Gene Expression and Transcription in Eukaryotes

Gene Expression & Transcription in Eukaryotes

• Eukaryotic genes are composed of coding sequences (exons) interrupted by non-coding sequences (introns).
• The average gene spans 10-16 kb of DNA.
• Each gene possesses exons, start signals, stop signals, and regulatory control elements.
• The human genome contains an estimated 30,000 to 120,000 genes.
• Genes are the fundamental units of inheritance determining all phenotypes.
• Genes contain instructions for specific proteins, providing the blueprint for cellular proteins.

Objectives

• Gene Structure
• Gene Expression
• Types of Genes
• Post-transcriptional modifications (RNA splicing, 3' polyadenylation, 5' capping)

Genes

• Genes are segments of DNA containing instructions for a specific protein.
• A promoter is a region of DNA initiating transcription of a specific gene. Promoters are located upstream of the sense strand. They consist of the conserved eukaryotic promoter regions which include the CAAT box, TATA box, GC box, and CAP site.
• The size of promoters can range from 100-1000 base pairs.
• Gene expression is the process converting genetic information into the structures/functions of a cell through the production of biological molecules (proteins or RNA).

Gene Expression

• Gene expression is controlled at various points during the sequence leading to protein synthesis.
• The DNA strand used as a template for transcription is called the antisense strand. The mRNA produced is a copy of the sense strand
• RNA polymerase is essential in the process of transcription.
• Transcription ends with a specific stopping sequence.

Central Dogma

• Transcription: DNA → RNA
• Translation: RNA→ Protein.

Types of Genes

• Constitutive Genes (Housekeeping Genes): These are expressed at a constant rate and are not subjected to regulation (e.g., enzymes of glycolysis).
• Regulated Genes: These are expressed only under certain conditions (e.g., insulin gene expression in the pancreas).

Flow of genetic information in Prokaryotes vs. Eukaryotes

• Prokaryotic transcription and translation occur in the cytoplasm simultaneously.
• Eukaryotic transcription occurs in the nucleus, while translation occurs in the cytoplasm.

Eukaryotic Promoter

• Promoter is a region of DNA initiating transcription.
• Promoters are located near the transcription start sites of genes on the same strand and upstream from the sense strand (towards the 5' region).

Eukaryotic Gene Promoter Sequences

• Sequences within the eukaryotic promoter region are recognized by RNA polymerase II.
• Sequences range from 45 to 55 bases long.
• Important elements in promoters include the TATA box and the Hogness box.

Enhancer Sequences

• Enhancers are regulatory DNA sequences influencing transcription.
• Enhancers can be located upstream, downstream, or within the coding region, and may be thousands of nucleotides away from the target gene.
• They bind to transcription factors (activators) increasing the likelihood of gene transcription.

Silencers

• Transcription factors (repressors) bind to areas called silencer regions to reduce the rate of transcription.

Transcription (Synthesis of RNA)

• Transcription is the process where DNA is transcribed into mRNA, carrying instructions for proteins.

Messenger RNA (mRNA)

• mRNA is used to create blueprints for protein production.
• mRNA carries information from DNA for protein synthesis.

Reverse Transcription

• Reverse transcription is the synthesis of DNA from an RNA template catalyzed by reverse transcriptase.
• Retroviruses (e.g., HIV, hepatitis C) utilize reverse transcription to insert their genetic material into the host genome.

Transcription in Eukaryotes (Second Step): Post-transcriptional modifications or gene processing

• The primary RNA transcript is modified into mature RNA.
• Modifications: 5' capping, 3' polyadenylation, and RNA splicing; occurring in the nucleus before translation in the cytoplasm.

Capping

• 7-methylguanosine (m7G) is added to the 5' end of mRNA.
• This protects the mRNA from degradation.
• A phosphatase enzyme is involved in capping as it removes the 5′ phosphate enabling the addition of the cap.

Cleavage and Polyadenylation

• The RNA molecule undergoes cleavage at the 3' end.
• Adenine residues are added to form a poly (A) tail, which protects the mRNA from degradation.

RNA Splicing

• Introns are non-coding sections removed from the primary RNA transcript.
• Exons are the remaining coding sections joined together.

Synthesis of Messenger RNA (mRNA) in Eukaryotes

• The primary transcribed mRNA molecule undergoes processing (5' capping, 3′ polyadenylation, splicing).
• Mature mRNA then exits the nucleus for translation.

Clinical Correlates

• Defects in mRNA splicing of the β-globin gene cause β-thalassemia.
• Mutations in splicing lead to aberrant transcripts that are degraded.
• Homozygous mutations result in severe transfusion-dependent anemia.

Synthesis of ribosomal RNA (rRNA)

• rRNA participates in ribosome formation.
• rRNA undergoes methylation and trimming before assembling with ribosome subunits.

Synthesis of transfer RNA (tRNA)

• tRNA translates the genetic code into proteins.
• RNA polymerase III is involved in producing tRNA.
• tRNA precursors undergo intron removal, nucleotide modifications, and addition of a CCA sequence at the 3’ end.

Dive into the world of eukaryotic gene expression and transcription. This quiz covers essential concepts such as gene structure, types of genes, and post-transcriptional modifications. Test your knowledge about how genes function as the fundamental units of inheritance and their role in protein synthesis.