meiosis and sexual life cycles

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Chapter 13. Meiosis and Sexual Life Cycles. Overview: Variations on a Theme. living organisms are distinguished by their ability to reproduce their own kind Genetics = the scientific study of heredity and variation Heredity = the transmission of traits from one generation to the next - PowerPoint PPT Presentation


  • Overview: Variations on a Themeliving organisms are distinguished by their ability to reproduce their own kindGenetics =the scientific study of heredity and variationHeredity = the transmission of traits from one generation to the nextVariation = demonstrated by the differences in appearance that offspring show from parents and siblings

  • Inheritance of Genesin a literal sense, children do not inherit particular physical traits from their parentsit is genes that are actually inheritedGenes are the units of hereditymade up of segments of DNApassed to the next generation via reproductive cells called gametes (sperm and eggs) somatic cells any other cell other than the gamete or its precursoreach gene has a specific location called a locus on a certain chromosome

  • Comparison of Asexual and Sexual Reproduction asexual reproduction - a single individual passes genes to its offspring without the fusion of gametesproduces an exact copy of the parent a clonedone through fission or mitosis following duplication of DNAclone = a group of genetically identical individuals from the same parentsexual reproduction - two parents give rise to offspring that have unique combinations of genes inherited from the two parents

  • Sets of Chromosomes in Human Cellsa life cycle = generation-to-generation sequence of stages in the reproductive history of an organisma karyotype is an ordered display of the pairs of chromosomes from a cellhuman somatic cells have 23 pairs of chromosomes the two chromosomes in each pair are called homologous chromosomes, or homologschromosomes in a homologous pair have the same length, the same centromere positionalso carry genes at the same loci controlling the same inherited characters

  • sex chromosomes = X and Yhuman females have a homologous pair of X chromosomes (XX)human males have one X and one Y chromosomegamete (sperm or egg) contains a single set of chromosomes haploid (n)humans, the haploid number is 23 (n = 23)in an unfertilized egg (ovum), the sex chromosome is Xin a sperm cell, the sex chromosome may be either X or Y

    Some Definitions You Know Already

  • remaining 22 pairs of chromosomes are called autosomeseach pair of homologous chromosomes includes one chromosome from each parenta diploid cell (2n) has two sets of chromosomeshumans, the diploid number is 46 (2n = 46)dogs, the diploid number is 72! (2n = 72)Drosophila 2n = 4

    Some Definitions You Know Already

  • in a cell in which DNA synthesis has occurred - each chromosome has been replicatedeach duplicated chromosome consists of two identical sister chromatids joined at a centromere

  • chromosome = organized structure of DNA, protein and RNA found in the nucleus or nucleoid regionsingle piece of coiled DNA containing genes, regulatory elements and other nucleotide sequencesassociated with DNA binding proteins for the packaging of the DNA and control of gene expressiontrue definition DNA organized into chromatintwo forms in interphase euchromatin (active form) and heterochromatin (inactive form)replicated DNA condenses during the early stages of mitosis and meiosis to form two sister chromatids joined at a centromere

    Lets Remind Ourselves Shall We?1. chromatid2. centromere3. p arm (short arm)4. q arm (long arm)

  • chromosome = organized structure of DNA, protein and RNA found in the nucleus or nucleoid regioncircular (prokaryotes) or linear (eukaryotes)prokaryotic chromosome known as a genophore not organized as chromatingenes organized into operons no intronssmaller, circular genophores = plasmidscircular chromosomes found in eukaryotic mitochondria and chloroplasts

    Lets Remind Ourselves Shall We?

  • centromere = point where the two sister chromatids joinphysical role act as the site of assembly for the kinetochoreindirect role control the separation of chromatidskinetochore complex group of proteins that attaches the centromere to the spindle microtubules and is responsible for chromatid separation (signals that all chromosomes are attached, aligned and are ready for separation)two kinetochore regionsinner plate: associates with the centromere DNA sequencesmodified histone proteins that interact with DNAouter plate: associates with the microtubules

    Centromeres** the sister chromatids of a chromosome are linked all along theirlength by cohesin proteins-cutting of cohesins happens during prophase until metaphase only point of connection is the cohesins in the centromere region

