1. Cellular Reproduction
How to preserve genetic information.

2. When and why do cells divide?
Cells divide when there is a chemical signal to do so.
Skin cells may divide in response to crowding. Certain cells send out a chemical signal that tells neighboring cells to divide.
Cells may divide in response to an injury, to mend damaged tissue.
Growth factors can signal cell division in children to lengthen bones and add other tissues.

3. Specialized cell membrane proteins signal cell division when growth factors are present.
binds to receptor
and stimulates
cyclin synthesis.
growth factor
receptor
(plasma
membrane)
cyclin
Cyclin activates
Cdk; active Cdk
stimulates DNA
replication.
cyclin-
dependent
kinase
(cytoplasm)
Cdk’s are always
present in the cell.

4. Mitosis
One-celled eukaryotic organisms, and individual cells in a multi-cellular organisms, reproduce by mitosis followed by cytokinesis.

5. The problem
Eukaryotic cells are often diploid: that is, they have two of each kind of chromosome.

6. Why do eukaryotic cells have TWO of each chromosome? G E H
Why do eukaryotic cells have TWO of each chromosome?
If a cell with two of each chromosome divides, how many chromosomes should end up in each of the two daughter cells?

7. Each daughter cell will have a different, unpredictable number.
A cell with 14 chromosomes divides by mitosis. How many chromosomes will each daughter cell have? 28 14 7
Each daughter cell will have a different, unpredictable number.

8. When is DNA replicated? Just before cell division.
In preparation for transcription and translation.
Any time.

9. Overview of Mitosis
After DNA is replicated, it is condensed into chromosomes and identical copies are sorted in the process of mitosis.
Mitosis assures that the two daughter cells have exactly the same DNA.

10. Warning: Confusing terminology ahead!
After cell division, the single strand is a chromosome again. (Again, think of it as a one-chromatid chromosome.)
Before cell division, a strand of DNA is a chromosome. (Think of it as a one-chromatid chromosome.)
During cell division, two identical copies of a DNA strand link together into a two-chromatid chromosome.

11. Animated cell cycle at http://cellsalive.com
Normal cell processes, including protein synthesis (transcription and translation).
telophase and
cytokinesis
anaphase
G1: cell
growth and
differentiation
metaphase
prophase
mitotic cell
division
G0: nondividing
G2: cell
growth
Under certain
circumstances, cell
may return to
cell cycle.
interphase
The cell prepares for division, which includes making specialized proteins (more transcription and translation).
S: synthesis
of DNA;
chromosomes
are duplicated
DNA is replicated ONLY during S-phase. Once replication has happened, the cell MUST divide.
Animated cell cycle at

12. Chromosomes appear late in G2 phase, just prior to mitosis.
Prior to Mitosis, DNA is replicated during the S-phase of the cell cycle.
Chromosomes appear late in G2 phase, just prior to mitosis.
How many cells in this slide of an onion root tip are actually dividing? How can you tell?

13. INTERPHASE
nuclear
envelope
chromatin
nucleolus
centriole
pairs
Late Interphase
Can we tell if a cell in Interphase is in G1, S, or G2 of the cell cycle?

14. DNA (2 nm diameter)
histone proteins
nucleosome: DNA wrapped
around histone proteins
(10 nm diameter)
coiled nucleosomes
(30 nm diameter)
chromosome:
coils gathered onto
protein scaffold
(200 nm diameter)
protein
scaffold
DNA coils
A strand (double helix) of DNA wraps around histone proteins to form chromosomes. This protects DNA from damage during cell division.

15. The structure of a condensed chromosome (before pairing).
genes
centromere
telomeres
The structure of a condensed chromosome (before pairing).

16. Make a prediction: during chromosome formation, which will pair up?
Homologous chromatids that carry the same genes but different alleles?
Sister chromatids that carry the same genes and the same exact alleles?
(Hint: the end goal is to get identical genetic information into the daughter cells.)

17. gene 1
gene 2
same alleles
different alleles
Homologous chromosomes are those that carry the same genes but may have slightly different information (such as dominant or recessive versions of a gene). Homologous chromosomes do not pair together. Chromosomes only pair with their identical sister chromatids.

18. sister chromatids
centromere
Identical (sister) chromatids pair up during Prophase, and join at a pinched-in point called the centromere.

19. duplicated
chromosome
(2 DNA double
helices)
sister
chromatids
The chromosome at the end of Prophase consists of two strands of condensed DNA. Each sister chromatid carries exactly the same information.

20. Which of these are genetically identical?
Homologous chromosomes
Sister chromatids
Neither of these

21. Which of these will pair up to form chromosomes in mitosis?
Homologous chromosomes
Sister chromatids
Chromatids pair at random, so it’s impossible to predict.

22. MITOSIS: Prophase
condensing
chromosomes
beginning of
spindle formation
Notice that these cells in prophase have barely visible chromosomes as DNA begins to condense.

23. MITOSIS: Metaphase
spindle
microtubules
Chromosomes, with their paired identical chromatids, move to the center of the cell.

24. MITOSIS: Anaphase
"free" spindle
fibers
Identical chromatids separate from one another and migrate to opposite poles of the cell.

25. MITOSIS: Telophase
chromosomes
extending
nuclear envelope
re-forming
Telophase completes Mitosis. Both poles of the cell now have identical DNA, and the cell can divide in half.

