The Genetic Code and Gene Expression

1. The Genetic Code: A Universal Triplet Code

• The genetic code is the set of rules by which information encoded within genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells.
• It is a universal code, meaning that virtually all organisms use the same code. This provides evidence for a common ancestor of all life.
• It is a triplet code: Each codon, which specifies an amino acid, consists of a sequence of three nucleotides (bases).
  • There are 4 possible bases (A, T/U, C, G).
  • Therefore, there are \(4^3 = 64\) possible codons.
• The genetic code is degenerate (or redundant):
  • Most amino acids are encoded by more than one codon.
  • This reduces the impact of mutations, as a change in a single base may still code for the same amino acid (silent mutation).
• Start codon: AUG (methionine) signals the start of translation.
• Stop codons: UAA, UAG, and UGA signal the end of translation. These codons do not code for an amino acid.
• The code is non-overlapping: Each nucleotide is part of only one codon.
• The code is read in a sequential manner.

KEY TAKEAWAY: The genetic code is a universal, triplet, degenerate code that dictates how DNA sequences are translated into proteins.

2. Gene Expression: From DNA to Protein

Gene expression is the process by which information encoded in a gene is used to synthesize a functional gene product (protein). It involves two main steps: transcription and translation. In eukaryotic cells, RNA processing is an essential intermediary step.

2.1 Transcription

• Transcription is the process of copying the genetic information from a DNA template into a messenger RNA (mRNA) molecule.
• Occurs in the nucleus (in eukaryotes).
• Enzyme involved: RNA polymerase.
• Steps:
  1. Initiation: RNA polymerase binds to the promoter region of the gene. The promoter is a specific DNA sequence that signals the start of the gene.
  2. Elongation: RNA polymerase moves along the DNA template strand, synthesizing a complementary mRNA molecule by adding RNA nucleotides. Uracil (U) is used instead of thymine (T) in RNA.
  3. Termination: RNA polymerase reaches a termination sequence on the DNA, signaling the end of the gene. The mRNA molecule is released.

2.2 RNA Processing (Eukaryotic Cells)

• Before mRNA can be translated in eukaryotes, it undergoes RNA processing in the nucleus.
• Three main steps:
  1. Capping: A modified guanine nucleotide (the “cap”) is added to the 5’ end of the mRNA. This protects the mRNA from degradation and helps ribosomes bind to it.
  2. Splicing: Introns (non-coding regions) are removed from the pre-mRNA, and exons (coding regions) are joined together. This process is carried out by a complex called the spliceosome.
  3. Polyadenylation: A poly(A) tail (a string of adenine nucleotides) is added to the 3’ end of the mRNA. This protects the mRNA from degradation and helps with export from the nucleus.
• The processed mRNA is now mature and ready to be translated.

2.3 Translation

• Translation is the process of synthesizing a polypeptide (protein) from the information encoded in mRNA.
• Occurs on ribosomes in the cytoplasm.
• Requires:
  • mRNA (messenger RNA): Carries the genetic code from DNA to the ribosome.
  • tRNA (transfer RNA): Brings the correct amino acid to the ribosome, according to the mRNA codon. Each tRNA has an anticodon complementary to a specific mRNA codon and carries the corresponding amino acid.
  • Ribosomes: Complexes of rRNA (ribosomal RNA) and proteins that facilitate the binding of mRNA and tRNA and catalyze the formation of peptide bonds between amino acids.
• Steps:
  1. Initiation: The ribosome binds to the mRNA at the start codon (AUG). The initiator tRNA carrying methionine binds to the start codon.
  2. Elongation: The ribosome moves along the mRNA, one codon at a time. For each codon:
     • A tRNA with the complementary anticodon binds to the mRNA.
     • The amino acid carried by the tRNA is added to the growing polypeptide chain via a peptide bond.
     • The ribosome translocates (moves) to the next codon.
  3. Termination: The ribosome reaches a stop codon (UAA, UAG, or UGA). There is no tRNA that corresponds to these codons.
     • A release factor binds to the stop codon, causing the polypeptide chain to be released from the ribosome.
     • The ribosome disassembles.
• The resulting polypeptide chain may then undergo further folding and modifications to become a functional protein.

EXAM TIP: Be able to describe the steps of transcription and translation in detail, including the roles of the key molecules involved (DNA, mRNA, tRNA, ribosomes, RNA polymerase).

3. Summary Table: Transcription vs. Translation

STUDY HINT: Create flowcharts to visualize the processes of transcription and translation. This will help you remember the sequence of events.

VCAA FOCUS: VCAA often asks questions about the roles of different molecules in transcription and translation and about the differences between these two processes.