THE GENETIC CODE

Universal Genetic Code

  • The universal genetic code dictates how information from DNA and RNA translates into proteins.

  • Focus is on interpreting data rather than memorizing the entire genetic code table.

Core Concept

  • It is a set of rules that dictates how information from DNA and RNA is converted into protein.

Universality and Exceptions

  • Shared by almost all organisms, some exceptions exist.

Composition of Codons

  • Composed of codons, which are three-letter nucleotide sequences.

  • These codons code for the 20 amino acids, which ultimately form proteins.

Non-Overlapping Nature

  • Codons are non-overlapping, meaning each nucleotide sequence codes for only one specific amino acid.

  • The reading frame determines how the sequence is interpreted.

Accessibility of Sequencing Data

  • Sequencing data, including entire genomes, is often available online.

  • Interpreting this data requires understanding how to identify start and end points.

Singlet Codes

  • Composed of the four bases: A, G, C, and U (in RNA).

DNA Tables

  • DNA tables may be used, where Thymine (T) replaces Uracil (U).

  • This implies a translation from DNA to RNA first.

Doublet Codes

  • Doublets consist of the first two letters of the code (e.g., AU).

  • Order is crucial; AU is different from UA.

  • 16 possible doublet codes exist.

Triplet Codes

  • Triplet codes consist of three letters, resulting in 64 possible outcomes.

  • This large variety contributes to life's diversity.

Redundancy

  • The code is redundant; multiple codons can code for the same amino acid.

Non-Overlapping Triplet Code

  • Combination of three bases code for a specific amino acid.

  • DNA's template strand is used to create mRNA.

Codon

  • A three-digit code on the mRNA.

  • Read in the 5' to 3' direction, based on the carbons on the sugar-phosphate backbone.

  • mRNA codons provide a specific code to produce a protein (e.g., UAC for tyrosine).

  • Codons are read sequentially without overlap (UAC followed by a new codon, not ACU).

Reading the Genetic Code

  • Genetic code tables can be represented in various formats, but the underlying concept remains the same.

Reading The Code

  • First Read: the first base is read from the left of the table.

  • Second Read: the second base is read from the top of the table.

  • Third Read: the third base is read from the right of the table.

  • Example: AUG codes for methionine (MET).

Start Codon

  • Methionine (AUG) is typically the start codon, initiating translation.

  • Example: CAU codes for histidine.

  • Some amino acids have multiple codons due to redundancy.

  • Leucine has six codons, serine has six codons, some initiate transcription/translation.

  • Consider Methionine as the start codon for this subject for now.

  • Can appear in middle of sequence

Stop Codons

  • Stop codons (e.g., UAA) signal the end of protein synthesis. Does not actually make an amino acid.  

Reading Frames

  • It is important to understand reading frames in order interpret genetic codes.

Reading Frames in English

  • Example: "The rat saw the fat cat and big dog and rat." Shifting the reading frame results in nonsense.

Reading Frames in Amino Acid Translation

  • Shifting the reading frame by one or two nucleotides can result in a completely different (often non-functional) protein.

  • There are three possible reading frames in the 5' to 3' direction.

Example

  • mRNA sequence: Multiple reading frames yield different amino acid sequences. Start codons determine the reading frame.

  • The region between the start and stop codons is the open reading frame, encoding the peptide or protein.

Mutations

  • Mutations are alterations in the sequence. They can be beneficial, detrimental, or neutral.

Point Mutation

  • A single mutation in a single nucleotide that will cause a change in a single amino acid change.

Silent Mutation

  • Change in the genetic code that does not change the amnio acid.

Nonsense Mutation

  • Results in a stop codon too early which does not produce protein.

Insertions and Deletions

  • Insertion: Add nucleotides

  • Deletion: Cut out nucleotides, can be detrimental or beneficial.

  • Frameshift Mutations: caused by adding one, two, deleting one, or two nucleotides.

Missense Mutation

  • A single amino acid is changed. Can have detrimental or beneficial impacts.

Duplications

  • Bigger changes, generally lead to additions where bunches of genes are copied.