c3 Cell Division

TERM Chromosome

A structure made of DNA and histones

DNA (deoxyribonucleic acid) A polynucleotide that contains the pentose sugar

deoxyribose.

Chromatid One of two identical parts of a chromosome held

together by a centromere

Centromere The centralize region joining two sister chromatids

Genome The complete complement of an organisms genes An organisms genetic material

Chromatin Content of the nucleus Consisting of protein, DNA and RNA, which form threads

Euchromatin Loosely packed chromatin

Heterochromatin Densely packed chromatin

Cytokinesis Division of the cytoplasm during cell division

Karyokinesis Division of the nucleus during cell division

Gene Functional unit of heredity

Composed of DNA which carries information from one generation to the next

Gamete A haploid cell, formed by meiotic cell division

of a germ cell

Required for sexual reproduction

The concept of cell division

Basic characteristics of cell division

Characteristics of cell survival, DNA and

chromosome

THE CONCEPT OF CELL DIVISION.

Modern cell theory states that All new cells are

derived from other cell.

All cells which comprise a human are derived,

through the cell division, from single zygote formed

by the fusion of two gametes.

These gametes in turn were derived from the division

of certain parental cell.

There are two basic types:

1.Mitosis which results in all daughter cells

having the same number of chromosomes as

the parent.

2. Meiosis which results in the daughter cells

having only half the number of

chromosomes found in the parent cell.

Cell division

involves the distribution of identical genetic material

(DNA) to two daughter cells.

A dividing cell

duplicates its DNA, allocates the two copies to

opposite ends of the cell

then split into daughter cells.

The cell cycle

Description of the four stages in the cell

cycle

General explanation events during the G1, S

and G2 followed by events in the mitotic

phase.

THE CELL CYCLE.

1. The complete sequence of events in the life of an individual diploid cell.

2. The four stages of the cell cycle;

i. G1

The first Gap Phase

ii. S Phase

iii. G2

The second Gap phase

iv. Mitotic Phase

i. G1 - The first growth

phase

The longest phase

Volume of cytoplasm

increase

Protein synthesis

Increase number of

organelles

ii. S phase

DNA synthesis phase

The cells DNA replicates

and non consist of two

identical chromatids.

iii. G2 - The second growth

phase

Energy stores are increased.

iv. Mitosis

This process of

nuclear division and

followed by division

of cytoplasm called

cytokinesis.

Mitosis

Explain the mitotic cell division using diagrams and photographs

Show the position of the chromosomes at each stage

State the changes in the chromosomes

Describe briefly the cytokinesis process and list the differences between cell division in animal and plant cells

Significances of mitosis

MITOSIS

2 phases : i. nucleus division ( karyokinesis )

ii. cytoplasm division ( cytokinesis )

4 stages: i. prophase

ii. metaphase

iii. anaphase

iv. telophase.

Mitosis in a generalized animal cell.

Prophase Chromosomes

visible as long, thin tangled threads.

shorten and thicken

comprise two chromatids joined at the centromere.

Centrioles migrate to opposite ends of poles of the cell

(except for plant).

microtubules develop and form a star-shaped structure called an aster.

Some of these microtubule, called spindle fibers, span the cell from pole to pole.

The nucleolus disappears

Nuclear envelope disintegrates

Metaphase The chromosomes

arranged themselves at

the metaphase plate,

and become attached

to certain spindle fibers

at the centromere.

Contraction of these fibers draws the

individual chromatids

slightly apart.

Anaphase The centromeres split and further shortening of the spindle fibers causes the

two chromatids of each chromosome to separate and migrate to opposite

poles.

The shorting of the spindle fibers is due to the progressive removal of the tubulin molecules of which they are made.

The energy for this process is provided by mitochondria which are observed to collect around the spindle fibers.

Telophase The chromatids reach their respective poles and a new nuclear envelope

forms around each group.

The chromatids uncoil and lengthen, thus becoming invisible again.

The spindle fibers disintegrate and nucleolus reforms in each new nucleus.

Cytokinesis division of cytoplasm

In Animal Cells

Occur by a process known

as cleavage.

The first sign of cleavage is

the appearance of cleavage

furrow. (Begins as a

shallow growth in the cell

surface)

Cytokinesis division of cytoplasm In Plant Cells Have walls but no cleavage furrow.

During telophase, vesicles derived from Golgi apparatus move along

microtubules to the middle of the cell

producing a cell plate.

The cell plate enlarges until its surrounding membrane fuses with the

plasma membrane along the perimeter

of the cell.

