mitosis and meiosis 1

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

PowerPoint Lectures forBiology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon

Lectures by Yemi Olabiran

Chapter 8

Cell Division: Mitosis and Meiosis


Two types of Cell Division

Cell Division is necessary in any life cycle

Mitosis and Meiosis are two types of divisions of the nucleus

Cytokinesis – division of the entire cell

Mitosis (asexual reproduction)

– Genetically identical cells produced

Meiosis

– Production of gametes. Offspring not genetically identical


• In sexual reproduction

– Fertilization of sperm and egg produces offspring

• In asexual reproduction

– Offspring are produced by a single parent, without the participation of sperm and egg


CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION

8.1 Like begets like, more or less

• Some organisms reproduce asexually

– And their offspring are genetic copies of the parent and of each other

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Figure 8.1A


• Other organisms reproduce sexually

– Creating a variety of offspring

Figure 8.1B


8.2 Cells arise only from preexisting cells

• Cell division is at the heart of the reproduction of cells and organisms

– Because cells come only from preexisting cells


8.3 Prokaryotes reproduce by binary fission

• Prokaryotic cells

– Reproduce asexually by cell division

Col

oriz

ed T

EM

32,

500

Prokaryotic chromosomes

Figure 8.3B


Prokaryoticchromosome

Plasmamembrane

Cell wall

Duplication of chromosomeand separation of copies

1

Continued elongation of thecell and movement of copies2

Division intotwo daughter cells

3

• As the cell replicates its single chromosome, the copies move apart

– And the growing membrane then divides the cells

Figure 8.3A


THE EUKARYOTIC CELL CYCLE AND MITOSIS

8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division

• A eukaryotic cell has many more genes than a prokaryotic cell

– And they are grouped into multiple chromosomes in the nucleus


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• Individual chromosomes contain a very long DNA molecule associated with proteins– And are visible only when the cell is in the

process of dividing• If a cell is not undergoing division

– Chromosomes occur in the form of thin, loosely packed chromatin fibers

Figure 8.4A


Haploid and Diploid CellsHaploid cells- one set of chromosomes

Diploid cells- two sets of chromosomes

Humans- haploid number(n) is 23 and diploid number (2n) is 46

Homologous Chromosomes

– Paternal and maternal chromosomes are homologous, they are similar but not identical.

– Each one carries the same genes but may have different versions (alleles) of these genes


• Before a cell starts dividing, the chromosomes replicate

– Producing sister chromatids joined together at the centromere

TEM

36,

000

Centromere(condensed DNA)

Sister chromatids

Figure 8.4B


• Cell division involves the separation of sister chromatids

– And results in two daughter cells, each containing a complete and identical set of chromosomes

Centromere

Chromosomeduplication

Sisterchromatids

Chromosomedistribution

todaughter

cellsFigure 8.4C


8.5 The cell cycle multiplies cells

• The cell cycle consists of two major phasesINTERPHASE

S(DNA synthesis)

G1

G2

Cytokin

esis

Mitosis

MITOTICPHASE (M)

Figure 8.5


• During interphase

– Chromosomes duplicate and cell parts are made

• During the mitotic phase

– Duplicated chromosomes are evenly distributed into two daughter nuclei


8.6 Cell division is a continuum of dynamic changes

• In mitosis, after the chromosomes coil up

– A mitotic spindle moves them to the middle of the cell


• The sister chromatids then separate

– And move to opposite poles of the cell, where two nuclei form

• Cytokinesis, in which the cell divides in two

– Overlaps the end of mitosis


• The stages of cell division

INTERPHASE PROPHASE PROMETAPHASE

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Chromatin

Centrosomes(with centriole pairs)

Nucleolus

Nuclearenvelope

Plasmamembrane

Early mitoticspindle

Centrosome

CentromereChromosome, consistingot two sister chromatids

Spindlemicrotubules

Kinetochore

Fragmentsof nuclearenvelope

Figure 8.6 (Part 1)


METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Daughterchromosomes

Nuclearenvelopeforming

Cleavagefurrow

Nucleolusforming

Figure 8.6 (Part 2)


8.7 Cytokinesis differs for plant and animal cells

• In animals

– Cytokinesis occurs by a constriction of the cell (cleavage) Cleavage

furrow

SE

M 1

40

Daughter cells

Cleavage furrow Contracting ring ofmicrofilaments

Figure 8.7A


• In plants

– A membranous cell plate splits the cell in two

Figure 8.7B

TEM

7,5

00

Cell plateforming

Wall ofparent cell

Daughternucleus

Cell wall New cell wall

Vesicles containingcell wall material

Cell plate Daughter cells


8.8 Anchorage, cell density, and chemical growth factors affect cell division

• Most animal cells divide

– Only when stimulated, and some not at all


8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction

• When the cell cycle operates normally, mitotic cell division functions in

– Growth

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Figure 8.11A


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• Replacement of damaged or lost cells

Figure 8.11B


Meiosis : Reduction Division

Gamete formation must involve some mechanism that reduces the number of chromosomes to half the number found in other cells

If not, chromosome number would double with each fertilisation becoming impossibly large after several generations!

