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RNA Sequencing and the Molecular Biology of Translation

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RNA Sequencing (RNA-seq)

Overview of RNA-seq Workflow

RNA sequencing (RNA-seq) is a powerful technique used to analyze the transcriptome, providing insights into gene expression, RNA processing, and the identification of small RNAs. The workflow involves several key steps:

  • Sample Preparation: Biological samples are collected under different conditions (e.g., disease vs. normal).

  • RNA Isolation: Total RNA is extracted from the samples, often focusing on polyadenylated (Poly(A) tail) RNA species.

  • cDNA Generation: RNA is reverse transcribed into complementary DNA (cDNA), fragmented, size-selected, and sequencing adapters are added.

  • Sequencing: High-throughput sequencing generates millions of paired-end reads, each representing a short fragment of the original RNA.

  • Mapping and Analysis: Reads are mapped to a reference genome, transcriptome, and predicted exon junctions to quantify gene expression and identify transcript variants.

Example: RNA-seq can identify small RNAs in the 18-30 nucleotide range by size selection, enabling the study of microRNAs and other non-coding RNAs.

RNA-seq Library Preparation Strategies

Different strategies are used to enrich for specific RNA populations and improve transcriptome coverage:

  • Total RNA: Includes all RNA species, but highly expressed ribosomal RNAs (rRNAs) can dominate the library.

  • rRNA Depletion: Removes rRNAs to increase representation of mRNAs and non-coding RNAs.

  • PolyA Selection: Enriches for polyadenylated mRNAs, reducing the presence of rRNAs and some non-coding RNAs.

Strategy

Highly Expressed

Lowly Expressed

Notes

Total RNA

rRNAs

lncRNAs, mRNAs

High rRNA content, broad transcript representation

rRNA Depletion

mRNAs, lncRNAs

Some small RNAs

Removes rRNAs, increases mRNA/lncRNA detection

PolyA Selection

mRNAs

lncRNAs (with polyA)

Enriches for polyadenylated transcripts

Additional info: These strategies are chosen based on the biological question and the RNA species of interest.

The Molecular Biology of Translation

Central Dogma: DNA → RNA → Protein

The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein. Proteins are synthesized by translating messenger RNA (mRNA) at ribosomes.

  • DNA: Contains genetic instructions in the form of nucleotide sequences.

  • RNA: Transcribed from DNA; mRNA carries coding information for protein synthesis.

  • Protein: Composed of amino acid chains assembled at ribosomes.

Ribosomes: Complexes of ribosomal RNAs (rRNAs) and proteins, functioning as ribonucleoprotein complexes to catalyze protein synthesis.

Example: The ribosome sculpture "Waltz of the polypeptides" at CSHL symbolizes the dynamic nature of translation.

Structure and Function of Ribosomes

Ribosomes are essential for translation and differ in composition between domains of life:

  • Subunits: Ribosomes consist of a large and a small subunit, each containing specific rRNAs and proteins.

  • Abundance: Ribosomes are highly abundant; a single bacterial cell may contain ~20,000 ribosomes, accounting for 25% of cell mass.

  • Polysomes: Multiple ribosomes can translate a single mRNA simultaneously, forming polyribosomes (polysomes).

Domain

Ribosome Size (Svedberg units)

rRNA Components

Protein Components

Bacteria (E. coli)

70S (50S + 30S)

16S, 23S, 5S

~55 proteins

Archaea (S. solfataricus)

70S

16S, 23S, 5S

~65 proteins

Eukaryotes

80S (60S + 40S)

18S, 28S, 5.8S, 5S

~80 proteins

Additional info: Svedberg units (S) measure sedimentation rate, reflecting size and shape.

Translation Process: Initiation, Elongation, and Termination

Translation is divided into three main stages:

  • Initiation: Assembly of the ribosome on the mRNA, recognition of the start codon, and recruitment of initiator tRNA.

  • Elongation: Sequential addition of amino acids to the growing polypeptide chain, mediated by tRNAs and elongation factors.

  • Termination: Recognition of stop codons by release factors, leading to release of the completed polypeptide.

Key Ribosome Sites:

  • P site (Peptidyl site): Holds the tRNA with the growing polypeptide chain.

  • A site (Aminoacyl site): Binds incoming aminoacyl-tRNA.

  • E site (Exit site): Releases tRNA after its amino acid has been added.

Translation Initiation: Bacteria vs. Eukaryotes

Translation initiation mechanisms differ between bacteria and eukaryotes:

  • Bacteria: Initiation involves the Shine-Dalgarno sequence on mRNA, pairing with 16S rRNA. The initiator tRNA carries formylmethionine (fMet).

  • Eukaryotes: Initiation requires the 5' methyl cap and poly(A) tail, with multiple initiation factors (eIFs) facilitating ribosome assembly.

Feature

Bacteria

Eukaryotes

Start Codon Recognition

Shine-Dalgarno sequence

5' cap and scanning mechanism

Initiator tRNA

fMet-tRNAfMet

Met-tRNAiMet

Ribosome Subunits

30S + 50S = 70S

40S + 60S = 80S

Translation Elongation and Termination

During elongation, amino acids are added to the polypeptide chain by peptidyl transferase activity, which is catalyzed by rRNA (not protein), supporting the RNA World hypothesis.

  • Elongation Factors: Proteins that facilitate tRNA entry, translocation, and release.

  • Termination: Occurs when a stop codon (UAA, UAG, UGA) enters the A site; release factors (RFs) promote polypeptide release and ribosome disassembly.

Polyribosomes and Efficiency of Translation

Translation is highly efficient, with multiple ribosomes translating a single mRNA simultaneously. This increases the rate of protein synthesis and allows rapid cellular responses.

  • Polyribosomes: Structures containing groups of ribosomes actively translating the same mRNA.

  • Efficiency: Enables high levels of protein production in both prokaryotic and eukaryotic cells.

Genetic Code and mRNA Structure

The genetic code is universal and specifies how nucleotide sequences in mRNA are translated into amino acid sequences in proteins.

  • Codons: Triplets of nucleotides in mRNA that specify amino acids.

  • Start Codon: AUG (methionine) signals the beginning of translation.

  • Stop Codons: UAA, UAG, UGA signal termination.

mRNA Structure: Includes 5' untranslated region (UTR), coding region, and 3' UTR. The coding region is flanked by start and stop codons.

Summary Table: Key Features of Translation

Feature

Description

Central Dogma

DNA → RNA → Protein

Ribosome Composition

rRNAs and proteins; two subunits

Translation Stages

Initiation, Elongation, Termination

Polyribosomes

Multiple ribosomes translating one mRNA

Genetic Code

Triplet codons specify amino acids

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