Chapter 5,( Meiosis Mitosis)Biof4

8/11/2019 Chapter 5,( Meiosis Mitosis)Biof4

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CELL DIVISION



The necessity for the production of new

cells :

- replace dead cells

- growth



New cell are produced by mitotic cell

division



Chromosomes and chromosomal

number

The cells in sexually reproducing organism

can be divided :

- somatic cells - mitosis

- reproductive cells / gametes – meiosis



Every cell has a thread like structures in its

nucleus called – chromosomes

The number of chromosomes in each cells

of each species is constant

The cells of a different species have a

different species have a different

chromosomal number



Onions – 16 chromosomes

Fruit fly – 8 chromosomes

Chromosomes in the nucleus exist in pairs- diploid ( 2n )

Gametes contains only half the number of

chromosomes – haploid



Somatic cells in human body – 46

chromosomes

- inherited from each parent

Gamete only has 23 chromosomes







Female – XX chromosomes

Male – XY chromosomes



Mitosis maintains the chromosomal number of

species and ensures genetic material is passed on

to the offspring

Each daughter cell that is formed through

mitosis receives genetic material (DNA )

from the parent cell

Mitosis doubles the number of cells

without changing the genetic content of

the cell



The sequence of bases in the DNA

encodes information for the synthesis of

enzymes (protein ) which are needed by

the cell



The cell cycle

The cell cycle is the period that extends

from the time a new cell is produced until

the time the cell completes a division

Two major phases :

1. interphase

2. mitotic cell division ( M phase )



G1 phase

G2 phase

S phase

(DNA replication



Interphase

G1

Cell acquire and synthesis the material for

cell division

Proteins and new organelles are

synthesised

If external condition are conducive forgrowth, cell enters the S phase



Chromosomes are extremely fine and

cannot be seen under light microscope (

chromatin )



S phase

Synthesis of DNA - replication

A duplicated chromosomes consist twoidentical sister chromatids



Chromosomes duplication and

condensation

Chromosomes

duplication

Chromosomes

condensesDuplicated

chromosomes

contains twoidentical DNA

double helix

Two chromatids joined

at centromere





Sister

chromatid

One

chromatid



G2

Cell continues to grow and remains

metabolically active

Cell accumulates energy and completes

its final preparations for division



Mitosis

Two major phases :

1. mitosis

2. cytokinesis



Mitosis – prophase

- metaphase

- anaphase- telophase



Prophase

- Chromosomes condense and tightly coiled

- Chromosomes become shorter, thicker,visible under light microscope

- Chromosomes consist of two sister

chromatids joined together at the

centromere







Spindle fibre is formed in the cytoplasm

End of prophase – nucleolus disappears,

nuclear membrane disintegrates



Metaphase

- The centromers of all chromosomes are

lined up on the equator of the cell

- Mitotic spindle are fully formed

- Sister chromosomes are still attached to

each other at the centromer







Anaphase

- Two sister chromatids separate at the

centromere

- Sister chromatids are pulled to the

opposite poles by the shortening of the

spindle fibre







The separate chromosomes are known as

daughter chromosomes

Anaphase ends when the chromosomes

reach the poles of the cell



Telophase

- Begins when both set of chromosomes

reach the opposite poles of the cell

- Chromosomes start to uncoil and revert to

their extended state again

- Spindle fibre disappear

- Nuclear membrane forms around each set

of chromosomes



Nucleolus reforms







Cytokinesis

- The cytoplasm of the cell divides to form

two daughter cells, each having one

nucleus



Actin filamentscontract to pull a

ring of the plasma

membrane inwards,

forming cleavage

furrow

Cleavage furrowpinches at the

equator of the

cell

The cleavage

furrow deepens

until the cell

separates into

two daughter cell

Cytokinesis in animal cell





Cytokinesis in plant cells

Membrane-

enclosed

vesicle joint to

form a cell plate

Cell plate grows

outwards until its

edges fuse with

the plasma

membrane

- New cell wall and

plasma membrane

are formed

The cell plate

divides the cell

into two daughter

cell



The importance of controlled

mitosis

The rate and timing of cell division isimportant for normal cell growth,development and maintenance

