Gene Expression Proteins Gene Structure

Gene Expression

  • Definition: Gene expression is the process of synthesizing a biomolecule encoded in a gene.

  • Protein-Coding Genes: Gene expression involves two major processes: transcription and translation.

Gene Structure

  • Gene: A gene is a region of DNA that contains instructions for making a functional biomolecule.

  • Number of Genes: Humans have approximately 20,000 protein-coding genes.

  • Genome Composition: Genes represent a small portion of the overall genome.

  • Intergenic Region: Regions between genes, often non-coding without direct instructions for proteins.

Overview of Gene Expression Processes

  • Types of RNAs in Gene Expression:

    • mRNA (Messenger RNA): Only mRNAs are translated into proteins.

    • rRNA (Ribosomal RNA): Functions in the structure and function of ribosomes but is not translated.

    • tRNA (Transfer RNA): Transfers amino acids during protein synthesis, also not translated.

    • Other RNAs: Various types that play roles in gene expression but are not translated into proteins.

  • Flow of Genetic Information:

    • The general flow is DNA → mRNA → protein, indicating that the encoded information in DNA is transcribed into mRNA, which is then translated into protein.

Protein-Coding Gene Expression

In Eukaryotic Cells

  • Transcription:

    • Occurs in the nucleus and involves the following steps:

      • RNA polymerase synthesizes a pre-mRNA transcript from DNA.

      • Pre-mRNA undergoes further processing (RNA splicing, capping, tailing) in the nucleus to form mature mRNA.

      • Processed mRNA is transported out of the nucleus via the nuclear pore into the cytoplasm.

  • Translation:

    • Takes place in the cytoplasm at ribosomes, involving:

      • The maturation and arrival of the processed mRNA at the ribosome.

      • tRNAs corresponding to mRNA codons deliver specific amino acids, facilitating polypeptide chain elongation until a stop codon is reached.

In Prokaryotic Cells

  • Transcription and Translation:

    • Both processes occur simultaneously in the cytoplasm, as prokaryotes lack a defined nucleus.

    • The mRNA may be synthesized and immediately translated into protein.

Examples of Proteins

  • Histone Proteins:

    • Function: Wrap around DNA to form nucleosomes, which package DNA and play a critical role in regulating gene expression.

    • Histone Modification:

      • Methylation: Makes DNA inaccessible; genes are inactive.

      • Acetylation: Makes DNA accessible; genes can be expressed.

  • Collagen:

    • An essential structural protein that accounts for at least 20% of all proteins in humans.

    • Structure: Comprises three interwoven chains, each about 1400 amino acids long.

  • Green Fluorescent Protein (GFP):

    • Isolated from jellyfish and utilized for tagging other proteins to visualize their expression patterns.

  • Vault Protein Complex:

    • Present in every cell, composed of multiple proteins forming a hollow structure. Its function remains largely unknown, but it appears to be conserved among eukaryotes.

Amino Acids and Protein Structure

  • Building Blocks of Proteins:

    • Amino Acids: There are 20 types of amino acids.

    • Structure: Each amino acid has a central carbon atom bound to an amino group, a carboxyl group, a hydrogen atom, and a variable R group (which distinguishes one amino acid from another).

Types of Amino Acids:

  • Classification:

    • 1. Nonpolar (Hydrophobic): e.g., Alanine (Ala), Valine (Val)

    • 2. Polar (Positively Charged/Basic): e.g., Lysine (Lys), Arginine (Arg)

    • 3. Polar (Negatively Charged/Acidic): e.g., Aspartic Acid (Asp), Glutamic Acid (Glu)

Peptide Bonds

  • Formation: Amino acids are linked by peptide bonds that form between the amino group of one amino acid and the carboxyl group of another.

Levels of Protein Organization

  • Primary Structure: Sequence of amino acids in a polypeptide chain.

  • Secondary Structure: The way in which the polypeptide chain folds into alpha helices or beta pleated sheets.

  • Tertiary Structure: Further folding of the polypeptide chain into a three-dimensional shape based on interactions between R groups.

  • Quaternary Structure: Combination of multiple polypeptide chains to form a functional protein.

Nucleic Acids vs Proteins

  • Nucleic Acids: DNA and RNA, composed of nucleotides; serve as information carriers.

  • Proteins: Comprised of amino acids; functions include catalysis (enzyme activity), structure, and signaling.

  • Structural Differences:

    • Nucleic acids have a sugar-phosphate backbone and nucleotide sequences.

    • Proteins contain amino acid sequences with distinct termini: amino- (N-) and carboxyl- (C-).

Biomolecule Attributes

  • Test Your Knowledge:

    • Attributes of Nucleic Acids vs. Proteins:

      • Building blocks for Proteins = amino acids; Nucleic Acids = nucleotides.

      • Types include DNA and RNA for nucleic acids.

      • Strands contain phosphodiester bonds in nucleic acids (DNA/RNA) and peptide bonds in proteins.

Gene Structure in Eukaryotes

  • Components of a Protein-Coding Gene:

    • Promoter: Region where RNA polymerase binds to initiate transcription.

    • Exons: Coding sequences of a gene that are expressed.

    • Introns: Non-coding sequences that are spliced out during mRNA processing.

    • UTR (Untranslated Regions): Sequences that are not translated into protein, located at both ends of the mRNA.

    • Exome: The sequence of all exons in a genome; in humans, it is about 1.5% of the total genome.

Gene Structure in Prokaryotes

  • Characteristics:

    • Prokaryotic genes do not contain introns.

    • The structure includes a promoter, transcribed region, and UTR but lacks complex processing.

Operons in Prokaryotes

  • Definition: An operon is a locus with multiple genes controlled by a single promoter and an operator region.

    • Operator: A regulatory sequence for repressor binding, preventing RNA polymerase from initiating transcription.