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The Continuity of Life:The Continuity of Life:Cellular ReproductionCellular Reproduction
Chapter 9Chapter 9
Cell Division
• The cell cycle describes the sequence ofactivities that occurs from one celldivision to the next.
• During cell division, one parent celldivides producing two daughter cells.
Parent Cell
DaughterCell
DaughterCell
Cell Division
• Cells divide for bothreproduction and growth.– Reproduction
• Asexual: offspring formedfrom a single parent.
• Sexual: offspring formed fromtwo parent cells.
– Growth and Development• Body cells grow and renew
through asexual reproduction.
Types of Cell Division
• Binary Fission– Prokaryotic Cell Division
• Mitosis– Eukaryotic Cell Division that gives rise to
non-sex (non-gamete) cells.• Meiosis
– Eukaryotic Cell Division that gives rise tosex (gamete) cells.
Prokaryotic Life Cycle
• Most of theProkaryote’s life isspent replicationits DNA andgrowing larger.
• Only a smallfraction of time isspent during celldivision.
Binary Fission1. Circular DNA isattached to theplasma membrane.
2. DNA is replicated,creating twoidentical copies.
3. The plasmamembraneexpands, separatingthe DNA copies.
4. The new plasmamembrane growninward at themiddle of the cell.
5. The parent celldivides into twodaughter cells.
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Prokaryotic Cell Division Eukaryotic DNAOrganization in the
Nucleus
• In Eukaryotes, DNA is kept in the nucleus.• The entire sequence of an organism’s DNA
is called its genome.• Within the nucleus, DNA is organized into
pairs of homologous chromosomes (eachchromosome in the pair is identical).– Humans have 23 homologous paired
(46 individual) chromosomes.
Eukaryotic DNAOrganization into
Chromosomes
• DNA issupercoiledabout itself.
EukaryoticChromosome
Structure
Genes: DNA sequencethat contains information.Telomeres: a repeatednucleotide sequenceadding stability.Centromere: Site ofattachment of duplicatedchromosomes during celldivision.Chromosomes are calledsister chromatids whenattached at thecentromere.
When not preparing for cell division, DNA formschromatin and is relatively inaccessible.
Eukaryotic Chromosome Structure
1 Chromosome
1 Duplicated Chromosome
CentromereGenes
Eukaryotic Chromosome Structure
1 Chromosome
1 Sister Chromatid
1 Duplicated Chromosome
1 Pair of Homologous Chromosomes
Chromosomes During MitosisThe Human
Genome• Each of our cells has all
46 chromosomes (23homologous pairs).– The genetic information
for all of our genes andproteins.
• Chromosome pair 23contains the sexchromosomes.– XX or XY.
• The remainingchromosomes areautosomes.– Same size and same
genetic compositionwithin the pair.
Eukaryotic Cell Cycle
Interphase: phasewhere the cellsgrows andduplicates itschromosomes.
Cell Division: parentcell divideschromosomes andintracellularcomponents intotwo daughter cells.
Eukaryotic Cell Cycle
• During cell division, cells under goseveral distinct phases of development.– Prophase– Metaphase– Anaphase– Telophase– Cytokinesis
Types of EukaryoticCell Division
• Mitosis– Eukaryotic Cell Division that gives rise to
non-sex (non-gamete) cells.
• Meiosis– Eukaryotic Cell Division that gives rise to
sex (gamete) cells.
Early Prophase - Late Prophase - Metaphase
Anaphase - Telophase - Cytokinesis
During late interphase, duplicated chromosomesare in their relaxed form, as chromatin, withinthe nucleus.
In early prophase, chromosomes condense andshorten.Spindle microtubules, protein fibers required forproper movement of chromosomes, begin to form.
By late prophase, the nucleus is being recycled.Spindle microtubules attach to the kinetochore,a protein structure at the centromere.
During metaphase, spindle fibers lengthen andshorten to line chromosomes at the equator.
In anaphase, sister chromatidsseparate, pulled by microtubulesattached at the kinetochore.“Free” spindle fibers interact tolengthen the cell.
After chromosomes have reached thepoles, during telophase, microtubules arerecycled.A new nuclear envelope forms around thechromosomes and cytokinesis occurs.