Different cells divide at differentfrequencies

Human skin – divide throughout their

lifespanNerve and muscle do not divide once theymature



The effects of uncontrolled mitosis

Cell that divides repeatedly without control

and regulation can produce cancer cells

Due to the severe disruption to the

mechanism that control the cell cycle

Cancer cell will form tumour

Lead to malfunction of tissues and

ultimately death



Factors that caused cancer :

1. Damage to DNA

2. Changes of genes

3. Radiation – UV, gamma rays

4. Chemical compound – tar

5. Carcinogenic – formaldehyde



The application of cloning

Cloning is the process of producing clones

or genetically identical copies of a cell,

tissue or organism through asexual

reproduction

Organisms that are produced have the

same content and chromosome number

as the parent organism





Tissue culture technique

Involves the growth of cells or tissues

outside the organisms in a suitabe culture

medium, which contains nutrients and

growth hormones or through in vitromethods

To produce plant and animal cells through

asexual reproduction



Different parts of plants can be cultured

- young shoots

- meristematic tissues

- leaves

- roots

- seeds

- embryos

- cells



-Small pieces of a plant’s leaf, shoot, bud ,

stem or root tissues are cut out

- these cut out plant tissues are called

explant-Enzymes are used to digest the cell walls

of tissues

-Naked cells without cell walls called

protoplasts

-The protoplasts are sterilised and placed

in a container which contains nutrient

solution

-A culture medium consist a complex

mixture of glucose, amino acids, minerals

and other substances required for thegrowth of tissues

- culture medium and apparatus must be

sterilised



- Protoplast begin to divide

- Cell division produces aggregates cells

and then develop into a callus

- Callus develop into somatic embryo

- Embryo develops into plantlet and

transferred into the soil for growth



Advantages of tissue culture

Thousand of young plants can be

produced

Have the same genetic content as parent

cells

A large number of identical plants can be

grown for commercial purposes



Advantages of cloning

Multiply copies of useful gene or clones

- Escherichia coli – produce bovine growth

hormone

- increase the quality of cow’s milk

- produce insulin – the gene that codes the

synthesis of human insulin is inserted into

the bacteria genome



Transgenic crops can be produced that

are resistant to herbicides, pests and

diseases

- high and better quality yields

- wheat, soya beans



Bacillusthuringiensis

Cotton plant

Resistant to the Bt.

