1. Sexual Reproduction:
Meiosis

2. What is Sexual Reproduction?
Combines genetic material from two parents to produce a new individual
Creates an increase in variation among the offspring
Populations can adapt to changing environments
The world is more enjoyable with variation
Costly – it takes twice as many individuals to produce an offspring

3. Homologous Chromosomes
Genetic material (DNA) is found in chromosomes
Each chromosome has a “twin” that:
contains the same genes
codes for the same traits
has the same shape
is the same size
Has the same location of the centromere
Has the same banding pattern when stained
One chromosome came from the father, the other came from the mother
These twins are called homologous.

4. Homologous Chromosomes
Cells that have two of each chromosome are called Diploid (2n).
This is the normal condition for most cells.
Cells that have only one of each chromosome are called Haploid (n).
This is half of the usual amount of DNA.
Homologous Chromosomes

5. N =
1 – 2 9 3 6 7 10
631 13 23 24 39

6. Homologous Chromosomes
Humans have 23 pairs of chromosomes – pairs of autosomes and one pair of sex chromosomes (number 23)
Autosomes are chromosomes that do not code for gender
In females they are all homologous
(both #23 are X chromosomes)
In males, the autosomes are homologous, but the sex chromosomes are not (one X, oneY)

7. Homologous Chromosomes
Karyotype
Display that arranges chromosomes in homologous pairs

8. Special Considerations:
1. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain?
2. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

9. Meiosis
Purpose:
Divide the nucleus and reduce the number of chromosomes in half
Ensure that each new cell has a complete set of chromosomes
Create cells to become gametes
Also called reduction division

10. Human Life Cycle Most of our cells are diploid (2n)
Reproductive cells called gametes are haploid (n)
They are produced in gonads (ovary and testes) from diploid gametocytes
Gametes

11. Two Stages of Meiosis Meiosis I and Meiosis II
A cell normally has two sets of chromosomes (2n)
In meiosis I the homologous chromosomes are separated
In meiosis II the sister chromatids are separated
Four cells are produced, each with one set of chromosomes (n).

12. Meiosis I The phases of meiosis I are similar to mitosis:
Prophase I, Metaphase I, Anaphase I, Telophase I
The formation of tetrads is called synapsis
Dyads are sister chromatids attached by the centromere

13. Meiosis I – Prophase I Chromosomes condense
Nuclear membrane disappears
Spindles form
Homologous chromosomes pair up creating tetrads.
This formation of tetrads is called synapsis
Crossing over occurs

14. What is Crossing Over? During synapsis of prophase I:
Homologous chromosomes tangle and trade parts of their DNA
Variation increases
Genes that are linked (on the same chromosome) can become separated

15. Metaphase 1 Tetrads line up at the equatorial plate.
Meiosis 1
Metaphase 1
Tetrads line up at the equatorial plate.
Either homologue can face either pole

16. Meiosis 1
Anaphase 1
Homologous pairs separate Sister chromatids (now called dyads) remain together.

17. Meiosis 1
Telophase 1
Two haploid daughter cells are formed Each daughter contains only one chromosome of the homologous pair

18. Prophase II, Metaphase II, Anaphase II, Telophase II
Meiosis II
After a phase called interkinesis (similar to G2), both cells formed in meiosis I go through phases again
Prophase II, Metaphase II, Anaphase II, Telophase II
Interkinesis

19. Prophase II
Meiosis 2
chromosomes condense, nuclear envelope fragments, spindles form

20. Meiosis 2
Metaphase II
Chromosomes align at the equatorial plate.

21. Meiosis 2
Anaphase II
Sister chromatids separate

22. Meiosis 2
Telophase II
Cell division is complete Four haploid daughter cells are produced.
The DNA in each cell is different

23. Meiosis I in an Animal Cell
DNA Replication
Homologous chromosomes separate, pulled to opposite poles by centromeric spindle fibers.
Anaphase I
Homologous chromosomes
pair during synapsis.
Prophase I
Meiosis I
2n = 4
Daughter cells have
one chromosome from
each homologous pair.
Telophase I
Homologous pairs
align at the metaphase
plate.
Metaphase I
Chromosomes still
consist of
two chromatids.
Interkinesis
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24. Meiosis II in an animal Meosis II Prophase II Metaphase II Anaphase II
Cells have one
chromosome
from each
homologous pair.
n = 2
Metaphase II
Chromosomes align
at the metaphase
plate.
Anaphase II
Daughter chromosomes
move toward the poles.
Telophase II
Spindle disappears,
nuclei form, and
cytokinesis takes
place.
Daughter Cells
Meiosis results
in four haploid
daughter cells.
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

25. Nondisjunction
Disjunction - the separation of tetrads or sister chromatids
Nondisjunction- no separation occurs

27. Turner Syndrome XO

28. Klinefelter Syndrome XXY

29. Down Syndrome
(trisomy 21)

30. Meiosis vs. Mitosis 2n 2n n n 2n
Two cells genetically identical to parent cell
Four cells not identical to parent

31. Two daughter cells per cycle Four daughter cells per cycle
Mitosis
Meiosis
One division
Two divisions
Two daughter cells per cycle
Four daughter cells per cycle
Daughter cells genetically identical
Daughter cells genetically different
Same chromosome number as parents
Chromosome number half that of parents
Occurs in somatic cells
Occurs in gametocytes
Throughout life cycle
Completed after sexual maturity
Used in growth, repair, asexual reproduction
Sexual reproduction, new gene combinations