CELL DIVISION

Chromatin

� Cell’s collection of DNA and associated proteins (histones)

� Appearance changes as cell divides

2

DNA and proteins arranged as cylindrical fiber

DNA

histone one nucleosome

Chromosome

-  supercoil or highly folded chromatin

chromatin

duplicated chromosome

Sex chromosomes -  function in sex determination XY, XX, ZZ, ZW Autosome -  chromosome that is not a sex chromosome and that

appears as a homologous pair in a somatic cell. -  22 pairs of autosomes (transmit all genetic traits

and conditions other than those that are sex-linked) -  Euchromosome -  chromosomes that are alike -  not involved in determining sex

Ploidy - term referring to the number or sets of chromosomes a. Diploid - 2 sets of chromosomes - 2N b. Haploid - one set of chromosomes - N

Polyploidy

-  condition when animals have more than two sets of chromosomes

- abnormal cell division

Cellular Reproduction:

-  growth in multicellular organisms is chiefly by multiplication of cells

-  Cell division

a. Embryonic development

b. Growth

c. repair of tissue

Figure 12.2 The functions of cell division

20 µm 100 µm 200 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Mitosisfunctionssister chromatidscell cycleStagescytokinesis

Cell division

an orderly sequence of events that extends from the time a cell divides to form two daughter cells to the time those daughter cells divide again

Cell division

Mitosis Meiosis somatic cells gametes 1 cell cycle 2 cell cycles

2 daughter cells 4 daughter cell Diploid haploid

Cell cycle

- period from the cell is produced until it completes

a. mitosis (body cells)

b. meiosis (egg and sperm)

-  consists of 2 major phases:

1. Interphase/Growth Phase “resting phase”

3 subphases:

a. G1

b. S

c. G2

2. M phase

The Cell Cycle � Growth Phase/Interphase (Resting

Phase) - longest - 90% of cell cycle � M Phase - 2 processes 1. division of nucleus 2. division of cytoplasm

INTERPHASE

G1 S

(DNA synthesis)

G2

G1

-  cells grow in size

S phase

-  Growth continues

-  DNA synthesis

chromatids

-  copy of the chromosome produced by

replication

G2

-  Cells continue to grow

-  Protein synthesis

-  assemble structures organelles, microtubules

- 

Figure 12.4 Chromosome duplication and distribution during cell division

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centrometers Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into twochromosomes and distribute

them to two daughter cells.

Mitosisfunctionssister chromatidscell cycleStagescytokinesis

Chromatid

- copy of the chromosome produced by replication

Parts of a chromatid:

Centromere

- point of attachment between (2) sister chromatids

Kinetochore

- plate-like trilaminar structure or disc-like protein complex that develops on each side of the centromere

The duration of the cell cycle phases varies considerably in different kinds of cells. For a typical rapidly proliferating human cell with a total cycle time of 24 hours, the G1 phase might last about 11 hours, S phase about 8 hours, G2 about 4 hours, and M about 1 hour.

2. M- Phase (Mitosis) - process by which a eukaryotic cell divides into two cells

2 steps: 1. karyokinesis “nuclear division” 2. cytokinesis “cytoplasmic division”

Phases: a. Prophase - period of preparation b. Metaphase – period of separation c. Anaphase – period of migration d. Telophase – period of reconstruction

Interphase

Animal  Cell   Plant  Cell  

Photographs  from:  h4p://www.bioweb.uncc.edu/biol1110/Stages.htm  

Early Prophase

�  chromosomes become visible

�  duplicated chromosomes begin to condense

�  centrioles radiate an array of microtubules called aster

Late Prophase

�  New microtubules are assembled

�  One centriole pair is moved toward opposite pole of spindle

�  Nuclear envelope breaks up

�  Spindle microtubules become attached to the two sister chromatids of each chromosome

Metaphase

�  Sister chromatids are lined up at the spindle equator

�  Chromosomes are maximally condensed

�  During Late metaphase sister chromatids begin to separate (period of separation)

Anaphase �  microtubules in the

mitotic spindle shorten

�  Sister chromatids of each chromosome are pulled apart

�  Once separated, each chromatid is a full pledged chromosome (daughter chromosome)

�  each pole has an identical complete set of chromosomes

Early Telophase

�  begins once the daughter chromosomes arrive at the opposite poles

�  mitotic spindle disassembles

�  nuclear envelope forms around each set of chromosomes

Late Telophase

�  cell begins to pinch at the middle

Cytoplasmic Division

�  usually occurs late telophase

�  Cytoplasmic division occurs after nuclear division

�  Two mechanisms

�  Cleavage furrow (animals)

�  Cell plate formation (plants)

Results of Mitosis

� Two daughter nuclei

� Each with same chromosome number as parent cell

Figure 12.9 Cytokinesis in animal and plant cells

Daughter cells

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm1 µmVesicles

forming cell plate

Wall of patent cell Cell plate

New cell wall

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

Mitosisfunctionssister chromatidscell cyclestagescytokinesis

Vesicle = small membrane-lined —bag“

Animal Cell Division

Cell Plate Formation

REMEMBER! Interphase  Prophase  Metaphase  Anaphase Telophase  Cytokinesis

IPMATC

I Pray More At The Church

Animal  Mitosis  -­‐-­‐  Review    

Interphase

Prophase

Metaphase

Anaphase

Telophase

Interphase