lecture 10-mitosis and meiosis

8/7/2019 Lecture 10-Mitosis and Meiosis

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Reproduction

Living organisms reproduce by two methods

Asexual reproduction

Offspring are identical to the original cell or organism

Involves inheritance of all genes from one parent

Sexual reproduction Offspring are similar to parents, but show variations in

traits

Involves inheritance of unique sets of genes from two

parents

Copyright 2009 Pearson Education, Inc.


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Cell Reproduction

Cellular reproduction is the process by which organisms

(unicellular, multicellular) reproduce

Cell arise from pre-existing cells

Cellular reproduction involves two important processes Growth

Cell division

Growth involves duplication of the cells DNA andcontents in cytoplasm

Cell division involves the separation of the cells DNA andcontent into daughter cells


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Eukaryotic chromosomes are composed ofchromatin

Chromatin = DNA + proteins

To prepare for division, the chromatin becomes highly compact,

and the chromosomes are visible with a microscope

Early in the division process, chromosomes duplicate Each chromosome appears as two sister chromatids,

containing identical DNA molecules

Sister chromatids are joined at the centromere

Chromatin



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Chromatin to Chromosomes


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Sister chromatids

Centromere


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The Cell Cycle

The process of cellular reproduction involves an orderedseries of steps - cell cycle

A cell spends most of its existence in interphase Most of interphase involves the normal activities and functions of

cell

Part of interphase is spent preparing the cell for cell division


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The cell cycle is an ordered sequence of events for celldivision

It consists of two stages

Interphase: duplication of cell contents

G1

growth, increase in cytoplasm Sduplication of chromosomes DNA replication

G2growth, preparation for division

Mitotic phase: division Mitosisdivision of the nucleus

Cytokinesis

division of cytoplasm

The cell cycle



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S

(DNA synthesis)G1

G2

INTERPHASE


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Mitosis and Cytokinesis

The separation of the sister chromatids during mitosis

produces two genetically identical daughter chromosomes.

The daughter cells produced by mitosis have the same

number of chromosomes as the original cell and each has

identical content


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Centromere

Chromosomeduplication

Sister

chromatids

Chromosomedistribution

todaughter

cells


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Mitosis progresses through a series of stages

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis often overlaps telophase

Mitosis



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The Spindle

In most eukaryotic cells, the separation of identicalchromatids during mitosis utilizes spindle fibers

The spindle fibers are organized by the centrosome


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Centrosomes

(with centriole pairs) Kinetochore

Early mitotic

spindle

Chromatin

INTERPHASE PROMETAPHASEPROPHASE

Centrosome Fragments

of nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Nuclearenvelope

Spindlemicrotubules

Nucleolus

Centromere


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Metaphaseplate

Nucleolusforming

METAPHASE TELOPHASE AND CYTOKINESISANAPHASE

Cleavagefurrow

Daughterchromosomes

NuclearenvelopeformingSpindle


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Human cells have 46 chromosomes. By the end of

interphase

How many chromosomes are present in one cell?

How many chromatids are present in one cell?

Questions!



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Phases of Mitosis in Animal Cells

Cytokinesis - Cytoplasm is divided into separate cells

- Occurs after mitosis or at the same time

with telophase

- It occurs differently in animal and plantcells

The cell cycle is completed after cytokinesis

Cytokinesis does not always occur!!!

If cytokinesis does not occur, the cell will be multinucleated(have multiple nuclei)


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Cytokinesis in Animal and Plant

Cells In animal cells begins during anaphase as a cleavagefurrow, an indentation of the membrane.

Actin filaments contracts until the cytoplasm is separated

between the two daughter cells

For plants it forms a cell plate (from vesicles that

transport cell material)


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Cleavage

furrow

Contracting ring ofmicrofilaments

Daughter cells

Cleavage furrow


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Cell plate Daughter cells

Cell wall

Vesicles containing

cell wall material

Daughternucleus

Cell plateforming

Wall ofparent cell

New cell wall


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Cell Cycle Checkpoints

The cell cycle has checkpoints that control

the progression of the cell cycle G1 checkpoint is important because passing this

point commits the cell to division - if DNA

damaged apoptosis can occur

G2 checkpoint is the point at which the cell cycle

pauses until DNA replication has been completed

M checkpoint mitosis when division pauses untilthe chromosomes are distributed accurately to the

daughter cells


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G1 checkpoint

Control

system

M

S

G2

G1

M checkpoint

G2 checkpoint

G0


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The Cell Cycle and Cancer

Cancer Cells Cancer cells divide rapidly and escape controls of the cell cycle

They spread to other tissues through the circulatory system

Growth is not inhibited by other cells, and tumors form

Benign tumors remain at the original site Malignant tumors spread to other locations by metastasis

How do we treat cancers?

