Meiosis and Sexual Reproduction Chapter 7. 7.1 Meiosis In some organisms haploid gametes join to...
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Transcript of Meiosis and Sexual Reproduction Chapter 7. 7.1 Meiosis In some organisms haploid gametes join to...
Meiosis and Sexual Meiosis and Sexual ReproductionReproduction
Chapter 7Chapter 7
7.1 Meiosis7.1 Meiosis In some organisms haploid gametes join to In some organisms haploid gametes join to
form a diploid zygote form a diploid zygote If gametes weren’t haploid imagine how the If gametes weren’t haploid imagine how the
number of chromosomes would increase of number of chromosomes would increase of timetime
Meiosis is the form of cell division that makes Meiosis is the form of cell division that makes diploid cells haploid when forming gametesdiploid cells haploid when forming gametes
There are 2 divisions in Meiosis – Meiosis I There are 2 divisions in Meiosis – Meiosis I and Meiosis IIand Meiosis II
Before Meiosis begins DNA is replicated so Before Meiosis begins DNA is replicated so cell has homologous chromosomescell has homologous chromosomes
Prophase IProphase I
Chromosomes Chromosomes condensecondense
Nuclear envelope Nuclear envelope breaks downbreaks down
Homologous Homologous chromosomes pair chromosomes pair upup
Crossing over occursCrossing over occurs Chromatids exchange Chromatids exchange
Metaphase IMetaphase I
Homologous pairs Homologous pairs move to equator by move to equator by spindlespindle
Anaphase IAnaphase I
Homologous pairs Homologous pairs separate and separate and pulled towards pulled towards polespoles
Chromatids do not Chromatids do not separate; still separate; still connected by connected by centromerescentromeres
Telophase ITelophase I
Individual Individual chromosomes chromosomes gather at polesgather at poles
Most organisms Most organisms now have now have cytokinesiscytokinesis
2 new cells formed2 new cells formed Chromosomes do Chromosomes do
not replicate againnot replicate again
Prophase IIProphase II
New spindle formsNew spindle forms
Metaphase IIMetaphase II
Chromosomes line Chromosomes line up along equator up along equator and are still and are still attached at attached at centromerescentromeres
Anaphase IIAnaphase II
Centromeres divide Centromeres divide Chromatids Chromatids
separate; Now separate; Now called called chromosomeschromosomes
Telophase IITelophase II
Nuclear envelope Nuclear envelope forms around forms around chromosome setschromosome sets
Meiosis in ActionMeiosis in Action
Meiosis and Genetic Meiosis and Genetic Variation Variation
Three key ways meiosis creates new Three key ways meiosis creates new genetic variationsgenetic variations Independent AssortmentIndependent Assortment Crossing OverCrossing Over Random FertilizationRandom Fertilization
Independent AssortmentIndependent Assortment Most organisms have Most organisms have
more than 1 more than 1 chromosomechromosome
In humans the gamete In humans the gamete gets 1 chromosome gets 1 chromosome from the 23 pairsfrom the 23 pairs
Which of the pair you Which of the pair you get is chanceget is chance
This random distribution This random distribution is called Independent is called Independent AssortmentAssortment
Mathematically 2Mathematically 223 23 = = over 8.3 million over 8.3 million combinationscombinations
Crossing OverCrossing Over Portion of 1 Portion of 1
homologous homologous chromosome is chromosome is broken off and broken off and exchanged with exchanged with the the corresponding corresponding portion of its portion of its homologous homologous partnerpartner
Random FertilizationRandom Fertilization
What egg and sperm meet is also What egg and sperm meet is also completely randomcompletely random
Mathematically that means that the Mathematically that means that the possible combinations of the sperm possible combinations of the sperm (about 8.3 million) can be multiplied (about 8.3 million) can be multiplied by the combinations for the egg by the combinations for the egg (about 8.3 million) giving 70 trillion (about 8.3 million) giving 70 trillion possible combinationspossible combinations
Meiosis generates variation in a Meiosis generates variation in a species very quicklyspecies very quickly
Pace of evolution is sped up due to Pace of evolution is sped up due to variationvariation
A + B = C, a new individual not A + B = C, a new individual not identical to its parentsidentical to its parents
Gametogenesis – Formation of Gametogenesis – Formation of GametesGametes
Spermatogenesis – Spermatogenesis – sperm production sperm production occurs in testesoccurs in testes Diploid cell Diploid cell
increases in size increases in size and becomes and becomes immature germ cellimmature germ cell
Meiosis I = 2 Meiosis I = 2 immature sperm immature sperm cellscells
Meiosis II = 4 Meiosis II = 4 haploid cellshaploid cells
Develop tails and Develop tails and become spermbecome sperm
Oogenesis – egg production Oogenesis – egg production occurs in ovariesoccurs in ovaries Diploid cell increases and Diploid cell increases and
becomes germ cellbecomes germ cell Meiosis I = after cytokinesis Meiosis I = after cytokinesis
cytoplasm is not equally dividedcytoplasm is not equally divided Smaller one called polar bodySmaller one called polar body
Meiosis II = larger cell divides Meiosis II = larger cell divides again, again cytoplasm not again, again cytoplasm not equally dividedequally divided
