Cell Division

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CHAPTER 3CELL DIVISION

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CELL DIVISION

3.1 The concept of cell division.3.2 The cell cycle.3.3 Mitosis.3.4 Meiosis.

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OBJECTIVES

At the end of this lesson, students should be able to:

Explain cell divisions. State the importance of cell division in

living organisms. Explain the stages in cell cycle.

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THE CONCEPT OF CELL DIVISION

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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 (sex cells).

These gametes in turn were derived from the division of certain parental cell.

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There are two basic types:

1.Mitosis which results in all daughter cells having the same number of chromosomes as the parent cell.

1.Meiosis which results in all daughter cells having only half the number of chromosomes found in the parent cell.

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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, and only then split into daughter cells.

The entire genetic material in a cell or individual organism that can be inherits is called genome. -In prokaryotes, the genome is often a single long DNA molecule.-In eukaryotes, the genome consists of several DNA molecules.

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Before the cell can divide, DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete genome.

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into chromosomes.

The DNA is associated with varies proteins that maintain the structure of the chromosome and help control the activity of the gene.

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Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

-Human somatic cells (body cells) have 46 chromosomes.-Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell.

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The major carrier of genetic information in eukaryotes are the chromosomes (within cell nucleus).

Chromosomes are made of chromatin (DNA – protein complex), and it is organized into a long and thin fiber.

The DNA is associated with varies proteins that maintain the structure of the chromosome and help control the activity of the gene.

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After a cell duplicates it’s DNA in preparation for division, the chromatin condenses : It becomes densely coiled and folded, making the chromosome much shorter and so thick.

Each duplicated chromosome has two sister chromatids, containing identical copies of the chromosome’s DNA molecule.

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In its condensed form, the chromosome has a narrow ‘waist’ at a specialized region called the centromere.

Each sister chromatid has a kinetochore of proteins & chromosomal DNA at the centromere.

The kinetochores of the joined sister chromatids face in opposite directions.

Campbell & Reece Sixth Edition m/s 217

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Mitosis, the division of a nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm.

These processes continue every day to replace dead and damaged cell.

Essentially, these processes produce clones - cells with the same genetic information.

Cell division enables organism to grow and repair damaged parts.

Meiosis – produce gametes – eggs or sperm cell and occur in gonad (tester and ovaries).

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Pairs of chromosomes This sort photograph is

called a karyotype.

Karyotype = The number and structure of the chromosome in the nucleus of a cell.

The last pair of chromosomes displayed are the sex chromosomes.

http://www.umanitoba.ca

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All the other chromosomes are termed autosomes.

Autosomes = Any of the chromosomes in a cell other than the sex chromosomes.

We can identify the sex of an individual by looking at the sex chromosomes.


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In female, the two sex chromosomes are alike and are termed X chromosomes.

In males, there is one X chromosome but the other is much shorter and is called the Y chromosome.

Human cells have 46 chromosomes - that’s 23 homologous pairs (homologous chromosomes).


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This total number of chromosomes is called the diploid number (2n).

Diploid = Having two sets of chromosomes, one set being derived from the female parent and the other from the male.


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Half your chromosomes came from your father (sperm) - called the paternal chromosomes.

The other half came from your mother (egg)

- called maternal chromosomes.

Each homologous pair is made up of one paternal and one maternal chromosomes.


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Gametes (sex cells) have half the normal diploid number.

This is called the haploid number, shown as (n).

So, human sperms will have 23 chromosomes and human eggs will have 23 chromosomes.

23http://www.bbc.co.uk

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THE CELL CYCLE

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OBJECTIVES

At the end of this lesson, students should be able to:

Explain the stages in cell cycle

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The cell cycle is the complete sequenceof events in the life of an individual diploid cell.

The cell cycle consists of two main phases:- mitotic (M) phase, or dividing phase- interphase, the nondividing phase.

The M phase includes mitosis and cytokinesis.1. Mitosis - Division of the nucleus.2. Cytokinesis - Division of the cytoplasm.

By Dr. Gary E. Kaiser

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Interphase accounts for 90% of the cell cycle.

