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 from DNA to RNA to protein. This process involves two main steps: transcription and translation.

• Transcription: The process by which the information in a DNA sequence is copied into messenger RNA (mRNA) using RNA polymerase.

• Translation: The process by which the sequence of the mRNA is decoded to build a polypeptide (protein) at the ribosome.

Key Point: Genetic information flows from DNA → RNA → Protein. This flow is generally irreversible (information does not flow from protein back to nucleic acids).

Example: The central dogma is illustrated as:

DNA --transcription--transcription--> RNA --translation--translation--> Protein

Transcription: Overview and Steps

Transcription is the synthesis of RNA using DNA as a template. It occurs in three main steps:

1. Initiation: RNA polymerase binds to the promoter region of DNA, unwinds the DNA, and begins RNA synthesis.

2. Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction by adding complementary RNA nucleotides.

3. Termination: Transcription ends when RNA polymerase reaches a terminator sequence (prokaryotes) or after RNA processing (eukaryotes).

Promoter: DNA sequence where RNA polymerase attaches and initiates transcription.

Terminator: DNA sequence signaling the end of transcription (mainly in prokaryotes).

Transcription in Prokaryotes vs. Eukaryotes

	Prokaryotes	Eukaryotes
RNA Polymerase	Directly binds promoter	Requires transcription factors to bind promoter
Termination	Terminator sequence	RNA processing after transcription

RNA Processing in Eukaryotes

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

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

• 3' Polyadenylation: Addition of a poly-A tail to the 3' end.

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

Alternative Splicing: Allows a single gene to code for multiple proteins by varying the combination of exons included in the final mRNA.

Types of RNA

There are several types of RNA, each with a specific function:

• Messenger RNA (mRNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• Ribosomal RNA (rRNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

• Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the coded mRNA message.

The Genetic Code

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

• Triplet Code: Each amino acid is encoded by a sequence of three nucleotides (codon).

• Redundant: Multiple codons can code for the same amino acid.

• Universal: Shared by almost all organisms.

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

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

Translation: Protein Synthesis

Translation is the process by which ribosomes synthesize proteins using the mRNA template. It occurs in three main steps:

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

2. Elongation: tRNAs bring amino acids to the ribosome, where they are added to the growing polypeptide chain.

3. Termination: When a stop codon is reached, release factors bind, and the completed polypeptide is released.

Ribosome Structure and Function

• Composed of a small and large subunit, each made of rRNA and proteins.

• Has three binding sites for tRNA: A (aminoacyl), P (peptidyl), and E (exit) sites.

Post-Translational Modifications

After translation, proteins may undergo further modifications, known as post-translational modifications (PTMs), which can affect their function:

• Methylation

• Acetylation

• Phosphorylation

• Glycosylation

• Hydroxylation

• Lipidation

• Sulfation

Mutations

Mutations are permanent changes in the DNA sequence. They can affect gene expression and protein function.

• Point Mutations: Change a single nucleotide (substitution).

• Frameshift Mutations: Insertions or deletions that alter the reading frame of the gene.

Mutations can be caused by errors in DNA replication or by mutagens (chemical or physical agents).

Comparison: Transcription vs. Translation

	Transcription	Translation
Product Formed	RNA Molecule	Protein
Macromolecule Change?	Nucleic Acid to Nucleic Acid	Nucleic Acid to Protein
Major Enzyme/Structure	RNA Polymerase	Ribosome
Location	Nucleus (eukaryotes)	Cytoplasm
Direction of Synthesis	5' to 3'	N-terminus to C-terminus

Key Equations and Concepts

• Base Pairing in Transcription: A pairs with U (in RNA), T pairs with A, C pairs with G, G pairs with C.

• Directionality: RNA is synthesized in the 5' to 3' direction.

Practice Example

Given a DNA template strand: 3'-TACGGCATG-5', the corresponding mRNA sequence is 5'-AUGCCGUAC-3'.

Summary Table: Types of Mutations

Type	Description	Effect
Silent	Base substitution with no amino acid change	No effect on protein
Missense	Base substitution changes one amino acid	May alter protein function
Nonsense	Base substitution creates a stop codon	Premature termination
Frameshift	Insertion/deletion alters reading frame	Usually nonfunctional protein

Additional info: This guide includes practice questions and diagrams to reinforce understanding of gene expression, transcription, translation, and mutations, as well as the genetic code and post-translational modifications.