Genetic Information Flow and Protein Synthesis

Transcription and Translation

Genetic information is stored in DNA and is expressed through the processes of transcription and translation, resulting in the synthesis of proteins. The sequence of nucleotides in DNA determines the sequence of amino acids in a protein.

• Transcription: The process by which a DNA template strand is used to synthesize a complementary mRNA strand.

• Translation: The process by which the mRNA sequence is decoded by ribosomes to produce a specific polypeptide (protein).

• Codon: A sequence of three nucleotides in mRNA that corresponds to a specific amino acid or stop signal during translation.

• Start Codon: AUG (codes for Methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal the end of translation.

Example: For the DNA template strand TAC CGT TTT AAC TTC AAA CCC ACT:

• Transcribe to mRNA: AUG GCA AAA UUG AAG UUU GGG UGA

• Translate to protein: Met-Ala-Lys-Leu-Lys-Phe-Gly-Stop

Genetic Mutations

Types of Mutations

Mutations are changes in the DNA sequence that can affect the structure and function of proteins. They can be classified based on their effect on the protein sequence.

• Substitution Mutation: One base is replaced by another, which may or may not change the amino acid sequence.

• Insertion/Deletion Mutation: Addition or removal of bases, which can cause frameshifts and often have more severe effects than substitutions.

Example: A single base substitution in the DNA template may result in a different mRNA codon and thus a different amino acid in the protein.

Synonymous vs. Nonsynonymous Mutations

Mutations can be categorized based on whether they alter the amino acid sequence of the protein.

• Synonymous (Silent) Mutation: A nucleotide change that does not alter the amino acid sequence due to the redundancy of the genetic code (often occurs at the third position of the codon).

• Nonsynonymous Mutation: A nucleotide change that does alter the amino acid sequence (missense or nonsense mutations).

Example: Changing the third base of a codon from C to T may still code for the same amino acid (synonymous), while changing the first or second base is more likely to result in a different amino acid (nonsynonymous).

Rates of Molecular Evolution

Detecting Natural Selection

The rate and type of mutations in a gene can provide evidence for natural selection acting on that gene.

• Negative Selection: Selection against deleterious mutations; synonymous mutations are more common than nonsynonymous.

• Positive Selection: Selection for beneficial mutations; nonsynonymous mutations are more common than synonymous.

Key Ratio:

• If (nonsynonymous > synonymous): Evidence for positive selection.

• If (synonymous > nonsynonymous): Evidence for negative selection.

Genetic Code Table

Codon to Amino Acid Mapping

The genetic code table is used to translate mRNA codons into amino acids. Each codon specifies a particular amino acid or a stop signal.

Application: Melanin Production Gene in Populations

Comparing DNA Sequences and Selection

Comparing gene sequences for melanin production in different populations can reveal the type of selection acting on the gene.

• Transcribe the DNA template to mRNA.

• Translate the mRNA to protein using the genetic code table.

• Determine if mutations are synonymous or nonsynonymous.

• Assess evidence for positive or negative selection based on mutation types.

Example: If a population's gene sequence has more nonsynonymous mutations compared to the ancestral gene, it may indicate positive selection for a new trait.

Practice Problems

Mutation Analysis and Protein Translation

Students are asked to:

• Transcribe given DNA template strands to mRNA.

• Translate mRNA sequences to protein sequences.

• Identify the type of mutation (synonymous or nonsynonymous).

• Interpret the evolutionary significance of observed mutations.

Example: For the DNA template TAC CGT TTT AAC TTC AAA CCC ACT:

• mRNA: AUG GCA AAA UUG AAG UUU GGG UGA

• Protein: Met-Ala-Lys-Leu-Lys-Phe-Gly-Stop

For a mutated template TAC CGT TTT AAC TTC AAA CCC ACT (with a base change), students determine the new mRNA and protein, and classify the mutation.

Summary Table: Synonymous vs. Nonsynonymous Mutations

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard biology curriculum.