During interphase the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division.


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Interphase has three subphases:

the G1 phase (“first gap”) – cell growth before DNA replication,

the S phase (“synthesis”) – DNA replication,

the G2 phase (“second gap”) – preparation for division.

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G1-THE FIRST GROWTH PHASE• Volume of cytoplasm

increase.• Protein synthesis. • mRNA, tRNA, rRNA &

ribosomal protein, tubulin and protein histone

• Synthesize cytoplasmic organelles such as mitochondria and ribosomes

• Increase number of organelles.


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S PHASEThe longest phase DNA synthesis phase.• Replication of DNA

occurs during the S phase,

• Resulting in the DNA content of the cell being double.

• Chromosomes replicate to produce two sister chromatid/ DNA replication.


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G2-THE 2ND GROWTH PHASE Energy stores are

increased (metabolic events occur).

Synthesis microtubule. Cell continue to grow in

size. Protein synthesis and org anelles production still

continue. ATP production. The cell completes

preparations for cell division.


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MITOSIS

This process of nuclear division and followed by division of cytoplasm called cytokinesis.


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Interphase of mitosis in bluebell cell

Interphase/prophase of cell mitosis. Image 1 of 6. Light micrograph of bluebell (Endymion sp.) cells undergoing cell division (mitosis). Four cellular nuclei (stained orange) are seen in the early stages of mitosis. The two nuclei at lower image are seen in interphase. This is the resting stage of the cell between divisions, the nucleus being seen as large and condensed. At upper image two cells are seen in prophase. At this stage the orange thread-like chromosomes are visible and progressively become shorter and fatter until they divide and separate into two "daughter" nuclei later in mitosis. Part of mitosis sequence.

(c) McGraw Hill Ryerson 2007

Chromatin vs. Chromatids

Interphase Late Interphase

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OBJECTIVESAt the end of this lesson, students should be able to:

Describe the four stages of the mitotic cell division.

Explain the behaviour of the chromosomes at each stage.

Briefly describe the cytokinesis process. Compare the cell division in animal and

plant cell. State the significance of mitosis.

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Mitosis involves two phases :

- nucleus division (karyokinesis) - cytoplasm division (cytokinesis).

Mitosis is divided into four distinct stages:- Prophase.- Metaphase.- Anaphase.- Telophase.

Credit: THOMAS DEERINCK, NCMIR/SCIENCE PHOTO LIBRARY

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PROPHASE

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•The chromosomes The chromosomes become visible as long & become visible as long & thin threads.thin threads.

The chromosomes start The chromosomes start to coil up and become to coil up and become shorter and thicker – two shorter and thicker – two chromatids joined at the chromatids joined at the centromere.centromere.

The centrioles move The centrioles move apart to opposite end apart to opposite end (poles) of the nucleus.(poles) of the nucleus.


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From each centriole, microtubules develop and form a star-shaped structure called an aster.

Some of these microtubule, called spindle fibers, cross 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.

44Campbell & Reece Sixth Edition m/s 218

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METAPHASE

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Chromosomes moves to the metaphase plate (cell’s midplane), the plane equidistant between the spindle poles

Kinetochores attach chromosomes to mitotic spindle and align them along metaphase plate at equator of cell


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ANAPHASE

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Begin as the sister chromatids separate.

Kinethochore microtubules shorten at kinetochore end

- chromosomes move to opposite poles.

The shorting of the spindle fibers is due to the progressive removal of the tubulin molecules of which they are made. Campbell & Reece Sixth Edition m/s 219

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


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TELOPHASE

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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 disintegrates and nucleolus reforms in each new nucleus.


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http://deskarati.com

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Cytokinesis – division of cytoplasm

1. In Animal Cells

Occur by a process known as cleavage.

Begins as a ring of actin microfilaments associated with the plasma membrane encircles the cell in the equatorial region.


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The ring contract, producing a cleavage furrow that gradually deepens and separates the cytoplasm into two daughter cells.

Each daughter cell has a complete nucleus.

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Cytokinesis in Animal Cells

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Cytokinesis – division of cytoplasm2. In Plant Cells

Occur by forming a cell plate in the equatorial region and growing laterally toward the cell wall.