larvae

Gene



- delayed ripening in tomatoes

Does not need pollinating agents

- propagation can take place at any

time

Cloned plants can produce flowers and

fruits within a shorter period

- less effort



Exercise

Describe the significance of mitotic cell

division in living organisms

A somatic plant cell has a chromosomes

number of 4. state how manychromosomes you would expect to find in

this plant at the prophase phase of mitosis



Control environmental pollution

- gene for the synthesis of lipase from

animals is inserted into the bacterial

genome - bacteria that clean up oil spill

- clean up toxic waste

Animal cloning

- cow that has been genetically enginered

to produce antibiotics in its milk



Disadvantages of cloning

The long term side effects to the humanhave not been determined

The long term effect and safety aspect of

releasing bacterial clones to theenvironment are not yet known

Clones do not show genetic variations

- clones can be wipe out easily bydiseases or drastic changes ofenvironment



May undergo mutation which can

endanger human and environment

Gene from transgenic crop may be

transferred to weeds that will becomeresistant to herbicides

Cloned animals have a shorter lifespan



Meiosis

A process of nuclear division that reduces

number of chromosomes in daughter cells

to half of the parent cell



If mitosis is the only mean of nuclear

division, each gamete produced by the

reproductive organs would contains a

complete sets of chromosomesEach offspring through the fertilisation of

the male and female gametes would have

twice the chromosomal number of theirparents



Sperm

46 chromosomes

Ovum

46 chromosomes

Zygote

92 chromosomes



Meiosis produces haploid (n) gametes

Contain half of the genetic material of the

parent cells

During sexual reproduction the fusion of

two gametes restores the complete

number of chromosomes forming a diploid

zygote



Sperm

23 chromosomes

Ovum

23 chromosomes

Zygote

46 chromosomes



Meiosis occurs in reproductive organs that

is in testes and ovaries

Plants – meiosis occur in the anthers and

ovaries



The process of meiosis

Meiosis I

Meiosis II



The stages of meiosis

Interphase

- The cells replicates its DNA /

chromosomes

- After replication each chromosomes

consists of two identical sister chromatids

- Chromosomes are not condense, not

visible under microscope



Animals – a pair of centrosomes are

formed in the cytoplasm

Centrossomes consist a pair of centrioles



Meiosis I



Meiosis I

Prophase I- Chromosomes begins to condense, shorter and

thicker

- Homologous chromosomes come together to

form pairs of bivalent through a process calledsynapsis

- Each bivalent consist of four part structure calledtetrad

- A tetrad consist of two homologouschromosomes, each of which is made up of twosister chromatids





Non sister chromatids exchange segments

of DNA in a process known as crossing

over

Crossing over resuts in new combinationof genes on a chromosomes

The points at which segments of

chromatids cross ver are called chiasma



End of prophase I – the nucleolus and

nuclear membrane disappear

Two pairs of centrioles migrate to the

opposite poles of the cell



Metaphase I

- Spindle fibre pull tetrads to the middle of

the cell

- Homologous chromosomes align at the

metaphase plate / equator of the cell

- One chromosomes of each pair is

attached from one pole and the other at

the opposite chromosome





Anaphase 1

- Homologous chromosomes are pull apart

by spindle fibre and move them to the

opposite pole

- Each chromosomes still consist of two

sister chromatids

- At the end of anaphase I each pole has

only two chromosomes





Telophase I- The chromosomes arrive at the poles

- Each pole has a haploid daughter nucleus

because it contains only one set ofchromosomes

- Spindle fibre disappear

- Nuclear membrane reappear to surroundeach group of chromosomes

- Nucleolus reappear in each nucleus





Cytokinesis occur simultenously withtelophase I

Most organism there are no interpase

between meiosis I and meiosis II





Prophase II

- Nuclear membrane disintegrate

- Spindle fibre reform in each daughter cell





Metaphase II

- The chromosomes each still made up of

two sister chromatids are positioned

randomly at the metaphase [late

- Sister chromatids is attached to the

spindle fibre at the centromere





Anaphase II

- The centromere of the sister chromatids

separate

- The sister chromatids of each

chromosomes are now individual

chromosomes

- Each individual chromosomes movestowards the opposite pole of the cell





Telophase II

- The nuceoli and nuclear membrane reform

- The spindle fibre break down

- Cytokinesis follows and four haploid

daughter cells are formed

- Each haploid cell contains half the number

of chromosomes and genetically different

from the parent diploid cells



Assignment



Assignment

Find out the differences between meiosis Iand meiosis II

State the differences between mitosis and

meiosis

The importance of meiotic cell



p

division

Ensure the diploid number ofchromosomes is maintained

Provides genetic variation

- the process of crossing over in

prophase I result in the

exchange of genetic material

- during metaphase I each pair ofchromosomes is arranged randomly

Appreciating the movement of



pp g

chromosomes during mitosis and meiosis

Asexual reproduction produces offspringthat are identical to the parent

Sexual reproduction produces genetic

variability

If meiosis does not occur properly the

gametes formed will have abnormal

number of chromosomes



47 chromosomes – Down’s syndrome- mental retardation

Radiation and chemicals are carcinogenic

that can disrupt the processes of mitosisand meiosis

Food that contains preservatives such as

sodium nitrite, benzene and formaldehydecan change the structure of DNAmolecule

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Chapter 5,( Meiosis Mitosis)Biof4