Gene Expression: From DNA to Protein

Gene Expression

Overview of Gene Expression

Gene expression is the process by which information encoded in genes is used to produce molecules that determine the phenotype of organisms. This involves decoding genetic information to synthesize functional proteins.

• Gene expression includes transcription, mRNA processing, and translation.

• Proteins produced through gene expression are responsible for cellular structure and function.

• Phenotypic traits are determined by the types and amounts of proteins synthesized.

Transcription

Definition and Process

Transcription is the synthesis of an RNA molecule from a DNA template. It is the first step in gene expression, where a segment of DNA that constitutes a gene is copied into RNA.

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

• RNA polymerase: The enzyme responsible for RNA synthesis.

• rNTPs: Ribonucleoside triphosphates, the building blocks of RNA.

• Transcription produces a single-stranded RNA molecule complementary and antiparallel to the DNA template strand.

Transcription Unit Structure

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• RNA-coding region: The part of the gene that is transcribed into RNA.

• Terminator: Sequence signaling the end of transcription.

Differences Between DNA and RNA

Structural and Functional Comparison

DNA and RNA differ in structure, function, and chemical composition.

Feature	DNA	RNA
Strands	Double-stranded	Generally single-stranded
Sugar	Deoxyribose	Ribose
Bases Used	Adenine (A), Thymine (T), Cytosine (C), Guanine (G)	Adenine (A), Uracil (U), Cytosine (C), Guanine (G)
Function	Stores genetic information	Encodes proteins, can function as an enzyme

Classes of RNA in Eukaryotes

Types and Functions of RNA Molecules

Multiple types of RNA exist in eukaryotic cells, each with specific functions.

Type of RNA	Location	Function
Ribosomal RNA (rRNA)	Cytoplasm	Structural and functional components of ribosomes
Messenger RNA (mRNA)	Nucleus and cytoplasm	Carries genetic code for proteins
Transfer RNA (tRNA)	Cytoplasm	Helps incorporate amino acids into polypeptide chain
Small nuclear RNA (snRNA)	Nucleus	Processing of pre-mRNA
Small nucleolar RNA (snoRNA)	Nucleus	Processing and assembly of rRNA
Small cytoplasmic RNA (scRNA)	Cytoplasm	Variable functions
MicroRNA (miRNA)	Cytoplasm	Inhibits translation of mRNA
Piwi-interacting RNA (piRNA)	Cytoplasm	Regulates gene expression

Additional info: rRNA, mRNA, and tRNA are directly involved in translation.

Messenger RNA (mRNA)

Role and Structure

mRNA carries genetic information that specifies the synthesis of a particular protein.

• Three mRNA bases in a row form a codon, which specifies a particular amino acid.

• Most mRNAs are 500–4,500 bases long.

• Differentiated cells produce specific mRNA molecules called transcripts.

• Transcripts are used to manufacture encoded proteins.

Key Terms

• Gene expression: The process by which genetic information is used to synthesize proteins.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide chain from mRNA.

• Codon: Sequence of three mRNA bases specifying an amino acid.

• Transcript: An mRNA molecule produced from a gene.

• Polypeptide: A chain of amino acids linked by peptide bonds.

Summary Table: DNA vs. RNA

Characteristic	DNA	RNA
Strands	Double-stranded	Single-stranded
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, enzymatic activity

Example: Central Dogma of Molecular Biology

• DNA is transcribed to mRNA.

• mRNA is translated to a polypeptide (protein).

Additional info: The central dogma describes the flow of genetic information from DNA to RNA to protein.