Meiosis and Sexual Life Cycles Chapter 13 Inheritance of Genes Genes are the units of heredity, and are made up
of segments of DNA Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosome
Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical
individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Sets of Chromosomes in Human Cells
Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes The sex chromosomes are called X and Y The 22 pairs of chromosomes that do not determine sex are called autosomes
The two chromosomes in each pair are called homologous chromosomes, or homologs The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes
For humans, the diploid number is 46 (2n = 46) A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a
single sex chromosome Behavior of Chromosome Sets in the Human Life Cycle Fertilization is the union of gametes (the sperm and the egg)
The fertilized egg is called a zygote and has one set of chromosomes from each parent Gametes are the only types of human cells produced by meiosis, rather than mitosis Plants and some algae exhibit an alternation of
generations This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis
Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte
In 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 Each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with
replicated chromosomes; it is called the reductional division In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with
unreplicated chromosomes; it is called the equational division A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the
parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is
virtually identical in meiosis II and mitosis Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are
responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II
(separation of sister chromatids) Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that
arises in each generation Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Crossing over Random fertilization
Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter
cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by
combining DNA from two parents into a single chromosome Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum
(unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
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