Human Genome: Gene Structure and Function – Study Notes

Human Genome: Gene Structure and Function

Objectives

This section outlines the key learning goals for understanding gene structure and function in the human genome.

• Discuss the main components of protein-coding genes

• Understand the flow of genetic information from DNA to mRNA to proteins

• Interpret the consequences of genetic variation that alters gene structure or function

Anatomy of a Gene

The structure of a typical protein-coding gene includes several distinct regions, each with specific roles in gene expression and regulation.

Main Components of a Gene

• Promoter: A DNA sequence where transcription factors and RNA polymerase bind to initiate transcription. The promoter is typically located upstream (5') of the transcription start site.

• Enhancers: Regulatory DNA sequences that bind specific transcription factors to increase the efficiency of transcription. Enhancers can be located far from the gene they regulate.

• Exons: Segments of a gene that are transcribed and retained in the mature mRNA; they encode the amino acid sequence of the protein.

• Introns: Non-coding intervening sequences that are transcribed but removed from the pre-mRNA during RNA splicing.

• 5' Untranslated Region (5' UTR): The region upstream of the coding sequence; important for ribosome binding and regulation of translation.

• 3' Untranslated Region (3' UTR): The region downstream of the coding sequence; often contains regulatory elements and the poly-A signal for mRNA stability.

Gene Structure Diagram

• Enhancers → Promoter → 5' UTR → Exon 1 → Intron 1 → Exon 2 → Intron 2 → Exon 3 → 3' UTR

Example: The beta-globin gene contains three exons and two introns, with regulatory elements upstream and downstream of the coding region.

Key Definitions

• Transcription Start Site: The location where RNA polymerase begins synthesizing RNA.

• Translation Start Site: The position in mRNA where the ribosome begins translating the nucleotide sequence into protein, usually marked by the start codon (AUG).

Consensus Sequences in Splicing

Splicing removes introns from pre-mRNA. Key consensus sequences are found at the exon-intron boundaries:

• 5' Splice Site: Typically contains the sequence GU at the start of the intron.

• 3' Splice Site: Typically contains the sequence AG at the end of the intron.

• Branch Point: An adenine (A) residue within the intron, important for lariat formation during splicing.

Table: Main Gene Components and Their Functions

Component	Function
Promoter	Initiates transcription by binding RNA polymerase and transcription factors
Enhancer	Increases transcription efficiency by binding regulatory proteins
Exon	Encodes amino acid sequence; retained in mature mRNA
Intron	Non-coding; removed during RNA splicing
5' UTR	Regulates translation initiation
3' UTR	Regulates mRNA stability and translation

Additional info:

• Mutations in promoters, enhancers, or splice sites can disrupt gene expression and lead to disease.

• Alternative splicing allows a single gene to produce multiple protein isoforms.