Two daughter cells result, each with its own plasma membrane. A new cell wall

arising from the contents of the cell

plate has formed between the daughter

cells.

Differences between mitosis in plant and animal

cells

Animal Cells Plant Cells

1. Involve aster or

spindle formation.

1. Do not form

centriols and lack

centriols.

2. Cytokinesis occurs

by the constriction of

microtubules cleavage furrow.

2. Occurs by the

growth of a cell plate

through the fusion of

vesicles.

Significance of mitosis

Genetic stability

Mitosis produce two nuclei which have the same number of chromosomes as the parent cell.

Daughter cells are genetically identical to the parent cell and no variation in genetic information

can be introduced during mitosis.

This result in genetic stability within populations of cells derived from the same parental cells.

Significance of mitosis

Growth

The number of cell within organism increases by mitosis and this is the basis of growth in multicellular organisms.

Cell replacement

Replacement of cells and tissues involves mitosis.

Regeneration

Some animal are able to regenerate whole parts of the body, such as legs in crustacea and arms in star fish. Production of the new cells

involve mitosis.

Asexual reproduction

Mitosis is the basis of asexual reproduction, the production of new individuals of a species by one parent organism.

Questions

A

B

Figure 1

Figure 1 shows the phase in two type of cell division and

each cell contain four chromosomes

(a) i. Name the phase in cell A (1M)

ii.State 2 events which occur before the phase

that you name in a(i). (2M)

iii. State the type of cell division in cell A (1M)

iv. State 2 significant of a(iii). (2M)

(b) i. Name the phase in cell B (1M)

ii. Give the reason for your answer in b(i). (1M)

(c) Name the organ which undergoes cell division in a(iii).

In plant : (1M)

In animal : (1M)

Meiosis

Explain the processes in Meiosis I and Meiosis II using diagrams and photographs

Show the position and changes of the chromosomes during each stage

Define chromatid, synapsis, bivalent, tetrad, chiasma, cross-over and centromere

Compare and contrast meiosis and mitosis

CONTENT OF MEIOSIS Meiosis (meio, to reduce)

a form of nuclear division in which the chromosome number is halved from

the diploid number (2n) to the haploid

number (n).

involves DNA replication during interphase in the parent cell,

followed by two cycle of nuclear divisions and cell division,

meiosis I

meiosis II

Thus a single diploid cell gives rise to four haploid cells.

Meiosis occurs during the formation of

sperms and eggs (gametogenesis) in animal and during spores formation in plants.

a continuous process but is

conveniently divided into prophase,

metaphase,

anaphase

telophase.

These stages occur in the first

meiotic division and again in the second meiotic division.

Prophase I Metaphase I Anaphase I Interphase

Meiosis

Telophase I Metaphase II Prophase II Anaphase II

Telophase II

Terms Sister chromatid

Two identical chromatid which are held together at the centromere.

Synapsis Pairing

Bivalent A pair of homologous chromosomes jointed by proteins of the synaptonemal complex and chiasma

Each chromosome consists of two chromatids and therefore, each bivalent consist of four chromatid.

Tetrad a pair of homologous chromosomes with four chromatids

Homologous chromosome A pair of chromosome which have the same pattern of genes along the chromosome but the nature

of the genes may differ.

One member of each pair comes from female parent and the other from male.

Chiasma The region of cross-over at which two non-sister chromatid are joined.

Crossing over An exchange of portions of chromatids between homologous chromosome.

Haploid A single set of unpaired chromosomes

Diploid Two sets of chromosomes are present, one set being derived from the female parent and the other

from male.

MEIOSIS I

MEIOSIS I

4 phase

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase I

The longest phase.

This phase can be

divided into 5 stages;

a) Leptotene

b) Zygotene

c) Pachytene

d) Diplotene

e) Diakinesis

i. Leptotene The beginning of first prophase of meiosis Chromatids can be seen and pairing begins

ii. Zygotene The second phase of the first prophase of meiosis Pairing (synapsis) of homologous chromosomes takes places Intimate contact is made between identical regions of

homologous, in a process involving proteins and DNA organized to form a synaptonemal complex.

iii. Pachytene Paired homologous chromosomes are fully contracted and

twisted around each other.

iv. Diplotene Paired homologous chromosomes begins to move apart They remain attached at a number of points (chiasma)

v. Diakinesis The period at the end of the first prophase of meiosis The separation of homologous chromosomes is almost complete

and crossing over has occurred.

Prophase I

All chromosomes are fully contracted and deeply stained;

The centrioles (if present) have migrated to the poles.

Chiasma and crossing over occurs.

