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Objectives
• Identify that amino acids are coded by
mRNA base sequences and are linked to
become proteins
• Describe how mRNA codons are
translated into amino acids
• Summarize the process of protein
synthesis
Review
• What was the purpose of transcription?
– To transcribe (copy) DNA into a disposable copy
called mRNA to leave the nucleus.
• What are the nitrogen bases of mRNA?
– A, U, G, C
• What is the sugar for mRNA?
– Ribose
• What are the 3 differences between DNA and
RNA?
– Ribose sugar, single stranded, Uracil instead of
Thymine
• What are the 3 different types of RNA?
– mRNA, rRNA, tRNA
Translation
• TRANSLATION: process that converts, or
translates, an mRNA message into a
protein (amino acid chain).
• How is mRNA translated??
– By reading the nitrogen base sequences in
groups of 3. These groups of 3 are called
Codons.
Codons
• Codon- a group of three nucleotides on
mRNA that code for a particular amino
acid
codon for
methionine (Met)
codon for
leucine (Leu)
• If you had the mRNA sequence:
UCGCACGGU
• The codons would be:
UCG CAC GGU
• What amino acids would be coded for?
– Let’s find out!
Finding Amino acids from Codon
• Reading the
mRNA codons
left to right
write the
amino acid
sequence of
the protein
translated from
the mRNA.
Codon to Amino Acid
UCG =
– Serine
CAC =
– Histidine
GGU =
– Glycine
Amino Acids
• There are 64 possible three-base codons that
can be created using the four nitrogen bases (4
x 4 x 4 = 64)
• There are 20 different amino acids
– There are multiple codons that code for the
same amino acids
• AUG= start codon for protein synthesis or
methionine. This means that translation always
begins with methionine (AUG).
• Three codons do not code for an amino acid so
are considered stop codons that signals the
end of an amino acid chain. - UAA, UAG, UGA
How does a cell interpret DNA?
• Write the following DNA sequence:
– GAC AAG TCC ACA ATC
• Write the mRNA sequence that would be
transcribed from this gene:
– CUG UUC AGG UGU UAG
• Reading the mRNA codons, write the
amino acid sequence of the polypeptide
translated from the mRNA
Codon to Amino Acid
CUG=
– Leucine
UUC=
– Phenylalanine
AGG=
– Arginine
UGU=
– Cysteine
UAG=
– Stop
• So proteins are determined by the chain of
amino acids that make them up
• But how do our protein makers know which
amino acids to add?
What’s Next…
Translation!
–
–
–
•
After mRNA LEAVES the NUCLEUS, it
takes DNA’s message to the RIBOSOME
(rRNA)
Proteins are made on the rRNA using mRNA
as a template.
So translation occurs in the rRNA, which is
located in the cytoplasm of the cell.
Three steps to translation
–
–
–
Initiation
Elongation
Termination
)
REMEMBER: mRNA is READ 3 letters at a time
in order to determine what protein is to be
made
o These three letters are called a CODON
o The CODON on the mRNA matches up with
the ANTICODON on the tRNA molecule
Initiation: The start of translation
• The small ribosomal subunit attaches to the
strand of mRNA at the 5` end.
• A tRNA molecule with the start or initiator
anticodon (UAC) attaches to the mRNA.
• The large ribosomal subunit attaches, locking
the initiator tRNA in the P (peptide) site.
Elongation
The amino acid chain gets longer
• The second tRNA molecule, with an anticodon
that is complementary to the second codon on
the mRNA, attaches in the A site of the large
subunit.
• A peptide bond is formed between the amino
acid of the tRNA in the A site and that of the P
site.
• The tRNA molecule in the P site is kicked out
and the one in the A site (with the growing
polypeptide chain) shifts into the P site.
• The process repeats many times until tRNA
reaches a stop codon.
Termination
The final step
• Toward the end of the coding sequence of
the mRNA, a termination (stop) codon is
encountered (UAG, UAA, or UGA).
• There are no tRNA molecules that match the
stop codons, but a special protein called a
release factor does recognize the sequence,
and attaches.
• This special protein causes the newly created
amino acid chain to be released.
• The ribosomal subunits disassemble.
• Translation is complete!!!!!!!!!!!!!!!!!
Amino Acid
Chain
Review
• 1. Initiation
• 2. Elongation
• 3. Termination
**Protein Synthesis in a Nutshell**
Transcription
-In NUCLEUS
-DNA copied to
mRNA
-mRNA to Ribosome
Translation
-In RIBOSOME
-Codon/Anticodon pair
-codon determines what
AA comes in
-AA combine-proteins
Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Translation
Nucleus
Phenylalanine tRNA
Methionine
Ribosome
mRNA
Start
codon
Lysine
mRNA
Translation Continued
Growing polypeptide
chain
Ribosome
tRNA
Lysine
tRNA
mRNA
Ribosome
Translation
direction