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

Gene Expression: From a Gene to a Protein

Introduction to Gene Expression

Gene expression is the process by which the information encoded in DNA is used to direct the synthesis of proteins, which in turn determine the observable traits (phenotype) of an organism. Proteins serve as the link between genotype (genetic makeup) and phenotype (physical traits).

• One Gene = One Polypeptide: Each gene typically encodes a single polypeptide, which may function as a protein or as part of a protein complex.

• Example: A gene for pigmentation encodes an enzyme required for pigment synthesis. A mutation in this gene can result in a lack of pigment.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a cell:

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into the amino acid sequence of a protein.

• Information Flow: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Transcription

Transcription is the process of copying a segment of DNA into RNA. In eukaryotes, this occurs in the nucleus.

• RNA Synthesis: RNA is synthesized complementary to the DNA template strand.

• Base Differences: RNA contains uracil (U) instead of thymine (T).

• Direction: RNA polymerase II synthesizes RNA in the 5' to 3' direction.

• Template Strand: Only one strand of DNA serves as the template for RNA synthesis.

Stages of Transcription

1. Initiation: RNA polymerase binds to the promoter sequence and unwinds DNA.

2. Elongation: RNA polymerase moves downstream, synthesizing RNA.

3. Termination: RNA polymerase releases the completed RNA transcript.

Transcription Initiation in Eukaryotes

• Promoter: A specific DNA sequence (often includes a TATA box) where transcription begins.

• Transcription Factors: Proteins that regulate the activity of the promoter and are required for RNA polymerase binding (found only in eukaryotes).

• Transcription Initiation Complex: The assembly of transcription factors and RNA polymerase at the promoter.

RNA Processing (Eukaryotes Only)

Before mRNA can be translated, it undergoes several modifications:

• 5' Cap: Addition of a modified guanine nucleotide (~30 bases).

• Poly-A Tail: Addition of a stretch of adenine nucleotides (~250 bases) at the 3' end.

• Functions:

  • Facilitates export of mRNA from the nucleus.

  • Protects mRNA from degradation.

  • Assists with ribosome attachment.

• UTR (Untranslated Region): Non-coding regions at both ends of the mRNA that do not code for protein.

Introns and Exons

• Exons: Coding regions that are expressed as protein.

• Introns: Non-coding intervening sequences that are removed during RNA splicing.

• RNA Splicing: Introns are cut out and exons are joined together to form mature mRNA.

• Domains: Each exon may encode a distinct functional domain of the protein.

Distinguishing RNA from DNA

Nucleic Acid	Sugar	Bases
RNA	Ribose	Adenine, Cytosine, Guanine, Uracil
DNA	Deoxyribose	Adenine, Cytosine, Guanine, Thymine

The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein.

• Codon: A sequence of three mRNA bases that encodes a single amino acid.

• Universal: Nearly all organisms use the same genetic code.

• Redundant: Multiple codons can encode the same amino acid (especially at the third "wobble" position).

• Not Ambiguous: Each codon specifies only one amino acid.

Feature	Description
Start Codon	AUG (codes for Methionine, initiates translation)
Stop Codons	UAA, UAG, UGA (signal termination of translation)
Number of Codons	64 possible codons (43)
Number of Amino Acids	20 standard amino acids

Translation

Translation is the process by which the sequence of bases in mRNA is converted into a sequence of amino acids, forming a polypeptide. This occurs on ribosomes in the cytoplasm or on the endoplasmic reticulum.

• mRNA: Carries the genetic code from DNA to the ribosome.

• tRNA: Transfer RNA molecules have anticodons that pair with mRNA codons and carry specific amino acids.

• Ribosome: Composed of rRNA and proteins; has two subunits (large and small) and three binding sites (A, P, E).

Stages of Translation

1. Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (carrying Met) at the start codon (AUG).

2. Elongation: tRNAs bring amino acids to the ribosome, matching their anticodons to mRNA codons. Peptide bonds form between amino acids, and the ribosome moves along the mRNA.

3. Termination: When a stop codon enters the A site, a release factor causes the ribosome to disassemble and release the completed polypeptide.

Reading Frame

• Start Codon (AUG): Establishes the correct reading frame for translation.

• Codons do not overlap: Each set of three bases is read sequentially.

Prokaryotic vs. Eukaryotic Gene Expression

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
RNA Processing	None	5' cap, poly-A tail, splicing
Translation	Can begin before transcription ends	Occurs after mRNA processing and export

Summary Table: Key Terms

Term	Definition
Gene Expression	Process by which DNA directs protein synthesis
Transcription	Synthesis of RNA from DNA template
Translation	Synthesis of protein from mRNA template
Codon	Three-base sequence in mRNA encoding an amino acid
Exon	Coding region of a gene
Intron	Non-coding region removed during RNA processing
Promoter	DNA sequence where transcription begins
tRNA	Transfer RNA, brings amino acids to ribosome
rRNA	Ribosomal RNA, structural and catalytic component of ribosomes

Additional info: The notes include diagrams and tables comparing DNA and RNA, illustrating the genetic code, and showing the steps of transcription and translation. These are essential for understanding how genetic information is converted into functional proteins in both prokaryotic and eukaryotic cells.