Reproduction

ReproductionLS Chapter 42 Types of ReproductionSexual and AsexualA=withoutAsexual means without sexAdvantages and disadvantages to eachSome organisms can do either or both, depending on circumstances.JellyfishMales release sperm, females release egg into the waterThey meet and fertilize and young jellyfish form on the sea floorThe young jellyfish then can reproduce asexually before becoming adults

Sexual ReproductionType of reproduction that requires two organismsDNA from each of the parents come together and form unique offspringThe female sex cell is the eggThe male sex cell is the spermThe process of them coming together is called fertilizationThe fertilized egg is called a zygoteThe zygote then goes through the cell cycle and becomes multicellular

ChromosomesIn interphase, DNA is in the form of chromatinIt winds around proteins into a tight structure called a chromatidThere are 2 of them, called sister chromatidsSister chromatids attach at the centromere and form a chromosome (X-shaped)Humans have 23 pairs of chromosomes (total of 46 chromosomes)1 from each pair come from mother, 1 from father I.E. 23 from mom, 23 from dad=23 pairs=46 chromosomesThe pair of chromosomes (1 from mom, 1 from dad) are called homologous chromosomesHomologous chromosomes have the same DNA, but may have different forms of a geneExample: One may have the gene for green eyes, the other may have the gene for brown eyes (much more to come on this in the genetics section)

MeiosisOccurs in sexually reproducing organismsSplits cells into the number of chromosomesImportant! What if cells didnt reduce their chromosome numbers?46 chromosomes from mom, 46 chromosomes from dad=92 chromosomes2x as many as you should haveVery, very, very similar to mitosisSame words are used, steps are the sameHappens twiceStages: Interphase, Meiosis I, Cytokinesis, Meiosis II, another cytokinesisWhile mitosis produces 2 cells, meiosis produces 4 cellsInterphaseExactly the same as in mitosisLongest phasePeriod of growth and developmentSame stagesG1 Phase-GrowthS Phase-DNA duplicatesG2 Phase-Growth and duplication of organellesMeiosisOccurs in 2 phasesMeiosis IProphase IMetaphase IAnaphase ITelophase ICytokinesisMeiosis IIProphase IIMetaphase IIAnaphase IITelophase IICytokinesis

Prophase IChromatin condenses into chromosomes (sister chromatids attached at the centromere)Homologous chromosomes (1 from mom and 1 from dad) attach and exchange genesThe nucleus breaks downCentrioles move towards poles of cellSpindle forms

Metaphase 1Spindle fibers attach to chromosomes at the centromereHomologous chromosomes line up in the equator of the cell

Anaphase IHomologous chromosomes are pulled apart1 chromosome moves to each end of the cell

Telophase I2 nuclei form1 around each set of chromosomesSpindle breaks downChromosomes unwind into chromatinCell begins to splitCytokinesis occursThe new cells enter a short resting phase before Meiosis II begins

Prophase IIMeiosis II happens in both cells from meiosis I at the same timeChromatin winds up into chromosomes (sister chromatids attached at the centromere)Nucleus breaks down Spindle forms

Metaphase IISpindle attaches to chromosomes Chromosomes line up in the center of the cellThis time there is only 1, so the chromosomes are single file

Anaphase IISister chromatids are pulled apart at the centromere

Telophase II4 nuclei formSpindles break downChromatids unwind into chromatinCells begin to divideAnother cytokinesis4 cells are produced

Results of MeiosisRecall that in Prophase I, the homologous chromosomes exchanged genesThis causes there to be 4 different cells produced from MeiosisContrast this with the 2 identical cells resulting from mitosisEach of the cells produced from meiosis have of the DNA as the parent cellIn males, meiosis results in sperm cellsIn females, meiosis results in egg cellsWhen the 2 cells fuse during fertilization, the DNA combines + = 1 total cellThis cell then undergoes mitosisAdvantages and disadvantages of Sexual ReproductionAdvantages:The offspring are unique due to meiosisExcept for twins, no 2 sexually-reproducing organisms have the same DNAThe genetic variation makes organisms more likely to adapt and evolve to changing environmental conditionsDisadvantages:SlowHumans take 9 months, some elephants can be 2 years!Requires energyNeed to find a mate Some organisms use a lot of energy finding a mate

Asexual ReproductionReproducing involving only one organismThe offspring is a clone of the parentIt is genetically identical to the parentAll offspring are also identical to one anotherSome asexually-reproducing organisms can exchange parts of DNA to increase genetic diversity a little4 main types:FissionBuddingRegeneration Vegetative PropagationFissionMany prokaryotic organisms reproduce this wayOne organism splits into 2Similar to mitosis

BuddingOffspring grows on the parent organismOccurs in yeast, hydras, and potatoes (though may be considered to be vegetative propagationdepends who you ask)

RegenerationOffspring grows from a piece of the parent organismHappens in planarians, sea stars, and hydras, and other animalsAnd Time Lords

Vegetative Propagation Similar to regenerationOnly for plantsOffspring grow from part of the parent plantStrawberries, potatoes, spider plants

Advantages and Disadvantages of Asexual ReproductionAdvantages:Dont need a mateDont need a lot of energy finding a mateQuickCan produce many offspring quickly Some bacteria can split every few minutes!Disadvantages:Low genetic diversityProne to extinction in changing conditionsUnable to adapt for the same reason.