Microcheck

RNA polymerase recognizes sequences in DNA called promoters and at those sites initiates synthesis of RNA using one strand of DNA as a template. The RNA is synthesized in the 5' to 3' direction and synthesis stops when RNA polymerase encounters a terminator. Translation occurs as ribosomes move along mRNA in the 5' to 3' direction, with the ribosomes serving as the structure that facilitates the joining of one amino acid to another. tRNAs carry specific amino acids, thus acting to decode the genetic code.

■ How does the orientation of the promoter dictate which strand is used as a template for RNA synthesis?

■ Explain why it is important for the translation machinery to recognize the correct reading frame.

■ Could two mRNAs have different nucleotide sequences and yet code for the same protein?

7.4 Differences Between Eukaryotic and Prokaryotic Gene Expression

Eukaryotes differ significantly from prokary-otes in several aspects of transcription and translation (table 7.4). For example, in eukaryotic cells, most mRNA molecules are extensively modified, or processed, in the nucleus during and after transcription. Shortly after transcription begins, the 5' end of the transcript is modified, or capped, by the addition of a methylated guanine derivative, creating what is called a cap. The cap likely stabilizes the transcript and enhances translation. The 3' end of the molecule is also modified, even before transcription

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Table 7.4 Major Differences Between Prokaryotic and Eukaryotic Transcription and Translation

Prokaryotes

Eukaryotes

mRNA is not processed.

A cap is added to the 5' end of mRNA, and a poly A tail is added to the 3' end.

mRNA does not contain introns.

mRNA contains introns, which are removed by splicing.

Translation of mRNA begins as it is being transcribed.

The mRNA transcript is transported out of the nucleus so that it can be translated in the cytoplasm.

mRNA is often polycistronic; translation usually begins at the first AUG that follows a ribosome-binding site.

mRNA is monocistronic; translation begins at the first AUG.

has been terminated. This process, called polyadenylation, involves cleaving the transcript at a specific sequence of nucleotides and then adding approximately 200 adenine derivatives to the newly exposed 3' end. This creates what is called a poly A tail, which is thought to stabilize the transcript as well as enhance translation. Another important modification is splicing, a process that removes specific segments of the transcript (figure 7.16). Splicing is necessary because eukaryotic genes are not always contiguous; they are often interrupted by non-coding nucleotide sequences. These intervening sequences, or introns, are transcribed along with the expressed regions, or exons, generating what is called precursor mRNA. The introns must be removed from precursor mRNA to form the mature mRNA that is then translated.

Eukaryotic DNA contains introns, which interrupt coding regions.

Eukaryotic DNA

Intron

Intron

Precursor mRNA

Transcription generates precursor mRNA that contains introns.

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