Gene Expression: From Gene to Protein (Chapter 17 Study Notes)

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Gene Expression: From Gene to Protein

Overview

Gene expression is the process by which information encoded in DNA is used to direct the synthesis of proteins, the molecules responsible for most cellular functions. This process involves two main stages: transcription and translation. The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein.

Concept 17.1: Genes Specify Proteins via Transcription and Translation

The Central Dogma

Central dogma: Genetic information flows from DNA → RNA → Protein.
Gene expression: The process by which DNA directs protein synthesis, including transcription and translation.

Stages of Gene Expression

Transcription: Synthesis of RNA using DNA as a template.
Translation: Synthesis of a polypeptide (protein) using information in mRNA.
Ribosomes: Cellular structures where translation occurs.

Gene Expression in Prokaryotes vs. Eukaryotes

In bacterial cells, transcription and translation occur in the cytoplasm, often simultaneously.
In eukaryotic cells, transcription occurs in the nucleus, and mRNA is processed before being exported to the cytoplasm for translation.

Cracking the Genetic Code

Codons: Triplets of Nucleotides

The genetic code is read in triplets called codons, each coding for a specific amino acid.
There are 64 possible codons ( combinations), 61 of which code for amino acids; 3 are stop codons (UAA, UAG, UGA) that signal termination of translation.
The start codon (AUG) codes for methionine and signals the beginning of translation.

Codon	Amino Acid	Function
AUG	Met	Start
UAA, UAG, UGA	None	Stop
UUU, UUC	Phe	Amino acid
GAA, GAG	Glu	Amino acid

Transcription: Synthesis of RNA from DNA

Steps of Transcription

Initiation: RNA polymerase binds to the promoter region (often includes a TATA box in eukaryotes) and unwinds DNA.
Elongation: RNA polymerase moves along the template strand, synthesizing RNA in the 5' to 3' direction.
Termination: Transcription ends when RNA polymerase transcribes a terminator sequence, releasing the completed RNA transcript.

RNA Polymerase Binding and Initiation

In eukaryotes, transcription factors help RNA polymerase II bind to the promoter.
The transcription initiation complex forms, allowing transcription to begin.

Post-Transcriptional Modification of mRNA

mRNA Processing in Eukaryotes

A 5' cap (modified guanine nucleotide) is added to the 5' end.
A poly-A tail (50–250 adenine nucleotides) is added to the 3' end.
These modifications protect mRNA and aid in export from the nucleus and translation.

RNA Splicing

Pre-mRNA contains introns (non-coding regions) and exons (coding regions).
Spliceosomes remove introns and join exons to produce mature mRNA.

Functional and Evolutionary Importance of Introns

Introns allow for alternative splicing, enabling a single gene to code for multiple proteins.
Protein domains: Modular regions of proteins often correspond to exons, allowing for evolutionary innovation.

Translation: Synthesis of Polypeptides from mRNA

The Role of tRNA

Transfer RNA (tRNA) brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.
tRNA has a cloverleaf structure (2D) and a compact 3D shape stabilized by hydrogen bonds.

Aminoacyl-tRNA Synthetases

Enzymes that attach the correct amino acid to its tRNA using ATP.
Specificity ensures accurate translation of the genetic code.

Ribosome Structure and Function

Ribosomes have two subunits (large and small) and three binding sites for tRNA: A site (aminoacyl), P site (peptidyl), and E site (exit).
They facilitate the matching of tRNA anticodons with mRNA codons and catalyze peptide bond formation.

Stages of Translation

Initiation: Small ribosomal subunit binds mRNA; initiator tRNA binds start codon; large subunit joins to form initiation complex.
Elongation: tRNAs bring amino acids; peptide bonds form; ribosome moves along mRNA (translocation).
Termination: Ribosome reaches stop codon; release factor binds; polypeptide is released; ribosome dissociates.

Mutations: Types, Causes, and Consequences

Point Mutations

A point mutation changes a single nucleotide in DNA, which can alter the resulting protein.
Example: Sickle-cell disease is caused by a point mutation in the β-globin gene, changing glutamic acid (Glu) to valine (Val).

Types of Mutations

Type	Description	Effect
Silent	Change in nucleotide does not alter amino acid	No effect on protein
Missense	Change in nucleotide alters amino acid	May affect protein function
Nonsense	Change creates a stop codon	Premature termination of protein
Insertion/Deletion	Addition or loss of nucleotides	May cause frameshift, altering downstream amino acids

Frameshift Mutations

Insertions or deletions that are not multiples of three nucleotides shift the reading frame, often resulting in nonfunctional proteins.

Consequences of Mutations

Mutations can be neutral, beneficial, or harmful depending on their effect on protein function.
Genetic diseases often result from harmful mutations that disrupt normal protein activity.

Summary Table: Key Steps in Gene Expression

Step	Location	Main Enzyme/Structure	Product
Transcription	Nucleus (eukaryotes), cytoplasm (prokaryotes)	RNA polymerase	Pre-mRNA (eukaryotes), mRNA (prokaryotes)
RNA Processing	Nucleus (eukaryotes)	Spliceosome, enzymes	Mature mRNA
Translation	Cytoplasm	Ribosome, tRNA	Polypeptide (protein)

Key Equations and Concepts

Number of possible codons:
Central dogma:

Example: Sickle-Cell Mutation

Normal β-globin DNA: CTC → mRNA: GAG → Protein: Glu
Mutant β-globin DNA: CAC → mRNA: GUG → Protein: Val

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