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