1. Meiosis

2. You have body cells and gametes
DNA in these
cells is passed on
to offspring
DNA in these
cells is NOT
passed on to
offspring

3. You have body cells and gametes
DNA in these
cells is passed on
to offspring
DNA in these
cells is NOT
passed on to
offspring

4. Ploidy: Number of sets of
chromosomes in a cell
 Haploid (n)-- one set chromosomes
 Diploid (2n)-- two sets
chromosomes
 Most plant and animal adults are
diploid (2n)
 Eggs and sperm are haploid (n)

5. Your cells have autosomes and
sex chromosomes

6. Summary of chromosome
characteristics
 Diploid set for humans; 2n = 46
 Autosomes - homologous
chromosomes, one from each
parent (humans = 22 sets of 2)
 Sex chromosomes (humans have 1
set of 2)
 Female-sex chromosomes are
homologous (XX)
 Male-sex chromosomes are non-
homologous (XY)

7. Mitosis
 The nucleus divides to form 2 nuclei
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis – the cytoplasm of the
cell is divided in half and the cell
membrane grows to enclose each
cell

8. Asexual (vegetative)
reproduction
 A form of duplication using only mitosis.
 Example,
 a new plant grows out of the root or
 a shoot from an existing plant.
 Produces only genetically identical offspring
since all divisions are by mitosis.
 Offspring called clones meaning that each is an
exact copy of the original organism
 Method of reproduction is rapid and effective
 Since the offspring are identical, there is no
mechanism for introducing diversity

9. Meiosis
2 divisions of the nucleus
 Meiosis I
 Meiosis II
A form of cell division that halves the
number of chromosomes when
forming specialized reproductive
cells, such as gametes or spores

10. Formation of Diploid & Haploid Cells
Mitosis Meiosis
2n 2n
2n 2n n n
n n n n
2 diploid cells produced
4 haploid cells produced

11. Sexual Reproduction
 Formation of new individual by a combination of two
haploid sex cells (gametes).
 Gametes for fertilization usually come from separate
parents
 Female- produces an egg
 Male - produces sperm
 The new individual is called a zygote, with two sets
of chromosomes (diploid).
 Meiosis is a process to convert a diploid cell to a
haploid gamete, and cause a change in the genetic
information to increase diversity in the offspring.

12. What is Meiosis I?
 Chromosomes in a diploid cell re-
segregate, producing four haploid
daughter cells.
It is this step in meiosis that
generates genetic diversity!

13. Prophase I
 homologous chromosomes
pair, a step unique to meiosis
 Chromosomal condensation
allows these to be viewed in
the microscope.
 Note that there are 2
chromosomes and 4
chromatids, with one
chromosome coming from
each parent.

14. Metaphase I
 Paired chromosomes (four
chromatids) align at the
equator
 The orientation is random,
with either parental
homologue on a side.
 This means that there is a
50-50 chance for the
daughter cells to get either
the mother's or father's
homologue for each
chromosome.

15. Anaphase I
 Paired chromosomes
separate.
 Chromosomes, each
with two chromatids,
move to separate
poles.
 Each of the daughter
cells is now haploid (23
chromosomes)

16. Telophase I
 Nuclear envelopes may
reform, or the cell may
quickly start meiosis II.

17. Cytokinesis
 Analogous to mitosis
where two complete
daughter cells form.

18. Meiosis I

19. Meiosis II
 chromatids of each chromosome are
no longer identical because of
recombination.
 chromatids separate from one
another

20. Meiosis II

21. Comparing Meiosis & Mitosis
 Chromosome behavior
 Mitosis: Homologous chromosomes independent
 Meiosis: Homologous chromosomes pair forming
bivalents until anaphase I
 Chromosome number- reduction in meiosis
 Mitosis- identical daughter cells
 Meiosis- daughter cells haploid
 Genetic identity of progeny:
 Mitosis: identical daughter cells
 Meiosis: daughter cells have new assortment of
parental chromosomes
 Meiosis: chromatids not identical, crossing over

22. Meiotic Errors
 Nondisjunction- homologues don't separate
in meiosis 1
 Usually embryo lethal
 Trisomy 21, exception leading to Downs syndrome
 Sex chromosomes
 Turner syndrome: monosomy X
 Klinefelter syndrome: XXY
 Translocation and deletion: transfer of a
piece of one chromosome to another or loss
of fragment of a chromosome.