33.3-2

mRNA Processing and Translation

1. Overview of RNA Transcription

  • The primary transcript is the initial RNA formed from the DNA template.

  • Contains the complement of all bases transcribed from DNA.

  • For protein-coding genes, the primary transcript contains the information needed to guide ribosomes in producing proteins.

2. Function of Messenger RNA (mRNA)

  • mRNA carries the genetic message from DNA to ribosomes for protein synthesis.

  • Sometimes called mature mRNA since it can bind to ribosomes.

3. RNA Processing in Eukaryotes and Prokaryotes

3.1 Prokaryotic RNA Processing

  • In prokaryotes, RNA processing does not occur.

  • The transcribed RNA is mRNA, which can immediately bind to ribosomes for translation.

  • Translation can start before transcription is complete due to the absence of a nucleus.

  • Prokaryotic primary transcripts can code for multiple proteins, often for sequential steps in metabolic pathways.

3.2 Eukaryotic RNA Processing

  • In eukaryotes, transcription occurs in the nucleus, while translation happens in the cytoplasm.

  • Primary transcripts require processing to become mRNA before they can be exported from the nucleus.

  • RNA processing involves three main modifications:

    • 5′ Cap Addition:

      • Addition of a modified nucleotide (7-methylguanosine) at the 5′ end.

      • Essential for ribosome recognition and translation initiation.

    • Poly(A) Tail Addition:

      • Addition of about 250 adenine nucleotides to the 3′ end, forming a poly(A) tail.

      • Increases mRNA stability and assists in the transport of mRNA out of the nucleus.

    • Splicing:

      • Removal of non-coding regions (introns) and joining of coding regions (exons).

      • Exons are segments that are expressed as proteins while introns are removed.

4. Importance of Introns and Splicing

  • Introns allow for alternative splicing, producing different mRNAs and proteins from a single primary transcript.

  • Approximately 90% of human genes contain at least one intron, with most genes featuring 6 to 9 introns.

  • Alternative splicing increases protein diversity, allowing different proteins to be produced from the same gene based on splicing variations.

  • Evolutionarily, this has enabled the development of proteins with new or varied functions.

5. Noncoding RNAs

  • Not all primary transcripts are processed into mRNA; some function independently as noncoding RNAs.

  • Noncoding RNAs have functions that do not involve translation into proteins.