Open Reading Frames, Introns, Exons, and Transcription

Open Reading Frames (ORFs)

An Open Reading Frame (ORF) is a continuous stretch of nucleotides in DNA or RNA that has the potential to code for a protein. It begins with a start codon (usually AUG) and ends with a stop codon (UAA, UAG, or UGA).

• Location: ORFs are found in mature mRNA, but can also be identified in pre-mRNA before splicing.

• Reading Frames: Each mRNA has three possible reading frames per strand, determined by the position at which translation begins.

• Identification: The start of an ORF is marked by a start codon, and the end by a stop codon.

• Consequences of Wrong Reading Frame: Translation in the wrong frame can result in nonfunctional or truncated proteins due to premature stop codons.

Example: In the coding region of the lacZ gene, the correct reading frame produces β-galactosidase, while a frameshift mutation can abolish enzyme activity.

Introns and Exons

Exons are coding regions of a gene that remain in the final mRNA after splicing, while introns are non-coding regions that are removed during RNA processing.

• Variation Among Organisms: Not all organisms have the same number of introns and exons. Prokaryotes typically lack introns, while eukaryotes (especially higher organisms) have many introns.

• Reason for Variation: The complexity of gene regulation and alternative splicing in eukaryotes leads to more introns and exons.

Example: The human β-globin gene contains two introns and three exons, while the corresponding gene in bacteria is uninterrupted.

Transcription and RNA Processing

Transcription is the process by which RNA is synthesized from a DNA template. The initial product is pre-mRNA, which contains both exons and introns.

• Steps:

  1. Initiation: RNA polymerase binds to the promoter region.

  2. Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

  3. Termination: Transcription ends at a terminator sequence.

  4. Splicing: Introns are removed, and exons are joined to form mature mRNA.

• Genetic Elements in Final mRNA: Only exons and untranslated regions (UTRs) remain; introns are excised.

Example: In eukaryotic cells, the pre-mRNA of the insulin gene undergoes splicing to produce mature mRNA that is translated into insulin protein.

Levels of Gene Expression and Genome Content

Levels of Gene Expression

• Genome: The complete set of genes (DNA sequence) in an organism.

• Exome: All the exons (coding regions) of a genome.

• Transcriptome: The set of all expressed genes, defined by RNA molecules present in a cell or tissue at a specific time.

• Proteome: The complete set of proteins produced in a cell or organism.

• Interactome: The catalog of all protein-protein interactions.

• Epigenome: All the chemical modifications (tags) on chromatin that regulate gene expression.

Example: The human genome contains about 20,000 protein-coding genes, but the proteome is much larger due to alternative splicing and post-translational modifications.

Genome Mapping and Variation

Types of Genome Maps

• Linkage Maps: Show genetic distance between loci based on recombination frequencies, not physical distance.

• Restriction Maps: Provide physical landmarks on DNA, measured in kilobases, using restriction enzymes (e.g., BamHI, SalI).

• Sequencing Maps: Constructed by sequencing and assembling DNA fragments; provide ultimate resolution for gene identification.

Example: The first genetic map for Drosophila melanogaster was based on recombination frequencies; modern sequencing maps allow direct identification of genes.

Linkage Maps and Recombination Frequency

• Linkage Maps: Developed by Thomas Hunt Morgan and Alfred Sturtevant; show how often genes are inherited together due to crossing over during meiosis.

• Recombination Frequency (RF): The farther apart two genes are, the higher the RF; however, RF is not directly proportional to physical distance.

Equation:

Example: If RF(B-C) = 13.2%, RF(A-B) = 6.4%, then RF(A-C) = 18.5% (not simply additive).

Restriction Maps

• Restriction Enzymes: Endonucleases that recognize specific DNA sequences (e.g., BamHI: GGATCC).

• Physical Mapping: DNA fragments generated by restriction digestion are measured in kilobases.

Example: A restriction map of a plasmid shows the locations of BamHI and SalI sites, allowing estimation of fragment sizes.

Sequencing Maps

• Genome Sequencing: Involves fragmenting DNA, sequencing each piece, and assembling the full sequence.

• Gene Identification: Based on known gene structure and sequence features; experimental verification is required.

Example: The Human Genome Project used sequencing maps to identify all protein-coding genes.

Genetic Polymorphism

Definition and Allele Frequency

• Genetic Polymorphism: The existence of two or more alleles at a locus, with allele frequency >1% in the population.

