Concept 17.4 Lecture outline

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Concept 17.4

Translation is the RNA-directed synthesis of a polypeptide:

a closer look

the process of translation, a cell translates a genetic message to build a

polypeptide.

The tRNA molecule is a

translator.

The

interpreter is transfer RNA (tRNA), which

transfers amino acids from the cytoplasmic pool to a growing polypeptide in a

ribosome.

○ A cell has all 20 amino acids available

in its cytoplasm, either by synthesizing them from scratch or by taking them up

from the

surrounding solution.

Each

type of tRNA molecule translates a particular mRNA codon into a particular

amino acid. Each tRNA molecule bears a specific

amino

acid at one end. At the other end of the tRNA is a

nucleotide triplet called an anfile:///Users/reem.albitar/Desktop/Screen%20Shot%202015-05-12%20at%2012.20.05%20PM.pngticodon, which base-pairs with a complementary

codon on mRNA. { fig. 17.15 The structure of transfer RNA (tRNA)}.

Codon

by codon, the genetic message is translated as tRNAs deposit amino acids in the

order prescribed, and the ribosome joins the amino acids into a chain. tRNA is a translator because it reads a

nucleic acid word (the mRNA codon) and interprets it as a protein word (the

amino acid).

other types of RNA, tRNA molecules are transcribed from DNA templates.

both bacterial and eukaryotic cells, each tRNA is used repeatedly, picking up

its designated amino acid in the cytosol, depositing the amino acid at the

ribosome, and returning to the cytosol to pick up another copy of that amino

acid.

tRNA molecule consists of a single strand of about 80 nucleotides folded back on

itself to form a three-dimensional structure. tRNA includes a loop containing the

anticodon and an attachment site at the 3¢ end for an amino acid.

Each

amino acid is joined to the correct tRNA by aminoacyl-tRNA synthetase.

○ The 20 different synthetases match the 20

different amino acids.

○ Each has active sites for only a specific

tRNA–amino acid

combination.

The

synthetase catalyzes a covalent bond between them in a process driven by ATP

hydrolysis.

○ The result is an aminoacyl-tRNA or

charged amino acid.

The

second recognition process involves a correct match between the tRNA anticodon

and an mRNA codon.

each anticodon had to be a perfect match to each codon, we would expect to find

61 types of tRNA, but the actual number is about 45, because the anticodons of

some tRNAs recognize more than one codon.

Such

versatility is possible because the rules for base pairing between the third

base of the codon and the anticodon are relaxed. This flexible base pairing is called

wobble.

○ Wobble explains why the synonymous codons

for a given amino acid most often differ in their third base, not in their

other bases.

The

ribosome is the site of translation.

Ribosomes

facilitate the specific coupling of the tRNA anticodons with mRNA codons during

protein synthesis.

A ribosome consists of a large and a

small subunit, each made up of proteins and ribosomal RNA (rRNA).

eukaryotes, the subunits are made in the nucleolus. rRNA genes are transcribed and the RNA is

processed and assembled with proteins imported from the cytoplasm. The subunits are exported via nuclear

pores to the cytoplasm.

both bacteria and eukaryotic cells, large and small subunits join to form a

functional ribosome only when they attach to an mRNA molecule.

○ Because most cells contain thousands of

ribosomes, rRNA is the most abundant type of cellular RNA.

Though

similar in structure and function, bacterial and eukaryotic ribosomes have

enough differences that certain antibiotic drugs (like tetracycline and

streptomycin) can inactivate bacterial ribosomes without inhibiting eukaryotic

ribosomes.

Each

ribosome has a binding site for mRNA and three binding sites for tRNA

molecules.

○ The P

site (peptidyl tRNA-binding

site) holds the tRNA carrying the growing polypeptide chain.

○ The A

site (aminoacyl tRNA-binding

site) holds the tRNA carrying the next amino acid to be added to the chain.

○ Discharged tRNAs leave the ribosome at

the E (exit) site.

The

ribosome holds the tRNA and mRNA in close proximity and positions the new amino

acid for addition to the carboxyl end of the growing polypeptide.

○ It then catalyzes the formation of the

peptide bond.

the polypeptide becomes longer, it passes through an exit tunnel in the ribosome’s large unit and is released to the

cytosol through the exit tunnel.

Evidence

supports the hypothesis that rRNA, not protein, is responsible for the

ribosome’s structure and function.

○ Proteins on the exterior support the

shape changes of the rRNA molecules as they carry out catalysis during

translation.

○ rRNA is the main constituent at the

interface between the two subunits and of the A and P sites, and it is the

catalyst for peptide bond formation.

○ A ribosome can be regarded as one colossal

ribozyme.

The

process of translation builds a polypeptide.

Translation

can be divided into three stages: initiation, elongation, and termination.

○ All three phases require protein

“factors” that aid in the translation process.

○ Both initiation and chain elongation

require energy provided by the hydrolysis of GTP.

Initiation brings together mRNA, a tRNA with the

first amino acid, and the two ribosomal subunits.

First,

a small ribosomal subunit binds with mRNA and a special initiator tRNA, which

carries methionine.

○ In bacteria, the binding occurs at a

specific RNA sequence, just upstream of the start codon, AUG.

○ In eukaryotes, the small subunit, with

the initiator tRNA already bound, binds to the 5¢ cap of the mRNA and then moves, or scans, downstream along the mRNA until it reaches the start codon

AUG, which signals the start of translation.

○ This establishes the codon reading frame

for the mRNA.

