Gene Expression

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein. This process is fundamental to all living organisms and explains how genes are expressed as traits.

• Transcription: The process by which RNA is synthesized from a DNA template, using DNA as the coding template.

• Translation: The process by which proteins are synthesized using the encoded message of RNA (mRNA).

• Gene expression: The overall process by which genotype becomes expressed as phenotype.

Example: DNA → RNA → Protein

Additional info: The transfer of information from DNA to RNA is reversible, but transfer from nucleic acid to protein is irreversible.

Transcription

Introduction to Transcription

Transcription is the process that builds RNA using DNA within a gene as the coding template. Genes are units of DNA that encode a product, usually a protein.

• Promoter: DNA sequence where transcription of a gene begins (site of RNA polymerase attachment).

• Terminator: DNA sequence where transcription ends.

Direction: Transcription occurs in the 5' to 3' direction.

Overview of Transcription

Genes have two strands: the template strand (used for RNA synthesis) and the coding strand (same sequence as the RNA, except T is replaced by U).

• RNA nucleotides pair with DNA nucleotides according to base-pairing rules: A-U, T-A, C-G, G-C.

Example: If the DNA coding strand is 5'-ATGCGT-3', the template strand is 3'-TACGCA-5', and the mRNA is 5'-AUGCGU-3'.

Steps of Transcription

Transcription consists of three main steps:

1. Initiation: RNA polymerase binds to the promoter on DNA. In prokaryotes, RNA polymerase binds directly; in eukaryotes, transcription factors are required.

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

3. Termination: Transcription ends when RNA polymerase reaches the terminator sequence, releasing the RNA molecule.

Eukaryotic RNA Processing & Splicing

RNA Processing

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

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

• Poly-A Tail: Addition of a string of adenine nucleotides to the 3' end.

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

Function: These modifications protect mRNA from degradation and help in export from the nucleus.

RNA Splicing

Splicing removes introns and joins exons to create mature mRNA. The spliceosome is the complex responsible for splicing.

• Alternative splicing: Allows a single gene to code for multiple proteins by varying the combination of exons.

Types of RNA

Major Types of RNA

Cells use several types of RNA, each with distinct functions:

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

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

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

Genetic Code

Genetic Code Table

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein.

• Each group of three nucleotides (codon) specifies one amino acid.

• The code is redundant; multiple codons can specify the same amino acid.

Example: The codon AUG codes for methionine and also serves as the start codon.

Translation

Introduction to Translation

Translation is the process by which proteins are synthesized using the encoded message of mRNA. Ribosomes are the cellular structures that perform translation.

• tRNA: Delivers amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Charged tRNA: tRNA attached to an amino acid.

Ribosome Structure and Binding Sites

Ribosomes consist of a small and large subunit, each made of proteins and rRNA.

• Prokaryotes: 70S ribosome (30S small + 50S large subunit)

• Eukaryotes: 80S ribosome (40S small + 60S large subunit)

Steps of Translation

Translation consists of three main steps:

1. Initiation: The small ribosomal subunit binds mRNA and initiator tRNA before the large subunit joins. The start codon (AUG) specifies methionine.

2. Elongation: Amino acids are added one by one to the growing polypeptide chain.

3. Termination: A stop codon is reached, triggering release factors to release the polypeptide.

Post-Translational Modification

Types of Post-Translational Modifications (PTMs)

After translation, proteins may undergo further modifications that affect their function.

• Methylation

• Acetylation

• Phosphorylation

• Glycosylation

• Lipidation

• Sulfation

Example: Glycosylation is the addition of sugar groups to a protein.

Transcription vs. Translation

Mutations

Types of Mutations

Mutations are permanent changes in the DNA sequence. They can affect transcription and translation, impacting protein structure and function.

• Point Mutation: Substitution of a single nucleotide.

• Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame.

• Nonsense Mutation: Changes a codon to a stop codon, terminating translation prematurely.

• Missense Mutation: Changes a codon to specify a different amino acid.

Additional info: Mutations can be caused by errors in DNA replication or by external factors such as chemicals or radiation (mutagens).