Gene Expression: From Gene to Protein (Chapter 17) – Study Guide

Gene Expression: From Gene to Protein

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system. It explains how genetic information is transferred from DNA to RNA and then to protein.

• DNA → RNA → Protein: Genetic information is transcribed from DNA to messenger RNA (mRNA), which is then translated into a polypeptide (protein).

• Prokaryotes vs. Eukaryotes: In prokaryotes, transcription and translation occur in the cytoplasm, often simultaneously. In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Eukaryotes also process RNA before translation.

Example: In Escherichia coli (a prokaryote), mRNA can be translated while it is still being transcribed. In humans (a eukaryote), mRNA must be processed and exported from the nucleus before translation.

How Genes Code for Polypeptides

Genes contain the instructions for building polypeptides, which are chains of amino acids that fold into functional proteins.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Transcription: The process of synthesizing RNA from a DNA template.

• Translation: The process of synthesizing a polypeptide using the information in mRNA.

The Genetic Code

The genetic code is the set of rules by which information encoded in mRNA is translated into proteins.

• 20 Amino Acids: The building blocks of proteins.

• Nucleotide Triplets (Codons): Each codon consists of three nucleotides and specifies one amino acid.

• Anticodon: A sequence of three bases on tRNA that pairs with a complementary mRNA codon.

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

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

Example: The mRNA codon UUU codes for the amino acid phenylalanine.

Universality and Redundancy of the Genetic Code

• Universal: The genetic code is nearly universal among all organisms, indicating a common evolutionary origin.

• Redundancy: Multiple codons can code for the same amino acid (degeneracy of the code).

Example: Both UCU and UCC code for serine.

RNA vs. DNA: Similarities and Differences

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic storage	Information transfer, catalysis

Transcription: Basic Concepts and Molecular Components

Transcription is the synthesis of RNA from a DNA template. It involves several key molecular components:

• RNA Polymerase: Enzyme that synthesizes RNA by reading the DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator: Sequence signaling the end of transcription (in prokaryotes).

• Transcription Factors: Proteins in eukaryotes that help RNA polymerase bind to the promoter.

RNA Processing in Eukaryotes

Before mRNA leaves the nucleus, it undergoes several modifications:

• 5’ Cap: Modified guanine nucleotide added to the 5’ end for stability and ribosome binding.

• Poly-A Tail: String of adenine nucleotides added to the 3’ end for stability and export.

• RNA Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons).

Introns vs. Exons: Introns are non-coding regions removed during splicing; exons are coding regions that remain in the mature mRNA.

Alternative Splicing

Alternative splicing allows a single gene to code for multiple proteins by varying which exons are included in the final mRNA.

• Significance: Increases protein diversity without increasing the number of genes.

Example: The human troponin T gene can produce different protein isoforms in different tissues via alternative splicing.

Translation: Molecular Components and Process

Translation is the synthesis of a polypeptide using the information in mRNA. It occurs in the cytoplasm and involves:

• mRNA: Carries the genetic code from DNA.

• tRNA: Brings amino acids to the ribosome; each tRNA has an anticodon complementary to an mRNA codon.

• Ribosomes: Complexes of rRNA and proteins that facilitate the coupling of tRNA anticodons with mRNA codons.

Structure of tRNA

• Amino Acid Attachment Site: The 3’ end of tRNA binds a specific amino acid.

• Anticodon: A three-nucleotide sequence that pairs with the corresponding mRNA codon.

Point Mutations and Their Effects

Point mutations are changes in a single nucleotide pair of a gene. Types include:

• Base Substitution: One base is replaced by another.

• Deletion: Removal of a nucleotide.

• Addition (Insertion): Addition of a nucleotide.

Effects of point mutations:

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid to another.

• Nonsense Mutation: Changes a codon to a stop codon, truncating the protein.

• Frameshift Mutation: Insertion or deletion shifts the reading frame, altering downstream amino acids.

Effect of Point Mutations on Polypeptides

Point mutations can alter the amino acid sequence of a polypeptide, potentially affecting its function.

• Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene, changing a single amino acid and altering protein function.

Summary Table: Types of Point Mutations

Mutation Type	Description	Effect on Protein
Silent	Base change does not alter amino acid	No effect
Missense	Base change alters one amino acid	May affect function
Nonsense	Base change creates stop codon	Truncated protein
Frameshift	Insertion/deletion shifts reading frame	Usually nonfunctional protein

Key Equations and Concepts

• Number of possible codons:

$4^3 = 64$

• There are 4 possible nucleotides and 3 positions per codon, resulting in 64 possible codons.

Additional info: The above guide expands on the provided outline with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.