Meiosis: Cell Division and Genetic Diversity

• Meiosis is a specialized cell division that halves the chromosome number.
• Four haploid cells are created, each genetically distinct from the parent cell.
• Meiosis is essential for sexual reproduction in eukaryotes.
• This ensures genetic diversity in offspring.

Purpose of Meiosis

• Meiosis serves two main purposes: halving chromosome number for haploid gametes and enabling genetic variation in gametes.
• Without it, fertilization would double the chromosome number each generation, leading to genetic instability.

Stages of Meiosis

• Meiosis includes two successive nuclear divisions: meiosis I and meiosis II.
• Each division has prophase, metaphase, anaphase, and telophase stages.

Meiosis I

• Meiosis I is a reductional division that separates homologous chromosomes.
• Prophase I has chromosomes that condense.
• Homologous chromosomes pair up via synapsis, forming tetrads or bivalents during prophase I, which is complex and lengthy.
• Crossing over, which occurs during prophase I, involves the exchange of genetic material between non-sister chromatids of homologous chromosomes, leading to genetic recombination.
• Metaphase I includes tetrads which align at the metaphase plate, with each chromosome attached to spindle fibers from opposite poles.
• During anaphase I homologous chromosomes separate, and they move toward opposite poles, while sister chromatids remain attached.
• Telophase I includes chromosomes arriving at the poles.
• Nuclear envelopes may reform, and the results of this is the cell divides into two haploid cells, each with duplicated chromosomes during telophase I.

Meiosis II

• Meiosis II is an equational division in which sister chromatids separate, similar to mitosis.
• Prophase II: Chromosomes condense, and the nuclear envelope breaks down if it reformed in telophase I.
• Metaphase II includes chromosomes which align at the metaphase plate, with sister chromatids attached to spindle fibers from opposite poles.
• Anaphase II includes sister chromatids which separate and move toward opposite poles, becoming individual chromosomes.
• Telophase II includes chromosomes which arrive at the poles.
• Nuclear envelopes reform, and the cells divide.
• There are four haploid daughter cells produced.

Genetic Variation in Meiosis

• Meiosis generates genetic variation through independent assortment, crossing over, and random fertilization
• Independent assortment: Random orientation of homologous chromosome pairs on the metaphase plate in metaphase I allows for different combinations of maternal and paternal chromosomes to be inherited in daughter cells
• Crossing over: Exchanges genetic material between homologous chromosomes, thereby creating new combinations of alleles on the same chromosome, increasing genetic diversity
• Random fertilization: Fusion of a randomly selected egg with a randomly selected sperm further increases genetic variation in offspring

Significance of Meiosis

• Meiosis is essential for sexual reproduction, ensuring genetic diversity in populations, which is crucial for adaptation and evolution
• By reducing the chromosome number, meiosis prevents the doubling of chromosomes during fertilization, maintaining a stable chromosome number across generations
• Errors in meiosis can lead to aneuploidy, a condition in which cells have an abnormal number of chromosomes, often resulting in genetic disorders such as Down syndrome

Genetic Recombination

• Genetic recombination is the process of creating new combinations of genes on a chromosome
• This occurs through crossing over during prophase I of meiosis, where homologous chromosomes exchange genetic material
• Recombination increases genetic diversity by shuffling alleles between homologous chromosomes

Alleles

• Alleles are different versions of a gene at a specific locus (location) on a chromosome
• Each individual inherits two alleles for each gene—one from each parent
• Alleles can be dominant or recessive, determining the phenotype (observable trait) of an individual

Genotype and Phenotype

• Genotype: The genetic makeup of an individual, including all the alleles they possess
• Phenotype: The observable characteristics of an individual, which are determined by their genotype and environmental factors

Dominance and Recessiveness

• Dominant allele: An allele that masks the expression of another allele (recessive allele) when both are present in an individual
• Recessive allele: An allele whose expression is masked by a dominant allele when both are present in an individual; recessive alleles are only expressed when an individual has two copies of the recessive allele

Homozygous and Heterozygous

• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa)
• Heterozygous: Having two different alleles for a particular gene (e.g., Aa)

Punnett Squares

• Punnett squares are diagrams use to predict the possible genotypes and phenotypes of offspring based on the genotypes of their parents
• They illustrate the combinations of alleles that can occur during fertilization

Mendelian Genetics

• Mendelian genetics are a set of principles outlining heredity, including the law of segregation and the law of independent assortment
• Law of segregation: During gamete formation, the two alleles for each gene separate, so that each gamete carries only one allele for each gene
• Law of independent assortment: The alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome

Linkage

• Genes located close together on the same chromosome tend to be inherited together; this phenomenon is known as linkage
• Linked genes do not assort independently, and their inheritance patterns deviate from Mendel's law of independent assortment
• Crossing over can separate linked genes, but the frequency of recombination is proportional to the distance between the genes on the chromosome

Mutation

• Mutation is a change in the DNA sequence
• Mutations can occur spontaneously or can be induced by mutagens (e.g., chemicals, radiation)
• Mutations can be harmful, beneficial, or neutral
• Mutations are the ultimate source of genetic variation, providing the raw material for evolution

Types of Mutations

• Point mutations: Changes in a single nucleotide base in the DNA sequence
• Deletions: Removal of one or more nucleotide bases from the DNA sequence
• Insertions: Addition of one or more nucleotide bases to the DNA sequence
• Chromosomal mutations: Large-scale changes in the structure or number of chromosomes

Nondisjunction

• Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis
• Nondisjunction can lead to aneuploidy, a condition in which cells have an abnormal number of chromosomes

Aneuploidy

• Aneuploidy is a condition in which cells have an abnormal number of chromosomes
• Aneuploidy examples:
  • Trisomy: Having an extra copy of a chromosome (e.g., trisomy 21, which causes Down syndrome)
  • Monosomy: Missing a copy of a chromosome (e.g., Turner syndrome, where females have only one X chromosome)

Meiosis is a specialized type of cell division that halves the chromosome number, producing four haploid cells. It consists of two successive nuclear divisions: meiosis I and meiosis II, each including prophase, metaphase, anaphase, and telophase stages. This process is essential for sexual reproduction, ensuring genetic diversity.