Lecture 4 - RNA Synthesis

Overview

  • This lecture focuses on the processes of transcription and translation, along with a review of DNA replication and the differences between prokaryotic and eukaryotic mechanisms.

Revision from Previous Lecture

Primosome

  • Definition: A primosome is a complex of key proteins responsible for adding RNA primers during the replication process.

  • Function: Particularly bound on the lagging strand, it synthesizes RNA primers at intervals leading to the formation of Okazaki fragments.

  • Components:

    • Single-stranded DNA-binding proteins: Stabilize separated strands of DNA.

    • DNA gyrase: A protein that unwinds DNA to allow replication to proceed.

DNA Polymerase III

  • Mechanism: DNA polymerase III includes an exonuclease activity that identifies and removes incorrectly added nucleotides.

    • The exonuclease activity operates in the 3' to 5' direction and recognizes poor hydrogen bonding due to incorrect base pairing, allowing for correction.

Eukaryotic vs Prokaryotic DNA Replication

  • Key Differences:

    1. Origins of Replication: Eukaryotes have multiple origins of replication, whereas prokaryotes have only one.

    2. Chromosome Structure: Eukaryotic chromosomes are linear and require mechanisms to maintain ends (telomeres), unlike prokaryotic circular DNA.

    3. Histone Modification: In eukaryotes, histones must be removed and later re-bound during replication due to the compact packaging of DNA.

Transcription

Overview of RNA and Gene Expression

  • DNA is the genetic material that stores genes. With few exceptions (like some viruses with RNA genomes), gene expression results in RNA transcripts and subsequently proteins.

  • Gene Products: Genes may produce RNA (non-coding RNAs) or proteins.

  • Central Dogma: The flow of genetic information follows the sequence: DNA → RNA → Protein.

Types of RNA

  • Major RNA Types:

    1. Messenger RNA (mRNA): Template that encodes polypeptide sequences determined by genes.

    2. Transfer RNA (tRNA): Functions to decode mRNA into a polypeptide chain.

    3. Ribosomal RNA (rRNA): Constituent of ribosomes and critical for protein synthesis.

  • Other RNA Types: Includes numerous non-coding RNAs that are implicated in regulation and may play roles in memory and the immune system.

  • Transcriptome: Refers to the complete set of RNA transcripts in a cell, which provides insights into cellular function and regulation.

Transcription vs DNA Replication

  • DNA replication involves complete replication of DNA, while transcription synthesizes RNA copies from DNA templates.

  • Direction of Synthesis: Both processes synthesize new strands in a 5' to 3' direction; transcription copies the template strand in the 3' to 5' direction.

  • Transcription occurs independently of the cell division cycle and is regulated based on cellular needs.

Structure of RNA

  • Differences from DNA:

    • RNA contains a hydroxyl group (-OH) which makes it less stable compared to DNA.

    • Bases: RNA uses uracil (U) in place of thymine (T).

Chemical Structure of RNA

  • Composed of ribonucleotides, differing from DNA's deoxyribonucleotides by the presence of an additional hydroxyl group.

  • Pairing: Uracil pairs with adenine (A) instead of thymine.

  • RNA is generally single-stranded, creating secondary structures important for its function.

Mechanism of Transcription

Process

  1. Initiation: RNA polymerase binds to promoter regions and unwinds DNA.

  2. Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction without needing a primer.

  3. Termination: Signal sequences dictate when transcription should stop.

Promoters and Binding Mechanisms

  • RNA Polymerases: Different types exist in prokaryotes and eukaryotes, with prokaryotes relying predominantly on one RNA polymerase for all types of RNA synthesis.

  • Promoter Elements: Specific DNA sequences include:

    • +1 Site: The start of transcription.

    • Upstream Elements: Regulatory sequences that help direct RNA polymerase binding and ensure proper initiation of transcription.

Prokaryotic Promoters

  • Sigma Factor: A protein that assists RNA polymerase in recognizing and binding to the promoter.

  • Promoters have highly conserved sequences such as the -10 (Pribnow box) and -35 regions for efficient transcription initiation.

Transcription Termination

  • Two main types of termination:

    1. Rho-Dependent: Involves the Rho protein that unwinds RNA from DNA.

    2. Rho-Independent: RNA forms a hairpin loop due to self-complementary sequences, causing RNA polymerase to pause and release.

RNA Processing in Eukaryotes

  • Primary Transcript: Newly synthesized RNA molecule before processing, often containing introns and exons.

  • Processing may include:

    • Cleavage of large molecules into smaller ones.

    • Joining of exons to form a continuous coding sequence.

    • Addition of a poly A tail.

Fate of RNA Molecules

  • mRNA: Used as a template for protein synthesis and is typically rapidly degraded.

  • tRNA: Serves specific roles in delivering amino acids during translation and can form various secondary structures.

  • rRNA: Integral to ribosome structure and function, facilitating protein synthesis.

Summary and Next Steps

  • Understand and describe key RNA characteristics, transcription initiation, elongation, termination processes, and RNA processing differences.

  • Future lectures will delve deeper into protein synthesis and examine more differences between transcription in prokaryotes and eukaryotes.

  • Final notes for clarity: Coding strands can differ based on directionality of genes, illustrating complexity in genetic regulation and expression.