During cytokinesis, the cell divides intotwo daughter cells, each containing anucleus and half of the existing cytoplasm.
The details ofcytokinesis differaccording to phylum. Cytokinesis in Animal Cells
• In animal cells, microfilaments attached to theplasma membrane form a ring around the equator.
• During cytokinesis, this spindle fiber ring contractsuntil the two cells are separated.
Cytokinesis in Plant Cells
• During plant cell cytokinesis, the golgi apparatusforms carbohydrate vesicles that migrate to theequator.
• These vesicles fuse to form the new cell wall.
Interphase begins again after cells haveseparated.Spindle microtubules are recycled, thenuclear membrane is completed, andchromosomes relax.
Phases ofMitosis
Mitosis1. Early Prophase: chromosomes
condense and microtubules appear.2. Late Prophase: nucleus breaks down,
spindle fibers attach to kinetochores.3. Metaphase: fibers line up the
chromosomes at the equator.4. Anaphase: sister chromatids separate
and microtubules elongate the cell.5. Telophase: chromosomes relax,
nuclear envelope starts to form,microtubules are recycled.
6. Cytokinesis: the cell divides in two.
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Interphase
Interphase
Mitosisin real time
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From The American Society for Cell Biology, http://cellimages.ascb.org/
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Metaphase
Transition, metaphase to anaphase
Anaphase
Mitosis vs. Meiosis
Mitosis is used forasexual reproductionof cells for growth andrepair.
Meiosis is used forsexual reproduction,making male andfemale sex cells.
Sexual reproduction leads tomixing of genetic material(exchanging genes).
Both mitosis and meisosis follow the same basic steps:
Interphase Prophase Metaphase Anaphase Telophase Interphase
Control of Cell Division
A growth factormolecule binds toa receptor whichupregulates cyclinproduction.
Cyclins bind toCdk proteins tostimulate DNAReplication.
Control of Cell Division
The cell hascheckpoints atevery major stepin cell division.
Chromosome Variability
• The parents may each give a different allele, thegene for a particular physical trait, to the offspring.– Examples: curly vs. straight hair, blue vs. brown eyes.
• Mixing of alleles leads to genetic diversity andvariability within the chromosome.
• In organisms whichundergo sexualreproduction, halfof the chromosomecomes from eachparent.
Meiosis• Two rounds of cell division to produce gametes,
sex cells with 1 copy of each chromosome.• Meiosis I produces diploid cells, possessing 2
copies of each chromosome.• Meiosis II produces haploid cells, or gametes,
those possessing 1 copy of each chromosome.
Meiosis I Meiosis II
• Meiotic cell divisionbegins like mitotic celldivision.
• During prophase I:chromosomes condenseand pair up, the nuclearenvelope begins tobreak down, and spindlemicrotubules begin toappear.
• During metaphase I,paired chromosomesalign at the equator,pairs attaching tospindle fibers fromopposite poles.
• In anaphase I,microtubules pullhomologous pairedchromosomes toopposite poles.
• Sister chromatids donot separate!
• Similar to mitotic telophase,microtubules stretch the celland disappear in telophase I.
• Cytokinesis occurs creatingtwo haploid cells with oneduplicated chromosomecopy in each.
• The second round fomeiosis behaves muchlike mitosis with half thenumber of chromosomes.
• In prophase II, thechromosome condensesand spindle fibers form.
• In metaphase II, spindlefibers attach to thekinetochore of each sisterchromatid, aligning themat the equator.
• Like mitosis, in anaphaseof meiosis II, sisterchromatids are pulledapart by lengtheningspindle fibers.
• Next, in telophase II,microtubules releasechromatids in the newlyforming nucleus andcytokinesis (cell division)occurs.
• At the end of meiosis, four newcells are formed.
• Each cell is a haploid, possessingone copy from each pair ofchromosomes (diploids have two).
A. Meiosis I Early ProphasePoles appear, chromosomes begin to
separate.
B. Meiosis I Late ProphaseNuclear envelope disintegrates,
microtubules appear.
C. Meiosis I Pro-MetaphaseSpindle microtubules are assembled.
D. Meiosis I MetaphaseMicrotubules attach to chromosomes
which align at the equator.
E. Meiosis I AnaphaseAttached microtubules shorten,
chromosomes are pulled thepoles.
