Protein Synthesis
When Watson and
Crick discovered
the structure of
DNA is was still
unclear how genes
actually worked.
The answer came
after the discover
of another nucleic
acid; RNA.
Chapter 13 in the textbook
RNA
• Type of organic
compound:
• Monomer:
• Circle each of the
following in the RNA
molecule
– Sugar (Ribose)
– Phosphate Group
– Nitrogenous Base
How is the structure of RNA different
than the structure of RNA?
RNA
RNA
How is RNA similar to DNA
• Both are nucleic acids
• Both have 4 different nitrogenous bases
• Both have a sugar phosphate backbone
How is RNA different than DNA
• Ribose sugar instead of deoxyribose
• RNA is generally single stranded
• RNA has uracil in place of thymine (U
instead of T)
RNA
Three Main Types of RNA
Type of
RNA
Abbreviation
Function
Messenger
RNA
mRNA
Copies the sequence of DNA bases in the
nucleus and travels to the ribosome
Ribosomal
RNA
rRNA
Makes up the ribosome
Transfer
RNA
tRNA
Transfers amino acids to the ribosome based
on the sequence of bases in the mRNA
Transcription
• Purpose: To copy a segment of DNA into mRNA.
• Location: Inside the nucleus.
• Process:
– RNA polymerase: an enzyme that unwind DNA helix
and uses base pairing rules to make mRNA from a DNA
template.
– Promoter: a specific
sequence of DNA that
RNA polymerase binds
to begin making
mRNA
Animation
Transcription
• Transcribe the DNA sequence below:
Transcription
RNA Editing (occurs between transcription and translation)
• 2% of the DNA in the nucleus is used to make proteins.
• Gene: sequences of DNA that code for proteins.
• Before the mRNA can leave the nucleus the DNA that is
used to make proteins must be separated from the DNA
that does not make proteins.
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Introns: Segments of DNA that do
not code for proteins. These are
removed before mRNA leaves the
nucleus
Exons: the segments of
DNA that code for a protein.
These are spliced together
once introns are removed
Transcription
RNA Editing
• Poly-A tail is added to the 3’ end of
the mRNA.
– Allows the mRNA to leave
through pores in the nuclear
membrane.
– Protects the mRNA from
enzymes as it leaves the nucleus.
• Guanine cap is added to the 5’ end of the mRNA.
– The guanine cap allows the mRNA to be recognized
by the ribosome.
Translation
The mRNA leaves the nucleus and travels to the ribosome.
Translation
• Purpose: To use the
information in the
mRNA sequence to
make proteins.
• Location: At the
ribosome (ribosomes
are made of rRNA).
Translation
Process:
• The ribosome reads the mRNA 3
nucleotides at a time.
• Codon: a group of 3 mRNA
nucleotides that corresponds to a
single amino acid.
• Genetic Code: The sequence of
nucleotides in DNA or RNA that
determines the specific amino
acid sequence in the synthesis of
proteins.
Translation
• Genetic Code
– There are 64 different
codon combinations
(4 X 4 X 4 = 64).
– There are only 20 different
amino acids, so there is
more is more than one
codon for each amino acid.
– Methionine is always the first amino acid in a protein.
(AUG is referred to as a start codon)
– There are three different stop codons which end
translation.
Translation
• Using the genetic code, the tRNA
molecule brings the correct amino
acids to the ribosome.
• Each molecule of tRNA has 2
parts.
– Anticodon: sequence of 3
nucleotides that matches up
with an mRNA codon.
– Amino acid: each tRNA
molecule can bind to only a
specific amino acid.
Translation
• Once the tRNA molecule arrives at the ribosome the
amino acid is removed.
• The amino acid is attached to the other amino acids at
the ribosome to form a polypeptide chain.
– Polypeptide: a polymer of
a protein
– Amino Acid: a monomer
of a protein
– Peptide bond: the chemical
bond that forms between
amino acids.
Translation
• The tRNA molecule leaves the ribosome to get another
amino acid.
