4.2 & 10.1 meiosis

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  • Outline of meiosis. Section 11.08
  • Crossing over in prophase I resulting in genetic recombination. Homologous chromosomes separate during meiosis, leading to the independent assortment of their alleles in the gametes. The gametes are either parental types with alleles AB and ab, or recombinants with alleles Ab and aB. Section 11.08
  • 4.2 & 10.1 meiosis

    1. 1. 4.2 & 10.1 Meiosis Topic 4 & 10 Genetics
    2. 2. 4.2 Meiosis  4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.  4.2.2 Define homologous chromosomes.  4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.
    3. 3. 4.2 Meiosis  Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase I. Names of the stages are required.  4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).  The characteristics of Down syndrome are not required.  4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
    4. 4. 4.2 Meiosis  Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase I. Names of the stages are required.  4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).  The characteristics of Down syndrome are not required.  4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
    5. 5. 4.2 Meiosis  4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.  4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred.
    6. 6. 10.1 Meiosis  10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.  10.1.2 Outline the formation of chiasmata in the process of crossing over.  10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.  10.1.4 State Mendel’s law of independent assortment.  10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
    7. 7. Fertilisation Male germ cell in testis (diploid) Female germ cell in ovaries (diploid) 46 46 Sperm-Gamete (haploid) Egg-Gamete (haploid) 23 23 Fertilisation zygote 46 Embryo 46 46 Foetus Mitosis Mitosis MeiosisMeiosis (diploid)
    8. 8. Haploid and Diploid  The nucleus of normal human body cells consist of 46 chromosomes or 23 pairs of chromosomes (2 of each chromosome).  This is referred to as the diploid number for humans (2n).  Gametes, sex cells, only have one set of chromosomes (23).  This is referred to as the haploid number for humans (n).  In diploid cells, each pair of chromosomes have the same genes, arranged in the same sequence (loci), but they do not necessarily have the same alleles of all of the genes.  They are therefore not identical but instead are homologous, Homologous pairs.
    9. 9. Reductive Division  The number of chromosomes in a cell can be reduced from diploid to haploid by the process of Meiosis.  Meiosis is described as Reductive Division.  Organisms that reproduce sexually have to halve their chromosome number at some stage in their life cycle because the fusion of gametes during fertilisation double it again (restores the diploid number).
    10. 10. Meiosis  Meiosis is a type of cell division which:  Results in the production of gametes (sex cells)  Occurs in germ cells in the gonads - diploid  Four gametes are produced from every germ cell  Each gamete has half the number of chromosomes as the original parent cell – haploid  Meiosis involves TWO divisions:  Meiosis I  Meiosis II  In sexual reproducing species, haploid cells must be formed by meiosis before fertilisation to ensure the diploid number of chromosomes in offspring is obtained.
    11. 11. Meiosis I Ref: Advanced Biology, Kent
    12. 12. Meiosis II Ref: Advanced Biology, Kent
    13. 13. Meiosis Meiosis I  Homologous chromosomes pair up. They are called a bivalent.  Non-sister chromatids cross over at points called chiasmata.  They may exchange genetic material – crossing over.  Homologous pairs line up at equator.  Maternal and paternal chromosomes of each pair line up independently of other pairs – independent assortment.  Homologous chromosomes separate and move towards opposite poles.  Two new cells form, each with half the original chromosome number. Meiosis II  New spindle apparatus forms.  Chromosomes line up at the equator in a single line.  Centromeres divide and sister chromatids move towards opposite poles.  Each cells divides, resulting in a total of four haploid cells.  Each cell formed is genetically unique due to crossing over and independent assortment.
    14. 14. Outline of meiosis.
    15. 15. Meiosis – Gamete Production  Meiosis is the name given to a specialised for of cell division which produces the gametes.  In animals this process occurs in organs called the Gonads.  In mature human females, eggs are produced by a process called Oogenesis in the Ovaries.  In mature human males, millions of sperm are produced daily by a process called spermatogenesis in the testes.
    16. 16. Meiosis – Gamete Production Ref: Biology Key Ideas
    17. 17. Sources of Variation in meiosis
    18. 18. 1. Crossing-Over (Recombination)
    19. 19. First meiotic division  Homologous chromosomes come together to form a bivalent  The bivalent has four chromatids (2 from each chromosome)  These can intertwine and exchange segments  This is called crossing over
    20. 20. Methods of genetic recombination: 1. crossing over
    21. 21. Homologous chromosomes align.
    22. 22. Crossing over  The point at which two chromatids cross is called a chiasma  Crossing over changes the mixture of genetic information carried on a chromatid  It produces genetic recombination  This makes an important contribution to the genetic variability that sexual reproduction introduces to a species
    23. 23. 2. Independent Assortment
    24. 24. Independent Assortment  Meiosis gives rise to genetic variation.  Variety in gametes is produced by how the bivalents line up on the equator during Metaphase I. Ref: Biology Key Ideas
    25. 25. Independent assortment  Refers to the random separation and assortment of non-homologous chromosomes during meiosis  2 chromosomes produces 4 combinations – 2n .  Humans have 23 pairs of chromosomes – 223 .  8 388 608 combinations  If you double that, because of each gamete, the total possible combinations is over 64 trillion.  Independent assortment alone is the source of significant variation
    26. 26. 3. Random fusion of gametes  8 388 608 combinations of different sperm (or eggs) due to independent assortment  Fertilisation is a random fusion of gametes. (random as to which egg fuses with which sperm).  The total possible combinations is over 64 trillion.
    27. 27. Mendel’s Law of Segregation  When gametes are produced, each gamete must receive a full complement of genes.  For this reason, the factors/alleles must separate so that only one factor/allele is present in each gamete.  Mendel’s Law of Segregation states: “The characteristics of a diploid organism are determined by alleles which occur in pairs. Of a pair of such alleles, only one can be carried in a single gamete”  Thus each gamete receives one complete set of alleles, and hence chromosomes:  ie: 23 chromosomes.  The two alleles of a gene are located on homologous chromosomes which move to opposite poles, causing segregation.
    28. 28. Non-disjunctions  Non-disjunctions are a form of chromosome mutation.  They occur when homologous chromosomes fail to separate properly during meiosis.  An extra chromosome is drawn to on pole, producing gametes with an extra chromosome and gametes with one less chromosome.  This is referred to as Trisomy.  Down’s syndrome is an example of trisomy 21.  Down’s syndrome people have an extra chromosome 21.
    29. 29. A Non-disjunction Leading to Down’s Syndrome Ref: Advanced Biology, Kent
    30. 30. Down’s Syndrome Ref: Advanced Biology, Kent A Down’s syndrome boy. A Karyotype of a Down’s Syndrome boy
    31. 31. Turner’s Syndrome Ref: Advanced Biology, Kent
    32. 32. 4.2 Meiosis  4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.  4.2.2 Define homologous chromosomes.  4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.
    33. 33. 4.2 Meiosis  Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase I. Names of the stages are required.  4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).  The characteristics of Down syndrome are not required.  4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
    34. 34. 4.2 Meiosis  Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase I. Names of the stages are required.  4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).  The characteristics of Down syndrome are not required.  4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
    35. 35. 4.2 Meiosis  4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.  4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred.
    36. 36. 10.1 Meiosis  10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.  10.1.2 Outline the formation of chiasmata in the process of crossing over.  10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.  10.1.4 State Mendel’s law of independent assortment.  10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.

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