D2.2.1 Gene expression Notes

Gene Expression Determines How Genes Effect the Phenotype

1. Imagine two siblings with the same genes: one spends all day outdoors in the sun, while the other stays inside.
2. Over time, one develops a tan while the other doesn't.
3. Even though their DNA is the same, differences in gene expression, how genetic instructions are activated respond to their environments, shaping their traits.

Gene Expression Occurs in Three Main Stages, Linking Genes to the Phenotype

1. Gene expression is the mechanism that bridges genotype (genetic code) and phenotype (observable traits).
2. It involves three key stages: transcription, translation, and protein function.

1. Transcription: DNA to RNA

1. The first step in gene expression is transcription, where a specific gene is "read" and converted into messenger RNA (mRNA).
2. This process occurs in the nucleus and involves three sub-stages:
   1. Initiation
      1. The enzyme RNA polymerase binds to a specific region of DNA called the promoter.
      2. The promoter acts as a "start signal," marking where transcription should begin.
   2. Elongation
      1. RNA polymerase unwinds the DNA and synthesizes a complementary RNA strand using one strand of DNA as a template.
      2. The RNA strand grows in the 5' to 3' direction, matching DNA bases with RNA bases:
         1. Adenine (A) pairs with Uracil (U) in RNA.
         2. Thymine (T) pairs with Adenine (A).
   3. Termination
      1. Transcription ends when RNA polymerase reaches a termination sequence on the DNA.
      2. The newly synthesized mRNA detaches and undergoes processing:
         1. A 5' cap and a poly-A tail are added to protect the mRNA and aid in its transport.
         2. Non-coding regions (introns) are removed through splicing, leaving only coding regions (exons).

2. Translation: RNA to Protein

1. The next step, translation, occurs in the cytoplasm on ribosomes, where the mRNA sequence is decoded to produce a polypeptide chain (a sequence of amino acids).
2. Translation involves three main stages:
   1. Initiation
      1. The ribosome binds to the start codon (AUG) on the mRNA, which codes for methionine.
      2. Transfer RNA (tRNA) molecules carry amino acids to the ribosome, matching their anticodons with the mRNA codons.
   2. Elongation
      1. The ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.
      2. Each tRNA delivers the correct amino acid by matching its anticodon with the codon on the mRNA.
   3. Termination
      1. Translation ends when the ribosome reaches a stop codon (UAA, UAG, or UGA).
      2. The ribosome releases the completed polypeptide, which detaches from the mRNA.

3. Protein Function: The Phenotypic Effect

1. Once translated, the polypeptide folds into a functional protein, which can perform various roles:
   1. Structural Proteins: Form part of the organism's physical structure (e.g., collagen in skin).
   2. Enzymes: Catalyze biochemical reactions (e.g., lactase breaks down lactose).
   3. Transport Proteins: Move molecules across cell membranes (e.g., hemoglobin carries oxygen).

Gene Expression Regulates Phenotype

1. The phenotype of an organism is shaped by which genes are expressed, where, and when.
2. Not all genes are active in every cell, gene expression is highly regulated to ensure the appropriate proteins are produced under the right conditions.

Why Is Gene Expression Important?

1. Adaptation to Environment: Gene expression responds to external factors like temperature, light, or diet. For instance, melanin production in skin increases with sun exposure.
2. Cell Differentiation: Specialized cells express unique genes to perform distinct functions (e.g., nerve cells vs. muscle cells).
3. Health and Disease: Abnormal gene expression can lead to diseases, such as cancer or metabolic disorders.

Gene Expression Drives Diversity

1. Gene expression demonstrates how organisms with the same genetic material can exhibit diverse traits.
2. For example:
   1. Identical twins may develop differently due to environmental factors influencing gene expression.
   2. A single organism can have cells as varied as neurons and liver cells, each expressing a unique subset of genes.