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DNA
The Secret of Life
Deoxyribonucleic Acid
• DNA is the molecule responsible for
controlling the activities of the cell
• It is the hereditary molecule
• DNA directs the production of protein
Structure of DNA
• In 1953, Watson and
Crick proposed that DNA
is made of two chains of
nucleotides held together
by nitrogenous bases.
• Watson and Crick also
proposed that DNA is
shaped like a long zipper
that is twisted into a coil
like a spring.
Structure of DNA
• Because DNA is
composed of two
strands twisted
together, its shape is
called double helix.
• A double helix
resembles a twisted
ladder.
Nucleotides
• DNA is made up of subunits called
nucleotides
• Nucleotides consist of the backbone,
which is made of sugar (deoxyribose) and
phosphate, as well as a nitrogenous
base
Nucleotide
Nitrogen base
Phosphate
Sugar
How Did DNA Get its Name?
• Based on what you just learned, how
do you think deoxyribonucleic acid
(DNA) got its name?
Nucleotides
• A nitrogenous base is a carbon ring structure
that contains one or more atoms of nitrogen.
• In DNA, there are four possible nitrogenous
bases: adenine (A), guanine (G), cytosine (C),
and thymine (T).
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Nucleotide Sequence
• The four nucleotides are represented by the first
letter in their name
– A – Adenine
– G – Guanine
– T – Thymine
– C – Cytosine
Nucleotide Sequence
• In DNA, Adenine always pairs with Thymine
• Cytosine always pairs with Guanine
Nucleotides
• Nucleotides stack on top of one another forming
the double helix, or twisted ladder
DNA Building
• Click here to link to a DNA building activity
online!
Go Get It Moment!
• When you hear “Move it!” you will have
30 seconds to come get one piece of
paper with an A, G, T, or C on it.
• Then find someone else in the room
whose letter correctly matches with
your nucleotide letter and stand by that
person.
• What are the questions?
• “Move it!”
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Go Get It Moment!
• When you hear “Double Helix”, you
will have 1 minute to make a
classroom DNA strand by standing
next to another pair of nucleotides
• What questions are there?
• “Double Helix”
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DNA Replication
• In order for cells to divide, DNA must
be able to make exact copies of
itself
– This process is known as DNA
Replication
DNA Replication
• DNA Replication occurs before
mitosis and meiosis
• Replication results in two identical
DNA daughter strands from one
mother strand
Process of DNA Replication
• The DNA strand is unzipped at the
hydrogen bonds by an enzyme
named helicase.
• Nucleotides in the nucleus then
find their corresponding
nucleotides on each of the two
open DNA strands and produce
two new DNA double helixes.
Link to DNA Replication
Animation
• Click here to see how DNA
Replication works
Protein Synthesis
• The main job for DNA is to direct
the production of protein
• Protein makes tissues and organs
and carries out the organism’s
metabolism
• Proteins are polymers (chains) of
amino acids
Protein Synthesis
• The sequence of nucleotides in
each gene contains information for
assembling the string of amino
acids that make up a single protein
• DNA sequence to make proteins!
RNA
• RNA is a nucleic acid composed of
nucleotides that is crucial in making protein
• There are three differences between DNA and
RNA
– RNA is a single strand
– The sugar in RNA is called Ribose instead of
DNA’s Deoxyribose
– Like DNA, RNA has 4 nitrogenous bases, but
instead of Thymine, Uracil is the 4th base
RNA as a Single Strand
• You recall that
DNA looks like a
twisted ladder
and is referred to
as a double helix
• RNA looks like
half a ladder
– There is only one
side to RNA
RNA Contains Ribose
• Remember how DNA got it’s
name?
– It is a nucleic acid with deoxyribose
as the sugar on the backbone
– Hence the name Deoxyribonucleic
acid
• Ribonucleic acid (RNA) has the
sugar ribose on it’s backbone
RNA Has Uracil
• Remember the base pairs in DNA?
