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Batterjee Medical College (BMC)

FAST (preparatory year)

Biology Department

2012-2013

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CELL DIVISION

Objectives

1. Define types of cell division.

2. Name the stages of the cell cycle and explain what happens during each stage.

3. Differentiate between Mitosis and Meiosis

� The cell cycle is the series of events that take place in a cell leading

to its division and duplication (replication). In cells with a nucleus

(eukaryotes), the cell cycle can be divided in two periods:

� Interphase: during which the cell grows, accumulating nutrients

needed for mitosis and duplicating its DNA.

� The mitosis (M) phase: during which the cell splits itself into two

distinct cells "daughter cells" and the final phase, cytokinesis, where

the new cell is completely divided.

TYPES OF CELL DIVISION:

A. The type of cell division in somatic body cells of eukaryotes is

known as mitosis, producing two identical cells and leaves the

daughter cells capable of dividing again.

B. For simple unicellular organisms such as Amoeba, one cell division

known as binary fission is equivalent to reproduction-- an entire new

organism is created.

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C. Another type of cell division present only in eukaryotes, called

meiosis, a cell is permanently transformed into a gamete and may not

divide again until fertilization. Meiosis is the process by which

gametes are generated for reproduction.

Some basic definitions are in order to better understand the

concept of cell division,

• Gene: Basic unit of heredity; codes for a specific trait.

• Genome: The total hereditary content of DNA of a cell or organism.

• Somatic cell: All body cells except reproductive cells.

• Gamete: Reproductive cells (i.e. sperm & eggs).

• Chromosome: Elongate cellular structure composed of DNA and

protein - they are the vehicles which carry DNA in cells.

• Diploid (2n): Cellular condition where each chromosome type is

represented by two homologous chromosomes.

• Haploid (n): Cellular condition where each chromosome type is

represented by only one chromosome.

• Homologous chromosome: chromosome of the same size and shape

which carry the same type of genes.

• Chromatid : One of two duplicated chromosomes connected at the

centromere.

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THE CELL CYCLE:

Resting Gap 0 G0 A resting phase where the cell has left the cycle

and has stopped dividing.

Interphase

Growth 1

(Gap 1) G1

Cells increase in size in G1. This phase is marked

by synthesis of proteins and enzymes that are

required in S phase for DNA replication.

Synthesis S DNA replication occurs during this phase.

Growth 2

(Gap 2) G2

During the gap between DNA synthesis and

mitosis, the cell will continue to grow. The

G2control mechanism ensures that everything is

ready to enter the M (mitosis) phase and divide.

Cell division Mitosis M Cell growth stops at this stage and cellular energy

is focused on division into two daughter cells.

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Nuclear division: Prophase–Metaphase–Anaphase and Telophase.

Cytoplasmic division: Cytokinesis.

MITOSIS (MITOTIC CELL DIVISION)

� Mitosis is a process of cell division which results in the production of two

daughter cells from a single parent cell. The daughter cells are identical to one

another and to the original parent cell.

� Mitosis maintains a constant amount of genetic material from cell generation to

cell generation.

� Why mitosis is performed by somatic body cells?

1. Growth. The number of cells within an organism increases by mitosis and this is

the basis of growth in multicellular organisms.

2. Cell Replacement. Cells are constantly sloughed off, dying and being replaced

by new ones in the skin and digestive tract. When damaged tissues are repaired,

the new cells must be exact copies of the cells being replaced so as to retain

normal function of cells.

STAGES OF MITOSIS

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Interphase (G2): Showing normal cell structure with

obvious nuclear membrane and chromatin materials in

thecytoplasmand duplicatedDNAcontent.

1- Prophase:

-The first and longest stage ofmitosis.

- Chromatincondenses to chromosomes.

- Each chromosome appear as two sister chromatids.

-Thenuclear envelopedissolves.

-The nucleolus in the nucleus disappears.

- Centrioles divide and migrate to oppositepoles.

-Aster rays and spindle fibers forms.

2- Metaphase:

- Chromosomes with Pairs of sister chromatids align

themselves at the equatorial plane of the cell

(metaphase plate).

3- Anaphase:- Anaphase begins when the duplicated centromeres of

each pair of sister chromatids separate, and the now-

daughter chromosomes begin moving toward opposite

poles of the cell due to the action of the spindle.

4- Telophase:

- Chromosome sets assemble uncoil at opposite poles, a

nuclear envelopeand nucleolus reforms.

� Cytokinesis: The division of the cytoplasm,

usually is in progress before nuclear division is complete

involves the formation of a cleavage furrow resulting in

thepinchingof the cell into two.

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MEIOSIS (MEIOTIC CELL DIVISION):

� Meiosis is a special type of nuclear division which segregates one copy of each

homologous chromosome into each new "gamete", the number of

chromosomes is reduced by half, and returned to the full amount when the two

gametes fuse during fertilization.