The regulation of the cell cycle is lost and uncontrolled cell divisionoccurs

Cancers are classified by their location

Carcinomas are cancers of the organs Sarcomas are cancers of the muscles

Leukemias are cancers of the blood


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Somatic cells have pairs of homologous chromosomes,receiving one member of each pair from each parent

Homologous chromosomes are matched in

Length

Centromere position Gene locations

A locus (plural, loci) is the position of a gene

Different versions of a gene may be found at the same locus on

maternal and paternal chromosomes

Somatic Cells



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The human sex chromosomes X and Y differ in size andgenetic composition

Pairs ofautosomes have the same size and genetic

composition

Applying Your Knowledge Humans have 46 chromosomes; how many homologous pairs

does that represent?

If there is one pair of sex chromosomes, how many pairs of

autosomes are found in humans?

Autosomes & Sex Chromosomes



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Sister chromatids One duplicatedchromosome

Centromere

Homologous pair of

chromosomes


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Meiosis

The homologous set of chromosomes is called the

diploid number (46)

The number of chromosome pairs an organism has is its

haploid number (23) Meiosis occurs in the sex organs producing gametes

(which one are these?) sperm and egg


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Haploid gametes (n = 23)

nn

Egg cell

Sperm cellFertilizationMeiosis

Multicellulardiploid adults

(2n = 46)

Mitosis anddevelopment

nn

22nn

Diploidzygote

(2n = 46)


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The Human Life Cycle

In males, meiosis is part of sperm production

(spermatogenesis)

In females, meiosis is part of egg production (oogenesis)

During fertilization, an egg and sperm unite to form azygote, restoring the chromosomes to 46 total


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Overview of Meiosis

Meiosis involves two cellular divisions

Meiosis I

Meiosis II

These two cellular divisions produce four daughter cells,each with one chromosome of each pair


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The Phases of Meiosis

The same four stages of mitosis occur during meiosis

Prophase

Metaphase

Anaphase Telophase

The four stages occur twice, once during meiosis I and

again during meiosis II


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Centrosomes(with centriolepairs)

PROPHASE I

Microtubules

attached to

kinetochore

INTERPHASE

Sites of crossing overMetaphaseplate

Spindle

MEIOSIS I: Homologous chromosomes separate

METAPHASE I

Sister chromatidsremain attached

ANAPHASE I

Nuclearenvelope

Sisterchromatids

Centromere(with kinetochore)

Homologouschromosomes separateChromatin

Tetrad


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PROPHASE I

MEIOSIS II: Sister chromatids separate

METAPHASE II ANAPHASE II

Cleavagefurrow

TELOPHASE II

AND CYTOKINESIS

Sister chromatidsseparate

Haploid daughtercells forming

TELOPHASE II

AND CYTOKINESIS


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Mitosis vs. Meiosis

Which characteristics are similar for mitosis andmeiosis? One duplication of chromosomes

Which characteristics are unique to meiosis?

Two divisions of chromosomes

Pairing of homologous chromosomes

Exchange of genetic material by crossing over

What is the outcome of each process?

Mitosis: two genetically identical cells, with the samechromosome number as the original cell

Meiosis: four genetically different cells, with half thechromosome number of the original cell


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Prophase

Metaphase IMetaphase

2n = 4

Tetradsalign at the

metaphase plate

Duplicatedchromosome(two sisterchromatids)

Parent cell(before chromosome duplication)


Chromosomesalign at the

metaphase plate

AnaphaseTelophase Sister chromatids

separate duringanaphase

Daughter cellsof mitosis

2n 2n

n


Site ofcrossing over

Tetrad formedby synapsis ofhomologouschromosomes

MEIOSIS

Prophase I

Anaphase I

Telophase I

MITOSIS

MEIOSIS I

Haploid

n = 2Daughtercells of

meiosis I

MEIOSIS II

n n n

Daughter cells of meiosis II

Homologouschromosomesseparate(anaphase I);

sister chroma-tids remaintogether

No furtherchromosomalduplication;sisterchromatidsseparate(anaphase II)


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Abnormal sex chromosomes

number Down syndrome is trisomy for chromosome 21

A Turner syndrome female has monosomy for the sex

chromosomes (XO)

A person with Klinefelter syndrome is an XXY male


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Down Syndrome trisomy for chromosome 21

lecture 10-mitosis and meiosis

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