1. Mitosis & Meiosis

2. Phases of the Cell Cycle
consists of two phases
Mitotic (M) phase = mitosis and cytokinesis)
Interphase = cell growth and copying of DNA in preparation for cell division
Interphase - about 90% of the cell cycle
can be divided into sub-phases
G1 phase -“first gap” – preparation for S phase
S phase “synthesis” – duplication of DNA
G2 phase - “second gap”- preparation for mitosis

3. Some terms to know -genome = all the DNA within the nucleus
-parent cell = cell about to undergo division (either mitosis or meiosis)
-daughter cell = cell that results from either mitosis or meiosis
-somatic cell = any cell within the body other than an egg or sperm
-a somatic cell has two complete sets of chromosomes
-one set is called the haploid number of chromosomes (n)
-therefore the somatic cell is said to be diploid (2n)
e.g. humans n = 23 (2n = 46)
-germ cell or gamete = sex cell
-gamete has only one set of chromosomes and is haploid

4. Cellular Organization of the Genetic Material
before the cell can divide – it must duplicate its DNA
REMINDER: eukaryotic chromosomes consist of chromatin, a complex of DNA and histone proteins
following replication, the DNA condenses into chromosomes
starts just prior to Mitosis
20 m
every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
e.g. humans – n=23
e.g. drosophila – n=2
e.g. dog – n=39

5. chromosome = organized structure of DNA and protein
chroma = color
soma = body
DNA is duplicated during the S phase of the cell cycle
then starts to condense just before mitosis
each condensed & duplicated chromosome is made of two sister chromatids
these chromatids will separate during cell division and be partitioned into daughter cells
chromatids are joined by a structure called a centromere
highly condensed region of the duplicated chromosome
role in attaching the duplicated chromosome to the spindle
0.5 m
Centromere
Sister chromatids
Figure 12.4 A highly condensed, duplicated human chromosome (SEM).

6. Mitosis is conventionally divided into five phases
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis begins during the latter stages of mitosis
For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.

7. Figure 12.7 Exploring: Mitosis in an Animal Cell

8. Mitosis
Prophase:
1. the replicated DNA begins to condense into sister chromatids joined at the centromere  (duplicated) chromosome
2. the centrioles (replicated at G2) move apart from each other
3. the spindle forms between the centrioles (made of microtubules)
4. the condensing, duplicated chromosomes attach to the spindle (via a kinetochore)
5. the nuclear envelope disintegrates
6. the nucleolus disappears
Spindle – structure that includes the
two centrioles, two asters and the spindle
microtubules than span the cell
Kinetochore – region of the centromere where
the chromosome attaches to the spindle
Prophase
Fragments of nuclear envelope
Nonkinetochore microtubules
Kinetochore
Kinetochore microtubule
Centromere
Kinetochore
Microtubules
Chromatid

9. Metaphase: centrioles are now at opposite ends of the cell and the spindle
is complete
1. the chromosomes move and line up along the equator of the cell = metaphase plate

10. Anaphase: shortest of the mitotic phases
1. the sister chromatid separate into daughter chromosomes
-one chromatid/chromosome moves toward one centriole of the cell,
the other the opposite
-pulled apart by the action of the spindle
** At the end of this phase – each end of the cell has equivalent numbers
of chromosomes – same number as the parent cell

11. Telophase: reverse of Prophase & takes place along with the event Cytokinesis
1. nuclear envelope reforms & two daughter nuclei result
2. the nucleolus reappear
3. the spindle disappears
4. daughter chromosomes uncoil  chromatin

12. Cytokinesis: division of the cytoplasm into the 2 daughter cells (cytokinesis = cell motion)
-mitosis can occur with cytokinesis  a single cell with 2 nuclei
-so actual cell division is cytokinesis!!!
-mitosis is actually the division of the nucleus = karyokinesis
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 m
-formation of a cleavage furrow that expands to split the cell into daughters

13. 10 m Cell plate 5 Telophase
Plant cell Cytokinesis: No cleavage furrow possible
-vesicles containing parts of the new cell wall form and migrate to the middle of the cell
-vesicles fuse together to produce a cell plate
-cell plate grows and eventually splits the parent into two daughter cells
(b) Cell plate formation in a plant cell (TEM)
Vesicles forming cell plate
Wall of parent cell
Cell plate
New cell wall
Daughter cells
1 m
10 m
Telophase 5
Cell plate
Figure Cytokinesis in animal and plant cells.

16. Meiosis
results in four haploid daughter cells with half the number of un-replicated chromosomes as the parental cell

17. The Stages of Meiosis
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Homologous chromosomes separate
Haploid cells with duplicated chromosomes
Sister chromatids separate
Haploid cells with unduplicated chromosomes
Interphase
Meiosis I
Meiosis II 2 1
after chromosomes duplicate in interphase, two divisions follow
Meiosis I: results in two haploid daughter cells containing half the number of duplicated chromosomes as the parent
parent – 46 duplicated chromosomes
daughter cell – 23 duplicated chromosomes
Meiosis II: results in four haploid daughter cells containing single chromatid/chromosomes
new “parent cell” – 23 duplicated chromosomes
daughter cell – 23 chromatid/chromosomes

18. Telophase I and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
Centrosome (with centriole pair)
Sister chromatids
Chiasmata
Spindle
Homologous chromosomes
Fragments of nuclear envelope
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Centromere (with kinetochore)
Metaphase plate
Microtubule attached to kinetochore
Chromosomes line up by homologous pairs.
Sister chromatids remain attached
Homologous chromosomes separate
Each pair of homologous chromosomes separates.
Cleavage furrow
Two haploid cells form; each chromosome still consists of two sister chromatids.
Figure 13.8 Exploring: Meiosis in an Animal Cell
Division in meiosis I occurs in four phases:
Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis

19. many similarities with mitotic prophase
Prophase I
many similarities with mitotic prophase
chromosomes begin to condense within the nucleus
the centrioles migrate and the spindle begins to forms
chromosomes attach to the spindle
BUT a unique event happens – synapsis = pairing of 2 homologous chromosomes
the paired chromosomes are called a tetrad
this pairing is necessary for a special event called crossing over
crossing over results in the exchange of genetic information between chromosomes

20. What are homologous chromosomes?
a set of one maternal chromosome and one paternal chromosome
e.g. maternal chromosome 1 + paternal chromosome 1
homologous, duplicated chromosomes pair up with each other inside a cell during meiosis

21. Metaphase I Metaphase Metaphase I
tetrads line up at the metaphase plate - with one chromosome facing each pole & the other chromosome facing the other pole
crossing over takes place during this phase
alignment is different from mitosis
Metaphase I
Metaphase

22. Crossing Over
introduces genetic variability by exchanging genetic information between homologous chromosomes
Anaphase II
Gametes
Metaphase II
Anaphase I
Metaphase I

23. Anaphase I the duplicated homologous chromosomes separate
each duplicated chromosome moves toward each pole - guided by the spindle apparatus

24. Telophase II and Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Sister chromatids separate
Haploid daughter cells forming
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Division in meiosis II also occurs in four phases
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Meiosis II is identical to mitosis
Figure 13.8 Exploring: Meiosis in an Animal Cell