Genetics and Gene Expression: Mendelian Inheritance, Transcription, and Translation

Genetics: Mendelian Inheritance and Gene Expression

Alleles and Genes

Genes exist in different forms called alleles, which are responsible for the variation in inherited traits. Each individual inherits two alleles for each gene, one from each parent.

• Dominant allele: Expressed when present; masks the effect of a recessive allele.

• Recessive allele: Only expressed when two copies are present (homozygous).

• Homozygous: Both alleles are the same (e.g., YY or yy).

• Heterozygous: Two different alleles (e.g., Yy).

Genotype refers to the genetic makeup (combination of alleles), while phenotype refers to the observable traits.

Mendelian Crosses and Ratios

• Monohybrid cross: Cross between two organisms differing in one trait.

• Dihybrid cross: Cross between organisms differing in two traits.

• Test cross: Used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive individual.

• Genotypic ratio: Ratio of different genotypes in offspring.

• Phenotypic ratio: Ratio of different phenotypes in offspring.

Example: In a monohybrid cross (Yy x Yy), the phenotypic ratio is typically 3:1 (dominant:recessive).

Mendel's Laws

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation, leading to genetic variation.

Monohybrid cross: Cross-fertilization between two organisms differing in one trait.

Probability in Genetics

• Probability rules can be used to predict the outcome of genetic crosses.

• Multiplication rule: Probability of two independent events both occurring is the product of their individual probabilities.

• Addition rule: Probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities.

Gene Expression: From DNA to Protein

Central Dogma of Molecular Biology

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

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of protein using the encoded message of mRNA.

Types of RNA

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

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

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Transcription: Steps and Regulation

• Initiation: RNA polymerase binds to the promoter region, unwinds DNA, and begins RNA synthesis.

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

• Termination: RNA polymerase reaches a terminator sequence and releases the RNA transcript.

In eukaryotes, the primary transcript (pre-mRNA) undergoes processing:

• 5' cap addition

• 3' poly-A tail addition

• Splicing to remove introns and join exons

RNA Splicing

Introns (non-coding regions) are removed from pre-mRNA, and exons (coding regions) are joined to form mature mRNA. Alternative splicing allows for different proteins to be produced from the same gene.

The Genetic Code

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. Each set of three nucleotides (codon) specifies an amino acid.

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

• The code is universal and redundant (more than one codon can specify the same amino acid).

Translation: Protein Synthesis

Translation occurs at the ribosome and involves decoding mRNA to build a polypeptide chain.

• Initiation: Small ribosomal subunit binds mRNA; initiator tRNA binds start codon; large subunit joins.

• Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons; peptide bonds form between amino acids.

• Termination: Stop codon is reached; release factor binds; polypeptide is released.

Ribosome Structure

• Composed of large and small subunits, each made of rRNA and proteins.

• Three binding sites for tRNA: A (aminoacyl), P (peptidyl), and E (exit).

Translation Table (Codon Chart)

The codon chart is used to determine which amino acid corresponds to each mRNA codon.

Codon	Amino Acid
AUG	Methionine (Start)
UUU, UUC	Phenylalanine
UUA, UUG	Leucine
UAA, UAG, UGA	Stop

Post-Translational Modification

After translation, proteins may undergo modifications such as phosphorylation, methylation, or acetylation, which affect their function and activity.

Regulation of Gene Expression

Prokaryotic Gene Regulation: Operons

In prokaryotes, genes are often organized into operons, which are groups of related genes regulated together by a single promoter and operator.

• Operator: DNA region where regulatory proteins bind to control transcription.

• Regulatory proteins: Can act as repressors or activators, affecting RNA polymerase binding.

Positive and Negative Gene Regulation

• Negative regulation: Repressor proteins inhibit gene expression by blocking transcription.

• Positive regulation: Activator proteins enhance gene expression by facilitating transcription.

Prokaryotic cells can rapidly change gene expression in response to environmental changes by regulating operons.

Eukaryotic Gene Regulation

• More complex than in prokaryotes; involves chromatin remodeling, transcription factors, and post-transcriptional modifications.

• Gene expression can be regulated at multiple stages: transcription, RNA processing, translation, and post-translation.

Mutations

Mutations are changes in the DNA sequence that can affect gene expression and protein function. They can occur spontaneously or be induced by environmental factors.

• Point mutations: Change in a single nucleotide.

• Frameshift mutations: Insertions or deletions that alter the reading frame.

Summary Table: Key Concepts in Genetics and Gene Expression

Concept	Description	Example
Allele	Different version of a gene	Y (yellow), y (green)
Genotype	Genetic makeup	YY, Yy, yy
Phenotype	Observable trait	Yellow or green peas
Transcription	DNA to RNA	mRNA synthesis
Translation	RNA to protein	Polypeptide synthesis
Operon	Cluster of genes under single promoter	lac operon

Additional info: These notes cover material relevant to Chapters 14 (Mendel and the Gene Idea), 16 (The Molecular Basis of Inheritance), 17 (Gene Expression: From Gene to Protein), and 18 (Regulation of Gene Expression) of a standard General Biology curriculum.