� Meiotic division occurs only in cells of male or female sex organs and

produces 4 haploid cells. (Oogenesis in ovary & Spermatogenesis in testis).

� Phases of Meiosis include two successive nuclear divisions, Meiosis I

(Reduction) and Meiosis II (Division).

In this drawing, the parent cell (2N) is diploid. It divides in the first division

(Meiosis I) into 2 daughter haploid cells (N). The daughter cells, which are

already haploid, then go through the second division (Meiosis II) into a total

of 4 daughter cells. These final daughter cells are haploid, act as gametes.

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MEIOSIS I:

1- Prophase I:

�Chromatincondenses into chromosomes.

�Chromosomes coil tightly & arevisible .

�Nuclear membrane& nucleolus disintegrate.

�Centrioles separate, migrateand Spindle fiber forms.

� Synapsis (joining) of homologous chromosomes occurs

making tetrads.

�Kinetochore fiber forms on each chromosome.

� Chromosomes in tetrad exchange fragments by a process

called crossingover.

2- Metaphase I:

� Paired homologues chromosomes line up side by

sidealongcell equator.

3- Anaphase I:� Anaphase I is when the tetrads of homologous

chromosomes (not sister chromatids) separate, and are drawn

to opposite poles by the spindle fibers (reduction ½ of

chromosomenumber).

.4- Telophase I:

� In telophase I only one set of (replicated) chromosomes

separate (no chromatid separation) is in each "cell". Depending

on species, new nuclear envelopes may or may not form.

� Two daughter cells are formed during telophase I. These

usually go immediately into the second meiotic cell division

(meiosis II).

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MEIOSIS II:

• •

1- Prophase II:

� In each haploid daughter cell (called Secondary

Spermatocytes or secondary oocytes ), a spindle forms.

Chromatidsareattached to the spindle.

�(Note: DNAis not copied before Meiosis II cell division).

2- Metaphase II:

�Chromosomes become aligned in the center of the cell

attached to spindle fibers.

�Each chromosome has two sister chromatids attached to the

spindle.

3- Anaphase II:�The sister chromatids separate and move to opposite poles.

Two sets of chromosomes are formed at each pole of each cell

(haploid=n).

4- Telophase II:

� The chromatids, now considered individual

chromosomes, arrive at the poles. Nuclear membrane

reforms, Nucleus reappears.

�Cytokinesis:

The process of meiosis is completed, producing

fourhaploiddaughtercells as a final result.

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IMPORTANT TERMS FOR MEIOSIS IN HUMAN:

• Reduces the number of chromosomes in new cells to half the number in the original cell

• New cells have a single copy of chromosomes (23 total) but are not identical to each other or the original parent cell

• Used for making gametes ( sperm and eggs) with the haploid or n number

• In meiosis, cells divide twice after a single DNA duplication • Meiosis I separates homologs & the Meiosis II separates sister

chromatids • Meiosis I stages are Prophase I, Metaphase I, Anaphase I, &

Telophase I • Meiosis II stages are Prophase II, Metaphase II, Anaphase II, &

Telophase II • Produces 4 haploid cells or gametes • When a sperm fertilizes an egg to form a zygote, the diploid

number of chromosomes is restored (23 + 23 = 46) • Egg cells or ova (ovum, singular) are larger , nonmotile cells • Gametoogenesis is meiosis producing eggs & occurs in the female's

ovaries. • Sperms contain less cytoplasm so they're smaller & have a

flagellum to swim to the egg. • Spermatogenesis is meiosis producing sperm cells & occurs in the

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HOMEWORK: CELL DIVISION

1. Mention the four stages of Mitosis in their order of occurrence.

1. ……………………………….

2. ……………………………….

3. ……………………………….

4. ……………………………….

5. Put (T) or (F) and correct the false statement:

1. In anaphase the nuclear envelope re-forms around the two sets of

chromosomes.

( )

2. Meiosis cell division goes on throughout the life in all parts of the

body.

( )

3. In prophase, the centromeres divide. At this point, each individual

chromosome goes from 1 chromosome with 2 chromatids to 2

chromosomes with one chromatid each.

( )

4. Mitosis is a reduction division.

( )

5. In humans, each cell contains 46 pairs of chromosomes.

( )

6. Males have two copies of the X chromosome, while females have one

X and one Y chromosome.

( )

7. Every person has two copies of each gene, one inherited from each

parent.

( )

8. Meiosis is the type of cell division by which germ cells (eggs and

sperm) are produced.

( )

9. Write 3 differences between Mitosis & Meiosis.

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10. In which phases the following events occur:

1. Formation of tetrads:

2. Separation of sister chromatids:

3. Separation of homologous chromosomes:

4. Duplication of genetic materials:

5. Formation of 2 separate cells:

6. Lining of the chromosomes at the metaphase plate:

7. Dissolving of nuclear membrane:

8. Crossing over:

V. What is the difference between oogenesis and spermatogenesis?

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VI. Write the title for each phase: