CH. 12.3 :
DNA, RNA, and Protein
Section Objectives:
•Relate the concept of the gene to the
sequence of nucleotides in DNA.
•Sequence the steps involved in
protein synthesis.
•Explain the different types of RNA
involved in protein synthesis
Genes and Proteins
• The sequence of nucleotides in DNA contain
information.
• This information is put to work through the production
of proteins.
• Proteins fold into complex, three- dimensional shapes
to become key cell structures and regulators of cell
functions.
• Thus, by encoding the instructions for making
proteins, DNA controls cells.
What are Genes
• You learned earlier that proteins are polymers of
amino acids.
• The sequence of nucleotides in your DNA is a
gene that contains the information for assembling
the string of amino acids that make up a single
protein. (instructions to make a protein)
• Proteins control an organism:
• Enzymes, Steroids, Structural Proteins etc.. Are all made
from the sequence of letters in your DNA
• Proteins are polymers made of amino acid monomers
• Where are proteins made?
• Ribosomes!!
Cell organization
• Remember:
– DNA is in the nucleus
• DNA contains genes = instructions for
making proteins
– Cells want to keep DNA in the nucleus where it
is protected
• “locked in the vault”
– How does the code for a protein get to a
ribosome if the DNA can’t leave the nucleus?
Passing on DNA information:
need RNA
• RNA like DNA, is a nucleic
acid
• RNA structure differs from
DNA structure in three ways.
– 1. Has ribose sugar instead of
deoxyribose (DNA)
Phosphate
group
Nitrogenous
base
(A, G, C, or U)
Uracil (U)
– 2. Replaces thymine (T) with
uracil (U)
– 3. Single stranded as opposed
to double stranded DNA
Sugar
(ribose)
RNA
• RNA has a different function than DNA
• Whereas DNA provides the instructions for
protein synthesis, RNA does the actual work of
protein synthesis.
• RNA Function: takes from DNA the instructions on
how the protein should be assembled, then—amino
acid by amino acid—RNA’s assemble the protein.
Types of RNA
3 types of RNA
• 1. Messenger RNA (mRNA), single, uncoiled strand
which brings instructions from DNA in the nucleus to
the site of protein synthesis (Ribosome).
• 2. Ribosomal RNA (rRNA), globular form, makes up
the ribosome –the construction site of proteins (site of
protein synthesis); binds to the mRNA and uses the
instructions to assemble the amino acids in the correct
order.
• 3. Transfer RNA (tRNA) single, folded strand that
delivers the proper amino acid to the site at the right
time. Has a specific anticodon that is complementary
to the sequence on the mRNA.
From gene to protein
protein
transcription
translation
Protein Synthesis: 2 step process
1. Transcription 2. translation
1.Transcription: DNA -> mRNA
• In the nucleus, enzymes make an RNA copy of a
portion of a DNA strand
The main difference between transcription and DNA
replication is that transcription results in the formation of one
single-stranded RNA molecule rather than a double-stranded
DNA molecule.
2. Translation:
mRNA -> Protein
• process of converting the information in a sequence
of nitrogenous bases in mRNA into a sequence of
amino acids in protein
Transcription
• Making mRNA from DNA
• DNA strand is the
template (pattern)
– match bases
• U:A
• G:C
• Enzyme
– RNA polymerase- splits the DNA, then attaches a
complementary strand of RNA. Afterwards the
DNA reattaches
Matching bases of DNA & RNA
• Double stranded DNA unzips
T G G T A C A G C T A G T C A T CG T A C CG T
Matching bases of DNA & RNA
• Double stranded DNA unzips
• RNA polymerase attaches at a promoter
which is a region of DNA that signals the
start of a gene.
T G G T A C A G C T A G T C A T CG T A C CG T
Matching bases of DNA & RNA
• Match complimentary RNA
bases to DNA bases on one of
the DNA strands
(T is replaced with U)
A
U
C
G
C
U
A
G
AG
G
U
C
A C C
RNA
polymerase
A
G
U
A
C
G
A
U
A
T G G T A C A G C T A G T C A T CG T A C CG T
C
Matching bases of DNA & RNA
• U instead of T is matched to A
DNA
TACGCACATTTACGTACGCGG
mRNA
AUGCGUGUAAAUGCAUGCGCC
RNA Processing
• Not all the nucleotides in the DNA of eukaryotic cells carry
instructions—or code—for making proteins.