The cell plate forms as a line of vesicles originating in the Golgi complex Campbell & Reece Sixth Edition m/s 222

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The vesicles contain materials to construct both a primary cell wall and middle lamella.

The vesicle membranes fuse to become the plasma membrane of each daughter cell.

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Mitosis in a generalized animal cell.

Campbell & Reece Sixth Edition m/s 218 & 219

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Mitosis in a generalized animal cell.

Campbell & Reece Sixth Edition m/s 218 & 219

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Phases of Mitosis in Plant Cells


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Differences between mitosis in plant and animal cells

Animal Cells Plant Cells

1. Centriols present. 1. Centriols absent.

2. Aster formed. 2. Lack of asters.

3. Cytokinesis occurs by the constriction of microtubules – cleavage furrow.

3. Occurs by the growth of a cell plate through the fusion of vesicles.

4. Occurs in all somatic cells.

4. Occurs mainly in meristem tissues.

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SIGNIFICANCE OF MITOSIS1. 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.

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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.

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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.

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OBJECTIVES At the end of the lesson, students should

be able to:

- Explain and compare the processes in meiosis I

and meiosis II.

- Explain the position and changes of the

chromosomes at each stage.

- Define chromatid, synapsis, bivalent, tetrad,

chiasma, crossing over and centromere.

- State the significance of meiosis.

- Compare meiosis and mitosis.

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INTRODUCTION

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Meiosis (meio, to reduce) is a form of nuclear division in which the chromosome number is halved from the diploid number (2n) to the haploid number (n).

Like mitosis, it involves DNA replication during interphase in the parent cell, but this is followed by two cycle of nuclear divisions and cell division, known as meiosis I (the first meiotic division) and meiosis II (the second meiotic division).

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

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Meiosis occurs during the formation of sperms and eggs (gametogenesis) in animal and during spores formation in plants.

Like mitosis, meiosis is a continuous process

These stages occur in the first meiotic division and again in the second meiotic division.

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MEIOSIS I

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Prophase I The longest phase.

This phase can be divided into 5 stages;a)Leptotene.

-Chromosomes appear as fine single threads.

-Spindle starts to form.

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b) Zygotene.-Pairing (synapsis) of homologous chromosomes take place.

-Each pair of homologous chromosome is called a bivalent/ tetrad.

http://bio1152.nicerweb.net

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Homologous chromosomes = Chromosomes having the same structural features. They have the same pattern of genes along the chromosome but the nature of the genes may differ

Bivalent (tetrad) - a pair of homologous chromosome; each chromosome consist of two chromatids and therefore each bivalent have four chromatids

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- The pairing of homologous chromosomes during prophase I of meiosis

Synaptonemal complex

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c) Pachytene.

-Homologous chromosomes are fully contracted and twisted around each other.

-the position of the chiasmata (chiasma;singular) become visible

-Crossing over takes place between non sister chromatids.

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Chiasma (pl. chiasmata)- the point at which paired homologous chromosomes remain in contact as they begin to separate during prophase I of meiosis, forming a cross shape (X shape).- at this point, crossing over occurs

Crossing over – the exchange of genetic material between non-sister chromatids during meiosis I; on factor that result in genetic variation

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d) Diplotene.

- Each homologous pair have separated from one another except at the chiasmata.

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e)Diakinesis.-Separation of homologous chromosomes is almost complete and the crossing over has occurred.

-The nucleoli and the nuclear membrane break down.

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By the end of prophase I:

-All chromosomes are fully contracted & deeply stained.

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

-The nucleoli & nuclear envelope has dispersed.

-Lastly the spindle fibres form.

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Prophase I is the longest and most complex stage in meiosis.

The key events are:

-The pairing of the homologous chromosomes (bivalents)

-The exchange of chromatid material at chiasmata (crossing over).

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Metaphase I

- The bivalents become arranged on the equator of the spindle, attached by their centromeres.

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Anaphase I

The paired homologous chromosomes separate and move to opposite poles.

This separate the chromosomes into two haploid sets, one set at each end of the spindle.