The nucleoli and nuclear envelope have dispersed.

Lastly, the spindle fibers form.

Metaphase I

The bivalents become arranged

around the

metaphase plate,

attached by their

centromeres.

Anaphase I

Spindle fibers pull homologous chromosomes, centromeres first, towards opposite poles of the spindle. This separate the chromosomes into two haploid sets, one set at each end of the spindle.

Telophase I

The arrival of homologous chromosomes at opposite poles marks the ends of meiosis I.

Halving of chromosome number has occurred but the chromosomes are still composed of two chromatids.

Spindle and spindle fibers usually disappear.

Cleavage (animals) or cell wall formation (plants) then occurs as in mitosis.

Prophase I Metaphase I Anaphase I Interphase

Meiosis

Telophase I Metaphase II Prophase II Anaphase II

Telophase II

MEIOSIS II

MEIOSIS II

4 phase

Prophase II

Metaphase II

Anaphase II

Telophase II

Interphase II

This stage is present usually in animal cell

and varies in length.

No further DNA replication occurs.

Meiosis II is similar to mitosis.

Prophase II

This stage is absent if interphase II is absent.

The nucleoli and nuclear envelopes disperse and the chromatids shorten

and thicken.

Centrioles, if present move to opposite poles of the cells and the end of

prophase II new spindle fibers appear.

They are arranged at right angles to the spindle of meiosis I.

Metaphase II

Chromosomes line up separately

around the equator

of the spindle.

Anaphase II

The centromeres divide and the

spindle fibers pull

the chromatids to

opposites poles,

centromeres first.

Sister chromatids separate

Telophase II

As telophase in mitosis but four haploid daughter cells are formed.

The chromosomes uncoiled, lengthen and become very indistinct.

The spindle fibres disappear and the centrioles replicate.

Nuclear envelope re-form around each nucleus which now posses half the number of chromosomes of the original parents cell (haploid).

Subsequent cleavage (animals) or cell wall formation (plants) will produce four daughter cells from the original single parent cell.

SIGNIFICANCE OF MEIOSIS

Halving the chromosome number ensures that when gametes with the haploid number fuse to form a zygote the normal diploid

number is restored.

Meiosis leads to increased variation because: When the haploid cells fuse at fertilization there is

recombination of parental genes.

During metaphase I, homologous chromosomes are

together at the equator of the spindle, but they separate into

daughter cells independently of each other.

Chiasma and crossing-over can separate and rearrange

genes located on the same chromosome.

Three events, unique to meiosis, occur during the first

division cycle

1. During prophase I, homologous chromosomes pair up in a process called synapsis.

A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.

Later in prophase I, the joined homologous chromosomes are visible as a tetrad.

At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.

Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.

2. During metaphase I homologous pairs of chromosomes,

not individual chromosomes are aligned along the metaphase plate.

In humans, you would see 23 tetrads.

3. During anaphase I, it is homologous chromosomes, not

sister chromatids, that separate and are carried to opposite poles of the cell.

Sister chromatids remain attached at the centromere until anaphase II.

The processes during the second meiotic division are virtually identical to those of mitosis.

COMPARISON BETWEEN MEIOSIS AND

MITOSIS

MITOSIS MEIOSIS

1. Occurs in soma cell. Occurs in gonad cells

(ovaries in females and

testes in males)

2. Conserves

chromosome number

(2n) replicated

chromosomes.

Reduces the chromosome

number by half (n) non-

replicated chromosomes.

*gonad : any of usually paired organs in animals that produced reproductive cell (gametes).


MITOSIS

MITOSIS MEIOSIS

3. By the end of prophase,

no synapsis occur to

form bivalent.

Synapsis occurs to form

bivalent at the homologous

chromosomes during

prophase I

4. No chiasma occurs so

there is no cross over. Some chiasma occurs to

form cross over.

Genetic variability is a result

from the cross over.


MITOSIS

MITOSIS MEIOSIS

5. The contain of genetic

in daughter cell is

identical in parental

cells.

The contain of genetic in

daughter cell is no identical

as the parental cell.

6. Two daughter cells

each diploid (2n)

Four daughter cells each

haploid (n)

7. Cytokinesis occurs

once.

Cytokinesis occurs once or

twice.

8. The daughter cell can

produces mitosis.

The daughter cell can

produces mitosis but not

meiosis.

MITOSIS PRODUCES 2 IDENTICAL DAUGHTER CELLS BUT

MEIOSIS PRODUCES 4 NON-IDENTICAL DAUGHTER CELLS

THATS ALL FOR

THIS TOPIC

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c3 Cell Division