1. Cell Reproduction
Chapter 8

2. Cell Reproduction
• As a cell prepares to divide, the DNA inside
the nucleus becomes organized into
chromosomes
• This is to ensure that both of the new cells get
all of the genetic information from the original
cell

3. Chromosome Structure
• Chromosomes are rod shaped structures
made of DNA and proteins found in the
nucleus of cells

4. Chromosome Structure
• Chromosomes consist
of 2 identical halves
called chromatids
– When a cell divides,
each of the two new
cells will receive one
chromatid
• Two chromatids are
attached at the
centromere

5. Chromosome Structure
• Between cell divisions, DNA is not so tightly
coiled
• This form is called chromatin

6. Chromosome Numbers
• Each species has a
characteristic number
of chromosomes in
each cell

7. Chromosome Numbers
• Animal chromosomes are categorized as
either sex chromosomes or autosomes
– Sex chromosomes determine the sex of an
organism
• In humans, sex chromosomes are X or Y
(females = XX, males = XY)
– All of the other chromosomes are autosomes

8. Chromosome Numbers
• Every cell of an organism produced by sexual
reproduction has two copies of each
autosome (one from each parent)
• The two copies are called homologous
chromosomes
– Same size and shape and carry genes for the same
traits

9. Chromosome Numbers
• Karyotype – picture of
the chromosomes in a
normal dividing cell
found in a human

10. Diploid and Haploid Cells
• Diploid – cells that have • Haploid – cells that only
2 sets of chromosomes have 1 set of
(46 total in humans) chromosomes (23 total
in humans)
• Only sperm and egg
cells are haploid

11. Section 2
CELL DIVISION

12. Cell Division in Prokaryotes
• Binary fission is the division of a prokaryotic
cell into two offspring cells

13. Cell Division in Eukaryotes
• Two types of cell division
– Mitosis results in new cells that are identical to
the original cell
– Meiosis occurs during the formation of gametes
(haploid reproductive cells)

14. The Cell Cycle
• A repeating set of
events in the life of a
cell
• Division is one phase of
the cycle
• Time between cell
divisions is called
interphase

15. Interphase
1. G1 phase – offspring cells grow to mature size
2. S phase – the cell’s DNA is copied
3. G2 phase – cell prepares for cell division
• Cells can exit the cycle and enter into the G0
phase to stop dividing (ex: fully developed
brain cells)

16. Stages of Mitosis

17. Prophase
• DNA condenses into
chromosomes
• Nuclear membrane
breaks down
• Centrosomes form and
microtubules grow from
them (called the mitotic
spindle)

18. Metaphase
• Chromosomes line up in
the middle of the cell
• Spindle fibers attach to
the centromere of each
chromosome

19. Anaphase
• Chromatids are pulled
apart and move to
opposite ends of the
cell

20. Telophase
• Chromosomes reach
opposite ends of the
cell
• Spindle fibers
disassemble
• Chromosomes unravel
• Nuclear membranes
reform

21. Cytokinesis
• Animal cells – • Plant cells – cell plate
cytoplasm divides in forms between new
two cells

22. Control of Cell Division
• A cell spends most of its time in interphase
• What tells the cells to exit interphase and
begin dividing?
• There are three main checkpoints that act as
“traffic signals” for the cell to divide or not to
divide

23. Control of Cell Division
1. Cell growth (G1)
checkpoint – controls
whether the cell will
divide
2. DNA synthesis (G2)
checkpoint – make
sure DNA was copied
properly
3. Mitosis checkpoint –
signals tell the cell to
exit mitosis

24. When Control Is Lost: Cancer
• If a mutation occurs in one of the genes that
regulates the cell cycle, cell growth and
division could be disrupted
• This disruption could lead to cancer – the
uncontrolled growth of cells

25. Section 3
MEIOSIS

26. Meiosis
• A process of nuclear division that reduces the
number of chromosomes in new cells to half
the number in the original cell

27. Formation of Haploid Cells
• Meiosis produces gametes, which are haploid
reproductive cells
• Cells preparing to divide by meiosis undergo
the G1, S, and G2 phases of interphase
– Cells begin meiosis with a duplicate set of
chromosomes, just like mitosis
• Cells undergoing meiosis divide twice,
resulting in 4 haploid (1n) cells

28. Two Stages of Meiosis
• First cell division = Meiosis I
– Prophase I, Metaphase I, Anaphase I, Telophase I
and Cytokinesis I
• Second cell division = Meiosis II
– Prophase II, Metaphase II, Anaphase II, Telophase
II and Cytokinesis II

29. Meiosis I

30. Meiosis II

31. Prophase I
• DNA coils tightly into chromosomes
• Spindle fibers appear
• Nuclear membrane disassembles
• Synapsis occurs - homologous chromosomes
line up next to each other
– Each pair is called a tetrad

32. Prophase I
• Crossing-over occurs – portions of chromatids
may break off and attach to adjacent
chromatids
• Genetic recombination results – genetic
material between maternal and paternal
chromosomes is exchanged

33. Prophase I

34. Metaphase I
• Tetrads line up along
the middle of the cell
• Spindle fibers attach to
the centromere of each
homologous
chromosome

35. Anaphase I
• Each homologous
chromosome moves to
an opposite end of the
cell
• Random separation of
homologous
chromosomes is called
independent
assortment and results
in genetic variation

36. Telophase I and Cytokinesis I
• Chromosomes reach
opposite ends of the
cell and cytokinesis
begins
• Two new cells are
produced, each
containing one
chromosome from each
homologous pair
(haploid – 1n)

37. Prophase II
• Spindle fibers form and
begin to move the
chromosomes toward
the midline of the
dividing cell

38. Metaphase II
• Chromosomes move to
the midline of the
dividing cell

39. Anaphase II
• Chromatids separate
and move toward
opposite ends of the
cell

40. Telophase II and Cytokinesis II
• Telophase II – nuclear
membrane forms
around the
chromosomes in each
of the four new cells
• Cytokinesis II – 4 new
cells are formed, each
with half of the original
cell’s number of
chromosomes

41. Development of Gametes
• In animals, the only
cells that divide by
meiosis are those that
produce gametes within
the reproductive organs
– In humans – testes
(males) and ovaries
(females)

42. Development of Gametes
SPERMATOGENESIS
• In the testes – male
gametes known as sperm
cells or spermatozoa are
produced
• One diploid cell divides
meiotically to form four
haploid cells called
spermatids – each
develops into a mature
sperm cells

43. Development of Gametes
OOGENESIS
• Production of mature egg
cells, or ova
• A diploid reproductive cell
divides meiotically to
produce one mature egg
cell (ovum) and three
polar bodies which will
degenerate – cytoplasm is
not evenly distributed in
cytokinesis

44. Sexual Reproduction
• Production of offspring through meiosis and
the union of a sperm and an egg
• Offspring are genetically different because
genes are combined in new ways
• Advantage: enables species to adapt rapidly to
new conditions