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Translation is a process that converts a message from one language into another. For example, a book may be translated from Spanish into English. Translation happens in cells, too. Cells translate an mRNA message into amino acids, the building blocks of proteins.

AminoacidsarecodedbymRnAbasesequences.Translation is the process that reads an mRNA message and turns it into a polypeptide. One or more polypeptides make a protein. An mRNA message can be translated into 20 different amino acids. How can just four nucleotides—A, U, C, and G—be translated into so many different amino acids? Just as the 26 letters of the alphabet can be used to form many more than 26 words, the four letters of RNA are put together in different combinations to form many different “words.”

Secondbase

firs

tba

se

Thirdbase

UUUphenylalanine(Phe)

UcU

serine (Ser)

UAU tyrosine (Tyr)

UGU cysteine (Cys)

U

UUc Ucc UAc UGc c

UUA leucine (Leu)

UcA UAA STOP UGA STOP A

UUG UcG UAG STOP UGG tryptophan (Trp) G

cUU

leucine (Leu)

ccU

proline (Pro)

cAU

histidine(His)

cGU

arginine (Arg)

U

cUc ccc cAc cGc c

cUA ccA cAA glutamine (Gln)

cGA A

cUG ccG cAG cGG G

AUUisoleucine

(Ile)

AcU

threonine (Thr)

AAU asparagine (Asn)

AGU serine (Ser)

U

AUc Acc AAc AGc c

AUA AcA AAA lysine (Lys)

AGA arginine (Arg)

A

AUG methionine (Met) AcG AAG AGG G

GUU

valine (Val)

GcU

alanine (Ala)

GAU aspartic acid (Asp)

GGU

glycine (Gly)

U

GUc Gcc GAc GGc c

GUA GcA GAA glutamic acid (Glu)

GGA A

GUG GcG GAG GGG G

GEnETiCCoDE:mRnACoDonS

CU GA CU GA CU GA CU GA

CU GA

CU GA

CU GA

CU GA

1

2

3

Suppose you want to deter-mine which amino acid is encoded by the cAU codon.

ThegeneticcodematcheseachmRnAcodonwithitsaminoacidorfunction.

Find the first base, c, in the left column.

1

Find the second base, A, in the top row. Find the box where these two intersect.

2

Find the third base, U, in the right col-umn. cAU codes for histidine, abbre-viated as His.

3

SEcTIoN

8.5 TranslationKEyConCEPT Translation converts an mRNA message into a polypeptide, or protein.

138 Holt McDougal Biology

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In the language of the genetic code, a “word” is made of three nucleotides and is called a codon. A codon codes for an amino acid. Multiple codons can code for the same amino acid. For example, the codons UCU, UCC, UCA, and UCG all code for serine.

In addition to codons that code for amino acids, there is a start codon that signals the start of translation. The start codon is AUG, and codes for the amino acid methionine. There are also three stop codons that signal the end of an amino acid chain.

For words to make sense, they must be read starting at the correct place, or with the correct reading frame. For example, the sentence “THE CAT ATE THE RAT” makes sense. “T HEC ATA TET HER AT,” which is made of the same letters, has a different reading frame and does not make sense. The same is true for codons. There are no spaces between codons, but the correct start site and the correct reading frame must be used for the message to make sense.

Use the chart to find the amino acid that is encoded by the codon GGG.

Aminoacidsarelinkedtobecomeaprotein.Remember that transcription makes three kinds of RNA—mes-senger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Proteins and rRNA form ribosomes. Ribosomes and tRNA form the machinery for translating mRNA to make proteins.

A tRNA molecule forms an L shape. Different tRNA molecules carry different amino acids. On one end of a tRNA molecule there is an anticodon that is complementary to a specific mRNA codon. For example, a tRNA molecule that has the anticodon CCC pairs with the mRNA codon GGG. The same tRNA molecule also carries the amino acid glycine.

aminoacid

anticodon

tRNA

A codon is a sequence of three nucleo-tides that codes for an amino acid.

VISUALVOCAB

Segment of mRNA

C U UU GA

codon for methionine (Met)

codon for leucine (Leu)

Codons are read as a series of three nonoverlapping nucleotides. A change in the reading frame changes the resulting protein.

Readingframe1

Readingframe2

C CCG GU UA A A GA

Arg Tyr Ser Ser

C CCG GU UA A A GA

Asp Thr Val

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Met Met

Leu Leu

Cys Cys

Met Met

Leu Leu

peptidebond

2

3The ribosome pulls the mRNA strand the length of one codon. The first tRNA is shifted into the exit site, where it leaves the ribosome and returns to the cytoplasm to recharge. The first site is again empty, exposing the next mRNA codon.

Met Met

Leu Leu

CU

U G

A

A

startcodon

incomingtRNA

mRNA

methionine

1The exposed codon in the first site attracts a complementary tRNA bearing an amino acid. The tRNA anticodon pairs with the mRNA codon, bringing it very close to the other tRNA molecule.

Met Met Arg Arg

Leu Leu Cys Cys

Cys Cys

U GA

The ribosome continues to translate the mRNA strand until it reaches a stop codon. Then it releases the new protein and disassembles.

cytoplasm

Translation occurs in the cytoplasm of both eukaryotic (illustrated) and prokary-otic cells. it starts when a tRNA carrying a methionine attaches to a start codon.

tRNAribosome

mRNA

nucleusaminoacid

The ribosome forms a peptide bond between the two amino acids and breaks the bond between the first tRNA and its amino acid.

leucine

TranslationconvertsanmRnAtranscriptintoapolypeptide. Theprocessconsistsofthreerepeatingsteps.

TRAnSLATion

The anticodon on the tRNA is complementary to the mRNA codon.

stopcodon

anticodon

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The process of translation happens in the cytoplasm of both prokary-otic and eukaryotic cells. The illustration on the previous page shows the process of translation in just one ribosome. In a cell, many ribosomes may translate the same gene at the same time to make many copies of the same protein.

What is the final product of the process of translation?

translation stop codon

codon anticodon

start codon

1. What does a start codon do? a stop codon?

2. What is the relationship between a codon and an anticodon?

3. Fill in the blanks to complete the concept map for the process of translation.

aminoacids

translationarelinkedtomake

arelike“words”

thatcodefor

throughtheprocessof

4. Refer to the genetic code table to find the amino acid that is encoded by the mRNA codon CCA. What bases would be found in the comple-mentary tRNA anticodon?

8.5 vocabularyCheck

Go back and highlight each sentence that has a vocabulary word in bold.

Mark It Up

8.5 TheBigPicture

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