Mitosis

CELL REPRODUCTION

Outline• Types of Cell Division• Structure of Chromosomes • Cell Division in Prokaryotes• Phases of cell cycle• Interphase• Mphase• Cytokinesis• Cell cycle-abnormalities

Cell Theory: all organisms consist of cells and arise from pre-existing cells

• Mitosis is the process by which new cells are generated for body tissues during development, growth, and tissue repair.– 2 cells are produced in one cycle of division– Daughter cells have ALL of the genetic material of the

parent cell• Meiosis is the process by which special cells called gametes

are generated for reproduction.– 4 cells are produced in two cycles of division– Daughter cells have HALF the genetic material of the

parent cell

SOMATIC Vs REPRODUCTIVE

GAMETES (n) – EGGS (OVA) & SPERM - MEIOSIS

(Somatic) OTHERS (2n) - MITOSIS

NUCLEAR Vs CYTOPLASMIC DIVISION

The state in which there are two copies of each chromosome present is known as diploid (2n).  

HHaploid (n)– only one copy of a chromosome is present

All of the somatic cells in our bodies are diploid cells The only cells in our bodies that are haploid are our gametes – eggs and sperm

Human somatic cells have 46 chromosomes diploid number (2n) = 46

23 from mom - 23 from dad

22 pairs are autosomes –true homologous pairs

1 pair is not necessarily homologous

- sex chromosomesfemales are XX

males are XY – a non homologous pair.

Meiosis

– form of cell division where there are two successive rounds of cell division following DNA replication

- produces haploid cells (n)

- start with 46 double stranded chromosomes (2n)After 1 division - 23 double stranded chromosomes (n) After 2nd division - 23 single stranded chromosomes (n) - occurs in our germ cells

– cells that produce our gametes - egg and sperm

Cell division

Cell division

All complex organisms originated from a single fertilized egg.

Every cell in your body started here, through cell division the numbers are increased

Cell then specialize and change into their various roles

Mitosis

• Mitosis is the process by which new body cell are produced for:– Growth– Replacing damaged or old

cells.

This is a complex process requiring different stages

Mitosis• All daughter cells contain the

same genetic information from the original parent cell from which it was copied.

• Every different type of cell in your body contains the same genes, but only some act to make the cells specialise – e.g. into nerve or muscle tissue.

BASIC GENETICS• Each cell in the human body contains two sets of 23

chromosomes

• Mitosis identically replicates this information

• Each cell therefore has the same genetic material

• Reproductive cells only have one set of chromosomes. These combine to make a new person with different genetic material to both parents

2 daughter cells identical to original

Parent cell

Chromosomes are copied and double in number

Chromosomes now split

Reproduction presents a major problem for cells and organisms: (how can information be transmitted faithfully to progeny)

I

II

III

IV

= one bit ofgenetic information

= one bit ofgenetic information

The information transfer problem becomes more challenging as more bits of information are incorporated into the organism

One of life’s solutions to this challenge: “Package” the

bits of information into single units called chromosomes

= one bit ofgenetic information

The nucleus

Nuclear membrane - double membrane

Chromosomes/Chromatin- genetic material/DNA

Gene - DNA segment

Chromosome--- DNA --- Gene

chromosomes

Packaging of genetic material in prokaryotes and eukaryotes

prokaryote cell

eukaryote cell

Structure of Chromosomes• Chromosomes are composed of a complex of

DNA and protein, chromatin.

• DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length.– forms nucleosome every 200 nucleotides

• DNA coiled around histone proteins

C Chromosome basics.TThe number of chromosomes present within the nucleus is a characteristic of the species. Chromosomes of humans and most other eukaryotic species occurs in pairs. Members of a chromosome pair are known as homologues.

