Chapter 14 RNA Molecules and RNA Processing

Chapter 14 RNA Molecules and RNA Processing

14.1 Many Genes Have Complex Structures

  • Gene Organization

    • The principle of colinearity: The sequence of nucleotides in DNA corresponds directly to the sequence of amino acids in proteins.

    • Noncolinearity is observed mainly in eukaryotic genes due to the presence of noncoding regions (introns).

    • Experiment involving the hybridization of DNA with the mRNA revealed interrupted coding sequences, indicating that genes often have introns.

    • Key points on Introns:

    • Eukaryotic genes often contain both exons (coding sequences) and introns (noncoding sequences).

    • DNA is often much longer than the mRNA it encodes; this disparity arises because introns are removed during RNA processing.

14.2 Messenger RNAs Modification after Transcription in Eukaryotes

  • Structure of mRNA

    • A mature mRNA consists of a 5′ untranslated region (5' UTR), a protein-coding region, and a 3′ untranslated region (3' UTR).

    • The 5' UTR includes the Shine-Dalgarno sequence, essential for ribosome binding.

  • Pre-mRNA Processing

    • Addition of 5′ cap (7-methylguanylate) to the 5′ end facilitates ribosome binding, increases stability, and promotes termination of transcription.

    • Addition of a poly(A) tail (composed of 50-250 adenine nucleotides) to the 3′ end increases stability and helps in mRNA export and translation.

  • RNA Splicing

    • Involves the removal of introns and joining of exons to create a continuous protein-coding sequence in mature mRNA.

    • Consensus sequences (5' splice site and 3' splice site) guide splicing with specific molecular machinery, including spliceosomes formed of snRNA and proteins.

    • Two-step process:

    1. Cleavage at the 5' splice site and formation of a lariat with the branch point.

    2. Cleavage at the 3' splice site and ligation of the adjacent exons.

14.3 Transfer RNAs and Their Modifications

  • Structure of tRNA

    • Common secondary structure is cloverleaf; contains anticodons that match codons in mRNA for amino acid attachment.

    • Modified nucleotides (e.g., ribothymine, pseudouridine) play significant roles in the stability and function of tRNA.

  • Processing of tRNA

    • Involves cleavage of precursor tRNA, removal of introns, and various base modifications.

14.4 Ribosomal RNA and Its Processing

  • Ribosome Composition

    • Composed of rRNA and proteins; bacterial ribosome is 70S (50S + 30S) while eukaryotic ribosome is 80S (60S + 40S).

    • Ribosomal RNA is synthesized as a precursor, then processed through methylation and cleavage to yield functional rRNAs.

14.5 Small RNA Molecules and Their Functions

  • RNA Interference (RNAi)

    • Small RNAs play roles in gene regulation by degrading mRNA or inhibiting translation.

    • Types of Small RNAs:

    • MicroRNAs (miRNAs): Regulate gene expression by base-pairing with target mRNAs.

    • Small interfering RNAs (siRNAs): Cause degradation of mRNAs from corresponding genes.

  • Key Differences between siRNAs and miRNAs:

    • siRNAs typically originate from mRNA, transposons, or viruses, while miRNAs are derived from distinct genes. Both are 21-25 nucleotides long.

14.6 Long Noncoding RNAs and Their Regulatory Role

  • Long Noncoding RNAs (lncRNAs)

    • Do not code for proteins but are involved in regulating gene expression and can act as decoys for other RNA molecules, such as miRNAs.

    • Enhancer RNAs: Transcribed from enhancers with roles in gene regulation.

This chapter covers the intricate processes of RNA processing and the structural complexity of RNA molecules across different organisms. Understanding these processes is crucial for grasping how gene expression is regulated at multiple levels.