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Unit 3 meiosis
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Unit 3 meiosis

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  • 1. BY L.N MASIA LIFE SCIENCES
  • 2.  In humans, somatic cells (body cells) have:  23 pairs of homologous chromosomes and  one member of each pair from each parent.  The human sex chromosomes (Gonosomes) X and Y differ in size and genetic composition.  The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. .
  • 3.  Homologous chromosomes are matched in:  length, Centromere  centromere position, and  gene locations (locus).  A locus (plural, loci) is the position of a gene.  Different versions (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.
  • 4.  Homologous chromosome pair Centromere
  • 5.  Humans and most animals and plants have diploid body cells.  That means they have two sets of chromosomes (homologous chromosome pair) one from each parent.  Diploid is written 2n.  It refers to the total number of chromosomes a cell can have.
  • 6.  Meiosis is a process that converts diploid nuclei to haploid nuclei.  Diploid cells have 2 sets of chromosomes.  Haploid cells have 1 set of chromosomes.  Meiosis occurs in the sex organs, producing gametes—sperm and eggs.  Fertilization is the fusion of a sperm and egg cell.  The zygote has a diploid chromosome
  • 7. Haploid gametes (n  23) A life cycle n Egg cell n Sperm cell Meiosis Ovary Fertilization Testis Diploid zygote (2n  46) 2n Key Multicellular diploid adults (2n  46) Mitosis Haploid stage (n) Diploid stage (2n)
  • 8.  All sexual life cycles include an alternation between  a diploid stage and  a haploid stage.  Why is meiosis so important? It produces haploid gametes which prevents the chromosome number from doubling in every generation. Produce gametes for fertilization.
  • 9. 3. MEIOSIS  Meiosis is a type of cell division that produces haploid gametes from diploid cells.  Two haploid gametes combine in fertilization to restore the diploid state in the zygote. 3
  • 10. SUMMERY OF THE MEIOSIS PROCESS 3
  • 11.  MEIOSIS I consisting of 5 phases:  Interphase I, Prophase I, Metaphase I, Anaphase I, Telophase I.  MEIOSIS II consisting of 4 phases  Prophase II, Metaphase II, Anaphase II, Telophase II.
  • 12.  Cell build up energy  DNA Replication (to make duplicated chromosomes  Cell doesn’t change structurally.
  • 13.  Events occurring in the nucleus:  Chromosomes coil and become individual chromo-somes, nucleolus and nuclear envelope disappear.  Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids)  Non-sister chromatids exchange genetic materi through the process of crossing over to ensure genetic variation.  Centrioli move to opposite poles with spindle fibers between them.
  • 14.  Genetic recombination is the production of new combinations of genes due to crossing over.  Crossing over is an exchange of genesbetween separate (non-sister) chromatids on homologous chromosomes.  Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment.  Genetic material are exchanged between maternal and paternal (nonsister)
  • 15. © 2012 Pearson Education, Inc.
  • 16.  Centrioli has reached the poles.  Homologous pairs align at the cell equator.  The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere. .
  • 17.  Spindle fibers contract.  Duplicated chromosomes move to opposite poles. .
  • 18.  Duplicated chromosomes have reached the poles.  A nuclear envelope and nucleolus re-forms around chromosomes.  Each nucleus now has the haploid number of chromosomes.  Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.
  • 19.  Follows meiosis I without chromosome duplication.  Each of the two haploid products enters meiosis II.
  • 20.  Chromosomes coil and become compact (if uncoiled after telophase I).  Nuclear envelope and nucleolus, if re-formed, dissappears again.  Centrioli move to opposite poles, forming spindle fibers between them.
  • 21.  Individual duplicated chromosomes align on the equator.  One chromosome per spindle fiber attached by means of kinetochore of centromere.  Centrioli has reached the poles.
  • 22.  Spindle fibers contract.  Duplicated chromosomes split in half (centromere dividing in 2)  Daughter chromosomes move to opposite poles.
  • 23.  Daughter chromosomes has     reached the poles. Two cells invaginate and form 4 daughter haploid cells (gametes) They uncoil and form chromatin. Nuclear envelope and nucleolus for around chromatin again. Centrioli for centrosome.
  • 24. Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Haploid daughter cells forming
  • 25.  Mitosis and meiosis both  begin with diploid parent cells that  have chromosomes duplicated during the previous interphase.  However the end products differ.  Mitosis produces two genetically identical diploid somatic daughter cells.  Meiosis produces four genetically unique haploid gametes.
  • 26.  Independent orientation at metaphase I  Random fertilization.  Crossing over of genes during prophase I © 2012 Pearson Education, Inc.
  • 27. 6. KARYOTYPE • A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. • Karyotypes allow for the observation of :  homologous chromosome pairs,  chromosome number, and  chromosome structure. © 2012 Pearson Education, Inc.
  • 28. Centromere Sister chromatids Pair of homologous chromosomes Sex chromosomes
  • 29.  An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.  A. Trisomy 21  involves the inheritance of three copies of chromosome 21 and  is the most common human chromosome abnormality.
  • 30. Down syndrome
  • 31.  Trisomy 21 produces a characteristic set of symptoms, which include:  mental retardation,  characteristic facial features,  short stature,  heart defects,  susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and  shortened life span.  The incidence increases with the age of the mother.
  • 32.  Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during:  meiosis I, if both members of a homologous pair go to one pole or  meiosis II if both sister chromatids go to one pole.  Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.
  • 33. MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Number of chromosomes n1 n1 n1 Abnormal gametes n1
  • 34. MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n1 n1 Abnormal gametes n n Normal gametes
  • 35.  Sex chromosome abnormalities tend to be less severe, perhaps because of  the small size of the Y chromosome or  X-chromosome inactivation.
  • 36.  In general,  a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and  the absence of a Y chromosome yields “femaleness.”
  • 37.  Chromosome breakage can lead to rearrangements that can produce:  genetic disorders or,  if changes occur in somatic cells, cancer.
  • 38.  a deletion, the loss of a chromosome segment,  a duplication, the repeat of a chromosome segment,  an inversion, the reversal of a chromosome segment, or  a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal.