recap

14.4 recap

Most eukaryotic genes contain noncoding sequences called introns, which are removed from the pre-mRNA transcript in the nucleus. The mature mRNA transcript is then exported through a nuclear pore to ribosomes in the cytoplasm, where translation takes place.

learning outcomes

You should be able to:

  • Describe the mRNA processing steps in eukaryotes.

  • Predict the outcome of a defect in mRNA processing in a eukaryotic cell.

  • Compare the features of a eukaryotic gene, noting introns, exons, and signals, with its mature mRNA transcript.

Question 1

How and why is the pre-mRNA transcript modified at the 5′ and 3′ ends during mRNA processing?

At the 5′ end, a “cap” of modified GTP is added. This facilitates binding of mRNA to the ribosome and protects the mRNA from hydrolysis by ribonucleases. At the 3′ end a poly A tail is added, with 100-300 A nucleotides. This assists in export of mRNA from the nucleus.

Question 2

Outline the steps involved in RNA splicing. What are the consequences if it does not happen correctly?

snRNP particles bind to consensus sequences at the 5′ and 3′ splice sites on pre-mRNA in the nucleus. The two ends approach each other, mediated by the snRNPs. mRNA is cut first at the 5′ end and then at the 3′ end, releasing the intron. The two exons are then joined.

Question 3

A gene that is 1,440 base pairs long codes for a polypeptide that is 192 amino acids long. Discuss the discrepancy.

192 amino acids would ne encoded by 576 nucleotides. Add start and stop codons for a total of 582 base pairs in DNA. The DNA has extra base pairs for the promoter and terminator of transcription, introns, and a sequence for mRNA binding to the ribosome.

Transcription and posttranscriptional events produce an mRNA that is ready to be translated into a sequence of amino acids in a polypeptide. We will turn now to the events of translation.