• Allele Frequency: Calculated as the proportion of a specific allele among all alleles at a locus in a population.

Equation (Hardy-Weinberg):

where and are the frequencies of two alleles.

Example: In a population of 10 people with 20 chromosomes, if 6 are "a" and 14 are "A", then , .

Neutral Polymorphisms vs Deleterious Mutations

• Neutral Polymorphisms: Sequence changes that do not affect protein function or organism fitness (e.g., silent mutations, changes in unconserved regions).

• Deleterious Mutations: Cause abnormal proteins and disease; usually have low allele frequency (<1%).

Example: Human eye color is a neutral polymorphism; sickle cell anemia is caused by a deleterious mutation.

Distribution of Polymorphisms

• Location: Most polymorphisms are found in non-coding regions, such as repetitive DNA, rather than exons.

• Reason: Non-coding regions tolerate more variation without affecting organism fitness.

Repetitive DNA and Transposons

Types of Repetitive DNA

• Highly Repetitive DNA: Short sequences (<100 bp) repeated thousands of times (e.g., satellite DNA, minisatellites, microsatellites).

• Moderately Repetitive DNA: Sequences repeated 10-1000 times; includes transposons and rRNA/tRNA genes.

Example: Short Tandem Repeats (STRs) are used in forensic analysis due to their high variability.

Transposons

• Transposons: Mobile genetic elements that can move within the genome and create additional copies of themselves.

• Impact: Can promote recombination and chromosomal rearrangements; major source of repetitive DNA.

Example: LINE and SINE elements in the human genome are transposons.

Types and Detection of Polymorphism

Types of Polymorphism

• Single Nucleotide Polymorphisms (SNPs): Variation at a single nucleotide position.

• Small-scale Insertions/Deletions: Addition or loss of a few nucleotides.

• Short Tandem Repeats (STRs): 1-6 nucleotides repeated 5-50 times.

• Transposons: Account for 45% of human genome.

Detection of Polymorphisms

• Sequencing: Direct determination of DNA sequence.

• Non-sequencing Approaches: Use restriction enzyme digestion or PCR to detect length differences in DNA fragments (e.g., STR analysis).

Example: Agarose gel electrophoresis separates DNA fragments by size after restriction digestion.

Applications of Polymorphisms

Parentage and Forensics

• Parentage Analysis: STR profiles are used to match children to parents.

• Forensics: DNA profiles from crime scenes are matched to suspects using polymorphism data.

Mapping Disease-Associated Mutations

• Genome-Wide Association Studies (GWAS): SNPs are screened across genomes of patients and controls to identify disease-associated variants.

Example: SNPs associated with heart disease or schizophrenia are identified by comparing patient and nonpatient DNA.

Genome Size and Organization

Genome Size Variation

• Definition: Total amount of DNA in the haploid genome.

• Variation: Genome sizes range from 106 bp in bacteria to 3.3 x 109 bp in humans.

Repetitive DNA and Genome Complexity

• Repetitive DNA: Larger genomes contain more repetitive DNA, not necessarily more genes.

• Transposons: Major source of repetitive DNA in eukaryotic genomes.

Organelle Genomes and Endosymbiosis

Non-nuclear DNA and Inheritance

• Non-Mendelian Inheritance: Traits controlled by organelle genomes (mitochondria, chloroplasts) are inherited maternally.

• Organelle DNA: Evolves at a different rate than nuclear DNA; usually no recombination between organelle genomes.

Example: Mitochondrial DNA analysis shows all humans descended from a single woman in Africa ~200,000 years ago.

Mitochondrial Genomes (mtDNA)

• Structure: Small, circular molecules resembling bacterial genomes; several copies per organelle.

• Variation: Number of protein-coding and RNA-coding genes varies by species.

Genes in Mitochondria

• Respiratory Complexes: mtDNA encodes components of the electron transport chain.

• Protein Synthesis Machinery: mtDNA encodes rRNAs and some tRNAs.

• Import of Proteins: Many mitochondrial proteins are encoded by nuclear DNA and imported into the organelle.

Endosymbiosis and Organelle Evolution

• Endosymbiosis Theory: Mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotic cells.

• Genome Reduction: Organelle genomes have lost many genes not necessary for independent life.

• Protein Transfer: Proteins encoded by nuclear genes require special targeting sequences for import into organelles.

Example: Sequence comparisons between mtDNA and bacterial DNA support a common evolutionary origin.