Overview: Variations on a Theme

¥                  Living organisms are distinguished by their ability to reproduce their own kind

¥                  Genetics is the scientific study of heredity and variation

¥                  Heredity is the transmission of traits from one generation to the next

¥                  Variation is demonstrated by the differences in appearance that offspring show from parents and siblings


Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

¥                  In a literal sense, children do not inherit particular physical traits from their parents

¥                  It is genes that are actually inherited


Inheritance of Genes

¥                  Genes are the units of heredity, and are made up of segments of DNA

¥                  Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)

¥                  Each gene has a specific location called a locus on a certain chromosome

¥                  Most DNA is packaged into chromosomes


Comparison of Asexual and Sexual Reproduction

¥                  In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes

¥                  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


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


Sets of Chromosomes in Human Cells

¥                  Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes

¥                  A karyotype is an ordered display of the pairs of chromosomes from a cell

¥                  The two chromosomes in each pair are called homologous chromosomes, or homologs

¥                  Chromosomes in a homologous pair are the same length  and shape and carry genes controlling the same inherited characters

¥                  The sex chromosomes, which determine the sex of the individual, are called X & Y

¥                  Human females have a homologous pair of X chromosomes (XX)

¥                  Human males have one X and one Y chromosome

¥                  The remaining 22 pairs of chromosomes are called autosomes

¥                  Each pair of homologous chromosomes includes one chromosome from each parent

¥                  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)

¥                  In a cell in which DNA synthesis has occurred, each chromosome is replicated

¥                  Each replicated chromosome consists of two identical sister chromatids

¥                  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

¥                  In an unfertilized egg (ovum), the sex chromosome is X

¥                  In a sperm cell, the sex chromosome may be either X or Y


¥                  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

¥                  The zygote produces somatic cells by mitosis and develops into an adult

¥                  At sexual maturity, the ovaries and testes produce haploid gametes

¥                  Gametes are the only types of human cells produced by meiosis, rather than mitosis

¥                  Meiosis results in one set of chromosomes in each gamete

¥                  Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number


The Variety of Sexual Life Cycles

¥                  The alternation of meiosis and fertilization is common to all organisms that reproduce sexually

¥                  The three main types of sexual life cycles differ in the timing of meiosis and fertilization

¥                  Gametes are the only haploid cells in animals

¥                  They are produces by meiosis and undergo no further cell division before fertilization

¥                  Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

¥                  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

¥                  Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

¥                  However, only diploid cells can undergo meiosis

¥                  In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring


Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid

¥                  Like mitosis, meiosis is preceded by the replication of chromosomes

¥                  Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

¥                  The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

¥                  Each daughter cell has only half as many chromosomes as the parent cell


The Stages of Meiosis

¥                  After chromosomes duplicate, two divisions follow

              Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes

              Meiosis II (equational division) sister chromatids separate

¥                  The result is four haploid daughter cells with unreplicated chromosomes

¥                  Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids

¥                  The sister chromatids are genetically identical and joined at the centromere

¥                  The single centrosome replicates, forming two centrosomes

¥                  Division in meiosis I occurs in four phases

              Prophase I

              Metaphase I

              Anaphase I

              Telophase I and cytokinesis


Prophase I

¥                  Prophase I typically occupies more than 90% of the time required for meiosis

¥                  Chromosomes begin to condense

¥                  In synapsis, homologous chromosomes loosely pair up, aligned gene by gene

¥                  In crossing over, nonsister chromatids exchange DNA segments

¥                  Each pair of chromosomes forms a tetrad, a group of four chromatids

¥                  Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred


Anaphase I

¥                  In anaphase I, pairs of homologous chromosomes separate

¥                  One chromosome moves toward each pole, guided by the spindle apparatus

¥                  Sister chromatids remain attached at the centromere and move as one unit toward the pole


Telophase I and Cytokinesis

¥                  In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

¥                  Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

¥                  In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

¥                  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

¥                  Division in meiosis II also occurs in four phases

              Prophase II

              Metaphase II

              Anaphase II

              Telophase II and cytokinesis

¥                  Meiosis II is very similar to mitosis


Prophase II

¥                  In prophase II, a spindle apparatus forms

¥                  In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate


Metaphase II

¥                  In metaphase II, the sister chromatids are arranged at the metaphase plate

¥                  Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

¥                  The kinetochores of sister chromatids attach to microtubules extending from opposite poles


Anaphase II

¥                  In anaphase II, the sister chromatids separate

¥                  The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles


Telophase II and Cytokinesis

¥                  In telophase II, the chromosomes arrive at opposite poles

¥                  Nuclei form, and the chromosomes begin decondensing

¥                  Cytokinesis separates the cytoplasm

¥                  At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes

¥                  Each daughter cell is genetically distinct from the others and from the parent cell

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

¥                             Three events are unique to meiosis, and all three occur in meiosis l

             Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

             At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

             At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

¥                  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)


Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

¥                  Mutations (changes in an organismÕs DNA) are the original source of genetic diversity

¥                  Mutations create different versions of genes called alleles

¥                  Reshuffling of alleles during sexual reproduction produces genetic variation


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 homologs into daughter cells independently of the other pairs

¥                  The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

¥                  For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes


Crossing Over

¥                  Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent

¥                  Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene

¥                  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

¥                  Crossing over adds even more variation

¥                  Each zygote has a unique genetic identity


The Evolutionary Significance of Genetic Variation Within Populations

¥                  Natural selection results in the accumulation of genetic variations favored by the environment

¥                  Sexual reproduction contributes to the genetic variation in a population, which originates from mutations