Meiosis involves two cell divisions that result in four haploid cells, each with half the number of chromosomes as the original cell. In meiosis I, homologous chromosomes pair up and separate, resulting in two cells each with one chromosome from each homologous pair. Sister chromatids remain attached. In meiosis II, sister chromatids separate, resulting in four haploid cells, each with an independent assortment of chromosomes. This process of meiosis and random fertilization leads to genetic variation between offspring.

1. Meiosis

2. Heredity
 Heredity - the transmission of traits from one
generation to the next
 Genetics – the scientific study of heredity and
hereditary variation
 Genes – hereditary units endowed from parents
 Segments of DNA
 Divided into Chromosomes
 46 in humans
 A gene’s specific location on a chromosome is called
its locus

3. Reproduction – 2 modes
 Asexual reproduction – a single individual
is the sole parent and passes copies of all
its genes to its off spring
 Sexual reproduction – two parents give
rise to offspring that have unique
combinations of genes inherited from
each parent

4. Asexual Reproduction
 1 parent
 Binary Fission in bacteria
 Single cell eukaryotes : mitotic cell division
 DNA is copied and divided equally between daughter
cells
 Multicellular organisms – Budding
 Hydra : Buds break off – are genetically identical to its
parent
 Each offspring in asexual reproduction is called
a clone

5. Figure 13.1 The asexual reproduction of a hydra

6. Sexual Reproduction
 2 parents
 Results in greater variation than asexual
reproduction
 Offspring vary genetically from siblings
and both parents
 Behavior of chromosomes during the sexual
lifecycle

7. Figure 13.2 Two families

8. Life Cycle
 Generation to generation sequence of
stages in the reproductive history of an
organism
 Interested in Sexual life cycles

9. Human Lifecycle
 Somatic cells (any cell but sperm or ovum cells)
have 46 chromosomes
 Can be visualized with a light microscope during
mitosis
 Are two of each type
 Arranged in pairs
 Karyotype –ordered display of an individuals
chromosomes
 Homologous chromosomes (homologues) –
chromosomes that make up a pair that have the
same length , centromere position and staining
pattern

10. Figure 13.3 Preparation of a human karyotype

11. Human Lifecycle
 Autosomes – somatic chromosomes
 If a gene for a trait is located at a particular
locus on a certain chromosome, then the
homologue of that chromosome will also have a
gene for the same trait at the same locus
 EXCEPTION: SEX CHROMOSOMES
 X and Y – only a small part are homologous
 Y is much shorter than the X
 X has few Y counterparts , Y is lacking many X genes
 XX (female) XY (male)

12. Karyotype
 The occurrence of homologous pairs of
chromosomes in our karyotype is a
consequence of our sexual origins
 A maternal set (23) and a parental set (23)

13. Figure 13.x3 Human female karyotype shown by bright field G-banding of
chromosomes

14. Figure 13.x5 Human male karyotype shown by bright field G-banding of
chromosomes

15. Sperm and Ova
 Have a chromosome count of 23
 22 autosomes – in a single set
 Plus a single sex chromosome (X or Y)
 HAPLOID (n)

16. Sperm and Ova – Sexual Intercourse
 A haploid sperm reaches and fuses with a
haploid ovum
 Fertilization of syngamy
 Results in a fertilized egg or zygote
 The zygote contains the two haploid sets of
chromosomes bearing genes representing the
maternal and paternal family lines
 Diploid (2n) - 2n = 46

17. Meiosis
 Differs from mitosis
 The process that halves the number of
chromosomes in a cell
 Occurs in Ovaries or Testes

18. A Variety of Sexual Lifecycles
 Human Life cycle
 Most fungi and some protists (including
some algae)
 Plants and some other species of algae

19. Figure 13.4 The human life cycle

20. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization
(syngamy)
Alternation
of
generations

21. Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
 Four daughter
chromosomes
 IMPORTANT: Homologous
chromosomes are different
than sister chromatids
 4 Haploid (n) cells instead
of 2 diploid cells (2n)

22. Meiosis I : Separates
Homologous Chromosomes
 Interphase
 Each of the chromosomes replicate
 The result is two genetically identical sister
chromatids which remain attached at their
centromeres
13-07a-InterphaseI.swf

