Meiosis and Sexual Life Cycles

Overview: Variations on a Theme

• Offspring resemble their parents more than they do unrelated individuals.

• Heredity: The transmission of traits from one generation to the next.

• Variation: Demonstrated by the differences in appearance that offspring show from parents and siblings.

• Genetics: The scientific study of heredity and variation.

Concept 10.1: Offspring Acquire Genes from Parents by Inheriting Chromosomes

• Children do not inherit particular physical traits from their parents in a literal sense.

• Genes: The units of heredity, consisting of segments of DNA.

• Gametes: Reproductive cells (sperm and eggs) through which genes are passed to the next generation.

• Chromosomes: Most DNA is packaged into chromosomes. For example, humans have $46$ chromosomes in their somatic cells.

• Somatic Cells: All cells of the body except for gametes and their precursors.

• Locus: A specific position on a certain chromosome where each gene is located.

Comparison of Asexual and Sexual Reproduction

• Asexual Reproduction: A single individual passes genes to its offspring without the fusion of gametes.

• Clone: A group of genetically identical individuals from the same parent, produced through asexual reproduction. Examples include Hydra (budding) and Redwoods.

• Sexual Reproduction: Two parents give rise to offspring that have unique combinations of genes inherited from both parents.

Concept 10.2: Fertilization and Meiosis Alternate in Sexual Life Cycles

• Life Cycle: The generation-to-generation sequence of stages in the reproductive history of an organism.

• Human Chromosome Sets:

  • Human somatic cells have $23$ pairs of chromosomes.

  • Karyotype: An ordered display of the pairs of chromosomes from a cell.

  • Homologous Chromosomes (Homologs): The two chromosomes in each pair. They share the same length, shape, and carry genes controlling the same inherited characters.

• Sex Chromosomes: The chromosomes referred to as $X$ and $Y$, which determine sex.

  • Females typically have a homologous pair of $X$ chromosomes ($XX$).

  • Males have one $X$ and one $Y$ chromosome.

• Autosomes: The remaining $22$ pairs of chromosomes (excluding sex chromosomes).

• Diploid Cell ($2n$): A cell containing two sets of chromosomes. For humans, the diploid number is $46$ ($2n = 46$).

• Haploid Cell ($n$): A cell containing a single set of chromosomes. For humans, the haploid number is $23$ ($n = 23$).

• Gamete Composition:

  • In an unfertilized egg (ovum), the sex chromosome is always $X$.

  • In a sperm cell, the sex chromosome may be either $X$ or $Y$.

• Fertilization: The union of gametes (sperm and egg).

• Zygote: The fertilized egg, which is diploid ($2n$) because it contains one set of chromosomes from each parent. It produces somatic cells by mitosis and develops into an adult.

• Meiosis: The process of cell division that results in one set of chromosomes in each gamete, occurring in the ovaries and testes at sexual maturity. Fertilization and meiosis alternate to maintain chromosome number.

The Variety of Sexual Life Cycles

• Animal Life Cycle: Gametes are the only haploid cells. They are produced by meiosis and do not divide further before fertilization. They fuse to form a diploid zygote that divides by mitosis to become a multicellular organism.

• Plants and Some Algae (Alternation of Generations):

  • Includes both diploid and haploid multicellular stages.

  • Sporophyte: The diploid organism that makes haploid spores by meiosis.

  • Gametophyte: The haploid organism that grows from a spore by mitosis and makes haploid gametes by mitosis.

• Most Fungi and Some Protists: The only diploid stage is the single-celled zygote; there is no multicellular diploid stage. The zygote produces haploid cells by meiosis, which then grow by mitosis into a haploid multicellular organism. This haploid adult produces gametes by mitosis.

• Commonalities:

  • Either haploid or diploid cells can divide by mitosis depending on the cycle.

  • Only diploid cells can undergo meiosis.

  • The halving and doubling of chromosomes in all cycles contribute to genetic variation.

Concept 10.3: Meiosis Reduces Chromosome Sets from Diploid to Haploid

• Meiosis is preceded by chromosome duplication.

• It consists of two sets of cell divisions: Meiosis I and Meiosis II.

• The result is four daughter cells (instead of two as in mitosis), each with half as many chromosomes as the parent cell.

• Sister Chromatid Cohesion: The close association of resulting sister chromatids along their lengths.

• Allele: Different versions of genes that homologs may have.

• Meiosis I (Separates Homologous Chromosomes):

  • Prophase I: Chromosomes condense; homologous chromosomes pair up gene by gene (synapsis). Crossing over occurs where nonsister chromatids exchange DNA segments at points called chiasmata.

  • Metaphase I: Homologous pairs (tetrads) line up at the metaphase plate; one chromosome faces each pole. Microtubules attach to kinetochores.

  • Anaphase I: Homologous chromosomes separate and move toward opposite poles. Sister chromatids remain attached at the centromere.

  • Telophase I and Cytokinesis: Each half of the cell has a haploid set of chromosomes (still consisting of two sister chromatids). Cytokinesis forms two haploid daughter cells.

• Meiosis II (Separates Sister Chromatids):

  • Very similar to mitosis. No chromosome duplication occurs between Meiosis I and II.

  • Prophase II: Spindle apparatus forms; chromosomes move toward the metaphase plate.

  • Metaphase II: Sister chromatids (no longer genetically identical due to crossing over) arrange at the metaphase plate and attach to spindle fibers.

  • Anaphase II: Sister chromatids separate and move as individual chromosomes toward poles.

  • Telophase II and Cytokinesis: Nuclei form, chromosomes decondense. The end result is four genetically distinct haploid daughter cells.

Crossing Over and Synapsis During Prophase I

1. Homologous pairs associate along their length, aligned allele by allele.

2. A zipper-like structure called the synaptonemal complex forms (synapsis).

3. DNA molecules of maternal and paternal chromatids are broken at matching points.

4. DNA breaks are closed so that paternal chromatid pieces are joined to maternal pieces and vice versa (crossover).

Comparison of Mitosis and Meiosis

• Mitosis: Conserves chromosome sets; produces two identical daughter cells.

• Meiosis: Reduces sets from two to one; produces four genetically different daughter cells.

• Unique Meiotic Events (all in Meiosis I):

  • Synapsis and crossing over in Prophase I.

  • Alignment of homologous pairs at the metaphase plate.

  • Separation of homologs during Anaphase I.

• Cohesins: In mitosis, they are cleaved at the end of metaphase. In meiosis, they are cleaved along arms in Anaphase I and at centromeres in Anaphase II.

Concept 10.4: Genetic Variation Contributes to Evolution

• Mutations: The original source of genetic diversity; they create different alleles.

• Reshuffling of Alleles: Occurs during sexual reproduction to produce further variation.

• Three Mechanisms of Genetic Variation:

  1. Independent Assortment: Homologous pairs orient randomly at Metaphase I. The number of possible combinations is $2^n$. For humans ($n = 23$), this exceeds $8$ million ($2^{23}$).

  2. Crossing Over: Produces recombinant chromosomes. In humans, an average of $1$ to $3$ crossover events occur per chromosome pair.

  3. Random Fertilization: Any sperm can fuse with any ovum. Combining independent assortment ($8.4$ million sperm options $\times$ $8.4$ million egg options) yields about $70$ trillion possible diploid combinations for a zygote.

• Evolutionary Significance: Natural selection favors genetic variations suited to the environment. Sexual reproduction facilitates the accumulation of these variations within a population.