Page 130 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

8.3 Translation: From mRNA to Protein 289

Figure 8.17 Different mRNAs can be produced from the 8.3 Translation: From
same primary transcript. Alternative splicing of the primary
transcript for the antibody heavy chain produces mRNAs that mRNA to Protein
encode different kinds of antibody proteins.
outside of gene
exon learning objectives
intron
intron in membrane-bound/ 1. Relate tRNA’s structure to its function.
exon in secreted
A poly-A addition sites 2. Describe the key steps of translation, indicating how
splice specific for membrane-bound each depends on the ribosome.

Antibody heavy-chain gene 3. List three categories of posttranslational processing
A A and provide examples of each.
1 2 3 4 5 6a 6b 7 8

Transcription Translation is the process by which the sequence of nucle­
otides in a messenger RNA directs the assembly of the
Primary transcript correct sequence of amino acids in the corresponding
Splicing for Splicing for polypeptide. Translation takes place on ribosomes that co­
membrane-bound secreted
antibody antibody ordinate the movements of transfer RNAs carrying spe­
cific amino acids with the genetic instructions of an
1 2 3 4 5 6a 7 8 1 2 3 4 5 6a 6b
5' AAAAAA 3' 5' AAAAAA 3' mRNA. As we examine the cell’s translation machinery,
mRNA mRNA
Exons that encode we first describe the structure and function of tRNAs and
membrane attachment ribosomes; and we then explain how these components
domain
interact during translation.
Membrane-bound antibody Secreted antibody

C terminus (Fig. 8.17). For the secreted antibody, only the Transfer RNAs Mediate the Translation
first six exons (including the right part of 6) are spliced to­ of mRNA Codons to Amino Acids
gether to make an mRNA encoding a heavy chain with a hydro­
philic (water­loving) C terminus. These two kinds of mRNAs No obvious chemical similarity or affinity exists between
formed by alternative splicing thus encode slightly different the nucleotide triplets of mRNA codons and the amino ac­
proteins that are directed to different parts of the body. ids they specify. Rather, transfer RNAs (tRNAs) serve as
adapter molecules that mediate the transfer of information
from nucleic acid to protein.
essential concepts
• Transcription is the process by which RNA polymerase The structure of tRNA
synthesizes a single-stranded primary transcript from a
DNA template. Transfer RNAs are short, single­stranded RNA molecules
• Transcription initiation requires a DNA sequence called 74–95 nucleotides in length. Several of the nucleotides in
tRNAs contain chemically modified bases produced by en­
the promoter that signals RNA polymerase to begin
copying. In eukaryotes, initiation requires an additional zymatic alterations of the principal A, G, C, and U nucleo­
DNA sequence called an enhancer. tides. Each tRNA carries one particular amino acid, and all
• During transcription elongation, RNA polymerase adds cells must have at least one tRNA for each of the common
nucleotides to the growing RNA strand in the 5′-to-3′ direction. 20 amino acids specified by the genetic code. The name of
• A terminator in the RNA transcript tells RNA polymerase a tRNA reflects the amino acid it carries. For example,
Gly
tRNA carries the amino acid glycine.
to cease transcription. As Fig. 8.18 shows, it is possible to consider the struc­
• In prokaryotes, the primary transcript is the messenger ture of a tRNA molecule on three levels.
RNA (mRNA).
• In eukaryotes, RNA processing after transcription 1. The nucleotide sequence of a tRNA constitutes the
produces a mature mRNA; the RNA transcript is modified primary structure.
by the addition of a 5′ cap and a poly-A tail, along with 2. Short complementary regions within a tRNA’s single
the excision of introns when exons are joined by splicing. strand can form base pairs with each other to create a
• Exons can be spliced together in alternative ways; alternative characteristic cloverleaf shape; this is the tRNA’s sec­
splicing produces different mRNA sequences and therefore ondary structure.
different polypeptides from the same primary transcript. 3. Folding in three­dimensional space creates a tertiary
structure that looks like a compact letter L.