28.5: Translation of RNA - Protein Biosynthesis

The primary cellular function of mRNA is to direct the biosynthesis of the thousands of diverse peptides and proteins required by an organism—as many as 150,000 in a human. The mechanics of protein biosynthesis take place on ribosomes, small granular particles in the cytoplasm of a cell that consist of about 60% ribosomal RNA and 40% protein.

The specific ribonucleotide sequence in mRNA forms a message that determines the order in which amino acid residues are to be joined. Each “word,” or codon, along the mRNA chain consists of a sequence of three ribonucleotides that is specific for a given amino acid. For example, the series UUC on mRNA is a codon directing incorporation of the amino acid phenylalanine into the growing protein. Of the 4³ = 64 possible triplets of the four bases in RNA, 61 code for specific amino acids and 3 code for chain termination. Table \(\PageIndex{1}\) shows the meaning of each codon.

Table \(\PageIndex{1}\): Codon Assignments of Base Triplets

		Third base (3′ end)
First base (5′ end)	Second base	U	C	A	G
U	U	Phe	Phe	Leu	Leu
	C	Ser	Ser	Ser	Ser
	A	Tyr	Tyr	Stop	Stop
	G	Cys	Cys	Stop	Trp
C	U	Leu	Leu	Leu	Leu
	C	Pro	Pro	Pro	Pro
	A	His	His	Gln	Gln
	G	Arg	Arg	Arg	Arg
A	U	Ile	Ile	Ile	Met
	C	Thr	Thr	Thr	Thr
	A	Asn	Asn	Lys	Lys
	G	Ser	Ser	Arg	Arg
G	U	Val	Val	Val	Val
	C	Ala	Ala	Ala	Ala
	A	Asp	Asp	Glu	Glu
	G	Gly	Gly	Gly	Gly

The message embedded in mRNA is read by transfer RNA (tRNA) in a process called translation. There are 61 different tRNAs, one for each of the 61 codons that specify an amino acid. A typical tRNA is single-stranded and has roughly the shape of a cloverleaf, as shown in Figure \(\PageIndex{1}\). It consists of about 70 to 100 ribonucleotides and is bonded to a specific amino acid by an ester linkage through the 3′ hydroxyl on ribose at the 3′ end of the tRNA. Each tRNA also contains on its middle leaf a segment called an anticodon, a sequence of three ribonucleotides complementary to the codon sequence. For example, the codon sequence UUC present on mRNA is read by a phenylalanine-bearing tRNA having the complementary anticodon base sequence GAA. [Remember that nucleotide sequences are written in the 5′ → 3′ direction, so the sequence in an anticodon must be reversed. That is, the complement to (5′)-UUC-(3′) is (3′)-AAG-(5′), which is written as (5′)-GAA-(3′).]

Figure \(\PageIndex{1}\): Structure of a tRNA molecule. The tRNA is a roughly cloverleaf-shaped molecule containing an anticodon triplet on one “leaf” and an amino acid unit attached covalently at its 3′ end. The example shown is a yeast tRNA that codes for phenylalanine. The nucleotides not specifically identified are chemically modified analogs of the four common nucleotides.

As each successive codon on mRNA is read, different tRNAs bring the correct amino acids into position for enzyme-mediated transfer to the growing peptide. When synthesis of the proper protein is completed, a “stop” codon signals the end, and the protein is released from the ribosome. This process is illustrated in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): A representation of protein biosynthesis. The codon base sequences on mRNA are read by tRNAs containing complementary anticodon base sequences. Transfer RNAs assemble the proper amino acids into position for incorporation into the growing peptide.

Worked Example \(\PageIndex{1}\): Predicting the Amino Acid Sequence Transcribed from DNA

What amino acid sequence is coded by the following segment of a DNA coding strand (sense strand)?

(5′) CTA-ACT-AGC-GGG-TCG-CCG (3′)

Strategy

The mRNA produced during translation is a copy of the DNA coding strand, with each T replaced by U. Thus, the mRNA has the sequence

(5′) CUA-ACU-AGC-GGG-UCG-CCG (3′)

Each set of three bases forms a codon, whose meaning can be found in Table \(\PageIndex{1}\).

Solution

Leu-Thr-Ser-Gly-Ser-Pro.

Exercise \(\PageIndex{1}\)

Add exercises text here.

1. Ala
2. Phe
3. Leu
4. Tyr

Answer

1. GCU, GCC, GCA, GCG
2. UUU, UUC
3. UUA, UUG, CUU, CUC, CUA, CUG
4. UAU, UAC

Exercise \(\PageIndex{2}\)

List anticodon sequences on the tRNAs carrying the following amino acids.

1. Ala
2. Phe
3. Leu
4. Tyr

Answer

1. AGC, GGC, UGC, CGC
2. AAA, GAA
3. UAA, CAA, GAA, GAG, UAG, CAG
4. AUA, GUA

Exercise \(\PageIndex{3}\)

What amino acid sequence is coded by the following mRNA base sequence?

CUU-AUG-GCU-UGG-CCC-UAA

Answer

Leu-Met-Ala-Trp-Pro-Stop

Exercise \(\PageIndex{4}\)

What is the base sequence in the original DNA strand on which the mRNA sequence in Problem 28-9 was made?

Answer

(5′) TTA-GGG-CCA-AGC-CAT-AAG (3′)