Ch 11:Cell and Molecular Biology Summary

Overview of the Central Dogma of Molecular Biology

  • The central dogma describes the flow of genetic information within a biological system.

  • Key processes:

    • Transcription: Formation of RNA from a DNA template.

    • Translation: Synthesis of proteins from mRNA.

Types of RNA

  • Main types of RNA:

    • Messenger RNA (mRNA): carries genetic information from DNA to the ribosome.

    • Ribosomal RNA (rRNA): combines with proteins to form ribosomes, the site of protein synthesis.

    • Transfer RNA (tRNA): transfers amino acids to ribosomes during translation.

    • Other types include snRNA, snoRNA, siRNA, piRNA, and miRNA.

RNA Polymerases and Transcription

  • RNA Polymerases:

    • Enzymes that synthesize RNA from a DNA template, functioning as DNA-dependent RNA polymerases.

    • Move in a 3' to 5' direction, synthesizing RNA 20-50 nucleotides per second.

  • Regulation of transcription frequency is crucial, involving a proofreading function.

Transcription in Prokaryotic Cells

  • Bacteria:

    • Have a single RNA polymerase composed of 5 subunits, forming a core enzyme.

    • σ factor is essential for binding the promoter, leading to transcription initiation.

    • The transcriptional complex undergoes conformational changes after incorporating 10 nucleotides.

Transcription in Eukaryotic Cells

  • Eukaryotes have three RNA polymerases:

    • RNA Polymerase I: Transcribes rRNA.

    • RNA Polymerase II: Transcribes mRNA and most small nuclear RNAs.

    • RNA Polymerase III: Transcribes tRNA and other small RNAs.

  • RNA processing involves trimming primary transcripts and includes operations unique to eukaryotes.

Ribosomal RNA (rRNA) Synthesis and Processing

  • Eukaryotic cells may contain millions of ribosomes, with rRNA composing over 80% of cellular RNA.

  • rDNA gene clusters are located in the nucleolus, contributing to ribosomal subunit formation.

Transfer RNA (tRNA) Synthesis

  • tRNAs are synthesized from clustered tRNA genes and processed by endonuclease ribonuclease P.

Messenger RNA (mRNA) Structure

  • Structure includes:

    • Continuous coding sequence.

    • 5' Methylated guanosine cap: essential for mRNA stability and initiation of translation.

    • 3' Poly A tail: protects mRNA from degradation.

  • mRNA is translated by ribosomes in the cytoplasm and has regulatory noncoding regions at both ends.

Split Genes and mRNA Processing

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

  • Splicing events remove introns from pre-mRNA, forming mature mRNA which is crucial for accurate protein synthesis.

Codons and Genetic Code

  • Codons are triplet sequences in mRNA that specify amino acids. The genetic code is nearly universal.

  • The first two bases of codons are invariant while the third can vary, allowing some flexibility in tRNA recognition.

Transfer RNA (tRNA) Structure and Function

  • tRNAs serve as adaptors between mRNA and amino acids during translation.

  • Each tRNA carries a specific amino acid and has a characteristic cloverleaf structure.

  • Wobble hypothesis: explains how the third base of a codon can vary without altering the specified amino acid.

tRNA Charging

  • The process of adding amino acids to tRNAs requires energy and is facilitated by aminoacyl-tRNA synthetases.

  • The reaction is a two-step process with proofreading capabilities to ensure accuracy.

Translating Genetic Information: Initiation

  • Translation involves the assembly of ribosomes around mRNA, requiring several initiation factors.

  • There are three steps in initiating translation:

    1. Binding of small ribosomal subunit to mRNA.

    2. Recruitment of aminoacyl-tRNA.

    3. Assembly of the initiation complex.

Ribosome Function during Translation

  • Ribosomes facilitate the selection and incorporation of amino acids into a polypeptide chain based on codon sequences.

  • They undergo mechanical changes driven by GTP hydrolysis, crucial for the translation process.

Translation Elongation and Termination

  • Elongation steps include:

    1. Aminoacyl-tRNA selection in the A site.

    2. Peptide bond formation between amino acids.

    3. Translocation of the ribosome along mRNA.

    4. Releasing deacylated tRNA from the E site after polypeptide formation.

  • Termination occurs when ribosome encounters stop codons, leading to the release of the completed polypeptide.

RNA Surveillance and Quality Control

  • Cells have mechanisms for detecting nonsense mutations and premature termination codons to maintain mRNA integrity.

Polyribosomes

  • Polyribosomes enable simultaneous translation of a single mRNA strand by multiple ribosomes, enhancing protein synthesis efficiency.

RNA Interference (RNAi)

  • RNAi involves small regulatory RNAs (siRNAs and miRNAs) that impede gene expression, potentially offering therapeutic avenues for disease.

CRISPR and Noncoding RNAs

  • CRISPR: A bacterial defense mechanism utilizing noncoding RNA to target and cleave invading genetic material.

  • Cas9: an RNA-guided enzyme that facilitates precise genetic edits and has applications in genome engineering.

Clinical Applications of RNA Interference

  • RNAi can be used as a therapeutic strategy by targeting genes for diseases such as Huntington's.

These notes summarize the essential processes involved in the central dogma of molecular biology, emphasizing the roles of various RNA types, transcription mechanisms, and the intricate processes involved in translation and regulation.