o o o o o o o o Chapter 13 Meiosis and Sexual Life Cycles Variations on a theme The transmission of traits from one generation to the next is called inheritance or heredity Along with inherited similarity there is also variation Genetics is the scientific study of heredity and hereditary variation Concept 13 1 Offspring acquire genes from parents by inheriting chromosomes Inheritance of genes Ch 13 Meiosis and Sexual Life Cycles 1 Parents endow their offspring with coded information in the form of hereditary units called genes Our genes program the specific traits that emerge as we develop from fertilized eggs into adults The genetic program is written in the language of DNA the polymer of four different nucleotides The transmission of hereditary traits has its molecular basis in the precise replication of DNA which produces copies of genes that can be passed from parents to offspring In animals and plants reproductive cells called gametes are the vehicles that transmit genes from one generation to the next Every species has a characteristic number of chromosomes Humans have 46 chromosomes in their somatic cells all cells of the body except the gametes and their precursors A gene s specific location along the length of a chromosome is called the gene s locus o Comparison of asexual and sexual reproduction Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves In asexual reproduction a single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes Mitotic cell division is the division in which the DNA is copied and allocated equally to two daughter cells The genomes of the offspring are virtually exact copies of the parent s genome An individual that reproduces asexually gives rise to a clone a group of genetically identical individuals In sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents Offspring of sexual reproduction vary genetically from their siblings and both parents 13 1 Summary Each gene in an organism s DNA exists at a specific locus on a certain chromosome We inherit one set of chromosomes from our mother and one set from our father In asexual reproduction a single parent produces genetically identical offspring by mitosis Sexual reproduction combines sets of genes from two different parents leading to genetically diverse offspring Concept 13 2 Fertilization and meiosis alternate in sexual life cycles A life cycle is the generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring Sets of chromosomes in human cells In humans each somatic cell has 46 chromosomes Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types The chromosomes are arranged in pairs starting with the longest chromosomes The resulting ordered display is called a karyotype same inherited characters The two chromosomes composing a pair have the same length centromere position and staining patters these are called homologous chromosomes or homologs Both chromosomes of each pair carry genes controlling the Ch 13 Meiosis and Sexual Life Cycles 2 The two distinct chromosomes referred to as X and Y is an important exception to the general pattern of homologous chromosomes in human somatic cells Human females have a homologous pair of X chromosomes XX but males have one X and one Y chromosome XY Because they determine an individual s sex the X and Y chromosomes are called sex chromosomes Other chromosomes are called autosomes We inherit one chromosome of each pair from each parent Thus the 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes a maternal and a paternal set The number of chromosomes in a single set it represented by n Any cell with two chromosome sets is called a diploid cell and has a diploid number of chromosomes abbreviated 2n For humans the diploid number is 46 2n 46 the number of chromosomes in our somatic cells Unlike somatic cells gametes contain a single set of chromosomes Such cells are called haploid cells and each has a haploid number of chromosomes n For humans the haploid number is 23 n 23 The set of 23 consists of the 22 autosomes plus a single sex chromosome An unfertilized egg contains an X chromosome but a sperm may contain an X or a Y chromosome o Behavior of Chromosome Sets in the Human Life Cycle The human life cycle begins when a haploid sperm from the father fuses with a haploid egg from the mother This union of gametes culminating in fusion of their nuclei is called fertilization The resulting fertilized egg or zygote is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines As a human develops into a sexually mature adult mitosis of the zygote and its descendant cells generate all the somatic cells in the body The only cells in the human body not produced by mitosis are the gametes which develop from specialized cells called germ cells in the gonads ovaries and testes In sexually reproducing organisms gamete formation involves a type of cell division called meiosis This cell division reduces the number of chromosomes from two to one in the gamete As a result from meiosis each human sperm and egg is haploid n 23 o The variety of sexual life cycles There are 3 main types of life cycles Humans and most other animals o In the type that occurs in humans and most other animals gametes are the only haploid cells o Meiosis occurs in germ cells during the production of gametes which undergo no further cell division prior to fertilization After fertilization the diploid zygote divides by mitosis producing a multicellular organism that is diploid Plants and some algae o o o o Plants and some species of algae exhibit a second type of life cycle called alternation of generations This type includes both diploid and haploid stages that are multicellular The multicellular diploid stage is called the sporophyte Meiosis in the sporophyte produce haploid cells called spores Unlike a gamete a haploid spore doesn t fuse with another cell but divides mitotically generating a multicellular haploid stage called the gametophyte Cells of the gametophyte give rise to gametes by mitosis Fusion of two haploid gametes at fertilization results in a diploid zygote