Meiosis and Sexual Reproduction Quiz

Meiosis and Sexual Reproduction

• Meiosis produces reproductive cells (gametes), such as sperm and egg.
• 4 genetically unique haploid (n) daughter cells are produced.
• Meiosis involves one round of DNA replication but two consecutive cell divisions.
• Meiosis 1 and meiosis 2 each have prophase, metaphase, anaphase, and telophase stages.

Synapsis

• During early prophase 1, homologous chromosomes pair up to form tetrads.
• Connected by synaptonemal complexes.

Mitosis

• Produces all other cell types (somatic cells).
• Creates 2 genetically identical diploid (2n) daughter cells.

Sexual Life Cycle

• Meiosis and fertilization make up the cycle.
• Zygote is formed when egg and sperm fuse.
• Diploid cells (2n) have 2 sets of chromosomes.
• Haploid cells (n) have only 1 set of chromosomes.
• Meiosis avoids the doubling of chromosomes in each generation.

Meiosis I

• Prophase I: Chromosomes condense, become visible, nuclear envelope disappears, spindle forms. Homologous chromosomes pair up (synapsis) forming tetrads. Crossing over (genetic recombination) occurs.
• Metaphase I: Homologous pairs align along the metaphase plate side-by-side. Microtubules (from opposite poles) attach to each homologue.
• Anaphase I: Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at the centromere.
• Telophase I: Nuclear envelopes may reform. Cytokinesis follows, creating two haploid daughter cells.

Meiosis II

• Prophase II: Nuclear envelopes dissolve (if reformed), spindles form.
• Metaphase II: Sister chromatids align along the metaphase plate. Microtubules from opposite poles attach to each sister chromatid.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Nuclear envelopes reform around each set, cytokinesis follows, producing 4 genetically unique haploid daughter cells.

Mendel's Principles

• Gregor Mendel's experiments on pea plants laid the groundwork for the chromosomal theory of inheritance.

• Traits have two distinct characteristics.

• Mendel used hybridizations (mating true-breeding individuals with different traits).

• Parental (P) generation: Initial mating plants.

• F₁ (first filial) generation: Offspring from the P generation.

• F₂ (second filial) generation: Offspring from the F₁ generation self-fertilization.

• Gene: A hereditary factor influencing a trait.

• Allele: An alternative version of a gene (e.g., purple or white flower color).

• Law of Segregation: Two alleles for a gene segregate during gamete formation, and are randomly reunited during fertilization.

• Law of Independent Assortment: Genes assort independently of each other during gamete formation.

Test Cross

• In a test cross, a dominant expressing organism is crossed with a homozygous recessive organism.
• If F₁ offspring are all heterozygotes, the dominant organism is homozygous.
• If F₁ offspring exhibit a 1:1 ratio of heterozygous and recessive homozygotes the dominant organism is heterozygous.

Extensions to Mendel

• Phenotypic plasticity: Environment alters the phenotype for the same genotype.
• Polygenic inheritance: Multiple genes influence one trait.
• Pleiotropy: One allele has multiple phenotypic effects.
• Multiple alleles: More than two alleles for a gene in a population (ex: blood types).
• Incomplete dominance: The heterozygote phenotype is a blend of the homozygous phenotypes.
• Codominance: Both alleles are fully expressed in the heterozygote.
• Epistasis: One gene obscures the effects of another gene.

Sex-linked traits

• Traits associated with sex chromosomes.
• X-linked: Present on the X chromosome only.
  • Males express X-linked traits more frequently (no counterpart for affected X allele on the Y)
  • Females (who are heterozygous) can act as carriers, exhibiting no phenotypic effect.

Chromosomal Basis of Inheritance

• Humans have 46 total chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).
• Y-chromosome: Consists of few active genes.
• Dosage compensation: Ensures equal expression of genes between sexes (even though their chromosome number differs, e.g., females have 2 X chromosomes and males have 1 X and 1 Y). Inactivation occurs in each female cell to avoid an excess of expression (a Barr body is formed.)
• Pedigree analysis: used to track traits through families showing dominant and recessive patterns.

DNA Replication

• DNA is replicated via semiconservative replication.

• Parental strands separate, and each serves as a template for a new daughter strand.

• Components:

  • Initiation
  • Elongation (new strands are synthesized by DNA polymerase)
  • Termination (replication is terminated)

• DNA replication requires: -DNA polymerase

  • Helicase -Primase -Ligase -SSBPs (single-strand binding proteins) -Topoisomerase

• Prokaryotes vs eukaryotes: Prokaryotes have one origin to replicate, eukaryotes have multiple.

Transcription

• Process of converting DNA to mRNA.
• Steps:
  • Initiation (binding of RNA polymerase)
  • Elongation (mRNA synthesis)
  • Termination (mRNA completion)
• Transcription in Eukaryotes utilizes 3 different RNA polymerases and requires a series of general transcription factors.

Translation

• Process of converting mRNA to protein.
• Steps: -Initiation (ribosome assembly) -Elongation (peptide bond formation) -Termination (termination sequence)
• Ribosomes are the sites for protein synthesis; they also require transfer RNA (tRNA). Each tRNA molecule brings the appropriate amino acid to the ribosome (based on the mRNA codons). A codon is a 3-base sequence that specifies one amino-acid.
• Transcription occurs in the nucleus, translation occurs in the cytoplasm (eukaryotes). In prokaryotes, both occur in the cytoplasm.

DNA Damage and Repair

• Enzymes repair mistakes and DNA damage during DNA replication and afterwards. -Mismatch repair -Photorepair (thymine dimers) -Excision repair

Test your knowledge on meiosis and its role in sexual reproduction with this quiz. Explore key concepts such as gamete formation, stages of meiosis, and the sexual life cycle. Understand the differences between meiosis and mitosis, and how they contribute to genetic diversity.