23. Prophase I
 Lasts longer and is more complex than
prophase in mitosis
 Chromosomes begin to condense and
homologues, each consisting of two sister
chromatids, pair up
 During Synapsis: A protein structure
attaches the homologous chromosomes
tightly together (synaptonemal complex)

24. Prophase I
 Later in prophase, when the synaptonemal
complex disappears, each chromosome pair
becomes visible in the microscope as a tetrad
 A cluster of four chromatids
 At various places along their length, chromatids
of homologous chromosomes are crisscrossed
 Occur at chiasmata
 Hold the homologous pairs together until anaphase I

25. Prophase I
 Other cellular components prepare for division
of the nucleus in a manner similar to that of
mitosis
 Centrosomes move away from each other and
spindle microtubules form between them
 The nuclear envelope and nucleoli disperse
 The spindle microtubules capture the kinetochores
that form on the chromosomes
 The chromosomes begin moving to the metaphase
plate
 Can last for days or longer (over 90% of meiosis)
13-07b-ProphaseI.swf

26. Metaphase I
 The chromosomes are now arranged on
the metaphase plate
 Still in homologous pairs
 Kinetochore microtubles from one pole of
the cell are attached to one chromosome
of each pair while microtubules from the
opposite pole are attached to the
homologue
13-07c-MetaphaseI.swf

27. Anaphase I
 The spindle apparatus guides the movement of
the chromosomes toward the poles
 Sister chromatids remain attached
 Move as a unit towards the same pole
 The homologous chromosome moves toward
the opposite pole
 Contrasts mitosis – chromosomes appear as
individuals instead of pairs (meiosis)
13-07d-AnaphaseI.swf

28. Telophase I
 The members of each pair of homologous
chromosomes continue to move apart
until they reach the poles of the cell
 Each pole now has a haploid
chromosome set but each chromosome
still has two sister chromatids

29. Cytokinesis
 Occurs simultaneously with telophase I
 Forms 2 daughter cells
 Plant cells – cell plate
 Animal cells – cleavage furrows
 NO FURTHER REPLICATION OF
GENETIC MATERIAL PRIOR TO THE
SECOND DIVISION OF MEIOSIS
13-07e-TelophaseICytokin.swf

30. Figure 13.7 The stages of meiotic cell division: Meiosis I

31. Figure 13.7 The stages of meiotic cell division: Meiosis II

32. Meiosis II : Separates sister chromatids
 Proceeds similar to mitosis
 THERE IS NO INTERPHASE II !

33. Prophase II
 A spindle apparatus forms and the
chromosomes progress toward the
metaphase II plate

34. Metaphase II
 The chromosomes are positioned on the
metaphase plate in a mitosis-like fashion
 Kinetochores of sister chromatids of each
chromosome pointing toward opposite
poles

35. Anaphase II
 The centromers of sister chromatids finally
separate
 The sister chromatids of each pair move
toward opposite poles
 Now individual chromosomes

36. Telophase II and Cytokinesis
 Nuclei form at opposite poles of the cell
and cytokinesis occurs
 After completion of cytokinesis there are
four daughter cells
 All are haploid (n) 13-07f-MeiosisIICytokin.swf

37. Figure 13.7 The stages of meiotic cell division: Meiosis II

38. Figure 13.8 A comparison of mitosis and meiosis

39. Figure 13.8 A comparison of mitosis and meiosis: summary

40. Origins of Genetic Variation
 As mentioned earlier, in species that
reproduce sexually, the behavior of
chromosomes during meiosis and
fertilization is responsible for most of the
variation that arises in each generation
 Independent assortment of chromosomes
 Crossing Over
 Random Fertilization

41. Figure 13.9 Independent Assortment

42. Figure 13.10 Crossing Over

43. Random Fertilization
 A human ovum plus a human sperm
 1 of 8 million combinations possible for each
ovum and sperm
 223
X 223
= over 70 billion combinations
 70 trillion possible combinations with out
considering crossing over
 YOU REALLY ARE UNIQUE!

44. Gametogenesis
 See handout for details

45. THE END OF MEIOSIS