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Chromosomes fertilization

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Fertilization2Fertilization occurs when the nucleus of a malereproductive cell combines with the nucleus of afemale reproductive cell The reproductive cells are called gametes In animals, the male gamete is the sperm cell and the female gamete is the ovum In flowering plants, the male gamete is a cell in the pollen grain and the female gamete is an egg cell in the ovule When the male and female gametes combine, the resulting cell is called a zygote 3. 3pollen nucleusPLANT nuclei combine cell division (mitosis) embryoegg cell formedspermsANIMAL nuclei combinecell division (mitosis)ovum embryo formed 4. 0.1 mm 4 pollen grainovaryovuleegg cell This sperm will fertilize the ovumthe pollen cell reaches ANIMALthe egg cell through a PLANTpollen tube 5. 5As a result of fertilization, the chromosomes from themale and female cells are combined in the same nuclearmembraneDo you see a problem with this? 6. 6The problem is that if the full complement of male andfemale chromosomes combine, the zygote andembryo would have twice as many chromosomesas its parentsIf 46 male chromosomes combined with 46 femalechromosomes, the offspring would have 92 chromosomesin their cellsAnd the next generation would have 184 chromosomes,and so onIn fact, when the gametes are formed, the number ofchromosomes is halved so that the zygote ends up withthe same number of chromosomes as its parents. 7. 7The type of cell division which gives rise to gametesis called At cell division, before the chromatids separate, the chromosomes are shared equally between the two daughter cells The following slides describe this process. Only two pairs of chromosomes are shown 8. Meiosis 1 8The chromosomes appear,shorten and thicken just asin mitosisThe blue chromosomesare from the male parent;the red chromosomesare from the female parentThe two long chromosomesand the two short chromosomesare called homologous pairs 9. Meiosis 2 9Homologous chromosomescome to lie closely alongsideeach other and behave likea single chromosome 10. Meiosis 310The nuclear membranedisappearsA spindle formsAt this stage it is thechromosomes (and notthe chromatids) which areseparatedThe cell starts to constrict 11. Meiosis 4 11One long chromosomeand one short chromosomego to either end of thedividing cellSo the cells now containonly two, rather than fourchromosomesBy this time the chromatidshave become clear 12. Meiosis 512Two new spindles form, atright angles to the originalspindleAt this stage, it is thechromatids whichseparate and pass toopposite ends of thecells The cell constricts in the plane at right angles to the first constriction 13. Meiosis 6 13Cell division is completed,forming four gameteseach with half the numberof chromosomes of theparent cell gametes 14. 14The number of chromosomes in the body cells is calledthe diploid numberThe number of chromosomes in the gametes is calledthe haploid number (half the diploid number)The diploid number in human cells is 46. The haploidnumber in the gametes is 23 15. 1523sperms produced 46 by meiosis23spermmother 23cell23 fertilizationzygoteovum23mothercell23 2346 46 ova produced by meiosis but only one develops to maturity 16. 16Cell division continues bymitosis, so all the cells willcontain 46 chromosomesearly embryo46 464646 46 46 46 46 46 46 46 4646 464646 46 46 17. Genes 17 gene for gene for brown eyes blue eyes Genes for any one characteristic occupy corresponding positions gene for on homologous chromosomes curly hair But they do not necessarily control the characteristic in the same way For example, one of the gene pair gene for responsible for eye colour mightstraight determine brown eyes and itshair partner determine blue eyes* 18. 18Usually only one of a gene pair will be expressed in anindividualA person inheriting the gene for brown eyes and the genefor blue eyes will have brown eyesThe gene for brown eyes is said to be dominant tothe gene for blue eyes. The gene for blue eyes is notexpressed in this individualThe gene for blue eyes is said to be recessive to thegene for brown eyes 19. Gene combinations19In the first stage of meiosis, the illustration (slide 10)showed one red and one blue chromosomegoing to each daughter cellOne gamete will receive thegene combination for brown Ceyes and curly hair. The otherwill receive the genes for blue Beyes and straight hair Bb c = gene for straight hair C = gene for curly hair c b = gene for blue eyes B = gene for brown eyes 20. 20It is just as likely that both blue chromosomeswill go to one daughter cell and both redchromosomes go to the otherOne gamete will receive thegenes B and c (brown eyesand straight hair) c B bThe other gamete will receivegenes b and C (blue eyes and Ccurly hair) 21. 21So, there could be 4 types of gamete with differentcombinations of the genes BC brown eyes, curly hair bcblue eyes, straight hairBc brown eyes, straight hair bCblue eyes, curly hair 22. Variation 22Meiosis not only halves the number ofchromosomes but can also rearrangethe genes This is one cause of the variations that occur in members of the same species 23. 23Rearrangement of genes can also takeplace at fertilizationA sperm may carry a gene for brown eyes (B) or a gene for blue eyes (b)An ovum may carry a gene for brown eyes (B)or a gene for blue eyes (b)At fertilization, four possible combinationscan occur 24. 24sperm ovum4 PossiblecombinationsB BBBBbfertilizationbB bbbbAlthough there are 4 possible combinations of genesBB, Bb and bB have the same effect of producing brown eyesOnly bb gives rise to blue eyes 25. Question 1Which of the following are gametes ?(a) sperms(b) dividing cells(c) ova(d) nuclei 26. Question 2In flowering plants, which of the following aregametes ?(a) egg cell(b) ovule(c) pollen grain(d) pollen cell 27. Question 3Which of the following occur in both mitosisand meiosis ?(a) chromatids separate(b) homologous chromosomes separate(c) nuclear membrane disappears(d) four cells are formed 28. Question 4What is the correct sequence of events in meiosis ?(a)(b) (c) (d) (e)(f)(a) a, b, d, c, e, f (c) b, d, a, c, e, f(b) b, a, d, c, e, f (d) a, b, d, c, e, f 29. Question 5Which of the following represent variationwithin a species ?(a) black cats and tabby cats(b) collie dogs and dachshunds(c) goldfinch and greenfinch(d) shire horses and race horses 30. AnswerCorrect 31. AnswerIncorrect

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