CAMPBELL and REECE Chapter 13

 Heredity: transmission of traits from 1 generation to the next  Variation: differences between members of same species  Genetics : scientific study of heredity & hereditary variation  Genes: discrete unit of hereditary information consisting of specific nucleotide sequence in DNA or RNA (some viruses)  Gametes: a haploid reproductive cell (ova, sperm)  Somatic Cells: any cell in multicellular organism except ova or sperm  Locus: specific place along length of a chromosome where a given gene is location

 genetic “program” written in language of DNA  DNA: the polymer of 4 nucleotides  most genes program cells to synthesize specific enzymes & proteins whose cumulative actions produces organism’s inherited traits

 vehicle used by plants & animals to transmit genes to next generation  male + female gamete unite during fertilization

 offspring are identical genetically to parent  used by unicellular & some multicellular eukaryotic organisms  cells of offspring derived by mitosis  aka clones  mutations can occur in asexual reproduction

 2 parents give rise to offspring that are genetically different from either parent

 is the generation-to-generation sequence of stages in reproductive history of an organism

 46 (23 pair) (2n)  22 pair autosomes  1 pair sex chromosomes  condense during mitosis so large enough to separate based on: size position of centromere patterns of colored banding produced by stains

 ordered display of chromosome pairs of a cell, arranged by size & shape  pairs of matching chromosomes have same same loci & are called homologous chromosomes or homologs

 Not homologous  determine individual’s sex: XX XY

DIPLOID #HAPLOID #  2 sets of chromosomes in cell: 1 set maternal source; 1 set paternal source  Humans = 46  Gametes have 1 set of chromosomes  Humans = 23

 haploid sperm + haploid ova fertilize  fusion of their 2 nuclei  fertilized egg called a zygote (diploid)  thru mitosis this fertilized egg will produce all somatic cells in mature adult  only cells not produced by mitosis are the gametes which develop from germ cells in gonads (ovary or testes)

 creates 1n gametes so when fertilization occurs  2n offspring  human life cycle typical of most sexually reproducing animals, plants, fungi, & protisits

 all sexual reproduction involves meiosis & fertilization but there is variation in timing  1. gametes are the only haploid cells humans & most other animals

 2. Alternation of Generations  plants & some species of algae  involves both diploid & haploid multicellular stages  Sporophyte : multicellular diploid state  Spores : haploid cells produced by sporophyte, divides by mitosis   Gametophyte : haploid multicellular stage that produces gametes (1n) which will fertilize producing 2n sporophyte

 3. Haploid Cells Predominant  occurs in most fungi & some protists  haploid gametes fuse & form diploid zygote  meiosis produces haploid cells (not gametes) which then either divided by mitosis forming multicellular haploid organisms or unicellular descendants

 starts with diploid cell with duplicated chromosomes  ends with separation of homologous chromosomes in 2 daughter cells which are each haploid but with duplicated chromosomes (reductional division)

 chromosomes condense  *homologs align along their lengths  paired homologs become physically connected to each other along their lengths by a “zipper-like” protein structure: synaptonemal complex  this state called synapsis

 Crossing Over : genetic rearrangement between nonsister chromosomes  involves exchange of segments of DNA  Chiasma : exists wherever cross over has occurred  spindle forms, nuclear envelope fragments, (as in prophase in mitosis)

 pairs of homologous chromosomes metaphase plate  both chromatids of 1 homolog attached to kinetochore microtubule from 1 pole; those of other homolog attached to kinetochore from other pole

 proteins that hold sister chromatids together breakdown allowing homologs to separate  homologs move toward opposite poles guided by spindles  sister chromatid cohesion persists at the centromere so duplicated chrtomosomes move to opposite poles

 each ½ of cell has complete haploid set of duplicated chromosomes  cytokinesis usually forms simultaneously w/telophase  forms 2 haploid daughter cells  in some species, chromosomes decondense & nuclear envelope reforms  No chromosome duplication occurs between Meiosis I & II

 starts with 2 haploid cells (with duplicated chromosomes)  sister chromatids separate  ends with 4 haploid daughter cells each genetically different than parent cell

 spindle apparatus forms

 chromosomes metaphase plate  2 sister chromatids no longer genetically identical (due to crossing over)  Kinetochores of sister chromatids attached to microtubules extending from opposite poles

 breakdown of proteins holding the sister chromatids centromere allows chromatids to separate  chromatids move toward opposite poles as individual chromosomes

 nuclei form  chromosomes condense  cytokinesis occurs  4 daughter cells each with haploid set of unduplicated chromosomes 4 daughter cells are genetically distinct from each other & from parent cell

1. Independent Assortment of Chromosomes 2. Crossing Over 3. Random Fertilization

 orientation of pairs of homologous chromosomes at metaphase I is random  each pair may orient with either its maternal or paternal homolog closer to a given pole  50% chance that a particular daughter cell of meiosis I will contain the maternal chromosome of a certain homolog (50% chance paternal)

 because humans have n = 23 the # of possibilities due to independent assortment is 2²³ or about 8.4 million possible combinations of maternal/paternal chromosomes in any 1 gamete

 produces recombinant chromosomes : individual chromosomes that carry genes derived from 2 different parents  in humans: average of 1 to 3 crossover events occur per chromosome pair depends on size of chromosome & position of centromere

 hill.com/sites/ /student_view0/ chapter11/meiosis_with_crossing_over.ht ml hill.com/sites/ /student_view0/ chapter11/meiosis_with_crossing_over.ht ml  a/meiosis-lg.mov a/meiosis-lg.mov

 each sperm or egg represents 1/84,000,000 possible combinations  zygote represents 1/70 trillion diploid combinations possible for these 2 parents

 ability of sexual reproduction to generate genetic diversity is considered 1 of most plausible explanations for evolutionary persistence of this process  genetic diversity very important in changing environment

 more advantageous because it ensures perpetuation of successful combinations of alleles  asexual reproduction less costly to organism in terms of nrg costs

 only reproduce asexually  rare  microscopic bdelloid rotifer  ~400 species living in variety of environments

 their existence does not disprove that sexual reproduction has evolutionary advantages  there are other mechanisms the rotifers use to increase their genetic variation if environment becomes very dry they form state of suspended animation during which rotifers exchange DNA with neighbors