Gene Expression: From Gene to Protein (Protein Synthesis) 17

Gene Expression: From Gene to Protein

Overview of Protein Synthesis

Protein synthesis is the process by which cells translate genetic information from DNA into functional proteins. This process involves two main stages: transcription and translation, and is fundamental to the flow of genetic information in all living organisms.

• Gene expression describes how genetic information is used to produce proteins.

• Transcription and translation are the two key steps.

• Prokaryotes and eukaryotes differ in the details of these processes.

When Protein Synthesis Occurs

Proteins are synthesized continuously in cells, but the timing and rate can vary depending on cell type and environmental conditions.

• Most active during the G1 phase of the cell cycle.

Types of RNA

Three main types of RNA are involved in protein synthesis, each with a distinct role.

• mRNA (messenger RNA): Carries genetic instructions from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Transcription (DNA → mRNA)

Transcription is the process by which a segment of DNA is copied into mRNA by the enzyme RNA polymerase.

• Template strand: The DNA strand used to synthesize mRNA.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription (e.g., TATA box in eukaryotes).

• Transcription factors: Proteins that help RNA polymerase bind and initiate transcription.

Initiation: Transcription factors + RNA polymerase = transcription initiation complex.

Elongation: RNA polymerase reads the DNA template 3'→5' and builds the mRNA 5'→3'.

Termination:

• In prokaryotes: Stops at terminator; mRNA is ready immediately.

• In eukaryotes: RNA polymerase II transcribes a polyadenylation signal; transcript is released.

RNA Processing (Eukaryotes)

In eukaryotes, the primary mRNA transcript undergoes several modifications before leaving the nucleus.

• 5' Cap: Modified guanine nucleotide added to the 5' end; promotes mRNA stability and ribosome binding.

• 3' Poly-A Tail: 50–250 adenine nucleotides added to the 3' end; promotes mRNA export and stability.

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

• Alternative splicing: Allows one gene to create multiple protein products.

Genetic Code and Codons

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

• Codon: A sequence of three mRNA nucleotides that specifies an amino acid.

• There are 64 codons; 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA).

• The code is universal and redundant (multiple codons can code for the same amino acid).

Translation (mRNA → Protein)

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

• Components: mRNA, tRNA, ribosome.

• Initiation: Start codon (AUG) is recognized; ribosome assembles.

• Elongation: tRNAs bring amino acids; peptide bonds form between amino acids.

• Termination: Stop codon triggers release factor; polypeptide is released.

Prokaryotes vs. Eukaryotes in Protein Synthesis

There are key differences in how prokaryotes and eukaryotes carry out transcription and translation.

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm (both transcription and translation)	Transcription in nucleus, translation in cytoplasm
RNA Processing	None	5' cap, poly-A tail, splicing
Timing	Simultaneous	Separate processes
Polyribosomes	Yes	Usually one per mRNA

Mutations

Mutations are changes in the DNA sequence that can affect protein synthesis.

• Point mutations: Affect a single nucleotide.

• Silent: No amino acid change.

• Missense: Different amino acid is incorporated.

• Nonsense: Stop codon is formed prematurely.

• Frameshift mutations: Insertions or deletions that alter the reading frame, often disabling the protein.

Splicing & Ribozymes

Spliceosomes are complexes that remove introns from pre-mRNA. Ribozymes are RNA molecules with catalytic activity.

• Ribozymes can catalyze reactions such as RNA splicing.

• RNA can fold into complex structures with catalytic groups and hydrogen bonds.

Importance of Introns

Introns, though non-coding, play important roles in gene regulation and protein diversity.

• Some introns regulate gene expression.

• Alternative splicing of introns increases protein diversity.

Operons (from Ch. 18 Overview)

Operons are clusters of genes under the control of a single promoter, common in prokaryotes.

• lac operon: Inducible; activated in the presence of lactose.

• trp operon: Repressible; turned off when tryptophan is present.

Additional info: Expanded explanations and definitions were added for clarity and completeness. Table comparing prokaryotes and eukaryotes was inferred from context.