D2.2A Gene expression and transcriptional regulation (HL)

Gene Expression: The Bridge From Genotype to 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 does not.
3. Their DNA is identical, so the difference comes from gene expression: how genetic instructions are switched on or off in response to the environment, shaping the traits we see.

Phenotypic Plasticity: One Genotype, Many Outcomes

The same genotype can produce different phenotypes depending on the conditions the organism experiences.

Gene Expression Runs in Three Stages: Transcription, Translation, and Protein Function

1. Gene expression is the process that turns a genotype (the genetic code) into a phenotype (observable traits).
2. It happens in three stages: transcription, translation, and protein function.

Transcription: DNA to RNA

1. In transcription, a gene is read and copied into messenger RNA (mRNA).
2. This happens in the nucleus and runs through three sub-stages.
3. Initiation
   1. The enzyme RNA polymerase binds to a region of DNA called the promoter.
   2. The promoter acts as a start signal, marking where transcription should begin.

1. Elongation
   1. RNA polymerase unwinds the DNA and builds a complementary RNA strand using one strand of DNA as a template.
   2. The RNA strand grows in the 5' to 3' direction, pairing each DNA base with the correct RNA base.
      1. DNA adenine (A) pairs with RNA uracil (U).
      2. DNA thymine (T) pairs with RNA adenine (A).
      3. DNA cytosine (C) pairs with RNA guanine (G).
      4. DNA guanine (G) pairs with RNA cytosine (C).