Mitosis/Cell division

mitosis is the process of cell division in whichone cell becomes two identical daughter cells

developmentrenewal regeneration

The cell cycle is the mechanism by which a cell duplicates its contents and then divides in two

cell grows and duplicates its contents

cell contents

cell divides in two

Each parental cell gives rise to two daughter cells on completion of each cell cycle

parental cell

daughter cell 1

daughter cell 2

Replication of chromosomes

• Replication is the process of duplicating a chromosome

• Occurs prior to division

• Replicated copies are called sister chromatids

• Held together at centromere

Binary Fission-prokaryotic cells

Cell Cycle

Prophase

Metaphase

Anaphase Telophase

The Stages The Stages of of MMitosisitosis

Interphase

REMEMBER!

Interphase

Prophase

Metaphase

AnaphaseTelophase

IPMAIPMATT

(Individuals Please Make All The Cells)

INTERPHASE

After a cell has divided, the two

new cells begin the process again,

the cells at this stage are in

Interphase.

• By late interphase, the chromosomes have been duplicated but are loosely packed.

• The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”).

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

PROPHASE• The chromatin (decondensed

DNA) in the nucleus, condenses to form pairs of chromosomes.

• The centrioles move to opposite ends of the nucleus.

• As this is happening the nucleolus

begins to break down

• Nuclear membrane begins to break down

Prophase

• Replicated chromosomes condense.

• Microtubules organize into a spindle

The Spindle

A spindle is a web type structure made up of microtubule fibers. It is essential for mitosis because it arranges the chromosomes into their correct positions in preparation for cell division.

Mitotic center

MicrotubuleA cell at metaphase a spindle

Chromosomes attached to spindle during nuclear division

METAPHASE

• The spindle becomes fully developed

• The nuclear membrane has completely gone

• The chromatid pairs are aligned along the center of the spindle (the EQUATOR)

• The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Anaphase

• Centromeres of sister chromatids separate

• Chromosomes move to opposite ends of the cell

• At anaphase, the centromeres divide, separating the sister chromatids.

• Each is now pulled toward the pole to which it is attached by spindle fibers.

• By the end, the two poles have equivalent collections of chromosomes.

TELOPHASE

Two new nuclei are formed when the chromosomes reach the opposite poles of the cell

The nuclear membrane is formed- the nucleolus reappears

The chromosomes disperse in the nucleus

Telophase

• Nuclear membranes form

• Spindle disappears

• Division of cytoplasm occurs (cytokinesis)

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

CYTOKINESIS

Literally means, division of the cytoplasm

Mitosis is the splitting of the nucleus.

Cytokinesis is the splitting of cytoplasm

Cytokinesis

• Cleavage of cell into two halves– animal cells

• constriction belt of actin filaments

– plant cells• cell plate

Cytokinesis

Cytoplasmic division

occurs after nuclear

division is complete.

Two cells are formed.

Cytokinesis in animals

A cleavage furrow forms that pinches the cell in two - the furrow represents a ring of actin and myosin filaments just under the plasma membrane.

Rat – epithelial cells

Mitosis – bone cell slides

1 2

3 4 5

Parent cellChromosomes copied

Copies separating 2 daughter cells

Cells split

Cytokinesis in plants

A cell plate made up of cell-wall components gradually forms in the middle of the cell.

So what’s the difference in Plants?

• Plant cells do not have a centriole

• Plant cells do not pinch in half.

• Cytoplasmic division is accomplished by a cell plate forming between 2 daughter cells

Plants

Cell Turnover – The speed of mitosis

One full cycle can vary between a couple of minutes to days.

For example skin and epithelial cells have a rapid turnover in the human body in order to replace the ones constantly being worn away.

Cells which make up organs such as the eye and the brain, need not multiply as often once they reach adult size.

Organs which need to produce new cells continuously have the highest turnover.

For example:-

Bone marrow-

producing replacement

blood cells

The testes - producing

spermatogonia

Tumors

Abnormalities can sometimes occur in cells which reproduce at a rapid rate, this in turn may lead to the formation of tumors.

Tumors of any type should be considered serious.

Although benign tumors do not usually cause a threat to a persons life, they can cause great inconvenience if not treated.

