Genetic Principles: Chapter 13 – Transcription

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Chapter 13: Transcription

Introduction to Transcription

Transcription is the process by which an RNA molecule is synthesized from a DNA template. This is a fundamental step in the central dogma of molecular biology, which describes the flow of genetic information from DNA to RNA to protein. Transcription is essential for gene expression and is tightly regulated in both prokaryotic and eukaryotic cells.

The Central Dogma and Classes of RNA

Central Dogma: DNA is transcribed into RNA, which is then translated into protein.
Many Classes of RNA Exist: Each class of RNA has a specific function in the cell.

Class of RNA	Cell Type	Location of Function in Eukaryotic Cells	Function
Ribosomal RNA (rRNA)	Prokaryotic and eukaryotic	Cytoplasm	Structural and functional components of the ribosome
Messenger RNA (mRNA)	Prokaryotic and eukaryotic	Nucleus and cytoplasm	Carries genetic code for proteins
Transfer RNA (tRNA)	Prokaryotic and eukaryotic	Cytoplasm	Helps incorporate amino acids into polypeptide chain
Small nuclear RNA (snRNA)	Eukaryotic	Nucleus	Processing of pre-mRNA
MicroRNA (miRNA)	Eukaryotic	Nucleus and cytoplasm	Inhibits translation of mRNA
Small interfering RNA (siRNA)	Eukaryotic	Nucleus and cytoplasm	Triggers degradation of other RNA molecules
Long noncoding RNA (lncRNA)	Eukaryotic	Nucleus and cytoplasm	Variety of functions
CRISPR RNA (crRNA)	Prokaryotic	-	Assists destruction of foreign DNA

Structure of RNA

RNA is a polymer of nucleotides, similar to DNA.
Each nucleotide consists of a ribose sugar, a phosphate group, and a nitrogenous base (adenine, uracil, cytosine, or guanine).
Nucleotides are joined by phosphodiester linkages.

Primary and Secondary Structure

Primary structure: Linear sequence of nucleotides.
Secondary structure: Short complementary regions within the RNA strand can pair to form structures such as hairpins and loops, which are critical for RNA function.
Secondary structures are more common in RNA than DNA due to its single-stranded nature.

Functional Importance of RNA Structure

Some RNAs, such as rRNA, act as ribozymes—RNA molecules with catalytic activity.
For example, rRNA catalyzes peptide bond formation during translation.

Transcription: Synthesis of RNA from DNA

All cellular RNAs are synthesized from DNA templates.
Some viruses can synthesize RNA from RNA.

Comparison of Transcription and Replication

Template-driven: Both use DNA as a template to synthesize a nucleic acid strand.
Location: In eukaryotes, both occur in the nucleus.
Enzymes: Both are catalyzed by polymerases (DNA polymerase for replication, RNA polymerase for transcription).
DNA unwinding: Both require unwinding of the DNA double helix.
Complementary base pairing: Both use base pairing to add nucleotides.
Directionality: Both synthesize new strands in the 5' → 3' direction.

Key Difference

Replication: Copies the entire DNA molecule.
Transcription: Only specific genes (parts of DNA) are transcribed into RNA as needed.

Genomic DNA and Gene Expression

The human genome contains about 20,000 protein-coding genes, which make up less than 2% of the genome.
More than 98% of the genome is noncoding DNA.
Transcription is highly selective; only genes whose products are needed are transcribed.

Major Components Required for Transcription

A DNA template
Ribonucleoside triphosphates (rNTPs) as raw materials
The transcription apparatus (proteins and enzymes necessary for RNA synthesis)

Discovery of the Transcription Template

Oscar Miller Jr. and Barbara Beatty (1969) used electron microscopy to show that RNA is transcribed from DNA templates in salamander oocytes.
They observed "Christmas tree" structures, where the trunk is DNA and the branches are nascent RNA transcripts.

Template Strand and Transcription Unit

Only one strand of the DNA double helix (the template strand) is transcribed for each gene.
Different genes may be transcribed from different DNA strands.

Transcription Unit

A transcription unit is a stretch of DNA that encodes an RNA molecule and includes regulatory sequences.
It consists of three critical regions:
1. Promoter: DNA sequence where transcription machinery binds and initiates transcription.
2. RNA coding region: Sequence that is copied into RNA.
3. Terminator: Sequence signaling the end of transcription.

Terms: Upstream and Downstream

Upstream: Direction opposite to transcription (toward the promoter).
Downstream: Direction in which transcription proceeds (toward the terminator).

The Substrate for Transcription

RNA is synthesized from ribonucleoside triphosphates (rNTPs).
Nucleotides are added one at a time to the 3' OH group of the growing RNA chain.
Direction of synthesis is 5' → 3'.
New RNA is complementary and antiparallel to the template strand.

The Transcription Apparatus

RNA polymerase is the enzyme responsible for RNA synthesis.
Accessory proteins regulate and assist the process at various stages.

Bacterial RNA Polymerase

Bacteria typically have one type of RNA polymerase, a large multimeric enzyme composed of several polypeptide chains.
The core enzyme consists of five subunits and catalyzes RNA elongation.
The sigma factor associates with the core enzyme to form the holoenzyme, which is required for promoter recognition and initiation.
After initiation, sigma factor dissociates from the core enzyme.

Eukaryotic RNA Polymerases

Most eukaryotic cells have three distinct RNA polymerases, each transcribing different classes of RNA:
- RNA polymerase I: Transcribes rRNA (except 5S rRNA).
- RNA polymerase II: Transcribes pre-mRNA and some small RNAs (e.g., snoRNA).
- RNA polymerase III: Transcribes tRNA, 5S rRNA, and some small RNAs.
All eukaryotic RNA polymerases are large multimeric enzymes with more than a dozen subunits.

Bacterial Transcription Stages

Initiation: Promoter recognition, formation of the transcription bubble, creation of first bonds between rNTPs, and escape of the transcription apparatus from the promoter.
Elongation: RNA polymerase synthesizes RNA by adding nucleotides to the 3' end.
Termination: RNA polymerase stops synthesis and releases the RNA molecule at the terminator sequence.

Promoters and Consensus Sequences

Promoters are essential for transcription initiation and determine the template strand.
Bacterial promoters contain consensus sequences at -10 (TATAAT, Pribnow box) and -35 (TTGACA) positions upstream of the transcription start site.
Consensus sequences represent the most common nucleotides at each position and are critical for promoter function.
Mutations in consensus sequences usually decrease transcription rate (down mutations), but up mutations can also occur.

Initiation Mechanism

The sigma factor binds the core RNA polymerase to form the holoenzyme, which recognizes and binds the -10 and -35 consensus sequences.
Binding causes DNA unwinding and positions the polymerase for RNA synthesis at the +1 site.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Number of RNA polymerases	One	Three (I, II, III)
Promoter structure	Simple, with -10 and -35 consensus sequences	Complex, with core and regulatory promoters
Accessory factors	Sigma factor	General transcription factors, mediator complex
Chromatin structure	Absent	Present; must be modified for transcription

Additional info:

In eukaryotes, transcription initiation requires chromatin remodeling and the assembly of a basal transcription apparatus at the core promoter.
General transcription factors (e.g., TFIID, TBP) are required for RNA polymerase II to initiate transcription.
Transcription is the first step in gene expression, followed by translation, where ribosomes use mRNA to synthesize proteins.