Cell Division Note

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3.0 CELL DIVISION (3 hours)

byAmran Md SaidMatriculation College of Pahang

Chapter Outlines:

3.1 The Concept of Cell Division

The importance of cell division in living organisms.• a phenomenon to determine the survival of organisms• distribution of indentical material to the daughter cells.• cell division results in the increase in number of cells

Basic characteristics:Cell division results in the increase in number of cellsCell division is the means whereby parents pass on genetic material to the daughter cells

Happens in 2 ways: Mitosis – in somatic cells Meiosis – in reproductive organs


Cell division is the means whereby parents pass on genetic material to the daughter cells

genetic material related with DNA and Chromosome

What is DNA and Chromosome?

• DNA – the basic building block of DNA consisting a sugar, a base, and a phosphate. • Nucleosomes – the basic structural unit of eukaryotic nuclear chromosomes, consisting

of two molecules each of the four core histones.• Chromatin – The piece of DNA-protein complex that is studied and analyzed. • Chromosome – The physical structures in which the genetic material of the cell is

organized.


Genetic Blueprints for Cells

FUNCTIONS OF:DNA - codes plans for making cells Chromosome - a single DNA molecule containing many genes human (46 chromosomes)Gene – each gene gives the directions for making 1 protein humans (approximately 2000 genes in each chromosome)

Parts of chromosomes

• Centromeres - region two chromosomes held are together before they are separated in mitosis

• Kinetochore - proteins bind to centromere and attach chromosome to spindle in mitosis • Chromatid - one of the two visibly distinct longitudinal subunits of all replicated

chromosomes that becomes visible btw early prophase and metaphase of mitosis

• Sister chromatids & centromeres

When do we expect to see chromatin & chromosomes?


Chromatin • thin threads comprises DNA and associated proteins • when eukaryotic cell is not dividing (interphase)

Chromosomes

• highly compacted structures from condensed chromatin.• seen at the time of cell division (prophase)

THE CELL CYCLE1.2 The Cell Cycle

• the period that extends from the time a new cell is produced till it completes its division is known as a cell cycle

• Four stages in the cell cycle• Events during the G1, S, G2 & M

The Cell Cycle

• orderly sequence of events • occurs from the time a cell is first formed• until it divides into two new cells.• Most of the cell cycle is spent in interphase.• Following interphase is the mitotic stage.• The four stages of the cell cycle;

i. G1 – The first growth phaseii. S Phaseiii. G2 – The second growth phaseiv. Mitosis

THE CELL CYCLE


The cell cycle is the complete sequence of events in the the period that extends from the time a new cell is produced till it completes its division is known as a cell cycleife of an individual diploid cell.

i. G1 – The first growth phase

The longest phaseVolume of cytoplasm increaseProtein synthesisIncrease number of organelles: mitochondria, ER, Golgy apparatus

ii. S phase

DNA synthesis phaseThe cell’s DNA replicates

iii. G2 – The second growth phase

Energy stores are increased.

1.3 Mitosis

At the end of this topic, students should be able to :• Describe the four stages of the mitotic cell division • Explain the behavior of the chromosomes at each stage• Briefly describe the cytokinesis process• Compare the cell division in animal and plant cells• State the significance of mitosis


Mitosis

• This is a process of nuclear division (karyokinesis) and followed by division of cytoplasm called cytokinesis.

Review of The Eukaryotic Cell Cycle

Mitosis

Definition of termsWhat are sister chromatids and non-sister chromatids?

Sister chromatids • duplicated chromosome• genetically identical

Non sister chromatids• Genetically non identical (one from maternal &

another from paternal)


Mitosis Process

Mitosis is divided into four distinct stages: prophase, metaphase, anaphase and telophase.(PMAT)

What happens during mitosis?

• Centromeres divide• sister chromatids separate • become daughter chromosomes.• End of mitosis, each chromosome consists of a

single chromatid.

What is mitotic cell division? is the division of somatic cells

produce cells that are an exact copy of parent cell

whereby the number of chromosomes stays constant 2n 2n.

it ensures genetic stability through generations.

PROPHASE

The chromosomes become visible, long, thin threads.

Gradually they shorten and thicken and each is seem to comprise two chromatids joined at the centromere.

From each centriole, 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.Collectively they form the spindle.

The nucleolus disappears and finally the nuclear envelope disintegrates, leaving the chromosomes within the cytoplasm of the cell.

2. METAPHASE

Centrosome positioned at opposite polesChromosomes move along spindle microtubules to equator of the cellAttach to spindle fibre by means of centromere

3. ANAPHASE

Sister chromatids


Separate & move towards opposite polesOne set of chromosomes moves along the spindle microtubule to each pole of the cell.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.

4. 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 CYTOKINESIS – division of cytoplasm– division of cytoplasmIn Animal CellsOccur by a process known as cleavage.The first sign of cleavage is the appearance of cleavage furrow.

CYTOKINESIS– division of cytoplasmCYTOKINESIS– division of cytoplasmIn 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. Cytokinesis in PlantsCytokinesis in Plants

Differences between mitosis in Differences between mitosis in plant and animal cellsplant and animal cellsSignificance of mitosisSignificance of mitosisGenetic 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.

2. Growth

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


3. Cell replacement

Replacement of cells and tissues involves mitosis.

4. 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.

5. Asexual reproduction

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

Stages of Mitosis (diagrams)

Mitosis – growth & developmentAsexual Reproduction by Mitosis (Protozoa)Asexual Reproduction by Mitosis (Hydra) Asexual Reproduction by Mitosis (yeast)

Explain the processes in Meiosis I and Meiosis II Explain the position and changes of the chromosomes during each stageDefine chromatid, synapsis, bivalent, tetrad, chiasma, cross-over and centromereState the significance of meiosis Compare and contrast meiosis and mitosis Meiosis requires two nuclear divisions and four haploid nuclei result.Humans have 23 pairs of homologous chromosomes, or 46 chromosomes total.

Process whereby a nucleus divides by two divisions into four nuclei, each containing half the original number of chromosomes.Consist of meiosis I and meiosis II.The period of time between meiosis I and meiosis II is called interkinesis .No replication of DNA occurs during interkinesis because the DNA is already duplicated.

Prophase I

The longest phase. This phase can be divided into 5 stages;

a) Leptoteneb) Zygotenec) Pachytened) Diplotenee) Diakinesis


Five Sequential StagesLeptotene - Chromosomes condenseZygotene - Synaptonemal complexPachytene - Crossing overDiplotene - Cell growthDiakinesis - Chromosomes recondense

Metaphase I

The bivalents become arranged around the equator of the spindle, 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.

Telophase I

Spindle disappear.Cleavage furrow (animals) or cell plate (plants) then occurs as in mitosis.

Interkinesis

a period of time between two nuclear divisions of a cell where no DNA replication occur.

Meiosis II is similar to mitosis.

Prophase II

This stage is absent if interkinesis 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.

Metaphase IIChromosomes line up separately around the equator of the spindle.


Anaphase IIThe centromeres divide and the spindle fibers pull the chromatids to opposites poles, centromeres first.

Telophase II

As telophase in mitosis but in meiosis 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).

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.

Chiasmata 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. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.

In humans, you would see 23 tetrads.

3. At 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.

Meiosis I:Prophase I - pairing of homologous chromosomesMetaphase I – homologous pairs line up at equatorAnaphase I – homologous chromosomes separateTelophase I – daughter cells are haploid

The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the haploid number of chromosomes.At the end of telophase II of meiosis II, there are four haploid daughter cells that are not genetically identical.At the end of mitosis , there are two diploid daughter cells that are identical.

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Cell Division Note