Protein Synthesis
Human Biology
DNA
• DNA
• Deoxyribonucleic Acid
• Twisted ladder or double helix
• Nucleotides
• Composed of alternating sugar
(Deoxyribose) and phosphate
molecules and
• Nitrogen bases
• Purines = adenine and guanine
• Pyrimidines = thymine cytosine
DNA
• Purines bond with Pyrimidines
• Complementary base pairs
• Adenine with Thymine
• Guanine with Cytosine
DNA
• Purines bond with Pyrimidines
• Complementary base pairs
• Adenine with Thymine
• Guanine with Cytosine
• Nucleoside
• Sugar bonding with a base
• Nucleotide
• Adding a phosphate to a nucleoside
• Phosphates attach to the 5’ carbon of
the sugar
Orientation of DNA
The carbon atoms on the sugar ring are
numbered for reference. The 5’ and 3’
hydroxyl groups (highlighted on the left)
are used to attach phosphate groups.

The directionality of a DNA strand is due to the orientation of the
phosphate-sugar backbone.
DNA is a double helix
P
T
A
P
G
5’
3’
P
C
DNA has directionality.
PP
P
C
Two nucleotide chains
together wind into a helix.
G
A
P
P
A sugar and phosphate
“backbone” connects
nucleotides in a chain.
T
P
P
G
Hydrogen bonds between
paired bases hold the two
DNA strands together.
C
P
P
3’
C
G
P
DNA strands are antiparallel.
5’
DNA
• A chromosome
• 23 pair = diploid
• 23 = haploid; sex cells
• Duplicating DNA structure tightly packed
around histone proteins to form a
nucleosome.
DNA
and
Genes
DNA
• DNA contains genes
• Genes are the codes for polypeptides
(proteins)
DNA
• Gene
• A series of bases that occupy a specific location (locus)
on a chromosome
• The code of a single protein or polypeptide
• Genetic Alphabet
• Triplet = Three nucleotides on DNA with their
corresponding base pairs making up the code of a single
amino acid
• Codon = Three successive nucleotides on RNA with their
corresponding base pairs making up the code of a single
amino acid
• 20 amino acids
• A series of amino acids makes up a protein
DNA
• Consists of 3 billion base pairs
• Codes for about 50 to 100,000 genes
• Genes may exist in alternate forms = alleles
• One allele from mom and one allele from dad
• Nucleotide changes or mutations may occur in a gene
• Sickle cell anemia
• In a healthy population, a gene may exist in multiple alleles
• Genetic Polymorphism = Multiple different forms at a
gene locus in a population
• Basis for DNA typing using MHC
Terminology
– Allele
» An alternate form of a gene
– Locus
» Location of a gene on a chromosome
– Gene
» Genetic code or “blueprint” for the cell to build one
particular protein
DNA
• DNA is located in the nucleus of
the cell
• Proteins are produced in the
cytoplasm of the cell using
ribosomes
Protein Synthesis
• DNA is very large and must
be copied in order to enter
the cytosol
• The DNA code is read by the
ribosome
• Amino acids are bonded to
make proteins
• http://www.youtube.com/watch?v=NJxobgkPEAo
RNA
• DNA contains the Genetic
Code
• RNA is the BLUEPRINT or
COPY of the Genetic Code
RNA Differs from DNA
• RNA has a sugar ribose
DNA has a sugar deoxyribose
Other Differences
•
•
RNA contains the base
uracil (U)
DNA has thymine (T)
RNA molecule is singlestranded
DNA is double-stranded
DNA
.
Three Types of RNA
• Messenger RNA (mRNA) copies
DNA’s code & carries the
genetic information to the
ribosomes
• Ribosomal RNA (rRNA), along
with protein, makes up the
ribosomes
• Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
Messenger RNA (mRNA)
• Carries the information for a
specific protein
• Made up of 500 to 1000
nucleotides long
• Sequence of 3 bases called codon
• AUG – methionine or start codon
• UAA, UAG, or UGA – stop codons
Ribosomal RNA (rRNA)
• rRNA is a single strand
100 to 3000 nucleotides
long
• Globular in shape
• Made inside the nucleus
of a cell
• Associates with proteins
to form ribosomes
• Site of protein
Synthesis
Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule with
attachment site at one end for an
amino acid
• Opposite end has three nucleotide
bases called the anticodon
Transfer RNA
amino acid
attachment site
U A C
anticodon
Codons and Anticodons
• The 3 bases of an anticodon are
complementary to the 3 bases of a
codon
• Example: Codon ACU
Anticodon UGA
Transcription
• The process of copying
the sequence of one
strand of DNA, the
template strand
• mRNA copies the template
strand
• Requires the enzyme RNA
Polymerase
Pathway to Making a Protein
DNA
mRNA
tRNA (ribosomes)
Protein
Transcription and
Translation
Transcription
• During transcription, RNA
polymerase binds to DNA and
separates the DNA strands
• RNA Polymerase then uses one
strand of DNA as a template
to assemble nucleotides into
RNA
Transcription
• Promoters are regions on
DNA that show where RNA
Polymerase must bind to
begin the Transcription of
RNA
• Called the TATA box
• Specific base sequences act
as signals to stop
mRNA Processing
• After the DNA is
transcribed into RNA,
editing must be done to
the nucleotide chain to
make the RNA functional
• Introns, non-functional
segments of DNA are
snipped out of the chain
mRNA Editing
• Exons, segments of DNA that
code for proteins, are then
rejoined by the enzyme ligase
• A guanine triphosphate cap is
added to the 5” end of the
newly copied mRNA
• A poly A tail is added to the 3’
end of the RNA
• The newly processed mRNA can
then leave the nucleus
Result of Transcription
CAP
New Transcript
copyright cmassengale
Tail
39
mRNA Transcript
•mRNA leaves the nucleus
through its pores and goes to
the ribosomes
copyright cmassengale
40
Translation
• Translation is the process of
decoding the mRNA into a
polypeptide chain
• Ribosomes read mRNA three
bases or 1 codon at a time and
construct the proteins
Transcription
Translation
copyright cmassengale
42
Ribosomes
• Made of a large and small
subunit
• Composed of rRNA (40%)
and proteins (60%)
• Have two sites for tRNA
attachment --- P and A
Step 1- Initiation
• mRNA transcript start
codon AUG attaches to
the small ribosomal
subunit
• Small subunit attaches
to large ribosomal
subunit
mRNA transcript
Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small subunit
A U G
C U A C U U C G
copyright cmassengale
Step 2 - Elongation
• As ribosome moves, two tRNA with
their amino acids move into site A and
P of the ribosome
• Peptide bonds join the amino acids
copyright cmassengale
Initiation
aa2
aa1
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
copyright cmassengale
Elongation
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
copyright cmassengale
aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
aa1
peptide bonds
aa4
aa2
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
peptide bonds
aa1
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
mRNA
terminator
or stop
codon
C A U G U U U A G
End Product –The Protein!
• The end products of protein synthesis is a
primary structure of a protein
• A sequence of amino acid bonded together
by peptide bonds
aa2
aa1
aa3
aa4
aa5
aa199
aa200
55
Messenger RNA (mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein Methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6