DNA to RNA

Learning Objectives

  1. Explain the concept of gene expression.

  2. Discuss how DNA is transcribed into RNA in detail.

  3. Describe the structure of RNA and the various types present in a cell.

  4. Differentiate between coding and non-coding RNA.

  5. Explain the role of specialized enzymes in transcription.

  6. Discuss transcriptional RNA modifications:

    • Polyadenylation.

    • Capping.

    • Splicing.

  7. Explain the role of exons and introns in gene expression.

Gene Expression Dynamics

  • All cells have the same DNA but express different proteins, leading to cell differentiation:

    • Muscle cells produce motor proteins for movement.

    • Neurons produce proteins for electrical impulse conduction.

Understanding Genes

  • Definition: The basic physical and functional unit of heredity, made up of DNA sequences.

  • Human Genome: Estimated 20,000 - 25,000 genes, varying from a few hundred to over 2 million bases in size.

Gene Structure

  • Components:

    • Promoter: Site where transcription initiates.

    • Coding Region: Actual gene segment that is expressed.

    • Terminators: Signals the end of transcription.

  • RNA polymerase unwinds DNA and synthesizes RNA complementary to the template strand.

  • Not all DNA codes for protein only genes

Regulation of Gene Expression

  • Gene expression can be influenced at various stages:

    • Chemical modifications of DNA/chromatin.

    • Transcription and post-transcriptional modifications.

    • Transport and degradation of RNA.

    • Translation and post-translational modifications.

Detection of Gene Expression

  • Techniques for detection include:

    • PCR & Real-Time PCR.

    • Gel Electrophoresis and Southern Blotting.

    • Microarrays and FISH (Fluorescence In Situ Hybridization).

The Transcription Process

Stages

  1. Initiation: RNA polymerase binds DNA and unwinds it.

  2. Elongation: RNA polymerase synthesizes RNA.

  3. Termination: The transcribed sequence triggers RNA polymerase to detach.

RNA Strand Characteristics

  • Template: Antisense strand (3' to 5') used for RNA synthesis.

  • Coding: Sense strand (5' to 3').

Differences Between Prokaryotic and Eukaryotic Transcription

  • Prokaryotes:

    • mRNA is mature and translated immediately.

    • Transcription and translation occur together.

    • mRNAs are polycistronic.

    • Half-life <15

  • Eukaryotes:

    • mRNA needs processing before translation.

    • Transcription and translation are separate.

    • mRNAs are monocistronic.

    • Half life few min-24 hours

RNA Molecule Types

  • mRNA (messenger RNA): Protein-coding RNA.

  • ncRNA (non-coding RNA): Includes various functional molecules like rRNA (ribosomal RNA), tRNA (transfer RNA), snRNA (small nuclear RNA), snoRNA (small nucleolar RNA).

  • RNAi (RNA interference): Regulation of gene expression.

Specialized Enzymes in Transcription

  • RNA Polymerases: Three types – RNA polymerase I, II, III,

    • RNA polymerase I: Transcribes rRNA.

    • RNA polymerase II: Transcribes mRNA.

    • RNA polymerase III: Transcribes both rRNA and tRNA.

  • Transcription Factors (TFs): Proteins that regulate gene expression by helping RNA polymerase bind to DNA.

Post-Transcriptional Modifications

  1. Capping: 5’ end of mRNA caps for stability and ribosome binding.

  2. Polyadenylation: Addition of a poly-A tail at the 3’ end for stability.

  3. Splicing: Removal of introns and joining of exons to form mature mRNA.

Summary of Gene Expression

  • The transcription process unzips DNA, creating a RNA transcript.

  • Capping and poly-A tails enhance mRNA stability for protein synthesis.

  • Introns are spliced out via spliceosomes, leaving mature mRNA.