Transcription

DNA Synthesis

REPLICATION

  • Essential biological process that duplicates DNA, employing nucleotides as building blocks of the strands.

RNA Synthesis

  • Process of transcribing DNA into RNA, facilitating various functions within the cell.

Protein Synthesis

  • Involves the translation of RNA into proteins made of amino acids, critical for cellular functions.

Transcription: From DNA to RNA

Definition of Transcription

  • Transcription is the mechanism by which cells copy DNA into RNA, specifically targeting the nucleotide sequence of specific genes when a protein is needed.

Central Dogma of Molecular Biology

  • Describes the flow of genetic information:

    • DNA → RNA → Protein

    • Gene: A segment of DNA that directs the production of a specific protein or functional RNA molecule.

RNA Overview

  • Ribonucleic acid (RNA) is:

    • Produced by the transcription of DNA.

    • Generally single-stranded and composed of covalently linked ribonucleotide subunits.

    • Serves numerous functions such as informational, structural, catalytic, and regulatory roles in cells.

Gene Expression

Definition

  • Gene expression is the process by which a gene creates a product useful to a cell or organism through the synthesis of a protein or RNA molecule.

Characteristics

  • Genes can exhibit varying levels of expression:

    • High levels: Rapid production of proteins.

    • Low levels: Slower production rate.

  • Genes can be transcribed multiple times, amplifying the expression and enabling fast protein synthesis.

  • Cells regulate translation speed, adding a layer of control.

RNA Structure and Function

Transcription Dynamics

  • RNA is a linear polymer made up of four different nucleotide subunits linked by phosphodiester bonds.

  • RNA differs from DNA in several key areas:

    • Chemical differences:

    • RNA employs the sugar ribose instead of deoxyribose.

    • RNA uses uracil in place of thymine.

    • Structural differences:

    • RNA molecules are predominantly single-stranded but can occasionally form double-stranded structures.

    • RNA molecules adopt diverse shapes necessary for functionality.

    • Functional differences:

    • Carries genetic information from DNA to protein.

    • Constitutes the core of ribosomes.

    • Transports amino acids for protein synthesis.

    • Some RNA molecules function as enzymes, known as ribozymes, or exhibit other specialized roles.

Transcription Process

Overview

  • Transcription generates RNA that is complementary to a strand of DNA, following several key components and steps:

    • Template Strand: Serves as a guide for RNA synthesis.

    • Coding Strand: Matches the sequence of the RNA transcript (with U substituting for T).

    • The specificity of the transcription is executed by the enzyme RNA polymerase.

Steps of Transcription

  • Initiation: RNA polymerase binds to the template DNA strand and unwinds a section to expose nucleotides.

  • Elongation: RNA polymerase synthesizes RNA by adding nucleotides one at a time, synthesizing in the 5' to 3' direction while reading in the 3' to 5' direction.

  • Termination: RNA polymerase encounters terminator sequences, leading to the release of the RNA transcript.

RNA polymerase Mechanism

  • RNA polymerases work collectively, allowing for multiple copies of RNA to be synthesized from the same gene concurrently.

  • Newly synthesized RNA is known as a RNA transcript, serving various cellular functions.

Types of RNA Produced

  • mRNA (messenger RNA): Carries genetic information coding for proteins.

    • In eukaryotes, it typically specifies one protein.

    • In prokaryotes, it can specify several proteins.

  • Noncoding RNAs: Functions without coding for proteins; vital for various cellular processes.

Phases of Transcription

Phases Overview

  • Initiation: Recognizing the start site and assembling necessary components for transcription.

  • Elongation: Sequentially adding nucleotides while using the template strand.

  • Termination: Detecting stop points for transcription and dissociation from DNA.

Prokaryotic Transcription

Initiation in Prokaryotes

  • Promoter: Specific DNA sequence indicating where RNA polymerase should bind to initiate transcription.

    • Orientation of promoter dictates template vs. coding strand implications.

    • Promoters include consensus sequences: -35 box and -10 box.

Role of Sigma Factor

  • Sigma Factor: A subunit of RNA polymerase in bacteria crucial for promoter recognition.

  • Steps in Prokaryotic Initiation:

    • Sigma factor initially binds RNA polymerase.

    • RNA polymerase loosely interacts with DNA and moves toward the promoter.

    • Binding and recognition of the -35 region facilitate the opening of the double helix.

Elongation in Prokaryotes

  • Sigma factor dissociates from RNA polymerase, which then proceeds to synthesize RNA by adding nucleotides (5' to 3' direction).

Termination in Prokaryotes

  • Terminator: Specific DNA sequence indicating RNA polymerase cessation point.

  • Interaction of the 3' end of the RNA transcript prompts dissociation from DNA.

Eukaryotic Transcription

Initiation in Eukaryotes

  • General Transcription Factors (GTFs): Essential proteins that bind to eukaryotic gene promoters for RNA polymerase II positioning.

  • Eukaryotic promoters possess diverse structures like:

    • Core Promoter: Includes components such as the TATA box.

    • Regulatory Sequences: Like GC and CAAT boxes that further dictate transcriptional control.

Elongation in Eukaryotes

  • Involves elongation factors that facilitate RNA polymerase II in accessing DNA sequences amid histones, synthesizing RNA in a 5' to 3' direction.

Termination in Eukaryotes

  • Gaining complexity, eukaryotic termination is contingent on the type of RNA polymerase:

    • RNA pol I: Stops upon encountering termination sequences.

    • RNA pol II: Lacks a defined stop signal and continues past the gene.

    • RNA pol III: Ceases transcription at stretches of U residues.

RNA Processing in Eukaryotes

Processing Overview

  • Post-transcriptional modifications are necessary for mRNAs before translation.

  • RNA processing occurs in steps:

    • Capping: Addition of a methylguanosine cap on the 5' end, ensuring protection, recognition, and processing for nuclear export.

    • Polyadenylation: Addition of numerous adenine nucleotides (poly-A tail) at the 3' end, enhancing stability and translation efficiency.

    • Splicing: Involves intron removal and exon joining leading to mature mRNA formation.

Splicing Mechanics

  • Introns: Noncoding sequences removed during processing.

  • Exons: Coding sequences retained in mature mRNA.

  • Spliceosome: Molecular complex facilitating intron excision by forming lariat structures, with ribozymes assisting the process.

  • Alternative Splicing: Method by which different proteins arise from a single gene due to varied splicing patterns.

Nuclear Export and Its Mechanism

  • Processed mRNA must exit the nucleus into the cytoplasm.

  • Successful export requires binding to specific proteins and sorting mechanisms to eliminate unnecessary fragments.

RNA Degradation

  • Eventually, mRNA undergoes degradation via Rnases. Lifespan varies according to transcript sequences, notably the 3' untranslated region.

Comparative Overview: Prokaryotic vs. Eukaryotic Transcription

Key Differences

  1. Prokaryotic Transcription:

    • Single type of RNA polymerase.

    • Sigma factor assists during transcription initiation.

    • Genes closely positioned with minimal obstructions.

  2. Eukaryotic Transcription:

    • Multiple RNA polymerases (I, II, III) each dictating unique RNA types.

    • Reliance on various transcription and elongation factors for function.

    • Genes separated by introns and higher order chromatin structures, adding complexity.

Summary Slide of Processes

Eukaryotes:

  • Involves intricacies like introns, mRNAs, transcriptional mechanisms, degradation, and capping.

Prokaryotes:

  • Illustrated as a comparatively straightforward transcription and degradation process.