15.1 The Genetic Code

The Genetic Code

DNA holds the blueprint for life, encoding instructions for building proteins. This process involves two key steps: transcription, where DNA's message is copied into RNA, and translation, where that RNA message is used to assemble amino acids into proteins.

The genetic code is a universal language that links DNA sequences to specific amino acids. It's characterized by its degeneracy (multiple codons often specify the same amino acid) and its near-universality across all living things, pointing to a shared evolutionary past.

Genetic Information and Protein Synthesis

Before diving into the details, it helps to nail down a few foundational terms.

• DNA (deoxyribonucleic acid) is the molecule that stores and transmits hereditary information from one generation to the next.
• Nucleotides are the building blocks of DNA and RNA. Each one consists of a sugar, a phosphate group, and a nitrogenous base.
• The genome is the complete set of genetic instructions in an organism.
• Protein synthesis is the process by which cells build proteins using the instructions encoded in DNA.
• Amino acids are the basic units of proteins, linked together in a specific sequence determined by the genetic code. There are 20 different amino acids used to build proteins.

DNA to Protein: Transcription and Translation

Getting from a gene in your DNA to a functional protein requires two major stages.

Transcription

Transcription takes place in the nucleus and produces a messenger RNA (mRNA) copy of a gene.

1. RNA polymerase binds to a promoter region on the DNA and unwinds the double helix.
2. One DNA strand serves as the template strand for RNA synthesis.
3. RNA polymerase reads the template strand and adds complementary RNA nucleotides (A pairs with U, C pairs with G).
4. The result is a single-stranded mRNA molecule that carries the gene's message out of the nucleus and into the cytoplasm.

Translation

Translation occurs in the cytoplasm on ribosomes, where the mRNA message is read and used to build a polypeptide (a chain of amino acids).

1. The mRNA binds to the small subunit of the ribosome.
2. Transfer RNA (tRNA) molecules, each carrying a specific amino acid, arrive at the ribosome.
3. Each tRNA has a three-nucleotide anticodon that is complementary to a codon on the mRNA.
4. The ribosome facilitates base pairing between the mRNA codon and the tRNA anticodon.
5. Once matched, the amino acid carried by the tRNA is added to the growing chain. Amino acids are joined by peptide bonds.
6. The ribosome moves along the mRNA, reading codon by codon, until it reaches a stop codon.
7. The completed polypeptide chain folds into a specific three-dimensional structure to become a functional protein (such as enzymes or hormones like insulin).

Codons and Amino Acid Specification

The genetic code defines the relationship between nucleotide sequences in mRNA and the amino acid sequences in a protein. The key unit of this code is the codon: a sequence of three mRNA nucleotides that specifies either a particular amino acid or a stop signal.

There are 64 possible codons total:

• 61 codons specify amino acids.
• 3 codons are stop codons (UAA, UAG, UGA) that signal the ribosome to end translation.
• AUG serves as the start codon and also codes for the amino acid methionine. Translation always begins here.

Here's how codons and tRNAs work together during translation:

• Each tRNA molecule has an anticodon that binds to a complementary codon on the mRNA.
• That same tRNA carries the specific amino acid corresponding to that codon.
• As the ribosome reads each codon, the matching tRNA delivers its amino acid, and the ribosome links it to the growing polypeptide via a peptide bond.

Genetic Code Characteristics

Three properties of the genetic code come up repeatedly on exams.

Degeneracy (Redundancy)

Most amino acids are specified by more than one codon. For example, leucine is coded for by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. This redundancy is protective because a mutation in the third position of a codon often still produces the same amino acid, reducing the impact of point mutations.

Universality

The genetic code is nearly identical across all domains of life: bacteria, archaea, and eukaryotes. This strong conservation suggests all living organisms share a common evolutionary origin. Some minor exceptions exist, particularly in mitochondrial genomes and a handful of single-celled organisms, but these are rare.

Wobble Base Pairing

The third nucleotide position in a codon is less strict in its base-pairing rules. This means a single tRNA can sometimes recognize more than one codon, as long as the first two bases match correctly. Wobble pairing is one reason why degeneracy works: fewer than 61 different tRNAs are needed to read all 61 sense codons.