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Meiosis

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Meiosis is going to help many people to know cells divide

Meiosis is going to help many people to know cells divide


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  • 1. MEIOSIS • Cebile Xaba • Student no: 201202916 • Professional Studies • Topic Meiosis
  • 2. MEIOSIS
  • 3. MEIOSIS
  • 4. MEIOSIS: WHY? • Mitosis divides one diploid cell to form two diploid cells • For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes. • If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome • 46 chromosomes from each parent would yield a 92 chromosome offspring • Meiosis allows for two divisions to divide a one diploid cell into four haploid cells.
  • 5. MEIOSIS: IMPORTANT VOCABULARY • Diploid- contains 2 sets of chromosomes • (2n = 46) • Example: human somatic cells • Haploid- contains 1 set of chromosomes • (n = 23) • Example: human gametes
  • 6. MEIOSIS ORGANISMS HAVE TENS OF THOUSANDS OF GENES THAT DETERMINE INDIVIDUAL TRAITS. A CHROMOSOME CAN CONTAIN A THOUSAND OR MORE GENES ALONG ITS LENGTH. DIPLOID AND HAPLOID CELLS IN THE BODY CELLS OF ANIMAL AND MOST PLANTS, CHROMOSOMES OCCUR IN PAIRS. ONE CAME FROM MALE PARENT AND FEMALE PARENT. A CELL WITH TWO OF EACH KIND OF CHROMOSOME IS CALLED A DIPLOID CELL (2N), NUMBER OF CHROMOSOMES.
  • 7. • Organisms produce gametes that contain one of each kind of chromosome. A cell containing one of each kind of chromosome is called a haploid cell and is said to contain a haploid, or n, number of chromosomes. • Homologous chromosomes The two chromosomes of each pair in a diploid cell are called homologous chromosomes. On homologous chromosomes, these genes are arranged in the same order, but because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other.
  • 8. WHY MEIOSIS? • When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. • Cell division that allows offspring to have the same number of chromosomes as their parents. This kind of cell division, which produces gametes containing half the number of chromosomes as parent’s body cell, is called Meiosis. • Meiosis occurs in the specialized body cells of each parent that produce gametes.
  • 9. THE PHASES OF MEIOSIS • Interphase- The cell replicates its chromosomes. The chromosomes are replicated during interphase that precedes meiosis I, also. • Prophase I – behaves in a similar way to one entering prophase of mitosis. DNA coils, homologous chromosomes line up with each other, gene by gene along their length, to form a four part structure called a tetrad.
  • 10. • A tetrad consists of two homologous chromosomes, each made up of two sister chromatids. The chromatids in a tetrad pair tightly. In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over. Crossing over results in a new combinations of alleles on a chromosome.
  • 11. • Crossing over can occur almost anywhere at random on a chromosome. This means that an almost endless number of different possible chromosomes can be produced by crossing over, providing additional variation to the variation already produced by the random assortment of chromosome. • This reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes, is called a genetic recombination.
  • 12. • Metaphase I- the centromere of each chromosome becomes attached to spindle fiber. Homologous chromosomes are lined up side by side as tetrads. • Anaphase I- The homologous chromosomes move to opposite ends of the cell. • Telophase I- The reverse order from the events of prophase I. The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells. However another cell division is needed because each chromosome is still doubled.
  • 13. THE PHASES OF MEISOIS II • The second division in meiosis is simply a mitotic division of the products of meiosis I. • Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II. Meiosis follows the same steps as the phases of mitosis.
  • 14. MEIOSIS II : THE EQUATIONAL DIVISION Prophase II (diploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells
  • 15. CHROMOSOME NUMBER • Organisms produce gametes (sex cells) that contain one of each kind of chromosome. • A cell with only one of each kind of chromosome is called haploid (n). • Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid. • Each species has a specific number of chromosomes. • Humans have 23 pairs (46 total) • Fruit Flies have 4 pairs (8 total) • Dogs have 39 pairs (78 total)
  • 16. HUMAN CHROMOSOMES • Humans have 23 pairs of chromosomes (46 total) • 22 pairs of autosomes • 1 pair of sex chromosomes • Half of each pair came from one parent and half came from the other parent
  • 17. TYPES OF NONDISJUNCTION • Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs) • Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome • Most zygotes with monosomy do not survive • One exception is the case of Turner’s Syndrome • Females have only one X chromosome instead of two • These people will still have female sexual characteristics but they will generally be underdeveloped
  • 18. MEIOSIS • Makes 4 cells genetically different from parent cell & from each other • Production of gametes = sperm & egg • Contain half the number of chromosomes = haploid (1N) • Called the “reduction” division = • Diploid Haploid • 2N 1N • Used in sexual reproduction 46 23
  • 19. MEIOSIS IS A SPECIAL KIND OF CELL DIVISION THAT HALVES THE NUMBER OF CHROMOSOMES IN GAMETES. THIS INSURES THAT THE CORRECT NUMBER OF CHROMOSOMES ARE PASSED ON. Copyright Pearson Prentice Hall
  • 20. HOMOLOGOUS CHROMOSOMES • Pair of chromosomes - maternal & paternal - similar in shape & size. • Homologous pairs – tetrads - carry genes controlling the same inherited traits. • Each locus - position of a gene - is in the same position on tetrads.
  • 21. VARIABILITY • Meiosis has a large role in maintaining variability in a species. • Through sexual reproduction, offspring are not simply replicas of one organism but a genetic combination of two organisms • Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring.
  • 22. Nondisjunction Examples Normal Example CHROMOSOMAL MUTATIONS • Errors can also occur during Meiosis. • Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction • This results in gametes with either an extra copy of a chromosome or no copy at all.
  • 23. CHROMOSOMAL MUTATIONS • Chromosomal mutations can happen when chromosomes break and do not repair correctly. pg. 306
  • 24. PG. 143 MEIOSIS VOCAB Word Homologous chromosomes Tetrads Crossing over Haploid Diploid Locus Spermatogenesis Oogenesis Description Picture
  • 25. REFFERENCES • meiosis-notes-1228089496174255-8 (1).pptRemoved • http://s3.amazonaws.com/ppt-download/meiosis-notes-12280894961742558.ppt?response-contentdisposition=attachment&Signature=39Oin4P0ITQb9GQBiS6sUVnDZmE%3D&E xpires=1394098865&AWSAccessKeyId=AKIAIW74DRRRQSO4NIKA.Accessed:06 March 2014. • http://www.slideshare.net/1slid/13-meiosis-text. Accessed :06 March 2014 • Campbell &Reece . (2010). Meiosis: Reduction Divission. 8th edition. Pearson Education; Pearson Benjamin Cummings.

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