Genome Content, Gene Mapping, and Genetic Polymorphism: Study Notes for Genetics

Open Reading Frames, Introns, Exons, and Transcription

Open Reading Frame (ORF)

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

• Location: ORFs are typically found in mature mRNA, not in pre-mRNA, as introns are removed during splicing.

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

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

• Consequences of Wrong Frame: Translation in the wrong reading frame can result in nonfunctional or truncated proteins due to incorrect amino acid sequence and premature stop codons.

• Example: A mutation that shifts the reading frame (frameshift mutation) can cause genetic diseases such as cystic fibrosis.

Introns and Exons

Exons are coding regions of a gene that remain in the final mRNA and are translated into protein. Introns are non-coding regions that are removed during RNA splicing.

• Variation Among Organisms: Not all organisms have the same number of introns and exons. Prokaryotes generally lack introns, while eukaryotes have variable numbers depending on species and gene complexity.

• Reason for Variation: Introns are thought to play roles in gene regulation and evolution, but their number and size can vary widely.

• Example: Human genes often contain multiple introns, while yeast genes have fewer.

Transcription and RNA Processing

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

• Steps:

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

  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.

• Final mRNA: Contains only exons and untranslated regions (UTRs); introns are absent.

• Example: The β-globin gene undergoes splicing to remove introns before translation.

Levels of Gene Expression and Genome Content

Levels of Gene Expression

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

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

• Transcriptome: The set of all RNA molecules expressed in a cell or tissue at a given time.

• Proteome: The complete set of proteins expressed in a cell, tissue, or organism.

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

• Epigenome: All chemical modifications (e.g., methylation) that regulate gene expression without altering DNA sequence.

Genetic Mapping and Genome Variation

Types of Genetic Maps

• Linkage Maps: Show genetic distance between loci based on recombination frequencies. Do not represent physical distances (base pairs).

• Restriction Maps: Indicate physical distances between markers using restriction enzymes that cut DNA at specific sequences.

• Sequencing Maps: Provide the ultimate map based on the actual DNA sequence; constructed by sequencing and assembly.

Linkage Maps

Linkage maps are constructed by analyzing how often genes are inherited together, which reflects their proximity on a chromosome.

• Recombination Frequency (RF): The farther apart two genes are, the higher the RF due to more frequent crossing over during meiosis.

• Calculation: RF is expressed as a percentage; for example, if RF(B-C) = 6.4% and RF(A-B) = 13.2%, then RF(B-C) + RF(A-B) > RF(A-C).

• Example: Drosophila melanogaster linkage maps show gene order and genetic distances.

Restriction Maps

Restriction maps use restriction enzymes (e.g., BamHI, SalI) to cut DNA at specific sites, allowing measurement of fragment sizes in kilobases (kb).

• Purpose: Provide physical landmarks for gene mapping and cloning.

• Example: A restriction map of a plasmid shows the locations of enzyme recognition sites.

Sequencing Maps

Sequencing maps are constructed by fragmenting DNA, sequencing the pieces, and assembling them into a complete genome 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 human genes.

Genetic Polymorphism and Mutation

Genetic Polymorphism

Genetic polymorphism refers to the existence of two or more alleles at a locus, with allele frequency >1% in the population. Polymorphisms can occur in coding or non-coding regions.

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

• Example: Human eye color is determined by multiple alleles with different frequencies.

• Equation: Hardy-Weinberg equilibrium for genotype frequencies: $p^2 + 2pq + q^2 = 1$

Neutral vs. Deleterious Mutations

• Deleterious Mutations: Cause abnormal proteins, disease, and reduced fitness; usually have allele frequency <1%.

• Polymorphisms: Mostly neutral, do not affect viability or fertility; allele frequency >1%.

• Example: COVID-19 variants started as rare mutations before becoming common.

Distribution of Polymorphisms

• Most polymorphisms are found in non-coding regions, such as repetitive DNA.

• Exons contain fewer polymorphisms due to functional constraints.

Repetitive DNA and Transposable Elements

Types of Repetitive DNA

• Non-repetitive DNA: Single copy per genome; indicator of complexity.

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

• Moderately Repetitive DNA: Longer sequences, repeated 10-1000 times; includes transposons and genes for tRNA/rRNA.

Transposons

Transposons are mobile genetic elements that can move to new locations in the genome and often make additional copies of themselves.

• Can promote recombination and chromosomal rearrangements.

• Major source of repetitive DNA; can drive genome evolution.

• Some transposons resemble retroviruses and encode reverse transcriptase.

Types and Detection of Polymorphisms

Types of Polymorphism

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

• Short Tandem Repeats (STRs): 1-6 nucleotide sequences repeated multiple times.

• Transposons: Account for 45% of human genome.

Detection Methods

• Sequencing: Direct identification of polymorphisms.

• Non-sequencing Approaches: Use restriction enzyme digestion and gel electrophoresis to detect fragment length differences.

• STR Analysis: PCR amplification of STR loci for parentage and forensic analysis.

Applications of Polymorphisms

Parentage and Forensics

• STR profiles are used for parentage testing and individual identification (e.g., crime scene analysis).

• High variation in STRs allows for unique genetic fingerprints.

• Probability of two unrelated individuals sharing the same STR profile is extremely low (e.g., 1 in 1014).

Mapping Disease-Associated Mutations

• Genome-wide association studies (GWAS) use SNPs to identify genetic variants associated with diseases.

• Comparison of SNP profiles between patients and controls reveals disease-specific variants.

Genome Size and Organization

Genome Size Variation

• Genome size is defined by the total amount of DNA in the haploid genome.

• Organisms vary widely in genome size, but larger genomes do not necessarily have more genes; they often have more repetitive DNA.

• Example Table:

Phylum	Species	Genome (bp)
Algae	Pyroneomas salina	6.6 x 106
Bacterium	E. coli	4.2 x 106
Yeast	S. cerevisiae	1.3 x 107
Mammal	H. sapiens	3.3 x 109

Human Genome Project

• Sequenced all 3.3 x 109 bases of human DNA.

• Estimated ~20,000 protein-coding genes and >100,000 proteins.

• Most of the genome is non-coding and contains many polymorphisms.

• Pie Chart: (Described)

  • Transposons: 45%

  • Introns: 24%

  • Other intergenic DNA: 22%

  • Simple repeats: 3%

  • Large duplications: 5%

Organelle Genomes and Non-Mendelian Inheritance

Organelle DNA

• Some DNA is found outside the nucleus, in organelles such as mitochondria and chloroplasts.

• Organelle DNA is inherited maternally (from the egg).

• Organelle genomes evolve at different rates and have lost many genes compared to their bacterial ancestors.

Mitochondrial Genomes (mtDNA)

• Small, circular DNA molecules; multiple copies per cell.

• Encode components of respiratory complexes and protein synthesis machinery (rRNAs, some tRNAs).

• Many mitochondrial proteins are encoded by nuclear DNA and imported into mitochondria.

• Example Table:

Species	Size (kb)	Protein-Coding Genes	RNA-Coding Genes
Fungi	12-100	8-14	9-24
Plants	60-240	27-42	20-28
Animals	16-17	13	24

Endosymbiosis and Organelle Evolution

• Sequence comparisons between mtDNA and bacterial DNA suggest a common origin via endosymbiosis.

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

• Transfer of proteins encoded by nuclear genes requires special targeting sequences.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard genetics curriculum.