– Adenine pairs with Thymine
– Guanine pairs with Cytosine
RNA Has Uracil
• In RNA
– Adenine pairs with URACIL
– Guanine pairs with Cytosine
Making Protein
• Protein production starts with DNA
• DNA passes instruction to RNA
• RNA carries out the work of linking
together chains of amino acids
Three Types of RNA
• There are three types of RNA
involved in Protein Synthesis
– Messenger RNA (mRNA)
– Ribosomal RNA (rRNA)
– Transfer RNA (tRNA)
Messenger RNA
• Messenger RNA (mRNA), brings instructions
from DNA in the nucleus to the cell’s factory
floor, the cytoplasm
– On the factory floor, mRNA moves to the assembly
line, a ribosome
– Remember that a ribosome is either a free-floating
small dot in the cell or is attached to the endoplasmic
reticulum making it “rough” ER
Ribosomal RNA
• The ribosome, made of Ribosomal
RNA (rRNA) binds to the mRNA and
uses the instructions to assemble the
amino acids in the correct order
Transfer RNA
• Transfer RNA (tRNA) is the supplier
• Transfer RNA delivers the amino
acids to the ribosome to be
assembled into a protein
Transcription
• The production of messenger RNA
(mRNA) is known as Transcription
• DNA acts as a template for the RNA
molecule
• To View a Link to Transcription Animation
Click Here.
Transcription
• The main difference between DNA
Replication and transcription is that
transcription results in one single strand of
RNA rather than an exact duplicate of a
double stranded DNA molecule
• Much of the information on the mRNA
strand codes for specific amino acids to
make protein
The Genetic Code
• A code is necessary to turn the language
of RNA into the language of amino acids
and proteins
• The four nitrogenous bases make up the
code
– The code letters are A, G, T, and C
• A set of three letters makes a “word”
called a Codon
– There are 64 possible codons
Codon
Amino Acids
• There are 20 different amino acids that
make up proteins
The Genetic Code
The Messenger RNA Genetic Code
First
Letter
U
Phenylalanine (UUU)
U
C
A
Phenylalanine (UUC)
Serine (UCU)
G
Cysteine (UGU)
Serine (UCC)
Tyrosine (UAC)
Leucine (UUA)
Serine (UCA)
Stop (UAA)
Leucine (UUG)
Serine (UCG)
Stop (UAG)
Leucine (CUU)
Proline (CCU)
Histadine (CAU)
Leucine (CUC)
Proline (CCC)
Histadine (CAC)
Arginine (CGC)
Leucine (CUA)
Proline (CCA)
Glutamine (CAA)
Arginine (CGA)
Leucine (CUG)
Proline (CCG)
Glutamine (CAG)
Arginine (CGG)
Isoleucine (AUU)
Threonine (ACU)
Asparagine (AAU)
Serine (AGU)
Isoleucine (AUC)
Threonine (ACC)
Asparagine (AAC)
Serine (AGC)
Isoleucine (AUA)
Threonine (ACA)
Methionine;Start
(AUG)
Threonine (ACG)
Lysine (AAA)
Lysine (AAG)
Cysteine (UGC)
Stop (UGA)
Tryptophan (UGG)
Arginine (CGU)
Arginine (AGA)
Arginine (AGG)
Alanine (GCU)
Aspartate (GAU)
Valine (GUC)
Alanine (GCC)
Aspartate (GAC)
Glycine (GGC)
Valine (GUA)
Alanine (GCA)
Glutamate (GAA)
Glycine (GGA)
Valine (GUG)
Alanine (GCG)
Glutamate (GAG)
Glycine (GGG)
Valine (GUU)
G
Second Letter
C
A
Tyrosine (UAU)
Glycine (GGU)
Third
Letter
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
The Genetic Code
• As you could see on the chart of amino
acids, there are more than one codon that
code for certain amino acids
– This results in fewer errors in protein
synthesis
The Genetic Code
• All organisms use the same genetic code
• This provides evidence that all life on earth
may have evolved from the same origin
Ribosomes
• The newly produced strand of mRNA
travels from the nucleus to the cytoplasm
where it attaches to a ribosome
• The ribosome is the actual site for protein
synthesis
Translation: From mRNA to
Protein
• The process of “reading” the three letter
“words” , or codons, is known as
translation
• Translation takes place at the ribosomes
in the cytoplasm
– The amino acids are free-floating in the
cytoplasm and congregate at the ribosome
during protein synthesis
The Process of Protein Synthesis
• The first codon is usually A-U-G,
methionine, which starts the production of
a new protein
• Following the start codon, the remaining
codons call for amino acids in the order in
which they appear on the mRNA strand
Transfer RNA (tRNA)
Amino acid
• At the ribosome, Transfer RNA
(tRNA) identifies the code
through translation and finds
the appropriate amino acid
matching the codons
– Each tRNA molecule only
attaches to one specific
amino acid
– The Anticodon on the
bottom of the tRNA
molecule corresponds with
the codons on the mRNA
strand
Transfer RNA molecule
Chain of RNA
nucleotides
Anticondon
Protein Synthesis Animation
– Click Here to Link to An Animation