13.1 How Does Meiosis Occur?

• The X and Y chromosomes are called sex chromosomes and are associated with an individual’s sex.

• Any chromosomes other than sex chromosomes are called autosomes.

• Chromosomes that are the same size and shape are called homologous chromosomes, or homologs and the pair is called a homologous pair.

• A gene is a segment of DNA, found at a specific place on a chromosome, that influences a trait.

• Biologists use the term allele to denote a particular version of a gene.

• Insects, humans, trees, and other organisms that have two versions (homologs) of each type of chromosome are called diploid (“double-form”).

• These cells and organisms are called haploid (“single-form”) .

• The number of distinct types of chromosomes in a given cell is called the haploid number.

• The number of chromosome sets is termed the cell’ s ploidy.

• A maternal chromosome comes from the mother, and a paternal chromosome comes from father

• The haploid number n indicates the number of distinct types of chromosomes present.

• Polyploid means that they have three or more of each type of chromosome in each cell.

• In animals, some or all of the haploid daughter cells go on to form egg cells or sperm cells in the process of gametogenesis

• The diploid cell that results from fertilization is called a zygote, and it is the first cell of a new individual.

• Life cycle is the sequence of events that occurs over the life of an individual, from fertilization to the production of offspring.

• Synapsis is a tight side-by-side pairing of homologous chromosomes along their corresponding regions.

• The homologs are held together by a network of proteins called the synaptonemal complex.

• The structure that  results from synapsis is called a bivalent

• Chromatids from different homologs are referred to as non-sister chromatids.

• They stay joined, however, by X-shaped structures called chiasmata

• The process of chromosome exchange is called crossing over. Crossing over is a fundamental part of meiosis.

13.2 Meiosis Promotes Genetic Variation

• Asexual reproduction is any mechanism of producing offspring that does not involve the production and fusion of gametes.

• Asexually produced offspring are clones-or exact genetic copies-of their parents.

• Sexual reproduction is the production of offspring through the generation and fusion of gametes.

• The two aspects of meiosis that promote variation are:

  • Separation and distribution of homologous chromosomes

  • Crossing over

• The creation of new combinations of alleles is called genetic recombination.

13.3 What Happens When Things Go Wrong in Meiosis

• Down syndrome, as the disorder came to be called, is observed in about one infant in every 700 live births in the United States.

• Nondisjunction is when the homologs or sister chromatids fail to separate, or disjoin.

• Monosomy is when there is only one copy of one of the chromosomes.

• Trisomy is when each cell has three copies of that chromosome

• Cells that have too many or too few chromosomes of a particular type are said to be aneuploid

• Errors are especially common in

  • microtubule attachment to kinetochores early in meiosis I

  • separation of chromosomes that have a single chiasma near their ends or their centromeres

  • the failure to form a chiasma

  • premature separation of sister chromatids.

• Primary oocytes, which are diploid precursors to eggs, enter meiosis I during embryonic development and arrest in prophase I until the female reaches sexual maturity

13.4 What Are the Benefits of Meiosis?

• More biologists are becoming convinced that sexual reproduction is helpful for two reasons:

  • Offspring are not doomed to inherit harmful alleles

  • the production of genetically varied offspring means that at least some may be able to resist rapidly evolving pathogens and parasites.