Gene Expression and Transcription in Eukaryotes

Gene Expression and Transcription in Eukaryotes

• Gene expression is the process to convert genetic information into the structures and functions of a cell by producing a protein or RNA molecule.
• Gene expression is controlled at various points during protein synthesis.
• The human genome contains an estimated 30,000 to 120,000 genes.
• Genes are the fundamental unit of heredity and determine phenotypes.
• Every gene carries instructions to make a specific protein, which are needed for cellular functions.
• Genes consist of several functional components, crucial for differing parts of gene expression.
• Eukaryotic genes have coding sequences (exons) interrupted by non-coding regions (introns).
• Average gene size is 7-10 exons spanning 10-16kb of DNA.
• The gene structure consists of exons, start signals, stop signals, and regulatory control elements.
• Gene expression is the process to transform genetic information into cellular structures and functions.
• Gene expression is regulated at multiple steps during protein synthesis.
• Central dogma: DNA → RNA → Protein (transcription and translation), crucial for protein production. 1- Transcription: DNA → mRNA; 2-Translation: mRNA → protein.

Types of Genes

• Constitutive genes (housekeeping genes) are expressed at constant rates and are not regulated. Examples include glycolysis enzymes.
• Regulated genes are expressed only under specific conditions. Examples include insulin production in pancreatic cells.

Gene Structure

• Genes are made of coding sequences (exons) and non-coding sequences (introns).
• Exons are the coding regions of a gene, and introns are non-coding sequences.
• Gene structure contains promoter regions and coding regions.
• Promoters are located upstream from the gene's transcription start sites.

Eukaryotic Promoter

• Promoters are DNA regions where transcription starts.
• They are near the gene's transcription start sites, upstream of the gene.
• Promoters are relatively short (100-1000 base pairs).
• Eukaryotic promoters often have conserved elements such as CAAT box, TATA box, GC box, and CAP site, recognized by transcription factors.

Enhancer Sequences

• Enhancers are regulatory DNA sequences that elevate the probability of a gene being transcribed.
• Enhancers can be located far from the gene they affect, upstream or downstream, or even within the gene's coding region.

Silencers

• Silencers are DNA sequences that decrease gene expression.
• The binding of repressors (transcription factors) to silencers reduces transcription rate.

Transcription (Synthesis of RNA)

• Transcription is the process of copying a sequence of DNA to produce RNA.
• Specific regions of DNA are transcribed into mRNA, used for protein synthesis.

Messenger RNA (mRNA)

• mRNA carries genetic information from DNA to the ribosomes for protein synthesis.
• DNA is the blueprint translated into an mRNA copy for protein creation.

Reverse Transcription

• Reverse transcription is the synthesis of DNA from an RNA template, catalyzed by reverse transcriptase.
• Retroviruses use reverse transcription to integrate their RNA genome into the host cell's DNA.

Transcription in Eukaryotes

• Transcription is a two-step process.
• The first step is initiation, in which RNA polymerase, with help of transcription factors, binds to the promoter and starts transcription.
• The second step is elongation, where the RNA polymerase continues transcribing the DNA to generate a primary mRNA transcript.
• The third step is termination, in which the RNA polymerase halts transcription at a stop signal in the DNA sequence.

RNA Polymerase

• RNA polymerases are enzymes that synthesize RNA from a DNA template.
• There are three main types of RNA polymerase in eukaryotes (I, II, and III) with different functions related to rRNA, mRNA, and tRNA transcription.

Transcription Factors

• Transcription factors are proteins that bind to DNA sequences and regulate transcription by influencing RNA polymerase binding

mRNA Processing (Post-transcriptional Modifications)

• RNA processing, or post-transcriptional modifications, changes the primary mRNA transcript into a mature mRNA.
• Pre-mRNA processing includes three major steps:
• 5' capping: adding a modified nucleotide to the 5' end to protect the RNA from degradation.
• 3' polyadenylation: adding a poly(A) tail to the 3' end for stability.
• RNA splicing: removing introns and joining exons.

Cleavage and Polyadenylation

• Cleavage and polyadenylation are processes that cut off the 3'-end of the pre-mRNA and add a poly(A) tail.
• The addition of poly(A) tail improves mRNA stability.

RNA Splicing

• Splicing removes non-coding introns from the pre-mRNA, joining coding exons to produce mature mRNA.

Synthesis of Ribosomal RNA (rRNA) and Ribosome Assembly

• Ribosomal RNA (rRNA) is a type of RNA that forms ribosomes, where proteins are synthesized.
• rRNA is transcribed and processed in the nucleolus.
• rRNA combines with ribosomal proteins to form ribosomes.
• Ribosomes play a crucial role in protein synthesis.

Synthesis of Transfer RNA (tRNA)

• Transfer RNA (tRNA) is a type of RNA that carries amino acids to ribosomes during protein synthesis.
• tRNA molecules are produced by RNA polymerase III.
• tRNA processing involves cleaving, modifying nucleotides, and adding a CCA sequence.
• tRNAs have distinctive structures with an anticodon region for matching with mRNA codons.
• These processes generate mature tRNA molecules suitable for protein synthesis.

Clinical Correlates

• Defects in mRNA splicing, such as those in the β-globin gene, are linked to β-thalassemia.
• These defects can lead to aberrant transcripts that impede protein production, resulting in various degrees of anemia depending on the type of mutation.

This quiz explores the fundamental processes of gene expression and transcription in eukaryotic cells. It covers the structure of eukaryotic genes, the significance of exons and introns, and the regulation of gene expression. Test your understanding of how genetic information is converted into functional proteins.