Meiosis and Genetics Terminology

• Meiosis

Genetics Terminology

• Sexually reproducing eukaryotes have two cell types, somatic cells and sex cells (gametes)
• A single germ cell divides into four unique daughter cells during meiosis
• Daughter cells possess half the number of chromosomes as the parent cell, making them haploid
• Ploidy refers to the number of sets of chromosomes in cells
• Haploid cells have one copy of each chromosome designated as "n"; gametes are haploid
• Diploid cell have two sets of chromosomes, having two of each chromosome designated as "2n"; somatic cells are diploid
• Diploid organisms receive one chromosome of each type from a female parent (maternal chromosomes) and another from a male parent (paternal chromosomes)
• Chromosomes exist in homologous pairs in diploid (2n) cells, with the exception of sex chromosomes (X,Y)
• Other chromosomes are known as autosomes and have homologues

Asexual Reproduction

• Many single-celled organisms reproduce by splitting or budding
• Multicellular organisms can reproduce asexually, which produces clones (offspring genetically identical to their parents)

Sexual Reproduction

• The fusion of two gametes produces a single zygote
• Sexual reproduction introduces greater genetic variation and allows genetic recombination
• A zygote has gametes from two different parents, with the exception of self-fertilizing organisms

Meiosis: Sex Cell (Gamete) Formation

• There are two divisions during meiosis: meiosis I and meiosis II
• During Prophase I, the chromosomes condense, and the nuclear envelope breaks down with crossing-over
• During Metaphase I, pairings of homologous chromosomes move to the equator of cells
• Homologous chromosomes move to opposite poles of the cell during Anaphase I
• The chromosomes gather at the poles of the cells and cytoplasm divides during Telophase I and cytokinesis
• In Prophase II, a new spindle forms around the chromosomes
• Metaphase II chromosomes line up at the equator
• Centromeres divide in Anaphase II and chromatids move to opposite poles of the cells
• A nuclear envelope forms around each set of chromosomes and the cytoplasm divides in Telophase II and cytokinesis
• Meiosis is not a cycle

Meiosis and Sexual Reproduction Life Cycle

• A diploid organism (2n) undergoes meiosis to produce gametes (egg and sperm) (n)
• The gametes then undergo fertilization (syngamy) to create a zygote (2n)

Genetic Variation in Diploid Organisms

• The fusion of sperm and egg results in unique offspring
• Offspring are a product of two individuals with differing genetic makeups
• Meiosis shuffles the genes to make the gametes of an individual genetically different from one another

Genetic Shuffling of Meiosis I

• There are events in meiosis I that shuffles the genes, in addition to a new combination of chromosomes resulting from fertilization
• Crossing over in Prophase I
  • Homologues break at identical locations and then rejoin with opposite partners
  • New combinations of alleles on individual chromosomes are created
  • Occurs randomly on every chromosome several times
  • Blends the inherited genes from parents
• Independent assortment in Metaphase I
  • There are equally probable arrangements of chromosomes at metaphase I

Spermatogenesis

• Spermatogenesis occurs as males produce sperm throughout life, starting after the onset of puberty, about 1,500 sperm each second
• Spermatogenesis progresses from spermatogonium to spermatocyte I to spermatocyte II to spermatid, then to spermatozoa

Oogenesis

• Oogenesis in females is probably completed before or soon after birth
• Three polar bodies develop during oogenesis as the mature ovum is generated
• Polar bodies have little cytoplasm and eventually degenerate

Mitosis vs. Meiosis

• Mitosis
  • 2n
  • Clone
  • There is the same genetic information in parent and daughter cell
• Meiosis
  • 1n
  • The daughter cells are different from the parent cell and each other
  • Daughter cells have half the equivalent number of chromosomes as a somatic cell
  • Genes are shuffled

Overview of meiosis, a type of cell division in sexually reproducing eukaryotes where a single germ cell divides into four unique haploid daughter cells. Includes definitions of ploidy, haploid, diploid and autosomes. Briefly contrasts this with asexual reproduction.