DNA, Protein Synthesis, and Mutations

Chapter 17: Nucleic Acids and Protein Synthesis

How DNA Codes for Proteins: Transcription and Translation

DNA contains the genetic instructions for synthesizing proteins, which are essential for cellular structure and function. The process by which DNA codes for proteins involves two main steps: transcription and translation.

• Transcription: The process where a segment of DNA is used as a template to synthesize messenger RNA (mRNA). The enzyme RNA polymerase reads the DNA sequence and creates a complementary mRNA strand.

• Translation: The mRNA strand travels from the nucleus to the ribosome, where it is read in sets of three nucleotides called codons. Each codon specifies a particular amino acid, which is brought to the ribosome by transfer RNA (tRNA). The amino acids are linked together to form a polypeptide chain, which folds into a functional protein.

Key Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Mutations and Their Effects on Protein Structure and Function

A mutation is a change in the nucleotide sequence of DNA. Mutations can occur naturally or be induced by environmental factors. A single base change, known as a point mutation, can have significant effects on protein structure and function.

• Example of a Mutation: Sickle cell anemia is caused by a single base substitution in the gene that codes for the beta chain of hemoglobin. The DNA codon GAG (which codes for glutamic acid) is changed to GTG (which codes for valine).

• Effect of a Single Base Change: This single amino acid substitution alters the structure of hemoglobin, causing the protein to form abnormal fibers. As a result, red blood cells become sickle-shaped and less efficient at transporting oxygen.

Key Equation (Example): $\text{Normal DNA:} \quad \text{GAG} \rightarrow \text{mRNA: GAG} \rightarrow \text{Amino Acid: Glutamic Acid}$ $\text{Mutated DNA:} \quad \text{GTG} \rightarrow \text{mRNA: GUG} \rightarrow \text{Amino Acid: Valine}$

Summary Table: Types of Point Mutations

Type of Mutation	Description	Possible Effect
Silent	Base change does not alter amino acid	No effect on protein function
Missense	Base change results in a different amino acid	May alter protein function (e.g., sickle cell anemia)
Nonsense	Base change creates a stop codon	Results in truncated, usually nonfunctional protein

Additional info: Mutations can be beneficial, neutral, or harmful, depending on their effect on protein function and the organism's environment.