CELL_DIVISION PHYSIOLOGY.................

Cell Division

1. Cell Division

= The process by which cells produce
offspring cells.
Omara Albert Martin, BDS(EQUSaT), Phdo
(IPI-MAYA), DE Asbl(Aesthetic Dentistry)
[phone]

2. Prokaryotic Cells

Binary fission = Division of prokaryotic cells into
two offspring cells.

Steps: 1. One chromosome, a circular piece of
DNA along with its proteins that is attached to
the cell membrane, makes a copy of itself
(replication).

2. The cell continues to grow until it is about
twice its original size.

3. The cell wall forms between the two

chromosomes and the cell splits into two new
cells.

3. @ cell replicates its DNA Cell wall

Cytoplasmic
membrane

Replicated
DNA

Nucleoid

© The cytoplasmic
membrane elongates,
separating DNA
molecules

© Cross wall forms;
membrane
invaginates

© cross wall forms
completely

© Daughter cells

Copyright © 2006 Pearson Education, Inc, publishing as Benjamín Cummings.

4. Eukaryotic Cells
« 1. Somatic cells of the body divide by the
process called the Cell Cycle. It consists of
Interphase, Mitosis and Cytokinesis.

Ge (277
HASE À

5. Interphase — Cell spends most of

its lifetime in interphase

G1 phase — the cell grows to mature size. The
cell has divided in the previous step and is
appx % of its mature size

S phase — A copy of DNA in the cell is made —
this is for the new cell

G2 phase - cell prepares for cell division by
making more organelles - it is going to need
enough for two new cells

Go — some cells go from G1 to GO. These cells
do not divide so they do not need a copy of
DNA ex. nerve cells

6. M phase or Mitosis = Nuclear division

process that divides the nucleus in half

* Results in 2 new cells with genetic material
that is identical to that of the original cell.

* The 2 offspring cells are about equal in size.

* Each receives a copy of the cells
chromosomes.

+ Each receives half of the cells cytoplasm and
organelles.

7. 4 phases:

« 1. Prophase - DNA shortens and coils into rod-
shaped chromosomes.

¢ Nucleolus and nuclear membrane breakdown
and disappear.

>
$

° Micotubules start to extend
from centriole. These mierotubles) > e

are now called Spindle Fibers I
$°

Prophase

8. 2. Metaphase

+ Spindle fibers move chromosome to center of
the dividing cell.

9. 3. Anaphase

* The chromatids of each chromosome separate
at the centromere and slowly move,
centromere first, toward the opposite poles of
the cell. (Now the chromatids are considered
individual chromosomes. )

ie me > N
<= =
Ÿ = HH
= ie

10. 4. Telophase

« After chromosomes reach opposite poles,
they uncoil.

+ The spindle fibers disassemble.

* The nuclear envelope forms around each set

of chromosomes and a nucleolus forms within
each.

Telophase

11. Cytokinesis = The cytoplasm divides.

* In animals, a cleavage furrow forms in the
middle of the two poles, pinching the cell into
2 new cells by way of microfilaments.

* In plants, a cell plate is formed by which

vesicles from the Golgi Apparatus fuse at the
midline of the cell.

12. 2. Reproductive cells divide by the
process of Meiosis.
Meiosis = Cell division that reduces the
number of chromosomes by half in new cells.
Starts after G,, S and G,.
Includes two divisions.
Only occurs in reproductive organs of animals.

Results in 4 new cells, each with half of the
original number of chromosomes.

13. Meiosis | : steps

1. Prophase 1 — DNA shortens and coils into
rod-shaped chromosomes.

Nucleolus and nuclear membrane disappear.
Spindle fibers form.

* Homologous chromosomes pair up ina
process called synapsis.

14. Synapsis = The pairing of homologous
chromosomes during meiosis.

Tetrad = A pair of homologous chromosomes lined up
so that the genes on one chromosome are near the
corresponding genes on the other homologue.
Crossing over = The exchange of pieces of
chromosomes that occurs during synapsis.

Crossing over allows genetic material to be exchanged
between maternal and paternal chromosomes and
results in genetic recombination.

Genetic recombination = Process of mixing up genetic
information which produces new combinations of it.

15. Synapsis: Pairing of
homologous chromosomes
-—

Paternal Maternal

+

Crossing over” }

16. 2. Metaphase |

* Tetrads randomly line up along the midline.
(ie. Some maternal chromosomes go to one
pole and some paternal chromosomes go to
the same pole.)

* Independent assortment = Random alignment
and separation of homologous pairs of
chromosomes.

17. Example of Independent Assortment

18. 3. Anaphase |

* Each homologous chromosome, made of 2
sister chromatids attached by a centromere,
move to opposite poles of the cell.

dyad —_£ I
Anaphase | > ,
& >,

Dee;

19. 4. Telophase |

+ Chromosomes reach the ends of the cell and
cytokinesis begins.

+ Meiosis | results in two haploid cells.

Telophase |

20. Meiosis Il —

* This starts with the products of Meiosis |; no
copying of DNA occurs.

* Sometimes it begins after nuclear membrane
forms and sometimes it begins without the
nuclear membrane reforming.

21. Meiosis Il : steps

« 1. Prophase Il - Spindle fibers form and move
chromosomes toward midline.

22. 2. Metaphase Il

+ Chromosomes move to midline.

23. 3. Anaphase II

+ Chromatids separate and move toward
opposite poles.

24. 4. Telophase Il

« Nuclear membrane reforms around
chromosomes of 4 new cells.

25. Cytokinesis Il

* Occurs next and 4 new cells are formed, each
with half of the original number of
chromosomes.

26. n of meiosis

Tat cell divisio

n of meiosis

——{2nd cell divisior

Meiosis

patera! @ suo Meiosis
Es Se =
ar

ges

Prophase 1

>| Metaphase 1

Anaphase 1

Telophase 1
.
Prophase2
2... 0/9)

27. 2 Forms of Meiosis in humans

« 1. Spermatogenesis = Production of sperm cells that
occurs in human male testes.

+ Spermatids mature into Spermatozo = mature
sperm cells.

An Ñ
so | | | |

CLR

28. 2. Oogenesis

+ = Production of mature egg cells that occurs in
human female ovaries.

+ Only one mature egg cell = ova is formed along with
three polar bodies due to the unequal division of the
cytoplasm. a Bu

29. Meiosis and Mitosis Compared