Transfer RNA

In order for amino acids to be available to go on line they must be brought to the ribosomes. While m-RNA seems to move without a helper, there is another kind of RNA that acts as a carrier for amino acids and brings them to the place of assembly: t-RNA. There is a different kind of t-RNA molecule for each kind of amino acid. An amino acid needed in the assembly of a polypeptide at the ribosome is escorted by a t-RNA molecule.

Transfer RNA molecules are much smaller than m-RNA molecules. Whereas an m-RNA molecule may have thousands of nucleotides, a t-RNA molecule has only seventy-three to ninety-three nucleotides. The amino acids required in a specified sequence are brought to the ribosomes one at a time by specific carrier molecules. Each molecule of t-RNA is three dimensional in appearance. When the crumpled aspect of the t-RNA molecule is flattened out, and the molecule is represented in a semi-diagrammatic fashion, it may appear as shown in figure 6-11.

SECOND LETTER

CO CE

SECOND LETTER

CO CE

U

C

A

G

u

UUU1 UUC. UUA UUG.

PHE LEU

UCU1 UCC UCA UCG.

SER

UAU1 UAC UAA UAG

?

TRP

U C A G

C

CUU1 CUC CUA CUG.

LEU

CCU1 CCC CCA CCG.

PRO

CAU" CAC. CAA" CAG.

HIS GLN

CGU CGC CGA CGG.

ARG

U C A G

A

AUU 1 AUC J AUA " AUG.

ILE MET

ACU " ACC ACA ACG.

THR

AAU ' AAC AAA " AAG J

ASN LYS

AGU1 AGC. AGA n AGG.

SER ARG

U C A G

G

GUU GUC GUA GUG_

VAL

GCU" GCC GCA GCG

ALA

GAU " GAC _ GAA1 GAG.

ASP GLU

GGU" GGC GGA GGG.

GLY

U C A G

ALA = Alanine ARG = Arginine ASA = Aspartic acid ASP = Asparagine CYS = Cysteine GLN = Glutamine GLU = Glutamic acid

GLY = Glycine HIS = Histidine ILE = Isoleucine LEU = Leucine LYS = Lysine MET = Methionine PHE = Phenylalanine

PRO = Proline SER = Serine THR = Threonine TRP = Tryptophan TYR = Tyrosine VAL = Valine

Figure 6-10 Matrix showing all the ways that the four bases adenine, guanine, cytosine, and uracil can be assembled in threes to make codons, the RNA triplets of m-RNA. The first letters of the triplets are listed in the vertical column at left; the second letters of the triplets are listed horizontally across the top; and the third letters of the triplets are listed in the vertical column at right. The bases indicated by the triplets are abbreviated in the matrix; full names are given below the matrix.

One portion of the t-RNA molecule is called the attachment site. This is where the amino acid that is to be transported hooks on. The attachment site is at the end of the molecule, and here the sequence of three bases is always the same: CCA. Another portion of the t-RNA molecule is called the recognition site. It tells what kind of amino acid is to be picked up. Here, a triplet of bases constitutes an anticodon, which can match up to the codon of the m-RNA molecule. When the t-RNA molecule arrives at the ribosome, it releases the amino acid that had been attached to the free-end, CCA position. The amino acid can then join to the growing peptide chain.

Attachment Site for Amino Acid

Attachment Site for Amino Acid

Picture Rna Transfer

tRNA Recognition Site (Anticodon) A

mRNAI

tRNA Recognition Site (Anticodon) A

mRNAI

Figure 6-11 At-RNA molecule is much shorter than other nucleic acid molecules. Transfer RNA is a carrier molecule that brings the needed amino acid to the site of assembly at the ribosomes. There is a different kind of t-RNAfor each amino acid. The amino acid to be carried is attached to the t-RNA molecule at the attachment site. Another part of the molecule is called the recognition site.

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