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In summary, there are several key features that distinguish prokaryotic gene expression from that seen in eukaryotes. These are illustrated in [link] and listed in [link] .

a) Diagram of prokaryotic cell with a plasma membrane on the outside. The DNA is in the cytoplasm and the mRNA is being copied at the same time that ribosomes are building proteins of the developing mRNA. B) Diagram of a eukaryotic cell with a plasma membrane an a nucleus. The DNA is in the nucleus and pre-mRNA is made during transcription; this is then process into mature mRNA. The mature mRNA then leaves the nucleus and enters the cytoplasm where translation takes place. This is when ribosomes bind to the mRNA and make proteins.
(a) In prokaryotes, the processes of transcription and translation occur simultaneously in the cytoplasm, allowing for a rapid cellular response to an environmental cue. (b) In eukaryotes, transcription is localized to the nucleus and translation is localized to the cytoplasm, separating these processes and necessitating RNA processing for stability.
Table titled: Comparison of Translation in Bacteria Versus Eukaryotes. Bacteria have 70s ribosomes made of a 30s (small subunit) with 16SrRNA subunit and a 50S (large subunit) with 5S and 23S rRNA subunits. Eukaryotic ribosomes are 80S with 40s (small subunit) with 18s rRNA subunit and 60S (large subunit) with 5S, 5.8S, and 28S rRNA subunits. The amino acid carried by the initiator tRNA is fMet for bacteria and Met for Eukaryotes. Bacteria have a Shine-Delgarno sequence in their mRNA while Eukaryotes do not. Transcription and translation is simultaneous in bacteria but not in eukaryotes.

Protein targeting, folding, and modification

During and after translation, polypeptides may need to be modified before they are biologically active. Post-translational modifications include:

  1. removal of translated signal sequences—short tails of amino acids that aid in directing a protein to a specific cellular compartment
  2. proper “folding” of the polypeptide and association of multiple polypeptide subunits, often facilitated by chaperone proteins, into a distinct three-dimensional structure
  3. proteolytic processing of an inactive polypeptide to release an active protein component, and
  4. various chemical modifications (e.g., phosphorylation, methylation, or glycosylation) of individual amino acids.
  • What are the components of the initiation complex for translation in prokaryotes?
  • What are two differences between initiation of prokaryotic and eukaryotic translation?
  • What occurs at each of the three active sites of the ribosome?
  • What causes termination of translation?

Key concepts and summary

  • In translation , polypeptides are synthesized using mRNA sequences and cellular machinery, including tRNAs that match mRNA codons to specific amino acids and ribosomes composed of RNA and proteins that catalyze the reaction.
  • The genetic code is degenerate in that several mRNA codons code for the same amino acids. The genetic code is almost universal among living organisms.
  • Prokaryotic (70S) and cytoplasmic eukaryotic (80S) ribosomes are each composed of a large subunit and a small subunit of differing sizes between the two groups. Each subunit is composed of rRNA and protein. Organelle ribosomes in eukaryotic cells resemble prokaryotic ribosomes.
  • Some 60 to 90 species of tRNA exist in bacteria. Each tRNA has a three-nucleotide anticodon as well as a binding site for a cognate amino acid . All tRNAs with a specific anticodon will carry the same amino acid.
  • Initiation of translation occurs when the small ribosomal subunit binds with initiation factors and an initiator tRNA at the start codon of an mRNA, followed by the binding to the initiation complex of the large ribosomal subunit.
  • In prokaryotic cells, the start codon codes for N-formyl-methionine carried by a special initiator tRNA. In eukaryotic cells, the start codon codes for methionine carried by a special initiator tRNA. In addition, whereas ribosomal binding of the mRNA in prokaryotes is facilitated by the Shine-Dalgarno sequence within the mRNA, eukaryotic ribosomes bind to the 5’ cap of the mRNA.
  • During the elongation stage of translation, a charged tRNA binds to mRNA in the A site of the ribosome; a peptide bond is catalyzed between the two adjacent amino acids, breaking the bond between the first amino acid and its tRNA; the ribosome moves one codon along the mRNA; and the first tRNA is moved from the P site of the ribosome to the E site and leaves the ribosomal complex.
  • Termination of translation occurs when the ribosome encounters a stop codon , which does not code for a tRNA. Release factors cause the polypeptide to be released, and the ribosomal complex dissociates.
  • In prokaryotes, transcription and translation may be coupled, with translation of an mRNA molecule beginning as soon as transcription allows enough mRNA exposure for the binding of a ribosome, prior to transcription termination. Transcription and translation are not coupled in eukaryotes because transcription occurs in the nucleus, whereas translation occurs in the cytoplasm or in association with the rough endoplasmic reticulum.
  • Polypeptides often require one or more post-translational modifications to become biologically active.

Fill in the blank

The third position within a codon, in which changes often result in the incorporation of the same amino acid into the growing polypeptide, is called the ________.

wobble position

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The enzyme that adds an amino acid to a tRNA molecule is called ________.

aminoacyl-tRNA synthetase

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True/false

Each codon within the genetic code encodes a different amino acid.

False

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Short answer

Why does translation terminate when the ribosome reaches a stop codon? What happens?

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How does the process of translation differ between prokaryotes and eukaryotes?

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What is meant by the genetic code being nearly universal?

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Below is an antisense DNA sequence. Translate the mRNA molecule synthesized using the genetic code, recording the resulting amino acid sequence, indicating the N and C termini.

Antisense DNA strand: 3’-T A C T G A C T G A C G A T C-5’

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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