26. MITOSIS: Cytokinesis
After Mitosis has finished sorting the chromosomes, cytokinesis takes place, dividing the cell into two new cells.

27. INTERPHASE
Before S phase, the cell was diploid (two copies of each chromosome). After cytokinesis, are the cells diploid or haploid?

28. The process of cytokinesis
1 Microfilaments form
a ring around the
cell's equator.
2 The microfilament
ring contracts,
pinching in the
cell's “waist.”
3 The waist
completely pinches
off, forming two
daughter cells.

29. Cytokinesis in plant cells
Golgi apparatus
cell wall
plasma
membrane
carbohydrate-
filled vesicles
1 Carbohydrate-filled
vesicles bud off the
Golgi apparatus and
move to the equator
of the cell.
2 Vesicles fuse to
form a new cell
wall (red) and
plasma membrane
(yellow) between
daughter cells.
3 Complete
separation of
daughter cells.

30. In which phase do chromosomes condense?
Prophase
Metaphase
Anaphase
Telophase

31. In which phase do sister chromatids separate from one another?
Prophase
Metaphase
Anaphase
Telophase

32. In which phase do chromosomes line up in the middle of the cell?
Prophase
Metaphase
Anaphase
Telophase

33. Meiosis
Meiosis is cell division that involves the reduction of chromosomes in a cell.

34. The problem:
When diploid organisms reproduce sexually, two cells must fuse and share genetic information.
The end result of sexual reproduction is a new diploid organism that has genetic information from both parents.

35. 2n n
meiotic
cell division 2n 2n n
fertilization
diploid
parental
cells
haploid
gametes
diploid
fertilized
egg
The cells from the parents must be haploid if their offspring is to be diploid.
While diploid cells hold two copies of each chromosome (one from each parent), haploid sex cells hold one copy of each chromosome.

36. Why does it produce four haploid cells?
homologous
chromosomes
sister
chromatids
Meiosis is reduction division. It begins with a diploid cell and produces haploid cells.
Why does it produce four haploid cells?

37. Meiosis also involves the cell cycle, and takes place after S phase of the cell cycle. DNA is replicated before meiosis.
G1: cell
growth and
differentiation
mitotic cell
division
G0: nondividing
G2: cell
growth
Under certain
circumstances, cell
may return to
cell cycle.
interphase
S: synthesis
of DNA;
chromosomes
are duplicated

38. First half of meiosis: separation of homologous chromosomes.
MEIOSIS I
Homologous chromosomes
pair and cross over.
Homologous chromosomes
line up in pairs.
Homologous chromosomes
move to opposite poles.
paired homologous
chromosomes
chiasma
spindle
microtubule
recombined
chromosomes
(a) Prophase I
(b) Metaphase I
(c) Anaphase I
(d) Telophase I
First half of meiosis: separation of homologous chromosomes.

39. Prophase I
Homologous chromosomes
pair and cross over.
paired homologous
chromosomes
spindle
microtubule
chiasma
Notice that four strands — maternal and paternal chromosomes and their identical sister chromatids — join into a single unit, called a tetrad.

40. protein strands
joining duplicated
chromosomes
direction of
“zipper”
formation
Protein strands “zip” the homologous chromosomes
together.

41. While in tetrads, homologous chromosomes often swap ends, further mixing up genetic information.
recombination
enzymes
chiasma
chiasma
Recombination
enzymes bind
to the joined
chromosomes.
Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid
(purple).
Recombination
enzymes and protein
zippers leave.
chiasmata remain,
helping to hold
homologous
chromosomes
together.

42. Tetrads line up in the center of the cell.
Metaphase I
Homologous chromosomes
line up in pairs.
Tetrads line up in the center of the cell.
recombined
chromosomes

43. Anaphase I
Homologous chromosomes
move to opposite poles.
Because homologous chromosomes separate (instead of identical sister chromatids), each pole of the cell gets a full set of chromosomes but different genetic information.

44. MEIOSIS II
(e) Prophase II
(f) Metaphase II
(g) Anaphase II
(h) Telophase II
(i) Four
haploid
cells
Meiosis II begins immediately after Meiosis I, with a short rest in between (no interphase in between). In Meiosis II, sister chromatids separate from one another.

45. Anaphase II
Metaphase II
In both cells, chromosomes line up in Metaphase II so that sister chromatids can separate in Anaphase II.

46. Telophase II
End
The result of meiosis is four haploid cells. Each has one copy of each chromosome, which may carry different versions of the same genes. Each gamete (sex cell) can have different genetic information.

47. In prophase I of meiosis:
Sister chromatids pair up.
Homologous chromatids pair up.
Homologous chromosomes pair up.
Nothing pairs up.

48. In anaphase I of meiosis:
Tetrads split, homologous chromosomes migrate to opposite poles.
All four chromatids migrate to four quarters of the cell.
Chromatids are randomly distributed.

49. The end product of meiosis is:
Two identical cells.
Two non-identical cells.
Four identical cells.
Four non-identical cells.

50. Each cell produced by meiosis is:
Diploid
Haploid
Tetraploid
A random ploidy

52. Recap
Mitosis divides one diploid cell and produces two diploid daughter cells. It is cell division used for growth and cell replacement.
Meiosis divides one diploid cell into four haploid cells. It is used in reproduction.