• Genes usually contain many long non-coding nucleotide
sequences, called introns, that are scattered among the coding
sequences.
• Regions that contain information are called exons because
they are expressed.
• When mRNA is transcribed from DNA, both introns and exons
are copied.
• The introns must be removed from the mRNA before it can
function to make a protein.
• Enzymes in the nucleus cut out the intron segments and paste
the mRNA back together.
• The mRNA then leaves the nucleus and travels to the
ribosome.
RNA Processing:simplified
• Noncoding segments called introns are spliced
out ( coding segment = exons)
Translation: From mRNA to Protein
• takes place at the ribosomes in the
cytoplasm.
Involves 3 types of RNA
1. Messenger RNA (mRNA) =carries
the blueprint for construction of a protein
2. Ribosomal RNA (rRNA) = the Ribosome
the construction site where the protein is
made
3. Transfer RNA (tRNA) =
the molecule delivering the proper amino
acid to the site at the right time
Genetic information written in codons is
translated into amino acid sequences
• Transfer of DNA to mRNA uses “language” of
nucleotides
– Letters: nitrogen bases of nucleotides (A,T,G,C)
– Words: codons : triplets of bases
( ex. AGC)- groups of 3
– Sentences: polypeptide chain
– The codons in a gene specify the amino acid
sequence of a polypeptide
The Genetic Code
• The nucleotide
sequence transcribed
from DNA to a strand
of messenger RNA
acts as a genetic
message, the
complete information
for the building of a
protein..
• Virtually all organisms
share the same
genetic code
Transfer RNA molecules serve as
interpreters during translation
• In the cytoplasm, a ribosome
attaches to the mRNA and
translates its message into a
polypeptide
• The process is aided by transfer
RNAs
• Each tRNA molecule has a
triplet anticodon on one end
and an amino acid attachment
site on the other
– Anticodon base pairs with codon
of mRNA
How does mRNA code for proteins
• mRNA leaves nucleus
• mRNA goes to ribosomes in cytoplasm
• Proteins built from instructions on mRNA
How?
mRNA
A C C A U G U C G A U C A GU A GC A U G GC A
aa
aa
aa
aa
aa
aa
aa
aa
How does mRNA code for
proteins?
DNA
TACGCACATTTACGTACGCGG
mRNA
AUGCGUGUAAAUGCAUGCGCC
?
Amino acids/
Met Arg Val Asn Ala Cys Ala
protein
How can you code for 20 amino acids with
only 4 DNA bases (A,U,G,C)?
mRNA codes for proteins in triplets
called codons
DNA
TACGCACATTTACGTACGCGG
codon
mRNA
AUGCGUGUAAAUGCAUGCGCC
?
protein
Met Arg Val Asn Ala Cys Ala
 Codon = block of 3 mRNA bases
The Genetic code
• For ALL life!
– support for a common
origin for all life
• Code has duplicates
– several codons for each
amino acid
– This “wiggle room” is
mutation insurance!
 Start codon


AUG
methionine
 Stop codons

UGA, UAA, UAG
How are the codons matched to
amino acids?
• The tRNA molecule has a complementary
codon to the mRNA called an anticodon
• There is a specific tRNA for each amino acid
mRNA to protein = Translation
• The working instructions  mRNA
• The reader  ribosome
• The transporter  transfer RNA (tRNA)
ribosome
mRNA
A C C A U G U C G A U C A GU A GC A U G GC A
U GG
tRNA
aa
aa
aa
U A C
tRNA
aa
A G
tRNA
aa
C
U AG
tRNA
aa
From gene to protein
protein
Transcription
DNA-> RNA
Translation
RNA-> protein
• DNA – TAC CAA GGA AGT GCG ATA CAT CGT AGC GGT
• mRNA• A.A. –