Lecture 7

MITOSIS MEIOSISPatricia Caldani MS

Mitosis

• Mitosis is the process in which the nucleus divides to form ________two__ new identical nuclei.

Why cells need to divide?

• Unicellular organism– Asexual reproduction

• Multicellular organism– Growth

– Repair

– Replacement

Why produce identical cells?

• Somatic cells contains 2 sets of chromosomes, maternal and paternal = diploid, 2n

• Gametes (sperm and ovum) have only __one_________ set of unpaired chromosomes = haploid, n

• Each organisms has its own fixed number of chromosomes, e.g. human has 46 chromosomes (23 pairs).

• Producing identical cells preserves the ___original genetic________ number of chromosomes.

Significance of mitosis

• Produces new cells for growth, repair and replace damaged or dead cells.

• Forms asexual reproduction in unicellular organisms.

• Ensures new cells are ____identical______ to parent cells.

• Preserves the ___diploid________ number of chromosomes of species.

G1S

G2Mitosis

telophaseanaphasemetaphaseprophase

Interphase

The Cell Cycle

Cytokinesis

Mitotic (M) phase

Series of cellular reproductive events in which duplication of cell content and cell division occur in an orderly sequence

Cell Cycle• Consist of 2 major phases:

Phase 1: Interphase

Phase 2: Mitotic phase (M phase)

• Interphase involves ___stages/phases________ for cell division– It has 4 subphases: G0 G1, S, G2

– G1, G2: cell growth, produce proteins and organelles, cell differentiation

– S: synthesis of DNA, duplication of chromosomes and replication of DNA

• M phase, the cell divides– Includes mitosis (nuclear division) and cytokinesis

(cytoplasmic division)

• Chromosomes are ___copied________ (# doubles)

• Chromosomes appear as threadlike coils ____chromatin_______ at the start

Cell membraneNucleus

Cytoplasm

Animal Cell Plant Cell

• Chromosomes condense• Centrioles (or poles) appear and begin to move to

__opposite_________ end of the cell. • Nuclear membrane disappear• Nucleolus disappear• Spindle fibers form between the poles.

CentriolesSister chromatids

Spindle fibers

Animal Cell Plant Cell

• Chromatids (or pairs of chromosomes) attach to the spindle fibers, line up on equator

• Each chromatid face __opposite_________ poles

• End when centromere divides

Centrioles

Spindle fibers

Animal Cell Plant Cell

• Chromatids (or pairs of chromosomes) separate and begin to move to opposite ends of the cell.

• Shortening of spindle fibre• Chromatids chromosomes (daughter)

Centrioles

Spindle fibers

Animal Cell Plant Cell

• Two new nuclei form. • Chromosomes appear as chromatin (threads

rather than rods).• 2 sets of chromosomes one at

each__end_______• Spindle fibres disappear• Nucleolus & nuclear membrane are formed• Mitosis ends. Nuclei

Nuclei

Chromatin

Animal Cell Plant Cell

• Cell membrane moves inward (constrict) around the __cytoplasm_________ of cell to create two daughter cells – each with its own nucleus with identical chromosomes.

CYTOKINESIS (ANIMAL)

CYTOKINESIS (PLANT)

• In plant cells, cytokinesis starts with formation of cell In plant cells, cytokinesis starts with formation of cell plate at the plate at the ______________________ of cell of cell

• Cell plate enlarges until it Cell plate enlarges until it ___________ ___________ with plasma with plasma membranemembrane

• New cell wall is produced and separates the two New cell wall is produced and separates the two daughter cells daughter cells

Animal Mitosis -- ReviewInterphase

Prophase

Metaphase

Anaphase

Telophase

Interphase

Plant Mitosis -- ReviewInterphase

Prophase

Metaphase

Anaphase

Telophase

Interphase

MITOSISANIMAL PLANT

Similarities:

Two identical cells are formed at the end of a divisionThey produce more cells and cause growth

Differences:

Centrioles found at each pole during mitosis

No centrioles(cortical microtubules)

Cells divide everywhere, all the time

Mainly localized in special regions called meristems to either elongate the tips of

stems and roots or expand the girth of the plant

Formation of cleavage furrow at the equatorial plane

Formation of cell plate that grow outwards

REGULATION OF CELL CYCLE

• The cell cycle is controlled by ___regulatory__proteins __ in the cytoplasm

• Each type of cell has its own timing

• Some has frequent division

Controlled Mitosis

• The ability of cell to divide at its own timing and rate

• Enables __normal_________ growth and development and maintenance for perpetuity of living things

Uncontrolled Mitosis

• If the genes that regulate the cell cycle are damaged or mutated, cell divide __abnormal___cell growth_____

• Cause: – Harmful ray: nuclear radiation, UV ray

– Viruses

– Carcinogenic chemicals: food additives, benzo (α) pyrene in cigarette smoke

The Effects of Uncontrolled Mitosis

• Unregulated and very fast division increase the number of abnormal cells

• Form an abnormal mass – tumour

• Types of tumour: – Malignant

• Invasive by metastasis

• Cancer – Benign

• Localize• Does not cause

serious problems • Remove by

___________

What is the treatment of What is the treatment of cancer?cancer?