CANCER

• uncontrolled cellular mitotic divisions

Meiosis – A Source of DistinctionMeiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Meiosis does two things -Meiosis does two things -

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copiestwo copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copysingle copy of every chromosome (haploid). of every chromosome (haploid).

This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.

In meiosis, In meiosis, one diploid cells produces four one diploid cells produces four haploid cells.haploid cells.

Why do we need meiosis?• Meiosis is necessary to halve the

number of chromosomes going into the sex cells

Why halve the chromosomes in gametes?

• At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

2) Meiosis 2) Meiosis scramblesscrambles the specific forms of the specific forms of each gene that each sex cell (egg or sperm) each gene that each sex cell (egg or sperm) receives. receives.

This makes for a lot of This makes for a lot of genetic diversitygenetic diversity. This . This trick is accomplished through trick is accomplished through independent independent assortmentassortment and and crossing-overcrossing-over..

Genetic diversity is important for the Genetic diversity is important for the evolution evolution of populations and speciesof populations and species..

Meiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell

4 gametes

1st division

2nd division

The Stages of Meiosis:

• aka: Reduction Division

Meiosis I : Separates Homologous

Chromosomes• Interphase– Each of the chromosomes

replicate– The result is two genetically

identical sister chromatids which remain attached at their centromeres

Prophase I

• During this phase each pair of chromatids match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.

• Extremely IMPORTANT!!! It is during this phase that crossing over can occur.

• Crossing Over is the exchange of segments during synapsis.

Metaphase I

• The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.– Still in homologous pairs

Anaphase I

• The spindle guides the movement of the chromosomes toward the poles– Sister chromatids remain attached– Move as a unit towards the same pole

• The homologous chromosome moves toward the opposite pole– Contrasts mitosis – chromosomes

appear as individuals instead of pairs (meiosis)

Telophase I

• This is the end of the first meiotic cell division.

• The cytoplasm divides, forming two new daughter cells.

• Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is ready for the second meiotic cell division

Cytokinesis

• Occurs simultaneously with telophase I– Forms 2 daughter cells

• Plant cells – cell plate• Animal cells – cleavage furrows• NO FURTHER REPLICATION OF

GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

Figure 13.7 The stages of meiotic cell division: Meiosis I

Meiosis II : Separates sister

chromatids

• Similar to mitosis

• THERE IS NO INTERPHASE II !

Prophase II

• Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

Metaphase II

• The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

Anaphase II

• The centromeres of sister chromatids finally separate

• The sister chromatids of each pair move toward opposite poles– Now individual chromosomes

Telophase II and Cytokinesis

• Nuclei form at opposite poles of the cell and cytokinesis occurs

• After completion of cytokinesis there are four daughter cells – All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Different Sex Cells (Gametes) – Independent AssortmentIndependent Assortment

Independent assortment produces 2Independent assortment produces 2nn distinct gametes, where n = the number distinct gametes, where n = the number of unique chromosomes. of unique chromosomes.

That’s a lot of diversity by this That’s a lot of diversity by this mechanism alone.mechanism alone.

In humans, n = 23 and 2In humans, n = 23 and 22323 = 6,000,0000. = 6,000,0000.

Another Way Meiosis Makes Lots of Different Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverSex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in the Way Chromosomes Uniquely Pair and Align in

Meiosis Meiosis

MitosisMitosis The first (and The first (and distinguishing) distinguishing) division of meiosisdivision of meiosis

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

X chromosomeX chromosomeY chromosomeY chromosome

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error

Chromosome pair

Meiosis error - fertilization

Should the gamete with the Should the gamete with the chromosome pair be fertilized chromosome pair be fertilized then the offspring will not be then the offspring will not be ‘normal’.‘normal’.

In humans this often occurs In humans this often occurs with the 21with the 21stst pair – producing pair – producing a child with Downs Syndromea child with Downs Syndrome

21 trisomy – Downs Syndrome

Can you see the extra 21st chromosome?

Is this person male or female?