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Telophase I

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

The chromatids generally decondense, the nuclear envelope may reorganize and cytokinesis may take place.

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

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MEIOSIS II

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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.

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Prophase II

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.

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Metaphase II Chromosomes line up

separately on the equator of the spindle.

Anaphase II The centromeres divide

and the spindle fibers pull the chromatids to opposites poles,

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Telophase II Four haploid (n) daughter

cells are formed.

The chromosomes uncoiled, lengthen and become very indistinct.

The spindle fibres disappear.

Nuclear envelop re-form.

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Chromatid.A thread like strand formed from a chromosome during early stages of a cell division. Each chromosome divides along its length into 2 chromatids, which are at first held together at the centromere.

Synapsis.The close association between homologous chromosomes that develops during the first prophase of meiosis.

Bivalent.The two sets of paired chromosomes lay alongside each other.

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Tetrad.The chromosome complex formed by the synapsis of a pair of homologous chromosomes (i.e four chromatids)

Chiasma (pl. chiasmata)The point at which paired homologous chromosomes remain in contact as they begin to separate during prophase I of meiosis, forming a cross shape (X shape).

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Crossing over

The exchange of genetic material between non-sister chromatids during meiosis I; on factor that result in genetic variation.

Centromere

A specialist constricted region of a chromatid; contain the kinetochore that attach to the spindle during cell division.

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SIGNIFICANCE OF MEIOSIS

Golfer Tiger Woods holds his newborn daughter Sam Alexis Woods as wife Elin kisses the baby. Woods has famously refered to his mixed-race identity as 'Cablinasian,' a word he derived from Caucasian, black, American-Indian and Asian. (WireImage, Gretchen Dow Mashkuri/Associated Press)

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1. Meiosis ensures the constant chromosomal number is maintained from one generation to the next. This is done by the fusion of haploid gametes to produce diploid organisms.

2. Meiosis is important in genetic variation by producing new combinations of chromosomes & new combinations of alleles at different genetic loci.

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COMPARE AND CONTRAST BETWEEN MEIOSIS AND MITOSIS

No. MITOSIS MEIOSIS

1. Occurs in soma cell. Occurs in gametes cells(ovaries in females andtestes in males)

2. Conserveschromosome number(2n) replicatedchromosomes.

Reduces the chromosomenumber by half (n) non-replicated chromosomes.

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NU.

MITOSIS MEIOSIS

3. By the end of prophase,no synapsis occur toform bivalent.

Synapsis occurs to formbivalent at the homologouschromosomes duringprophase I

4. No chiasma occurs sothere is no cross – over.

Some chiasma occurs to formcross – over.Genetic variability is a resultfrom the cross – over.

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No. MITOSIS MEIOSIS

5. The contain of genetic indaughter cell is identicalin parental cells.

The contain of genetic indaughter cell is no identicalas the parental cell.

6. Two daughter cells eachdiploid (2n)

Four daughter cells eachhaploid (n)

7. Cytokinesis occursonce.

Cytokinesis occurs once ortwice.

8. The daughter cell can produces mitosis.

The daughter cell canproduces mitosis but notmeiosis.

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MITOSIS PRODUCES 2 IDENTICAL DAUGHTER CELLS BUT MEIOSIS PRODUCES 4 NON-IDENTICAL DAUGHTER CELLS


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111Campbell & Reece Sixth Edition m/s 242

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COMPARISON OF MEIOSIS I WITH MITOSIS

Meiosis I Mitosis

Prophase I

Pairing of homologouschromosome

Prophase

No pairing ofchromosomes

Metaphase I

Bivalent at metaphaseplate

Metaphase

Chromosomes at metaphase plate

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COMPARISON OF MEIOSIS I WITH MITOSIS

Meiosis I Mitosis

Anaphase I

Homologous of eachbivalent separate andchromosomes move topoles

Anaphase

Sister chromatidsseparate, becomingdaughter chromosomesthat move to the poles.

Telophase I

Two haploid daughtercells

Telophase

Two daughter cells, identical to the parentcell.

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BYE……BYE…….

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