Brain Brain cancercancer

Breast Breast cancercancer

Mitosis Application: CLONING

• The process to produce genetically identical cells / tissues / organisms / population from the mitosis of a single parent cell (clone)

• Naturally: asexual reproduction, vegetative propagation (plant)

• Artificially: cloning (animal), tissue culture (plant)Why?

– Increase quantity

– Improve quality

– Ensure uniformity of traits

Animal Cloning Cloning is carried Cloning is carried out by replacing out by replacing nucleus of an nucleus of an _________donor___________donor_____ ___ egg cell with the egg cell with the nucleus of a diploid nucleus of a diploid cellcell

ADVANTAGES OF CLONING

• rapid multiplication, mass production in short time

• genetically identical, desirable traits inherited and propagated

• therapeutic cloning, replace damaged or diseased tissues / organs

• fetal cell cloning, early detection of genetic defects

• no seeds / difficult to germinate, propagate endangered or rare species

DISADVANTAGES OF CLONING

• no genetic variation, susceptible to changes and new diseases, extinction

• possibility of mutations, harmful effects of GMF• grow old quickly, shorter lifespan• exclusion of certain species, decrease

biodiversity• traits transfer to wild species, creation of

superweeds

Name the phase:

• 1. 2.

• 3. 4.

•

What is Meiosis?

• Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell. 

• Meiosis enables organisms to reproduce sexually.

• Meiosis involves two divisions producing a total of four daughter cells.

Summary of the Phases of Meiosis

• meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1

• meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2

• In the first meiotic division, the number of cells is doubled but the number of chromosomes is not.

Animals

Plants

Meoisis Metaphase1 and Metaphase 2Independent Assortment

Meoisis-Prophase

The homologous chromosomes pair up as the The homologous chromosomes pair up as the chromosomes coil upchromosomes coil up

The nuclear membrane begins to disintegrateThe nuclear membrane begins to disintegrate

Centrosomes begin moving apart.Centrosomes begin moving apart.

• Synapsis (joining) of homologous chromosomes produces tetrads (also called _bivalents_____________).

• The two chromosomes may exchange fragments between non-sister chromatids by a process called __crossing over____________

Meoisis-Metaphase

Bivalents become aligned in the Bivalents become aligned in the __center____________ of the cell and __center____________ of the cell and are attached to spindle fibers.are attached to spindle fibers.

Independent Assortment

Random arrangement of pairs of chromosomes.Random arrangement of pairs of chromosomes.

Meosis-Anaphase I

Anaphase I begins when homologous chromosomes Anaphase I begins when homologous chromosomes ______separate________.______separate________.

Telophase I

The nuclear envelope reforms and nucleoli reappear.The nuclear envelope reforms and nucleoli reappear.This stage is absent in some species.This stage is absent in some species.

Meoisis-Prophase II

Meiosis-Metaphase II

Meiosis-Anaphase II

Meiosis-Telophase II

Daughter Cells

Variation

• Sexual reproduction promotes variation because each gamete (sperm or egg) contains a mixture of genes from two different parents.

• Crossing-over and Independent assortment promote variation.

• Variation is necessary for __natural selection____________.

Significance of Meoisis

• Genetic Diversity through Crossing Over:• Prophase I

 • Genetic Diversity through Independent Assortment:• During Metaphase I

Disadvantages-Aneuploidy

Name The Phase

• 1. 2.

• 3. 4.

Review• Understand why cells duplicate

• What is interphase? Follow the phases of interphase

• Describe the major events of mitosis and cytokinesis and understand what occurs at each stage-KNOW THE ORDER

• Discuss how mitosis relates to cancer

• Understand Cloning

• Understand why we need meiosis

• Describe the major events that occur during the separate divisions of meiosis-KNOW THE ORDER

• Describe how mitosis and meiosis differ

• Understand significance of synapsis and crossing over

• Be able to read a slide and identify the stage

• Describe Binary Fission

• Describe Aneuplody