BIOLOGY II

SBU 3023

HOW CELLS REPRODUCE

Overview of cell division mechanism, introducing cell

cycle, a closer look at mitosis, division of the cytoplasm

CONTENT

Overview of cell division mechanism

Introducing cell cycle

A closer look at mitosis, division of the cytoplasm.

Somatic cells have 2 sets of chromosomes: contains a

diploid number of chromosomes (2n).1 set inherited from

each parent.

Human: have 23 pairs of chromosome, 2n = 46 22 pairs are autosomes & 1 pair sex-chromosome

1400 nm

700 nm 300 nm

Condensed

chromosome

Condensed chromatin

Extended chromatin

Nucleosomes

DNA double helix

Packed nucleosomes

2 nm

Histone 11 nm

DNA wound around

a cluster of histone

molecules

30 nm

Scaffolding

protein

30 nm

fiber

CELL CYCLE Cell cycle - The period that extends from the time

a new cell is produced until the time the cell completes a cell division.

Two major phases in cell cycle are:

Interphase (Cells undergo rapid growth and

preparing for cell division).

M phase

Upon the completion of interphase, cells will enter

mitosis phase. In mitosis, cell division is further

divided into two steps:

Step 1: Mitosis (nuclear division-synthesized

chromosomes are equally divided into two daughter

chromosomes)

Step 2: Cytokinesis (cytoplasmic division-this step

take place right after the nuclear division to form 2

daughter cells.)

Cell grows

Cell prepares to divide

CELL CYCLE

INTERPHASE

A cell spends approximately 90 percent of its cycle in

interphase.

Cells carry out their normal functions and preparing

for cell division. S

The period between division, divided into 3 sub

phases (G1, S and G2):

G1- cells grow rapidly and new organelle are synthesis

S - synthesis of DNA and chromosomes are replicated

G2 - cells prepare for mitosis, synthesis protein and

mitotic spindle begin to form

MITOSIS

Mitosis is a division of the nucleus to produce two new

daughter cells containing chromosomes identical to

the parent cell.

MITOSIS

Mitosis is a continuous process and divided into 4 main phases based on the appearance and behavior of the chromosomes.

1.Prophase, P

2.Metaphase, M

3.Anaphase, A

4.Telophase, T

Centromere

Centrioles

MITOSIS-PROPHASE Early prophase

Chromosome condense and appear shorter and thicker and become visible in a light microscope

Each chromosome now consists of a pair of sister chromatids joined together at centromere.

Nucleolus disappears & nuclear membrane disintegrates.

Paired centrioles move to opposite ends of the cell

Late prophase

Nuclear membrane disappears

Spindle form

MITOSIS-METAPHASE

Spindle fibres are fully form

Sister cromatids line up at the spindle equator/ metaphase plate.

Two sister chromatids are still attached to one another at the centromere.

At the end of metaphase, the centromers divide.

MITOSIS-ANAPHASE

Anaphase begin with the separation of the centromers

The sister cromatids are drawn to opposite poles of the cell by shortening of spindle fibres.

Once separated, chromatids are referred to as daughter chromosome.

End anaphase: the poles of the cell have complete and equivalent sets of chromosome.

MITOSIS-TELOPHASE

Telophase begin when the two sets of daughter chromosomes have reached the two poles of the cell.

The spindle fibres disintegrate, the nuclear membrane forms around each set of daughter chromosomes and the nucleoli reappear.

The nucleolus also re-forms in each nucleus.

The chromosomes uncoil and become less visible under the light microscope.

Mitosis process is now completed.

This process take place immediately when nucleolus formed.

Different in animals and plants. Animal cells:

the cytoplasm contracts to pull the plasma membrane inwards, forming groove called a cleavage furrow

In plant cells, cytokinesis in cells is markedly different. A cleavage furrow does not form. Instead, membrane-enclosed

vesicle gather at a plant cell’s equator between the two nuclei.

Vesicle fuse to form a cell plate

CYTOKINESIS

Growth- allows a zygote to produce more cell in order

to grow

Repair and replacement- allow the multicellular

organism maintain its tissues, example skin cells and

blood

Asexual reproduction- clone

THE IMPORTANCE OF MITOSIS

Controlled mitosis: the ability of the cell to divide at

its own rate and time.

Crucial to normal growth, development and

maintenance for the perpetuity of living things.

Examples:

in plants controlled mitosis occurs at shoots results in

growth of plants.

in human Malphigian layer of skin divides actively to

replace dead and damaged cells.

THE IMPORTANCE OF CONTROLLED MITOSIS

Mutation is the change in the DNA structure of the

cell.

This change in the DNA corrupts the coded genetic

instructions for mitosis control.

This leads to uncontrolled mitosis, which is the non-

stop division of cells, producing a mass of new

daughter cells, called tumor.

THE EFFECTS OF UNCONTROLLED MITOSIS

IN LIVING THINGS

Genetic- some forms of cancer like prostate, colon, breast, skin, ovary are suspected to be inherited from the parents

Carcinogens- these are chemicals which affect genetic activity and cause cancer, e.g. of carcinogen a diesel exhaust, cigarette smoke, hair dyes, soot, arsenic, benzene and formaldehyde.

Radiation- excess exposure to x-ray, gamma-rays and ultra violet rays lead to increase cancer risk.

Viruses- some viruses (such as the HPV and HIV-1) cause cancer.

OTHER CAUSES OF TUMOR/CANCER

Mitosis is used to:

improve the quality

produce new species (speciation)

ensure uniformity in the traits.

Cloning: process of artificially creating a new

individual that is genetically identical to an existing

individual.

Technique in application of mitosis in cloning

tissue culture.

APPLICATION OF MITOSIS IN CLONING

APPLICATION OF MITOSIS IN CLONING