CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Mitosis – division of a cell to produce two identical cells; used mainly for growth and cell replacement

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION

• Mitosis – division of a cell to produce two identical cells; used mainly for growth and cell replacement

• Meiosis – division of cells to produce 4 daughter cells (gametes) with a single set of chromosomes

Copyright © 2009 Pearson Education, Inc.

8.1 Like begets like, more or less

– 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


Sexual reproduction produces offspring with unique combinations of genes

– Cell division perpetuates life– Cell division is the reproduction of cells

Growth and reproduction of organisms are made possible through cell division


Cell division results in genetically identical daughter cells

• Most cell division results in daughter cells with identical genetic information, DNA

• A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Terms

• All the DNA in a cell constitutes the cell’s genome

• DNA molecules in a cell are packaged into chromosomes


Terms

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

• Somatic cells (nonreproductive cells) have two sets of chromosomes. Most all cells in body

– They are diploid. (2n)

• Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells

– They are haploid. (1n)Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Terms

• Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division

• In preparation for cell division, DNA is replicated (duplicated) and the chromatin condenses so you can see the chromosomes

• Each duplicated chromosome has two sister chromatids, which separate during cell division


Mitosis – division of a cell to produce two identical cells; used mainly for growth and cell replacement


The cell cycle is an ordered sequence of events for cell division

It consists of two stages– Interphase: duplication of cell contents

including DNA– G1—growth, increase in cytoplasm

– S—duplication of chromosomes

– G2—growth, preparation for division

– Mitotic phase: division

– Mitosis—division of the nucleus

– Cytokinesis—division of cytoplasm

8.5 The cell cycle multiplies cells


S

(DNA synthesis)G1

G2

Cytokinesis

Mito

sis

INTERPHASE

MITOTICPHASE (M)

Cell cycle

Centrosomes(with centriole pairs)

Early mitoticspindle

Chromatin

INTERPHASE PROPHASE

Centrosome

Plasmamembrane

Chromosome, consistingof two sister chromatids

Nuclearenvelope

Nucleolus

Centromere

Interphase

– In the cytoplasm

– Cytoplasmic contents double

– In the nucleus

– Chromosomes duplicate during the S phase

8.6 Cell division is a continuum of dynamic changes


Chromatid

One duplicatedchromosome

Centromere

One unduplicatedchromosome

– Chromosome Monitor: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?



Prophase

– In the nucleus

– Chromosomes coil and become compact



Metaphaseplate

Nucleolusforming

METAPHASE TELOPHASE AND CYTOKINESISANAPHASE

Cleavagefurrow

Daughterchromosomes

NuclearenvelopeformingSpindle

Metaphase

– Chromosomes align at the cell equator

– Chromosome Monitor:By the end of metaphase

– How many chromosomes are present in one human cell?

– How many chromatids are present in one human cell?



Anaphase – Sister chromatids separate at the centromeres– Daughter chromosomes are moved to opposite

poles of the cell– Chromosome Monitor:

By the end of anaphase – How many chromosomes are present in one

human cell?– How many chromatids are present in one human

cell?



Telophase– The cell continues to elongate– The nuclear envelope forms around

chromosomes at each pole, establishing daughter nuclei

– Chromatin uncoils – Chromosome Monitor:

By the end of telophase – How many chromosomes are present in one

nucleus within the human cell?– Are the nuclei identical or different?



Cytokinesis

– Cytoplasm is divided into separate cells

– The cell begins to look like a figure 8 because cytokinesis is occuring



Cleavagefurrow

Contracting ring ofmicrofilaments

Daughter cells

Cleavage furrow

Mitosis

Correctly label phases of mitosis in this onion root tip specimen.

– Mitosis produces genetically identical cells for – Growth– Replacement– Asexual reproduction

8.11 Review: Mitosis provides for growth, cell replacement, and asexual reproduction


MEIOSIS AND CROSSING OVER


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

8.12 Chromosomes are matched in homologous pairs


Sister chromatids One duplicatedchromosome

Centromere

Homologous pair ofchromosomes

Identify homologous pairs of chromosomesOne from mother; one from father

Identify duplicatedchromosomesIdentify sister chromatids

8.17 Homologous chromosomes can carry different versions of genes

Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes

– Homologous chromosomes may have different versions of a gene at the same locus

– One version was inherited from the maternal parent, and the other came from the paternal parent

– Homologues do not end up in the same cell at the end of meiosis so, gametes will receive either the maternal or paternal version of the gene


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

Pairs of autosomes have the same size and genetic composition

Chromosome Monitor:

– 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?

