
instead has the base called uricil which
bonds with adenine.
• RNA has many functions in the cell, most
importantly it is critical in the process of
protein synthesis
Protein Synthesis
The Central Dogma
• DNA holds the CODE to make Proteins
• RNA transfers the CODE to the Ribosome
• Proteins are strings of amino acids coded
for by DNA
• Proteins are the “workhorses” of the cell
Structure, enzymes, communication, etc.
Transcription
• The DNA
message is
transcribed into
an mRNA
message by
complementary
base pairing in
the nucleus.
Transcription Steps
1. DNA strands split
2. mRNA creates a complementary strand
to one of the DNA strands
3. mRNA leaves the nucleus for translation
Translation
• The mRNA then
goes into the
cytoplasm to give
the code for a
protein to the
ribosome. tRNA
molecules will
bring the correct
amino acid to the
ribosome.
Translation Steps
1. mRNA goes to cytoplasm
2. Ribosome attaches to start codon on mRNA
3. tRNA (contains 3 letter code and amino acid)
base pairs with mRNA
4. Amino acids from base pairs are connected to
form a chain
5. Once stop codon is reached, chain of amino
acids stops forming and leaves forming a
protein
DNA
Mutations
• Sometimes mistakes can be made in the
copying process of DNA. Any change in
the original sequence of base pairs is
called a mutation.
Mutations
• What happens when a mistake is made in
replication of DNA?
• Most mutations are NOT helpful!! But every so
often, a beneficial mutation occurs (black moths)
The Black Moth
• Peppered moths have a mutation making them
appear mostly black
• In the 19th century, more black moths were
found to exist then the original peppered type
• Having black wings found to be advantageous
because of industry producing soot that was
black during this time
Types of Mutations
Point Mutations – the change in one single
nucleotide base.
substitution – one letter replaces another
in the sequence. (ex. A->G)
insertion – one letter is added to the
sequence. (ex. a new T is added)
deletion – one letter is removed from the
sequence. (ex. a G is removed)
• Make a substitution in the following DNA
ATTGCGTGTG
Make an insertion in the original sequence
ATTGCGTGTG
Make a deletion in the original sequence
ATTGCGTGTG
Larger Mutations
• Inversions – when 2 or more bases
(letters) are switched into reverse order.
• Duplications – when bases are repeated in
the original order more than one time.
• Translocations – when bases are moved
from one area of the sequence to another.
Make an 3 base inversion mutation in the following
DNA sequence…
ATTGCGTGTG
Make a duplication of the inversion here
ATTGCGTGTG
Make a translocation of 3 bases
ATTGCGTGTG