• The ribosome move to the next mRNA codon an d a
new tRNA molecule with a amino acid will move into
the ribosome.
• Stop Codon: a codon in
mRNA that cause s the
ribosome to release and
stops translation.
Translation
Summary of Translation
1. mRNA binds to a ribosome
2. As each mRNA codon passes through the ribosome, a tRNA
molecules with a complimentary anticodon will travel to the
ribosome.
3. The tRNA brings the proper amino acid to the ribosome
according to the genetic code.
4. A peptide bond is formed between the amino acids in the
ribosome.
5. Old tRNA molecules leave as new tRNA molecules bring more
amino acids.
6. The chain of amino acids will grow until a stop codon is reached.
Mutations
Fantasy
Reality
Mutations
Mutations: a permanent and heritable change in
the nucleotide sequence of a gene.
• Subsitution Mutation: One base is substituted
for a different base.
– Also known as a point mutation
Mutations
Point or Substitution mutations can affect the protein in
various ways.
• Silent Mutation:
– The mutated codon still codes for the same amino acid.
– Since there are 64 different codons but only 20 different
amino acids – more than one codon can code for the
same amino acid.
– What affect do silent mutation have on the protein.
Example
– What happens to the protein when the DNA sequence
AGA is mutated to become AGC?
Mutations
• Missense Mutation:
– The mutated DNA nucleotide changes the amino acid
in the protein.
– How is the protein affected?
Example:
– Sickle cell disease: the sixth
codon in the protein
hemoglobin, GAG, is mutated
to GTG.
– How does this mutation
change the amino sequence
in hemoglobin?
Mutations
• Nonsense Mutations
– the mutation changes a codon for an amino acid to
one of the STOP codons (TAA, TAG, or TGA).
– How is the protein affected?
Example
– Cystic Fibrosis is caused by a mutation in the gene that codes
for the CFTR gene which is made of 1408 amino acids.
– the substitution of a T for a C at nucleotide 1609 converted a
glutamine codon (CAG) to a STOP codon (TAG).
– The protein produced by this patient had only the first 493
amino acids of the normal chain of 1480 and could not
function.
Mutations
• Insertions and Deletions: One bases is inserted to
removed from a DNA sequence.
– Also called a Frameshift mutation:
nucleotides are read 3 nucleotides at
a time (the reading frame).
Removing or adding 1 or 2
nucleotides will change the grouping
of the nucleotides.
– How does a
frameshift mutation
affect then protein?
Short Answer
• If mRNA is just a copy of the DNA, explain why the
process of transcription is necessary. (3 points)
• Explain how the mRNA molecule is changed after it is
transcribed but before it is translated. (3 points)
• Explain the process of translation. (5 points)
• In a point mutation, explain why a substitution usually
does not cause a major change in the protein but an
insertion or deletion usually change the entire protein. (4
points)
Chromosomal Mutations
• Chromosomal mutation: mutation that
changes the number or structure of
chromosomes.
– Deletion: The loss of all or part of
a chromosome
– Duplication: A segment is repeated
– Inversion: part of the chromosome
is reverse from its usual direction.
– Translocation: one chromosome
breaks off an attaches to another
chromosome.
Mutations
Mutagens: chemicals or physical agents that
cause mutations.
• Examples: X-rays and UV light
• Mutations are random; some are beneficial
and some are harmful.
Translation
Genetic Code – the sequence of A, C, G and U
used to read the mRNA sequence.
• The genetic code is read three letters at a time.
• Each group of three mRNA nucleotides is
called a codon.
• Each codon corresponds to a single amino acid.
So if you give me an amino acid sequence can I
figure out the exact sequence of amino acids needed?
STEPS
1. Take the DNA and transcribe it into mRNA
Example:
mRNA:
TAC ATA CTA GCG ACT
AUG UAU GAU CGC UGA
2. Take the mRNA sequence and decode it
using the codon chart.
AUG = MET
UAU = TYR
GAU = ASP
CGC = ARG