Forms another polar bodyForms another polar body Polar bodies will diePolar bodies will die
Ovum (pl. = ova) is larger cellOvum (pl. = ova) is larger cell More cytoplasm means more More cytoplasm means more
stored energystored energy
7.2 Sexual Reproduction7.2 Sexual Reproduction
Asexual ReproductionAsexual Reproduction Single parent passes on copies to offspring called Single parent passes on copies to offspring called
“clone”“clone” Identical to parentIdentical to parent Prokaryotes use binary fissionProkaryotes use binary fission
Many eukaryotes Many eukaryotes use asexual use asexual reproductionreproduction Fission – parent into Fission – parent into
2 equal sized 2 equal sized offspringoffspring
Fragmentation Fragmentation Body into several Body into several
piecespieces Missing parts grow Missing parts grow
backback Budding – new split Budding – new split
off from old; may off from old; may stay attachedstay attached
Genetic DiversityGenetic Diversity Asexual reproduction is simplest methodAsexual reproduction is simplest method In stable environment can produce many In stable environment can produce many
offspring in short time’offspring in short time’ Don’t need to waste energy making gametes Don’t need to waste energy making gametes
or find a partneror find a partner DNA varies very littleDNA varies very little In changing environment may not be able to In changing environment may not be able to
adaptadapt
Sexual ReproductionSexual Reproduction 2 parents form 2 parents form
reproductive cells with reproductive cells with half the number of half the number of chromosomeschromosomes
Diploid Mother makes Diploid Mother makes haploid gametehaploid gamete
Diploid Father makes Diploid Father makes haploid gametehaploid gamete
Haploid gametes make Haploid gametes make diploid offspring diploid offspring different from parentsdifferent from parents
Evolution of Sexual Evolution of Sexual ReproductionReproduction
May have come as a way for protists May have come as a way for protists to repair DNAto repair DNA
Only diploid cells can repairOnly diploid cells can repair Protists form diploid cells under Protists form diploid cells under
stressstress Many enzymes that repair DNA are Many enzymes that repair DNA are
involved in meiosisinvolved in meiosis
Sexual Life Cycle in Sexual Life Cycle in EukaryotesEukaryotes
Life Cycle – span from 1 generation to Life Cycle – span from 1 generation to nextnext
All sexually reproductive organisms All sexually reproductive organisms have a diploid-haploid-diploid lifecyclehave a diploid-haploid-diploid lifecycle
Only 3 Types of Sexual Life CyclesOnly 3 Types of Sexual Life Cycles Only difference is which phase Only difference is which phase
becomes multicellularbecomes multicellular
Haploid – simplest and most Haploid – simplest and most commoncommon
Zygote is the only diploid cell and it Zygote is the only diploid cell and it undergoes meiosis immediatelyundergoes meiosis immediately
Haploid cells in multicellular organism Haploid cells in multicellular organism produce gametes through mitosisproduce gametes through mitosis
Fusion – gametes fuse to form zygote Fusion – gametes fuse to form zygote and meiosis followsand meiosis follows
Meiosis allows for damage repairMeiosis allows for damage repair 2 homologous chromosomes are lined 2 homologous chromosomes are lined
up, special repair enzymes remove up, special repair enzymes remove damaged sections of DNA and fill in damaged sections of DNA and fill in gapsgaps
Protists, Fungi, AlgaeProtists, Fungi, Algae
Haploid Life CycleHaploid Life Cycle
DiploidDiploid
Adults are diploid, inherit from 2 Adults are diploid, inherit from 2 parentsparents
Diploid cell then meiosis then haploid Diploid cell then meiosis then haploid gametegamete
Gametes join during fertilization Gametes join during fertilization forming diploid zygoteforming diploid zygote
Zygote divides by mitosisZygote divides by mitosis Gametes are the only haploid cellsGametes are the only haploid cells
Diploid Life CycleDiploid Life Cycle
ParthenogenesisParthenogenesis New individual forms from New individual forms from
unfertilized eggunfertilized egg Hypothesized that Hypothesized that
mothers chromosomes mothers chromosomes are copied instead of are copied instead of getting fathers getting fathers chromosomeschromosomes
Produces clonesProduces clones Honeybees provide Honeybees provide
drones this way (haploid)drones this way (haploid) Whiptail lizards are all Whiptail lizards are all
femalesfemales Doesn’t seem possible in Doesn’t seem possible in
mammalsmammals Only mammal clones are Only mammal clones are
twins from split eggtwins from split egg
Alternation of GenerationsAlternation of Generations Regularly alternates between haploid and Regularly alternates between haploid and
diploiddiploid Plants – diploid phase called sporophyte Plants – diploid phase called sporophyte
produces sporesproduces spores Spore forming cells produce haploid spore that Spore forming cells produce haploid spore that
can form adult without another cellcan form adult without another cell Gametophyte is haploid phaseGametophyte is haploid phase
Produces gametes by mitosisProduces gametes by mitosis Gametes fuse and produce diploid phaseGametes fuse and produce diploid phase
Diploid – Haploid – Diploid – Haploid – etc…Diploid – Haploid – Diploid – Haploid – etc… Moss – haploid spores develop at top of Moss – haploid spores develop at top of
sporophyte stalksporophyte stalk