Cell Division & Its Regulation: (Mitosis & Meiosis) (Outline)• Why do cells divide?• New Terms: gamete, zygote, somatic cells, germ cells, haploid, and diploid. • Types of eukaryotic cell division- Mitosis & Meiosis• Cell division of somatic cells and germ line cells.• Comparison of Mitosis & Meiosis in terms of # of cell divisions, # of daughter

cells, DNA content in comparison to mother cell, haploid/diploid state of daughter cells, genetic sameness or variability of daughter and mother cells.

• Cell cycle and its phases; Interphase and the Mitotic phase and events and outcomes of subphases of Interphase: G1, S, G2

• State of chromatin in non-dividing and dividing cells: looses chromatin and condensed chromosome.

• The Mitotic phases: (a) Mitosis: Prophase, Metaphase, Anaphase, Telophase, and (b) Cytokinesis.

• Cytokinesis in animal and plant cells.• Cell Cycle control checkpoints: G1, G2, & M. Cancer cells. • Meiosis and events and outcomes of its two divisions. • Sources of variability in sexually reproducing organisms

Sperm cell (haploid)

NucleicontainingDNA

Egg cell (haploid)

Fertilized eggwith DNA fromboth parents

Embryo cells with copies of same inherited

DNA Offspring with traitsinherited fromboth parents

Zygote (diploid)

Eukaryotic multi-cellular organisms• From gametes -sperm or egg (haploid)• Fertilization of sperm and egg produces zygote

(diploid) • Somatic cells- Body cells of multi-cellular

organism (diploid)

Cell Division & Its Regulation

Key Roles of Cell Division• .• .

Purpose: distribution of genetic material to daughter cells

Types of cell division

Two types in eukaryotic cells:Mitosis produces genetic sameness

(asexual reproduction)Meiosis produces genetic variability

(sexual reproduction)

Eukaryotic Cell division1. Asexual reproduction

(Identical cells)- Unicellular/ Amoeba

- Some multi-cellular eukaryotes plants and some animals like hydra, by budding cells

Eukaryotic Cell division ( cont’d)

2. Sexual reproduction (gametes, non-identical cells- for genetic variety of offspring)

Most multi-cellular organisms have both asexual and sexual reproduction

Cell Reproduction in HumansSomatic cells (sameness)Germ line cells of the gonads (variability)

Mitosis produces 2 genetically identical cellsMeiosis produces 4 genetically non-identical cells each with ½ the number of chromosomes Mitosis and

development

Multicellulardiploid adults

(2n = 46)

Diploidzygote

(2n = 46) 2n

Meiosis Fertilization

Egg cell

Sperm cell

n

Haploid gametes (n = 23)

n

Cell Division• One mother cell divides into two identical

cells following an ordered sequence of events (Cell Cycle)

• Summary of event of dividing cells• Replicate the genetic material• Manufacture additional cellular content• Divide the nucleus• Separate the cytoplasm

Overall Comparison of Mitosis and Meiosis

Interphase with gaps for growthMitosis- division of the nucleusCytokinesis- division of the cytoplasmwww.cellsalive.com

Cell Cycle Phases

Phases of cell Cycle

InterphaseChromosomes duplicate and cell parts are made

MitosisDuplicated chromosomes are evenly distributed

into two daughter nuclei

State of DNA inside a living cell

In a non dividing and in a dividing cell• Packaging of long strands of DNA in small

nucleus (loose chromatin: non-dividing).

• Condensed chromosomes in a dividing cell.

• Chromosomes are visible only when the cell is in the process of dividing during the mitotic phase

Chromatin

Figure 2.3

Replication of Chromosomes

Chromosomes are replicated during S phase prior to mitosis

The result is two sister chromatids held together at the centromere

The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis. It arises from the centrosome.

Figure 2.3

MitosisUsed for growth and repair

Consists of a single division that produces two identical daughter cells

A continuous process divided into 4 sub-phases

- Prophase- Metaphase- Anaphase- Telophase

http://highered.mheducation.com/sites/0072437316/student_view0/chapter11/animations.html#

- Centrosome- a pair of centrioles, microtubule organizing center (MOC).

- Spindle fibers- mirotubules (tubulin)- Nuclear membrane- Nucleolus- Loose chromatin- Condensed chromosome –two sister

chromatids held by centromerewww.cellsalive.com /mitosis http://www.biology.arizona.edu/cell_bio/tutori

als/cell_cycle/cells3.html

Mitosis in a fish blastula

Mitosis in an onion root

Cytokinesis in animal and plant cells

Cleavagefurrow

SEM

140

×

Daughter cells

Cleavage furrow Contracting ring ofmicrofilaments

TEM

7,

500 ×

Cell plateforming

Wall ofparent cell

Daughternucleus

Cell wall New cell wall

Vesicles containingcell wall material

Cell plate Daughter cells

CytokinesisAnimal cells Plant Cells

Cleavage furrowCell plateMicrofilament (actin) and myosin contracting ringGolgi-derived vesicles

Frequency of cell division varies with the type of cell

• Very oftenSkin cellsBone marrowLining of stomach and intestines

• SometimesLiver cells

• Do not divide in mature animal Nerve cells

The Cell Cycle Control SystemThe sequential events of the cell cycle

are directed by a distinct cell cycle control system of regulatory proteins

G2 checkpoint

Control system

M checkpoint

G1 checkpoint

G1

S

G2M

Three major checkpoints are found in the G1, G2, and M phases

www.cellsalive.com

Density-dependent inhibition of cell division

Anchorage dependence of cells

Mortality of cells-limited numbers of cell divisions

Behavior of normal and cancer cells in cell

culture

Tumor

Glandulartissue

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

Cancer cells spread throughlymph and blood vessels toother parts of the body.

Lymphvessels

Bloodvessel

Figure 8.10

Cancer cells- grow out of cell cycle control

- form masses called tumors- malignant tumors spread and invade other tissues

MEIOSIS AND CROSSING OVERGenetic Variability

Chromosomes are matched in homologous pairs

– The somatic (body) cells of each species contain a specific number of chromosomes

– For example human cells have 46 chromosomes making up 23 pairs of homologous chromosomes

Chromosomes are matched in homologous pairs which carry genes for the same characteristics at the same place, or locus

Chromosomes

Centromere

Sister chromatids

Figure 8.12

Meiosis

• Two consecutive cell divisions, meiosis Iand meiosis II

• Results in four daughter cells• Each final daughter cell has only half as

many chromosomes as the parent cell

Meiosis animation• Stages of Meiosis

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/meiosis.html

• www.cellsalive.com / Meiosis

The results of crossing over during meiosis

Sources of variation in sexually reproducing organisms

1. Independent assortment of chromosomes during meiosis

2. Crossing over during meiosis3. Random fertilization

The results of independent assortment of homologous chromosome pairs at the metaphase plate in meiosis I

Chromosome Abnormalities involving chromosome numbers are caused by non-disjunction of

- Homologous chromosomes during Meiosis I- Sister chromatids during Meiosis II