F. Meiosis I TelophaseFree microtubules lengthen and
cytokinesis begins.
Meiosis I
Meiosis I in real time
From The American Society for Cell Biology, http://cellimages.ascb.org/
Meiosis II in real time
From The American Society for Cell Biology, http://cellimages.ascb.org/
Cell Types formed duringMitosis and Meiosis
• Diploid cell:– Formed during Mitosis and Meiosis I.– Contains a full set of homologously
paired chromosomes (two copies ofeach).
• Haploid cell:– Formed only during Meiosis II.– Contains only one copy of the
chromosomes (half the number allother cells have).
– Also called a gamete, a sex cell (eggand sperm).
A Comparison:Mitosis and Meiosis
Chromosome Attachment in Mitosis
• In Mitosis, spindle microtubules attached toeach individual chromatid at the kinetochore,
pulling sisterchromatids apartduring anaphase.
Each new cell two complete copies of thechromosomes, one from each sister chromatid.
Chromosome Attachment in Meiosis
In Meisosis I,spindle fibersattached to the
Each new cell receives two complete copies of the chromosomein the form of one of the pair of duplicated chromosomes.
kinetochore separating duplicatedchromosomes, not individual
chromatids of the chromosome, during the firstanaphase.Meiosis II attachment is the same as mitosis.
Counting Chromosomes• If “n” is the number of unique chromosomes in an
organism:– A diploid cell contains 2n chromosomes, a pair of each
chromosome.– A haploid cell contains n chromosomes, one copy of each.
• Humans have 23 unique chromosome pairs.– A muscle cell contains 46 chromosomes (23 pairs).– A female egg contains 23 chromosomes (one of each).
Diploid Parental Cells
Haploid Gametes Female Egg and Male Sperm Cell
Diploid Fertilized Egg
Introducing Genetic Variability• In organisms that undergo meiosis, genetic
recombination can occur.– The formation of new combinations of alleles on a
chromosome.
• Sexual organisms receive half of theirchromosomes from the mother and half fromthe father when two haploid gametes combine(egg and sperm).
• During prophase I of meiosis,pairs of chromosomes alignand crossing over of thechromosomes occurs.
Chromosome Diversity ThroughCrossing Over
• Homologouschromosomes pair up,side by side.
• Enzymes joinhomologous sectionsof the chromosome,like a zipper.
Chromosome Diversity ThroughCrossing Over
• Recombination enzymes bind to the joinedchromosomes.
• The enzymes nick the DNA strands, joining oppositestrands together, creating a chiasma, the site of across-over event.
• New gametes produced during meiosis now possessa mixture of genes (alleles) from both parents of theorganism.
More in Chapter 12
Introducing More Genetic Variability
• Genetic variability can also come aboutin the mixing of chromosomes.
• Example: Mosquito, 3 chromosomes
During meiosis, chromosomes arerandomly mixed and separated intonewly formed cells.
Maternal Chromosomes
Paternal Chromosomes
Introducing More Genetic Variability
• Genetic variability can also come aboutin the mixing of chromosomes.
• Example: Mosquito, 3 chromosomesMaternal Chromosomes
Paternal Chromosomes
Introducing More Genetic Variability
• Genetic variability can also come aboutin the mixing of chromosomes.
• Example: Mosquito, 3 chromosomesThere are 2n
possible sets ofchromosomes.
For the mosquito,n=3, so there are23=8 chromosomecombinations.
Meiosis I
Possible sets of chromosomes after meiosis.
Eukaryotic Cell CycleAll eukaryotic cells
go through phasesof interphase (cellgrowth and DNAreplication) andcell division(mitosis andmeiosis).
When do organismsundergo meiosisinstead of mitosis?
Eukaryotic Life Cycles Life Cycle of a EukaryoteHumans, and most animals, only undergo meiosis forsexual reproduction: Diploid lifestyle.
Life Cycle of a EukaryotePlants spend half of their time as a haploid, half as adiploid: Alternation of Generations.
Homework
Chapter 9:
Mitosis and Meiosis are essentially identical exceptfor two stages of cell division.
a. What are those two stages?b. What happens to the chromosomes in these
two stages (from part a) during the three types ofcell division: Mitosis, Meiosis I, and Meiosis II?
