Meiosis and other factors affecting genetic variability

Meiosis is crucial for creating genetic diversity in sexually reproducing organisms. The majority of eukaryotic organisms, whether multicellular or unicellular, rely on meiosis and fertilization as part of their reproduction. Sexual reproduction depends on the fusion of two gametes, each carrying a single set of chromosomes. When gametes combine, they form a zygote—a fertilized egg that restores the diploid state by containing two sets of chromosomes. (Haploid cells have one set of chromosomes, while diploid cells have two.) To maintain a stable chromosome number across generations, diploid cells must undergo a process that reduces their chromosome sets by half before forming gametes; otherwise, each successive fertilization would continuously double the chromosome count. This reduction is achieved through a specialized type of nuclear division essential for sexual reproduction.

By producing haploid gametes, meiosis reshuffles alleles through two processes: independent assortment and crossing over:

• Independent assortment occurs during metaphase I, when homologous chromosome pairs align randomly along the metaphase plate, resulting in numerous possible combinations of maternal and paternal chromosomes in the gametes.

• Crossing over, which happens during prophase I, involves homologous chromosomes forming tetrads (synapsis) and exchanging segments of DNA at the chiasmata, introducing novel allele arrangements into each chromosome.

Segregation and linkage

Independent assortment randomizes which copy of each homologous chromosome is passed to the offspring, but linkage can limit this mixing for genes on the same chromosome.

• Crossing over over involves the physical exchange of genetic material between homologous chromosomes. It mitigates linkage, especially when genes are far apart, whereas closely spaced genes are more likely to be inherited together.

Recombination

During prophase I, homologous chromosomes pair up in a process called synapsis, forming a structure known as a tetrad.

The protein complex known as the synaptonemal complex holds these homologous chromosomes together, facilitating proper alignment. At specific sites along the tetrad, called chiasmata, crossing over occurs, whereby segments of chromatids are exchanged.

Double crossover events can vary in outcome: in a 2-strand double crossover, the chromatids initially exchange segments and then reverse the exchange, leading to no net recombination; in a 3-strand double crossover, one chromatid participates in both exchanges, producing two recombinant chromatids; and in a 4-strand double crossover, all four chromatids are involved, resulting in four recombinant products.

• Single crossovers: two of the four chromatids swap alleles at a given locus, resulting in two recombinant chromatids and two non-recombinant chromatids
• Double crossovers can yield different outcomes:
  • a 2-strand double crossover might restore original arrangements (0% recombination)
  • in a 3-strand double crossover, one chromatid participates in both exchanges, producing two recombinant chromatids
  • in a 4-strand double crossover, all four chromatids are involved, resulting in four recombinant products.

Sex chromosomes and cytoplasmic (extranuclear) inheritance

Eukaryotes generally use an XX (female) and XY (male) system, with the Y chromosome carrying few genes. Sex-linked traits reside mostly on the X chromosome. Red-green color blindness, hemophilia, Duchenne muscular dystrophy, and Fragile X syndrome are all examples of inherited conditions found primarily in biological males, though the traits are carried by biological females.

Inheritance can also occur outside of nuclear DNA. Cytoplasmic inheritance refers to the exclusively maternal transmission of organellar DNA (e.g., mitochondria).

Mutations

A mutation is a change in the DNA sequence independent of normal recombination. It can arise randomly from replication errors or be induced by chemical or physical mutagens.

Types of mutations:

• Random mutation refers to spontaneous alterations in the DNA sequence, which can arise from exposure to radiation, chemical agents, or errors during DNA replication.
• Translation error occurs when mistakes in protein synthesis lead to the production of an altered protein, even if the DNA sequence itself remains intact.
• Transcription error happens when an incorrect RNA transcript is produced from an otherwise normal DNA sequence, potentially leading to faulty protein expression.
• Base substitution is a type of mutation where one nucleotide (A, T, G, or C) is replaced with a different nucleotide.
• Inversion occurs when a segment of a chromosome detaches and reattaches in the reverse orientation.
• Addition (insertion) involves the introduction of an extra nucleotide into the DNA sequence.
• Deletion is the loss of a nucleotide from the DNA sequence.
• Both insertion and deletion mutations can lead to a frameshift mutation, altering the reading frame of the genetic code. Certain large-scale rearrangements can reshuffle entire chromosome segments.
• Translocation happens when a chromosome fragment detaches and reattaches to a different location, either within the same chromosome or on another chromosome.
• Mispairing refers to incorrect base pairing, where A fails to pair with T, or G does not correctly pair with C.

Depending on environmental conditions, mutations may be advantageous (enhancing fitness, such as albino moths in the early Industrial Revolution in England blending in better with ash-covered trees) or deleterious (reducing fitness, such as limb deformations that cause an animal to be less likley to outrun predators).

Mutagens are agents that cause mutations, and carcinogens are mutagens capable of triggering changes that lead to cancer.

Inborn errors of metabolism- Some hereditary metabolic disorders, like Phenylketonuria (PKU), arise from specific mutation-driven enzyme deficiencies.

Genetic drift

Separate from natural selection, genetic drift randomly alters allele frequencies within a population due to chance events, sometimes overshadowing adaptive forces.

Bottleneck effect: when an event greatly reduces the population so that a small sample is left to reproduce, traits that were much less common in the larger population but happen to occur in the remaining group often become very common in the future generations as they repopulate

Founder effect: when a small portion separates from a larger group or herd and resettles in a new area, smaller diversity in the traits and genes of the “colonists” results in future generations that show less genetic diversity than the members of the species at large.

Synapsis, crossing over, and genetic diversity

By allowing homologous chromosomes to pair and exchange segments (synapsis and crossing over), meiosis underlies the extensive genetic variation that fuels evolution and contributes to the unique genetic makeup of each individual.