8.12 Chromosomes are matched in homologous pairs


Meiosis is a process that converts diploid nuclei to haploid nuclei

– Diploid cells have two homologous sets of chromosomes

– Haploid cells have one set of chromosomes

– Meiosis occurs in the sex organs, producing gametes—sperm and eggs

Fertilization is the union of sperm and egg

– The zygote has a diploid chromosome number, one set from each parent

8.13 Gametes have a single set of chromosomes


Haploid gametes (n = 23)

nEgg cell

Sperm cellFertilizationMeiosis

Multicellulardiploid adults

(2n = 46)

Mitosis anddevelopment

n

2n

Diploidzygote

(2n = 46)

The human life cycle

Like mitosis, meiosis is preceded by interphase

– Chromosomes duplicate during the S phase

Unlike mitosis, meiosis has two divisions

– During meiosis I, homologous chromosomes separate

– The chromosome number is reduced by half

– During meiosis II, sister chromatids separate

– The chromosome number remains the same

8.14 Meiosis reduces the chromosome number from diploid to haploid


Events in the nucleus during meiosis I

– Prophase I


– Homologous chromosomes come together as pairs by synapsis

– Each pair, with four chromatids, is called a tetrad

– Nonsister chromatids exchange genetic material by crossing over



Genetic recombination is the production of new combinations of genes due to crossing over

Crossing over involves exchange of genetic material between homologous chromosomes

– Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over

– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids

8.18 Crossing over further increases genetic variability


Animation: Crossing Over

Centromere

Chiasma

Tetrad

– Chromosome Monitor:Human cells have 46 chromosomes. At the end of prophase I

– How many chromosomes are present in one cell?

– How many chromatids are present in one cell?



Events in the nucleus during meiosis I

– Metaphase I– Tetrads align at the cell equator

– Anaphase I– Homologous pairs separate and move toward

opposite poles of the cell



Centrosomes(with centriolepairs)

PROPHASE I

Microtubulesattached tokinetochore

INTERPHASE

Sites of crossing over Metaphaseplate

Spindle

MEIOSIS I: Homologous chromosomes separate

METAPHASE I

Sister chromatidsremain attached

ANAPHASE I

Nuclearenvelope

Sisterchromatids

Centromere(with kinetochore)

Homologouschromosomes separateChromatin

Tetrad

Events in the nucleus during meiosis I – Telophase I

– Duplicated chromosomes have reached the poles

– A nuclear envelope forms around chromosomes in some species

– Each nucleus has the haploid number of chromosomes

– Chromosome Monitor:After telophase I and cytokinesis





Meiosis II follows meiosis I without chromosome duplication

Each of the two haploid products enters meiosis II

Events in the nucleus during meiosis II

– Prophase II





– Metaphase II

– Duplicated chromosomes align at the cell equator

– Anaphase II

– Sister chromatids separate and chromosomes move toward opposite poles




– Telophase II

– Chromosomes have reached the poles of the cell

– A nuclear envelope forms around each set of chromosomes

– With cytokinesis, four haploid cells are produced

– Chromosome Monitor:After telophase II and cytokinesis





PROPHASE I

MEIOSIS II: Sister chromatids separate

METAPHASE II ANAPHASE II

Cleavagefurrow

TELOPHASE IIAND CYTOKINESIS

Sister chromatidsseparate

Haploid daughtercells forming

TELOPHASE IIAND CYTOKINESIS

Comparing Mitosis and Meiosis

Mitosis

Meiosis

Comparing Mitosis and Meiosis

Mitosis v meiosis

http://www.pbs.org/wgbh/nova/miracle/divide.html

Which characteristics are similar for mitosis and meiosis?

– One duplication of chromosomes

Which characteristics are unique to meiosis?

– Two divisions of cells: meiosis I and meiosis II to create 4 cells instead of 2

– Occurs to create haploid gametes

– Pairing of homologous chromosomes for crossing over and across the metaphase plate in metaphase I

– Exchange of genetic material by crossing over

8.15 Mitosis and meiosis have important similarities and differences


What is the outcome of each process?– Mitosis: two genetically identical cells, with the

same chromosome number as the original cell

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

8.15 Mitosis and meiosis have important similarities and differences


Mitosis:

Loss of Regulation/Cancer


Cells anchor todish surfaceand divide.

When cells haveformed a completesingle layer, theystop dividing (density-dependent inhibition).

If some cells arescraped away, theremaining cells divideto fill the dish with asingle layer and thenstop (density-dependent inhibition).

G1 checkpoint

Controlsystem

M

S

G2

G1

M checkpoint

G2 checkpoint

G0

Cancer cells have genetic mutations that allow them to override these checkpoints and multiply indeterminately

Cancer cells escape controls on the cell cycle

– Cancer cells divide rapidly, often in the absence of growth factors

– They spread to other tissues through the circulatory system

– Growth is not inhibited by other cells, and tumors form

– Malignant tumors spread to other locations by metastasis

– Benign tumors remain at the original site

Growing out of control, cancer cells produce malignant tumors


Cells anchor todish surfaceand divide.

