Gene Expression and Mutation

Genes

  • Definition of Genes: Genes are the units of heredity, composed of a set of nucleotide sequences that encode the amino acid sequence of enzymes or other proteins.

  • Function of Genes:

    • Most genes code for proteins.

    • Some genes code for the production of specific types of RNA, which are essential for protein synthesis.

Conventional Perspective on Genes

  • One Gene, One Polypeptide: The conventional view states that one gene codes for one polypeptide.

  • Enzymes:

    • Enzymes are made up of one or more polypeptide strands that work together to catalyze reactions.

    • Enzymes are responsible for synthesizing all cell components, mediating the assembly of nucleic acids, and synthesizing proteins, carbohydrates, fats, and lipids.

  • Role of DNA and Enzymes: By specifying enzymes, DNA dictates the organism’s traits and overall biology.

Mendel’s Laws

Mendel’s First Law: Segregation of Unit Characters

  1. Discrete Alternative Forms: Traits encoded by genes are specified by alternative alleles, which do not blend in heterozygotes.

  2. Gamete Formation: In heterozygous diploid individuals, the two alternative alleles segregate during gamete formation.

  3. Equal Probability: Each gamete has an equal chance of carrying either allele from a gene pair.

Mendel’s Second Law: Law of Independent Assortment

  • Independent Assortment Principle: Genes assort independently typically because they are located on different chromosomes.

  • Prophase I of Meiosis: The pairing of homologous chromosomes and crossing over can cause genes on the same chromosome to assort independently.

  • Independence Frequency: The likelihood of independent assortment of genes on the same chromosome is directly proportional to the spatial distance between them; greater distances increase independence likelihood.

Alleles

  • Definition: Alleles are one of the two or more alternative forms of a gene.

  • DNA Composition: Alternative alleles consist of different nucleotide sequences, which may result in variations in the amino acid sequence coded by the gene.

Gene Expression

Transcription

  • Definition: Transcription is the initial phase of gene expression where an mRNA copy is synthesized from a DNA template.

  • Initiation: RNA polymerase binds to a sequence of nucleotides on the coding DNA strand, starting at the beginning of the gene.

Translation

  • Definition: Translation is the second stage of gene expression where ribosomes synthesize a polypeptide based on the nucleotide sequence of mRNA, directing amino acid order in the peptide chain.

Mutations

  • Definition: Mutations are any changes in the genetic message of a cell.

  • Point Mutations: These are changes in a gene's coding sequence that affect one or a few nucleotides.

  • Chromosomal Rearrangements: Include three primary types:

    1. Transposition: Individual genes relocate within a chromosome, potentially altering gene expression.

    2. Translocation: A segment of one chromosome becomes attached to another chromosome, impacting gene expression.

    3. Inversion: A section of a chromosome becomes reversed; these usually do not affect gene expression but influence recombination.

  • Heritability: Only mutations in germ cells are hereditary and can impact evolution. Mutations in somatic cells affect the individual and are not passed to offspring.

Types of Point Mutations

  1. DNA Damage: Can be classified into three categories:

    • A. Ionizing Radiation: High-energy radiation (e.g., x-rays, gamma rays) can create free radicals that break phosphodiester bonds in DNA.

    • B. Ultraviolet Radiation: UV rays can cause thymine and cytosine bases in DNA to absorb energy, leading to covalent bonds between adjacent pyrimidines (pyrimidine dimers), which block DNA replication if unrepaired.

    • C. Chemical Mutagens: Chemicals that alter DNA bases to modify their pairing behavior.

  2. Spontaneous Mispairing: Occur naturally without mutagens:

    • A. Isomerization: Nucleotide bases shift to alternative conformations, causing mismatches during replication.

    • B. Slipped Mispairing: Causes loops in the DNA strands which may result in deletions of bases during realignment.

    • C. Frame Shift Mutation: If mispairing occurs within a codon, it disrupts the normal reading frame and alters the entire sequence from that point.

Biological Significance of Mutation for Evolution

  • Mutations in germ cells are crucial for evolution, providing the genetic variation necessary for natural selection.

  • Evolution requires alternative alleles at a gene locus created by mutations, which lead to variations in protein functions.

  • The interactions of alternative alleles through mutation processes (e.g., transposition, translocation) also aid in genetic recombination, further influencing evolutionary outcomes.