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. • What is a karyotype and when is it used? Lab • Meiosis and events and outcomes of its two divisions. • Sources of variability in sexually reproducing organisms

Sperm cell (haploid)

Nuclei containing DNA

Egg cell (haploid)

Fertilized egg with DNA from both parents

Embryo cells with copies of same inherited

DNA Offspring with traits inherited from both 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 division 1. 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 Humans Somatic cells (sameness)

Germ cells of the gonads (variability)

Mitosis produces 2 genetically identical cells

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

development

Multicellular diploid adults

(2n = 46)

Diploid zygote

(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

Interphase with gaps for growth Mitosis- division of the nucleus Cytokinesis- division of the cytoplasm www.cellsalive.com

Cell Cycle Phases

Phases of cell Cycle

Interphase Chromosomes duplicate and cell parts are made Mitosis Duplicated chromosomes are evenly distributed

into two daughter nuclei

Overall Comparison of Mitosis and Meiosis

State of DNA inside a living cell In a non-dividing cell- DNA (2-3 m) is coiled as

Chromatin (DNA + proteins/histones)

In a dividing cell- chromatin condenses to form chromosomes (Chromatin + scaffold proteins)

State of DNA inside a living cell

http://www.biostudio.com/demo_freeman_dna_coiling.htm

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

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

An apparatus of microtubules that controls chromosome movement during mitosis

Arises from the centrosome

Figure 2.3

Mitosis Used for growth, repair, and replacement

Consists of a single division that produces two identical daughter cells

A continuous process divided into 4 sub-phases

- Prophase - Metaphase - Anaphase - Telophase

- 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 centromere

www.cellsalive.com /mitosis

http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html

Mitosis in a fish blastula

Mitosis in an onion root

Cytokinesis in animal and plant cells

Cleavage furrow

SEM

140

×

Daughter cells

Cleavage furrow Contracting ring of microfilaments

TEM

7,

500 ×

Cell plate forming

Wall of parent cell

Daughter nucleus

Cell wall New cell wall

Vesicles containing cell wall material

Cell plate Daughter cells

Cytokinesis Animal cells Plant Cells

Cleavage furrow Cell plate

Microfilament (actin) and myosin contracting ring

Golgi-derived vesicles

Frequency of cell division varies with the type of cell

• Very often Skin cells Bone marrow Lining of stomach and intestines • Sometimes Liver cells • Do not divide in mature animal Nerve cells

The Cell Cycle Control System The 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

G2 M

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

www.cellsalive.com

The Cell Cycle Control

• Progression through cell cycle is controlled by regulatory proteins

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

Glandular tissue

A tumor grows from a single cancer cell.

Cancer cells invade neighboring tissue.

Cancer cells spread through lymph and blood vessels to other parts of the body.

Lymph vessels

Blood vessel

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 OVER Genetic 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

Genetic Variability

Offspring acquire genes from parents by inheriting chromosomes

Meiosis

• Two consecutive cell divisions, meiosis I and meiosis II

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

many chromosomes as the parent cell

Meiosis animation

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

• www.cellsalive.com / Meiosis

• Stages of Meiosis http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter12/animations.html#

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 meiosis 3. 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