When cells haveformed a completesingle layer, theystop dividing (density-dependent inhibition).

If some cells arescraped away, theremaining cells divideto fill the dish with asingle layer and thenstop (density-dependent inhibition).

Cancer -cells will not stop

dividing

It is estimated that 1,437,180 people will be diagnosed with and 565,650 men and women will die of cancer in 2008 in the US

No cure for cancer because of the high variability of the disease between type and individuals

Growing out of control, cancer cells produce malignant tumors

Copyright © 2009 Pearson Education, Inc.SEER Cancer Statistics

Tumors

Benign tumor is a contained mass of gradually multiplying cells resembling its tissue of origin and is seldom fatal

Can grow to be so large because they don’t metastasize

Ex. (Neither ex is always benign)

Teratoma – can grow teeth and hair

Lipoma – tumor of the adipose tissue

Man with Thigh TumorThe Gordon Museum

http://images.google.com/imgres?imgurl=http://www.uphs.upenn.edu/path/web_docs/p200/GYN200/GYN0308.jpeg&imgrefurl=http://www.digitalspy.co.uk/forums/showthread.php?t=918782&usg=__HTmV8s2ARjPFt-BbII8QfnAsd6E=&h=370&w=500&sz=39&hl=en&start=20&um=1&tbnid=DLM_9HhFQO8K7M:&tbnh=96&tbnw=130&prev=/images?q=teratoma&um=1&hl=en&rls=com.microsoft:en-us&sa=N

ALTERATIONS OF CHROMOSOME NUMBER AND

STRUCTURE


Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis

– During Meiosis I

– Both members of a homologous pair go to one pole

– During Meiosis II

– Both sister chromatids go to one pole

Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes

8.21 Accidents during meiosis can alter chromosome number


Nondisjunctionin meiosis II

Normalmeiosis I

Gametes

Number of chromosomes

n + 1 n – 1 n n

Abnormal chromosome number: aneuploidy

Trisomy 21 involves the inheritance of three copies of chromosome 21

– Trisomy 21 is the most common human chromosome abnormality

– Chromosome 21 is one of the smallest human chromosomes (containing a relatively small number of genes). This provides an explanation as to why this condition is not fatal, while a trisomy involving another, larger autosome that controls many more genes would most likely be fatal.

An extra copy of chromosome 21 causes Down syndrome


Down syndrome An imbalance in chromosome number

causes Down syndrome, which is characterized by Characteristic facial features Susceptibility to disease Shortened life span Mental retardation Variation in characteristics

The probability of conceiving a baby with trisomy 21 increases with age of mother At maternal age 20 to 24, the probability is 1/1562

35 - 39 the probability is 1/214

>45 the probability is 1/19

Average lifespan of about 45-50 years

Most begin to suffer from dementia if they live into their 40’s or 50’s

Christopher Joseph Burke (Life Goes on and Touched by an Angel)(age 43)

http://en.wikipedia.org/wiki/File:Chris_Burke.jpg

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Age of mother

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20 40353025 5045

First described in 1942

Klinefelter's syndrome is the most common sex chromosome disorder

The principal effects are development of small testicles and reduced fertility

A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms

The condition exists in roughly 1 out of every 500 males

Cannot fix chromosmal abnormality, but testosterone treatment is an option for some individuals who desire a more masculine appearance and identity

Klinefelter’s syndrome (XXY)


17 year old twins with Klinefelter’s- exhibit gynecomastia and present with high pitched voices.

http://www.mdconsult.com/das/book/body/0/0/1555/f4-u1.0-B978-1-4160-2911-3..50026-1..gr60.jpg

More aneuploidy

Male

Taller than fathers and brothers

Some studies claim are more aggressiveand likely to be in prison (debatable)

Commonly go through life without diagnosis

XXX

Female

taller than average

Commonly go through life without diagnosis


XYY

Only syndrome in humans where having 45 chromosomes is not fatal

Usually have the genotype X0

First described in 1938

Occurring in 1 out of every 2500 girls

There are characteristic physical abnormalities, such as short stature, lymphoedema, broad chest, low hairline, low-set ears, and webbed neck

Exhibit gonadal dysfunction with amenorrhea and infertility

Approximately 98% of all fetuses with Turner syndrome result in miscarriage

Turner syndrome (XO)


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http://www.mdconsult.com/das/pdxmd/media/1206/6120625/large.jpg

http://odlarmed.com/wp-content/uploads/2009/01/turner.jpg

Documents

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Mitosis – division of a cell to produce two identical cells; used mainly for growth and cell replacement