Gene Expression: From Gene to Protein – Detailed Study Notes

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Gene Expression: From Gene to Protein

Overview: The Flow of Genetic Information

Gene expression is the process by which genetic information encoded in DNA is used to direct the synthesis of proteins, which perform most cellular functions. This process involves two major stages: transcription and translation.

Transcription: The synthesis of RNA from a DNA template.
Translation: The synthesis of a polypeptide using the information in mRNA.
Central Dogma: The concept that genetic information flows from DNA to RNA to protein.

Concept 17.1: Genes Specify Proteins via Transcription and Translation

Genes contain instructions for building proteins, which are expressed through transcription and translation. The relationship between genes and proteins was established through studies of metabolic pathways and genetic mutations.

One Gene–One Polypeptide Hypothesis: Each gene codes for a single polypeptide.
Beadle and Tatum's Experiments: Demonstrated that specific genes are responsible for specific enzymes in metabolic pathways.
Genetic Code: The sequence of nucleotides in DNA determines the sequence of amino acids in proteins.

Example: Mutations in genes encoding enzymes can lead to metabolic disorders, such as alkaptonuria.

Transcription and Translation: The Two Main Processes Linking Gene to Protein

Transcription and translation are the fundamental processes by which genetic information is converted into functional proteins.

Transcription: Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes), producing messenger RNA (mRNA).
Translation: Occurs in the cytoplasm, where ribosomes synthesize polypeptides using mRNA as a template.
RNA Polymerase: Enzyme that synthesizes RNA from DNA during transcription.

The Genetic Code

The genetic code is a set of rules by which information encoded in DNA is translated into proteins. It is nearly universal among organisms.

Codon: A sequence of three nucleotides in mRNA that specifies an amino acid.
Triplet Code: Three consecutive bases specify one amino acid.
Start Codon: AUG (methionine) signals the start of translation.
Stop Codons: UAA, UAG, UGA signal the end of translation.

Codon	Amino Acid
AUG	Methionine (Start)
UAA	Stop
UAG	Stop
UGA	Stop
UUU	Phenylalanine
GGC	Glycine
Additional info:	There are 64 possible codons for 20 amino acids.

Concept 17.2: Transcription – DNA-Directed Synthesis of RNA

Transcription is the process by which RNA is synthesized from a DNA template. This process involves initiation, elongation, and termination.

Initiation: RNA polymerase binds to the promoter region of DNA.
Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction.
Termination: Transcription ends when RNA polymerase reaches a terminator sequence.

Formula:

Concept 17.3: Eukaryotic Cells Modify RNA After Transcription

In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

5' Cap: Addition of a modified guanine nucleotide to the 5' end.
Poly-A Tail: Addition of 50–250 adenine nucleotides to the 3' end.
RNA Splicing: Removal of non-coding regions (introns) and joining of coding regions (exons).

Example: Alternative splicing allows a single gene to code for multiple proteins.

Concept 17.4: Translation – RNA-Directed Synthesis of a Polypeptide

Translation is the process by which ribosomes synthesize polypeptides using mRNA as a template. This process involves initiation, elongation, and termination.

tRNA: Transfer RNA brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.
Ribosome: The molecular machine that facilitates the coupling of tRNA anticodons with mRNA codons.
Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA.
Elongation: Amino acids are added one by one to the growing polypeptide chain.
Termination: Occurs when a stop codon is reached; the polypeptide is released.

Formula:

Folding and Modification of Proteins

After translation, polypeptides fold into their functional three-dimensional shapes and may undergo further modifications.

Chaperone Proteins: Assist in the proper folding of polypeptides.
Post-Translational Modifications: Addition of chemical groups (e.g., sugars, phosphates) to proteins.

Signal Peptides and Protein Targeting

Signal peptides direct newly synthesized proteins to specific destinations within the cell, such as the endoplasmic reticulum (ER), mitochondria, or nucleus.

SRP (Signal Recognition Particle): Recognizes signal peptides and directs ribosomes to the ER.
Secretory Proteins: Enter the ER for further processing and transport.

Concept 17.5: Mutations – Changes in DNA Can Affect Protein Structure and Function

Mutations are changes in the genetic material that can alter protein structure and function. They can occur spontaneously or be induced by environmental factors.

Point Mutation: A change in a single nucleotide pair.
Silent Mutation: Does not affect the amino acid sequence.
Missense Mutation: Changes one amino acid to another.
Nonsense Mutation: Changes an amino acid codon to a stop codon, terminating translation prematurely.
Insertions/Deletions: Addition or loss of nucleotide pairs, which can cause frameshift mutations.

Type of Mutation	Effect
Silent	No change in protein function
Missense	Change in one amino acid; may affect function
Nonsense	Premature stop codon; truncated protein
Frameshift	Altered reading frame; usually nonfunctional protein

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

What is a Gene?

A gene is a region of DNA that can be expressed to produce a functional product, either a polypeptide or an RNA molecule. The definition of a gene has evolved with advances in molecular biology.

Classical Definition: A gene is a unit of heredity that encodes a specific trait.
Molecular Definition: A gene is a sequence of DNA that is transcribed and translated to produce a functional product.

Additional info: Some genes encode functional RNAs (e.g., rRNA, tRNA) rather than proteins.

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