The

union of mRNA, initiator tRNA, and a small ribosomal subunit is followed by the

attachment of a large ribosomal subunit, forming the translation initiation complex.

○ Proteins called initiation factors bring all these components together.

○ Energy in the form of a GTP molecule is

invested in the formation of the initiation complex.

the elongation stage of translation,

amino acids are added one by one to the previous amino acid at the C-terminus

of the growing chain.

○ Each addition involves the participation

of protein elongation factors and

occurs in three-step cycles as each amino acid is added to the preceding one.

Energy

expenditure occurs in the first and third steps.

○ Codon recognition requires hydrolysis of

one molecule of GTP, which increases the accuracy and efficiency of this step.

○ One more GTP is hydrolyzed to provide

energy for the translocation step.

The

mRNA is moved through the ribosome in one direction only, 5¢ end first; this is equivalent to the

ribosome moving 5¢ ® 3¢ on the mRNA.

○ The ribosome and the mRNA move relative

to each other, unidirectionally, codon by codon.

The

elongation cycle takes less than a tenth of a second in bacteria and is

repeated as each amino acid is added to the chain until the polypeptide is

completed.

The

empty tRNAs that are released from the E site return to he cytoplasm, where

they will be reloaded with the appropriate amino acid.

Termination occurs when one of the three stop codons

reaches the A site of the ribosome.

○ A release

factor binds to the stop codon and causes hydrolysis of the bond between

the polypeptide and its tRNA in the P site.

○ This frees the polypeptide, which is

released through the exit tunnel of the ribosome’s large subunit.

The

translation complex disassembles.

○ Breakdown of the translation assembly

requires the hydrolysis of two more GTP molecules.

ribosome requires less than a minute to translate an average-sized mRNA into a

polypeptide.

single mRNA may be used to make many copies of a polypeptide simultaneously as

multiple ribosomes, polyribosomes or

polysomes, trail along the same

mRNA.

○ Polyribosomes can be found in bacterial

and eukaryotic cells.

Folding

and modification of a protein follows translation.

During

and after synthesis, a polypeptide spontaneously coils and folds to its

three-dimensional shape.

○ The primary structure, the order of amino

acids, determines the secondary and tertiary structure.

chaperone protein (chaperonin) helps the polypeptide fold correctly.

addition, proteins may require post-translational

modifications before doing their particular job.

These

modifications may require additions such as sugars, lipids, or phosphate groups

to amino acids.

○ Enzymes may remove one or more amino

acids from the leading (amino) end of the polypeptide chain.

some cases, a single polypeptide chain may be enzymatically cleaved into two or

more pieces.

○ In other cases, two or more polypeptides

may join to form a protein with quaternary structure.

Signal peptides target

some eukaryotic polypeptides to specific destinations in the cell.

Two

populations of ribosomes, free and bound, are active participants in protein

synthesis.

○ Free ribosomes are suspended in the

cytosol and synthesize proteins that reside and function in the cytosol.

○ Bound ribosomes are attached to the

cytosolic side of the endoplasmic reticulum (ER) or to the nuclear envelope.

○ Bound ribosomes make proteins of the

endomembrane system as well as proteins secreted from the cell.

Although

bound and free ribosomes are identical in structure, their location depends on

the type of protein they are synthesizing.

Translation

in all ribosomes begins in the cytosol, but a polypeptide destined for the

endomembrane system or for export has a specific signal peptide region at or near the leading end.

○ The signal peptide consists of a sequence

of about 20 amino acids.

A signal recognition particle (SRP) binds

to the signal peptide and attaches it and its ribosome to a receptor protein in

the ER membrane.

○ The SRP, which consists of a protein–RNA

complex, functions as an escort to bring the ribosome to a receptor protein

built into the ER membrane.

○ The receptor is part of a multiprotein

translocation complex.

Protein

synthesis resumes, with the growing polypeptide snaking across the membrane

into the ER lumen via a protein pore.

○ An enzyme usually cleaves the signal

polypeptide.

Secretory

proteins are released into solution within the ER lumen, but membrane proteins

remain partially embedded in the ER membrane.

Other

kinds of signal peptides are used to target polypeptides to mitochondria,

chloroplasts, the nucleus, and other organelles that are not part of the

endomembrane system.

○ In these cases, translation is completed

in the cytosol before the polypeptide is imported into the organelle.

○ While the mechanisms of translocation

vary, each of these polypeptides has a “zip code” that ensures its delivery to

the correct cellular location.

Bacteria

also employ signal sequences to target proteins to the plasma membrane for secretion.

Although

bacteria and eukaryotes carry out transcription and translation in similar

ways, they differ in cellular machinery and in the details of the processes.

Bacterial

and eukaryotic RNA polymerases differ significantly from each other.

Transcription

is terminated differently in bacteria and eukaryotes.

Bacterial

and eukaryotic ribosomes differ slightly.

Initiation

of translation is slightly different in bacteria and eukaryotes.

Gene

expression in eukaryotes differs from that of bacteria because of the greater

compartmental organization of the eukaryotic cell.

○ In the absence of a nucleus, a bacterial

cell can simultaneously transcribe and translate the same gene and the new

protein quickly diffuses to its operating site.

eukaryotes, the nuclear envelope segregates transcription from translation and

provides a compartment for extensive RNA processing between these processes.

○ This provides additional steps whose

regulation helps coordinate the elaborate activities of a eukaryotic cell.

Concept 17.4 Lecture outline

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