Gene Expression

Gene Expression

Driving Questions

  • What determines the shape of a protein molecule, and why is its shape important?

  • What are the steps of gene expression, and where in the cell do they occur?

  • How can organisms be genetically modified to produce recombinant proteins?

  • What are some pros and cons of genetically modified organisms?

Concept of Genes and DNA

  • DNA serves as a "recipe book" for proteins, but not all recipes are utilized at all times.

  • A small number of possible proteins are synthesized in any given cell at any given time.

  • Despite having the same DNA in all cells of the body, cells exhibit different functions and appearances.

Phenotypes and Genotype

  • Various plant traits include:

    • Flower color: Purple vs White

    • Flower position: Axial vs Terminal

    • Seed color: Yellow vs Green

    • Seed shape: Round vs Wrinkled

    • Pod shape: Inflated vs Constricted

    • Pod color: Green vs Yellow

    • Stem length: Tall vs Dwarf

  • Phenotype determination relies on dominance of traits:

    • Dominant Traits: Freckles, Widow's peak, Free earlobe.

    • Recessive Traits: No freckles, Straight hairline, Attached earlobe.

Genes and Chromosomes

  • Genes are located on chromosomes, with each chromosome carrying a distinct set of genes.

  • The process of synthesizing a protein from a gene is termed gene expression.

Structure of a Gene

  • Genes consist of two essential parts:

    • Regulatory Sequence: Functions as an on-off switch for the gene.

    • Coding Sequence: Dictates the amino acid sequence of the resulting protein.

Central Dogma of Biology

  • Gene expression is defined as the process of converting the genetic information from the coding sequence of a gene into a functional protein.

Differences between DNA and RNA

  • DNA (Deoxyribonucleic Acid):

    • Components include: Cytosine (C), Guanine (G), Adenine (A), Thymine (T).

    • Structure: Double helix with a deoxyribose sugar and a phosphate backbone.

  • RNA (Ribonucleic Acid):

    • Components include: Cytosine (C), Guanine (G), Adenine (A), Uracil (U) (which replaces Thymine in RNA).

    • Structure: Typically single-stranded with ribose sugar and a phosphate backbone.

Transcription Process

  • RNA polymerase is the enzyme responsible for copying a strand of DNA into a complementary strand of mRNA by binding to the regulatory sequence.

  • The process unfolds as:

    • RNA polymerase unwinds DNA to reveal the coding sequence.

    • The mRNA is synthesized from the DNA template, adhering to base-pairing rules:

    • Adenine (A) pairs with Uracil (U), Guanine (G) pairs with Cytosine (C).

  • The completed mRNA strand detaches, and the DNA rewinds into its double helix structure.

  • The mRNA leaves the nucleus, while the DNA remains unchanged in the chromosome.

Translation Process

  • Defines as converting mRNA into a polypeptide (
    a sequence of amino acids).

  • The mRNA acts as a template that contains codons (triplets of nucleotides) which specify amino acids.

  • There are 61 codons coding for different amino acids, with 3 codons serving as stop signals, and AUG signaling the start of translation.

Ribosomes and tRNA during Translation

  • Translation occurs at ribosomes, which have binding sites for mRNA and tRNA. Energy from ATP drives translation.

  • Transfer RNA (tRNA) molecules provide a crucial role by matching specific amino acids to their corresponding mRNA codons while their structure allows pairing between anticodons and mRNA codons.

Key Points of Translation

  • Ribosomes move along mRNA, reading codons to assemble amino acids into a polypeptide chain:

    • Each codon on mRNA complements an anticodon on tRNA.

    • The specified amino acid is added to the growing chain when the correct tRNA binds to the mRNA.

  • As the ribosome advances, the completed amino acid chain undergoes folding into its three-dimensional shape.

Protein Biology

  • Proteins are macromolecules composed of repeating amino acid subunits, playing vital roles in muscle contraction, catalyzing chemical reactions, and immune responses.

  • There are 20 different amino acids, each with a unique chemical side group contributing to their functions and structural properties.

Importance of Amino Acids in Protein Folding

  • The specific sequence of amino acids determines the protein’s three-dimensional shape.

  • Alteration in the amino acid sequence can significantly affect the protein's function.

Mutation Effects

  • Mutations refer to changes in the nucleotide sequence of DNA which can impact protein synthesis:

    • For instance, a mutant CFTR channel may fail to perform its function correctly, leading to physiological consequences.

Example of a Hemoglobin Mutation

  • Normal hemoglobin DNA sequence can differ from a mutated sequence, affecting the mRNA translation and the resultant amino acids.

    • Such mutations can lead to conditions like sickle-cell disease, emphasizing the consequences of genetic alterations on health.

Genetic Modification and Applications

  • Genetically modified organisms (GMOs) can produce recombinant proteins, such as spider silk, which exhibits exceptional toughness and has vast potential in applications such as:

    • Stronger skin grafts

    • Bulletproof vests

    • Biocompatible scaffolds for tissue growth

  • Ethical Considerations: While GMOs can offer groundbreaking advancements, they also raise debates regarding ecological impacts and human health implications.