Mitosis & Meiosis

Cell Division

• like begets like (more or less)• reproduction occurs at the cellular level

– have to give genetic material to new cell

• asexual• sexual• haploid = one set of DNA (1n)• diploid = 2 sets of DNA (2n)

Asexual

• production of genetically identical offspring by a single parent

• 1 parent cell (2n) → 2 daughter cells (2n)• DNA is identical• aka: cloning• whole organisms• cells within an organism (growth & repair)

Sexual

• production of genetically unique offspring– genetic contribution from 2 different parents

• need to reduce the amount of DNA by half– creates gametes (egg & sperm)

• gamete + gamete → new organism• (1n) + (1n) → (2n)

Prokaryotes: Binary Fission

• single chromosome• circular• small and uncomplicated• stimulus to divide: lots of food

– get too large: own waste = death– need ATP to:

• make new set of DNA• make new cell ‘stuff’

Figure 8.2B

Prokaryotic chromosomes

Eukaryotes: Mitosis• larger, more complex cells• large numbers of chromosomes

– chromosomes in pairs: 1 from mom & 1 from dad

• DNA is unwound most of the time: chromatin• duplicate in this form = 4 sets of DNA

• time to divide, need to organize: wind up into chromatids

Eukaryotes: Mitosis

• cells spend most of their time in Interphase– G1 phase: doing it’s thing but able to start dividing

• G0 phase: doing it’s thing but can’t divide

– S phase: synthesis (duplicate DNA & centrosome)– G2 phase: really getting ready to divide

http://aprenderasbiologia.blogspot.com/2011/01/ciclo-celular.html

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http://www.youtube.com/watch?v=PvDlilBgoSs

http://www.biologyjunction.com/cell_division_notes.htmhttp://usmle-review.org/cell-cycle.php

Eukaryotes: Mitosis

• Prophase– chromatin condenses into sister chromatids– mitotic spindle forms

• microtubules attach to the centrosomes & centromere

– nuclear membrane breaks down– mitotic spindle starts pushing chromatids toward

middle of ce

Eukaryotes: Mitosis

• Metaphase– all the centromeres are lined up at equatorial plane

• Anaphase– checkpoint– ATP used to push and pull centromeres toward poles– poles pushed away from each other too– fastest phase

Eukaryotes: Mitosis

• Telophase– ‘reverse’ of prophase

• chromatids unwind• spindle breaks down• nucleus reforms

• Cytokinesis– contractile ring of actin & myosin forms (hoodie)– plants form a cell plate → cell wall– make more ‘stuff’

Figure 8.6A

CytokinesisCleavage

furrow Contracting ring ofmicrofilaments

Daughtercells

Cleavagefurrow

Mitosis: Summary

• start with 1 2n parent• end up with 2 2n daughter cells• DNA in daughter cells is IDENTICAL

– to each other– to parent cell

Meiosis

• start with 1 2n parent cell• produce 4 1n gametes

– DNA is UNIQUE• each gamete is different than the others• each gamete is different than the parent

Meiosis

• somatic cells (body cells)– have 2 sets of chromosomes– called homologous pairs

• code for same things, (like eye color) but may have different information (blue or brown)

• 22 pairs of austosomes• 1 pair of sex chromosome (XX in female; XY male)

Meiosis

• gametes (1n)– have 1 set of chromosomes– meant to pair up with another gamete (1n) to

create a 2n organism

• gamete + gamete → zygote (fertilization) (1n) + (1n) (2n)

gametes created through process of MEIOSIS

Meiosis

• DNA is copied ONCE• cells divide TWICE

• cross-over: homologous chromosomes swap material = increased genetic diversity

Mom Dad

http://medakagb.lab.nig.ac.jp/Homo_sapiens/index.html

Mom Dad

http://medakagb.lab.nig.ac.jp/Homo_sapiens/index.html

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Figure 8.12B

A pair ofhomologouschromosomesin a diploidparent cell

A pair ofduplicatedhomologouschromosomes

Sisterchromatids

1 2 3

INTERPHASE MEIOSIS I MEIOSIS II

Figure 8.11

Pair of homologouschromosomes

Locus

Centromere

Sisterchromatids

One duplicatedchromosome

http://click4biology.info/c4b/4/gene4.2.htm

Meiosis

• Interphase:– chromosomes duplicated– centrosomes duplicated

Meiosis

• Prophase I:– chromatin coils up– synapsis: 2 sister chromatids come together (tetrad)

• cross over occurs• can take months

– centrosomes go to poles– spindle foms– nuclear envelope breaks down

Meiosis

• Metaphase I:– tetrads are aligned across from each other at equator

– sister chromatids across from each other– not all centromeres along equator

Meiosis

• Anaphase I:– sister chromatids pulled away from each other– uses ATP to push/pull chromatids toward poles– uses ATP to push poles away from each other

Meiosis

• Telophase I and Cytokinesis:– nuclear membrane reforms– chromatids uncoil into chromatin– some species skip this– short interphase (rest)

Meiosis

• Prophase II:– centosomes duplicate– spindle forms– chromosomes moved toward equator

• Metaphase II:– centromeres aligned on equator

Meiosis

• Anaphase II:– centromeres of sister chromatids pulled apart– ATP used to push/pull chromatids toward poles– ATP used to push poles away from each other

• Telophase II & Cytokinesis:– nuclei reform– DNA uncoils

Figure 8.14

Prophase

Metaphase

Duplicatedchromosome(two sisterchromatids)

MITOSIS

Parent cell(before chromosome duplication)

Chromosomeduplication

Chromosomeduplication

Site ofcrossingover

2n 4

Chromosomesalign at themetaphase plate

Tetrads (homologouspairs) align at themetaphase plate

Tetrad formedby synapsis ofhomologouschromosomes

Metaphase I

Prophase I

MEIOSIS I

AnaphaseTelophase

Sister chromatidsseparate during

anaphase

2n 2n

Daughter cells of mitosisNo furtherchromosomalduplication;sisterchromatidsseparate duringanaphase II

n n n n

Daughter cells of meiosis II

Daughtercells of

meiosis I

Haploidn 2

Anaphase ITelophase I

Homologouschromosomesseparate duringanaphase I;sisterchromatidsremain together

MEIOSIS II

Figure 8.19A_1

Trisomy 21

Figure 8.20A_s1

Nondisjunction

MEIOSIS I

Figure 8.20A_s2

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Figure 8.20A_s3

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Gametes

Number ofchromosomes

Abnormal gametes

n 1 n 1 n 1 n 1

Figure 8.20B_s1

Normalmeiosis I

MEIOSIS I

Figure 8.20B_s2

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Figure 8.20B_s3

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametesNormal gametes

n 1 n 1 n n