  • centromere = point where the two sister chromatids jointwo types: Point and RegionalPoint: smaller and more compact (e.g. yeasts)DNA sequence is necessary for the formation of the centromerebind to specific kinetochore proteinsRegional: most centromeres (e.g. humans)DNA sequence contributes to formation of the centromerenot an actual defined sequence of DNA but is an array of repetitive sequences of satellite DNA that are similar to one anotherDNA of the centromere is heterochromatin formheterochromatin form is critical for the adherence of cohesin proteins in that regionend up with a high concentration of cohesins in the centromere region

    Centromeres1. chromatid2. centromere3. p arm (short arm)4. q arm (long arm)

  • so what makes up a centromere?1. DNA regions two strands of DNA interact through base pairing DNA is heterochromatin2. cohesion proteins high concentrationinteracts with the DNA3. kinetochore inner and outer platesfor attachment to the microtubules and chromatin separation


  • centromere positionsMetacentric: typical X shaped chromosometwo arms (p and q) are equal in lengthhumans chr 1 & 3Submetacentric: arms are unequalp is shorterAcrocentric: p arm is so short it is hard to observehumans: chr 13,14,15,21,22 and YTelocentric: centromere is located at the end of the chromosomenone in humansHolocentric: the entire length of the chromosome acts as the centromereplants and many invertebrates (nematodes)

    Centromereschromosome 1chromosome 13

  • the centromere position in humans is inheriteddaughter chromosomes will assemble their centromeres in the same positions as the parent chromosomeepigenetic nothing to do with DNA sequencehas to do with interactions with histonesrole of H3 histone ? H3 in the histones that interact with the centromere region is replaced with a related protein called Centromere Protein A/CENP-A (humans)


  • Centromeres Hows this for cool?the tension receptors in the interzone sense the attachment of the kinetochore to microtubulescdc20 promotes the activation of the Anaphase Promoting Complex (APC) a group of proteins that initiates anaphase & triggers the separation of chromatids during anaphase

  • It gets even cooler! Yes it does!cdc20 promotes the activation of the Anaphase Promoting Complex (APC) a group of proteins that initiates anaphase & triggers the separation of chromatids during anaphaseAPC is a ubiquitin ligase promotes the attachment of ubiquitin to protein targets degradation ubiquitin is attached to S and M cyclins degradation results in transition from metaphase to anaphaseubiquitin is also attached to a protein complex made of proteins called securin & separaseubiquitin dependent degradation of securin releases separase from the complex separase now targets the cohesin protein complexes that link chromatids

    once separated - dynein motor proteins in the outer plate walk the separated chromatids along the microtubule to the opposite centrioles of the cell


  • Fertilization is the union of gametes - the sperm and the eggthe gametes have one set of chromosome - haploidproduced via meiosis from a diploid germ cellgametes are the only types of human cells produced by meiosis rather than mitosishas one set of chromosomes from each parent the zygote produces somatic cells by mitosis and develops into an adultfertilization and meiosis alternate in sexual life cycles to maintain chromosome number

    Behavior of Chromosome Sets in the Human Life Cycle

  • The Variety of Sexual Life Cyclesthe alternation of meiosis and fertilization is common to all organisms that reproduce sexuallythe three main types of sexual life cycles differ in the timing of meiosis and fertilization

  • gametes are the only haploid cells in animalsproduced by meiosis undergo no further cell division before fertilization fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organismAnimals

  • plants and some algae exhibit an alternation of generationstheir life cycle includes both diploid and haploid multicellular stagesdiploid organism - called the sporophyte - makes haploid spores by meiosisthe spore grows (via mitosis) into a haploid organism called a gametophytegametophyte bears the reproductive parts of the organism OR is the reproductive partgametophyte makes haploid gametes by mitosisfertilization of gametes produces a new diploid sporophyte


  • in most fungi and some protists, the only diploid stage is the single-celled zygotethere is no multicellular diploid stagethe zygote produces haploid spores by meiosiseach haploid cell grows by mitosis into a haploid multicellular organism - fungusthe haploid adult produces gametes by mitosisFungi

  • depending on the type of life cycle, either haploid or diploid cells can divide by mitosisin animals diploid cells undergo mitosis so that the organism grows into a diploid organismspecialized diploid germ cells undergo meiosis to produce gametesgamete fusion produces a new diploid organismin plants & fungus haploid spores undergo mitosis so the organism grows into a haploid organism (e.g. gametophyte)haploid organism produces gamet


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