Gene Expression: Transcription and Translation (Chapter 18 Study Notes)

Chapter 18: Gene Expression

Overview of Gene Expression

Gene expression is the process by which genetic information encoded in DNA is converted into functional products, primarily proteins. This involves two major steps: transcription and translation, which together ensure the flow of genetic information from DNA to RNA to protein.

• Transcription: The synthesis of RNA from a DNA template. The genetic "language" remains the same (nucleic acid to nucleic acid).

• Translation: The synthesis of protein from an RNA template. The genetic "language" changes from nucleic acid (RNA) to amino acid (protein).

Key Terms:

• DNA (Deoxyribonucleic Acid): The hereditary material in cells, composed of nucleotides containing deoxyribose sugar and the bases adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA (Ribonucleic Acid): A nucleic acid similar to DNA but contains ribose sugar and uracil (U) instead of thymine.

• mRNA (Messenger RNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• rRNA (Ribosomal RNA): A structural and functional component of ribosomes.

• tRNA (Transfer RNA): Brings amino acids to the ribosome during translation.

Example: The lactase gene is expressed in intestinal cells to produce the enzyme lactase, which digests lactose in milk.

Directional Flow of Genetic Information

The central dogma of molecular biology describes the directional flow of genetic information:

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used as a template to synthesize proteins.

• Reverse Transcription: In some viruses, RNA is used to synthesize DNA (e.g., retroviruses).

• RNA Replication: Some viruses replicate their RNA genomes directly.

Diagram: DNA → RNA → Protein (with possible reverse transcription and RNA replication in special cases).

Structural Differences Between DNA and RNA

DNA and RNA differ in their sugar components and nitrogenous bases:

• DNA: Contains deoxyribose sugar and thymine (T).

• RNA: Contains ribose sugar and uracil (U).

Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic storage	Information transfer, protein synthesis

Transcription: Mechanism and Stages

Transcription is the process of synthesizing RNA from a DNA template. It involves several stages:

• 1. RNA Polymerase Binding: RNA polymerase binds to a specific DNA sequence called the promoter.

• 2. Initiation: RNA synthesis begins at the transcription start site, usually a purine (often adenine).

• 3. Elongation: RNA polymerase moves along the DNA, unwinding the double helix and synthesizing RNA in the 5' to 3' direction. Each nucleotide is added to the 3' end of the growing RNA chain.

• 4. Termination: RNA synthesis ends when RNA polymerase encounters a termination signal. In bacteria, this may involve a hairpin loop structure or the rho factor.

Equation:

$\text{DNA template} + \text{NTPs} \xrightarrow{\text{RNA polymerase}} \text{RNA} + \text{PP}_i$

Example: In Escherichia coli, the -10 (TATAAT) and -35 (TTGACA) sequences are important promoter elements for RNA polymerase binding.

Transcription in Prokaryotes vs. Eukaryotes

While the basic mechanism of transcription is conserved, there are important differences between prokaryotic and eukaryotic transcription:

• Prokaryotes: Transcription and translation are coupled; mRNA can be translated while it is still being synthesized.

• Eukaryotes: Transcription occurs in the nucleus, and mRNA undergoes processing (capping, splicing, polyadenylation) before export to the cytoplasm for translation.

• Multiple RNA Polymerases: Eukaryotes have three main RNA polymerases (I, II, III), each transcribing different types of RNA.

Table: Eukaryotic RNA Polymerases

Polymerase	Location	Transcribes
RNA Polymerase I	Nucleolus	rRNA (except 5S)
RNA Polymerase II	Nucleus	mRNA, some snRNA
RNA Polymerase III	Nucleus	tRNA, 5S rRNA, other small RNAs

Promoters and Control Elements in Eukaryotes

Eukaryotic promoters are more complex than those in prokaryotes and include several key elements:

• Initiator (Inr) Sequence: Defines the transcription start point.

• TATA Box: Consensus sequence (TATA followed by two or three A/Ts), located about 25 nucleotides upstream of the start site.

• B Recognition Element (BRE): Located slightly upstream of the TATA box.

• Downstream Promoter Element (DPE): Located about 30 nucleotides downstream from the start site.

• Enhancers: Distant regulatory elements that increase transcription by looping DNA to bring transcription factors close to the promoter.

Example: The MCM6 enhancer regulates the LCT (lactase) gene, affecting lactase expression in intestinal cells.

Transcription Factors and Preinitiation Complex

Transcription factors (TFs) are proteins required for RNA polymerase binding and initiation of transcription. They interact with promoter and enhancer elements to regulate gene expression.

• General Transcription Factors: Required for transcription of all nuclear genes (e.g., TFIID, which includes the TATA-binding protein).

• Preinitiation Complex: A large assembly of proteins, including RNA polymerase and TFs, that forms at the promoter before transcription begins.

Example: Reduced transcription factor binding to the enhancer can lead to epigenetic silencing of the LCT gene.

Termination and RNA Processing in Eukaryotes

Termination of transcription in eukaryotes is governed by specific signals and is followed by RNA processing:

• Polyadenylation: Addition of a poly(A) tail (a string of adenine nucleotides) to the 3' end of mRNA, which is important for stability and export.

• RNA Cleavage: The transcript is cleaved at a specific site before polyadenylation.

• Additional Processing: Includes 5' capping and splicing to remove introns.

Equation:

$\text{Pre-mRNA} \xrightarrow{\text{cleavage, polyadenylation}} \text{mature mRNA}$

Example: Eukaryotic mRNAs receive a poly(A) tail at their 3' end after transcription by RNA polymerase II.

Epigenetic Regulation of Gene Expression

Gene expression can be regulated epigenetically through DNA methylation and chromatin remodeling, affecting the accessibility of genes to transcription machinery.

• DNA Methylation: Addition of methyl groups to DNA, leading to increased chromatin condensation and gene silencing.

• Chromatin Remodeling: Proteins and enzymes alter chromatin structure to regulate gene accessibility.

Example: Developmental downregulation of the LCT gene after weaning is associated with DNA methylation and reduced enhancer activity.