Organic Chemistry

056 CHAPTER 27 Nucleosides, Nucleotides, and Nucleic Acids

amino acid

transcription

modification

of RNA

1

5 translation

mRNA

nucleus

cytoplasm

rRNA

mRNA

DNA

tRNA binds an

amino acid

initial RNA

transcript

nuclear

envelope

tRNA

2

4

addition of

protein to RNA

3

ribosome

growing polypeptide

chain

growing polypeptide chain. Serine was specified by the AGC codon because the

anticodon of the tRNA that carries serine is GCU (Remember that a

base sequence is read in the direction, so the sequence of bases in an an-

ticodon must be read from right to left.) The next codon is CUU, signaling for a

tRNA with an anticodon of AAG That particular tRNA carries leucine.

The amino group of leucine reacts in an enzyme-catalyzed nucleophilic acyl substi-

tution reaction with the ester on the adjacent tRNA, displacing the tRNA. The next

codon (GCC) brings in a tRNA carrying alanine. The amino group of alanine dis-

places the tRNA that brought in leucine. Subsequent amino acids are brought in one

at a time in the same way, with the codon in mRNA specifying the amino acid to be

incorporated by complementary base pairing with the anticodon of the tRNA that

carries that amino acid.

Protein synthesis takes place on the ribosomes (Figure 27.17). The smaller subunit

of the ribosome (30S in prokaryotic cells) has three binding sites for RNA molecules.

It binds the mRNA whose base sequence is to be read, the tRNA carrying the growing

peptide chain, and the tRNA carrying the next amino acid to be incorporated into the

protein. The larger subunit of the ribosome (50S in prokaryotic cells) catalyzes peptide

bond formation.

(3¿-GAA-5¿).

5 ¿ ¡ 3 ¿

(3¿-UCG-5¿).

Figure 27.17N

1. Transcription of DNA occurs in
   the nucleus. The initial RNA
   transcript is the precursor of all
   RNA: tRNA, rRNA, and mRNA.


2. The initially formed RNA often
   must be chemically modified
   before it acquires biological
   activity. Modification can entail
   removing nucleotide segments,
   adding nucleotides to the - or
   -ends, or chemically altering
   certain nucleotides.


3. Proteins are added to rRNA to
   form ribosomal subunits. tRNA,
   mRNA, and ribosomal subunits
   leave the nucleus.


4. Each tRNA binds the appropriate
   amino acid.


5. tRNA, mRNA, and a ribosome
   work together to translate the
   mRNA information into a
   protein.

3 ¿

5 ¿