Meiosis halves the number of chromosomes during gamete formation


MEIOSIS AND CROSSING OVER

8.12 Chromosomes are matched in homologous pairs

• The somatic (body) cells of each species

– Contain a specific number of chromosomes

• For example human cells have 46

– Making up 23 pairs of homologous chromosomes


• The chromosomes of a homologous pair

– Carry genes for the same characteristics at the same place, or locus

Chromosomes

Centromere

Sister chromatidsFigure 8.12


8.13 Gametes have a single set of chromosomes

• Cells with two sets of chromosomes

– Are said to be diploid

• Gametes, eggs and sperm, are haploid

– With a single set of chromosomes


• Sexual life cycles

– Involve the alternation ofhaploid and diploid stages

Figure 8.13


8.14 Meiosis reduces the chromosome number from diploid to haploid

• Meiosis, like mitosis

– Is preceded by chromosome duplication

• But in meiosis

– The cell divides twice to form four daughter cells


• The first division, meiosis I

– Starts with synapsis, the pairing of homologous chromosomes

• In crossing over

– Homologous chromosomes exchange corresponding segments

• Meiosis I separates each homologous pair

– And produce two daughter cells, each with one set of chromosomes


• Meiosis II is essentially the same as mitosis

– The sister chromatids of each chromosome separate

– The result is a total of four haploid cells


• The stages of meiosis

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes (with centriole pairs)

Sites of crossing over

Spindle

Microtubulesattached to kinetochore

Metaphaseplate

Sister chromatids remain attached

Nuclearenvelope Chromatin

Sisterchromatids Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

Figure 8.14 (Part 1)


PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

MEIOSIS II: Sister chromatids separate

Figure 8.14 (Part 2)


Mitosis Meiosis

Parent cell(before chromosome replication)

Chromosome replication

Chromosome replication

Chromosomes align at themetaphase plate

Tetradsalign at themetaphase plate

Sister chromatidsseparate during anaphase

Homologous chromosomesseparate duringanaphase I;sister chromatidsremain together

No furtherchromosomalreplication; sisterchromatidsseparateduringanaphase II

Prophase

Metaphase

AnaphaseTelophase

Duplicated chromosome(two sister chromatids)

Daughter cellsof mitosis

2n 2n

Daughtercells of

meiosis I

n n nn

2n = 4

Tetrad formedby synapsis ofhomologouschromosomes

Meiosis i

Meiosis ii

Prophase I

Metaphase I

Anaphase ITelophase I

Haploidn = 2

Daughter cells of meiosis II

8.15 Review: A comparison of mitosis and meiosis

Figure 8.15


8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

• Each chromosome of a homologous pair

– Differs at many points from the other member of the pair


• Random arrangements of chromosome pairs at metaphase I of meiosis

– Lead to many different combinations of chromosomes in eggs and sperm

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probablearrangements of chromosomes at

metaphase I

Possibility 1 Possibility 2

Figure 8.16


• Random fertilization of eggs by sperm

– Greatly increases this variation


8.17 Homologous chromosomes carry different versions of genes• The differences between homologous chromosomes

– Are based on the fact that they can bear different versions of a gene at corresponding loci

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

ec

EC

White Pink ec

ec

EC

EC

Meiosis

BlackBrown

Chromosomes of the four gametes

Eye-colorgenes

Coat-colorgenes

Brown coat (C); black eyes (E) White coat (C); pink eyes (e)

Figure 8.17AFigure 8.17B


8.18 Crossing over further increases genetic variability

• Genetic recombination

– Which results from crossing over during prophase I of meiosis, increases variation still further

Figure 8.18A

ChiasmaTetrad

Centromere

TEM

2,2

00


Coat-colorgenes

Eye-colorgenes

Tetrad (homologous pair of chromosomes

in synapsis)

C E

c e

C E

c e

C E

c e

Chiasma

C E

C e

c E

c e

C E

C e

c E

c e

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

• How crossing over leads to genetic variation Breakage of homologous chromatids1

Joining of homologous chromatids2

3Separation of homologous chromosomes at anaphase I

4Separation of chromatids at anaphase II and completion of meiosis

Figure 8.18B

mitosis and meiosis 1

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