Cell Cycle and Cell Cell Cycle and Cell DivisionDivision

Prokaryotic Cells - BacteriaProkaryotic Cells - Bacteria

E. coliE. coli is a bacteria that resides in our colon and is a bacteria that resides in our colon and can be beneficial to us – our major source of can be beneficial to us – our major source of vitamin K that is necessary for proper vitamin K that is necessary for proper coagulationcoagulation

Have only 1 copy of their DNA, considered to be Have only 1 copy of their DNA, considered to be haploid, any change in the sequence may be haploid, any change in the sequence may be readily seen in the readily seen in the phenotypephenotype – observable – observable characteristicscharacteristics

Binary FissionBinary Fission E coli can grow and E coli can grow and

duplicate every 20 duplicate every 20 minutes minutes

DNA is attached to the DNA is attached to the cell membrane and after it cell membrane and after it duplicates, it separates as duplicates, it separates as the cell enlargesthe cell enlarges

When cell is double in When cell is double in size, the cell divides by size, the cell divides by adding a plasma adding a plasma membrane and cell wall membrane and cell wall between the 2 cellsbetween the 2 cells

Escherichia coliEscherichia coli Genome Genome

DNA is circular and encodes ~ 3000 proteinsDNA is circular and encodes ~ 3000 proteins Bacteria with unusual growth properties have Bacteria with unusual growth properties have

been identified – contain mutations that been identified – contain mutations that changed the phenotypechanged the phenotype Rifampicin growth because of mutation in the RNA Rifampicin growth because of mutation in the RNA

polymerase that allows for mRNA synthesis while in polymerase that allows for mRNA synthesis while in presence of the drugpresence of the drug

Bacteria can pass DNA back and forth between Bacteria can pass DNA back and forth between one anotherone another Mixed 2 mutant types together and found that Mixed 2 mutant types together and found that

bacteria can grow on media without either amino acidbacteria can grow on media without either amino acid

Gene TransferGene Transfer

Bacterial mating or Bacterial mating or conjugationconjugation – can be – can be achieved in bacteria that contain achieved in bacteria that contain plasmidsplasmids – small, circular dsDNA that is separate – small, circular dsDNA that is separate from rest of chromosomefrom rest of chromosome

Plasmids can replicate independently from Plasmids can replicate independently from the bacterial chromosomethe bacterial chromosome

F plasmidF plasmid or fertility plasmid allows for or fertility plasmid allows for mating and gene transfermating and gene transfer

Bacterial MatingBacterial Mating

Bacteria with F plasmid (donor) will encounter a Bacteria with F plasmid (donor) will encounter a bacteria without the F plasmid (recipient) and bacteria without the F plasmid (recipient) and establish a cytoplasmic bridge to pass on the F establish a cytoplasmic bridge to pass on the F plasmidplasmid

Now both are donor bacteria Now both are donor bacteria Method of passing on antibiotic resistanceMethod of passing on antibiotic resistance

IntegrationIntegration The plasmid can The plasmid can

integrate into the integrate into the chromosome and chromosome and when bacteria when bacteria conjugate they can conjugate they can pass on chromosomal pass on chromosomal genesgenes Increases genetic Increases genetic

variationvariation

TransformationTransformation

Bacteria can pick up DNA from surroundings Bacteria can pick up DNA from surroundings Take advantage of this in molecular biology labs Take advantage of this in molecular biology labs

to replicate DNA fragments of choiceto replicate DNA fragments of choice

DNA in the Eukaryotic CellDNA in the Eukaryotic Cell

A cell’s DNA is divided into a set of A cell’s DNA is divided into a set of chromosomes – each being a long linear chromosomes – each being a long linear DNA associated with proteins to help fold DNA associated with proteins to help fold into chromatininto chromatin

Also associated with proteins involved in Also associated with proteins involved in gene expression, DNA replication and gene expression, DNA replication and DNA repairDNA repair

ChromosomesChromosomes

With exception of the sex With exception of the sex chromosomes (X and Y) humans have chromosomes (X and Y) humans have 2 similar copies of each chromosome 2 similar copies of each chromosome one from our mother (one from our mother (♀)♀) one from our father (one from our father (♂)♂)

Called Called homologous chromosomeshomologous chromosomes One instance of non-homologous One instance of non-homologous

chromosomes are males because of chromosomes are males because of their X and Ytheir X and Y

Chromosomes are similar but not Chromosomes are similar but not identicalidentical

Chromosomal BandsChromosomal Bands

Set of human Set of human chromosomes is chromosomes is called a called a karyotypekaryotype

The staining The staining patterns can allow patterns can allow geneticists to geneticists to identify areas of identify areas of abnormalitiesabnormalities

Types of ChromosomesTypes of Chromosomes

Mitotic chromosomes are those that are Mitotic chromosomes are those that are visible during mitosis – condensedvisible during mitosis – condensed

Interphase chromosomes are loose and Interphase chromosomes are loose and string-like - string-like - chromatinchromatin

Components of ChromosomesComponents of Chromosomes

3 important DNA sequences needed for 3 important DNA sequences needed for accurate DNA copying, complete genome accurate DNA copying, complete genome to new daughter cellsto new daughter cellsDNA replication originDNA replication origin – a place to start DNA – a place to start DNA

replication, multiple sitesreplication, multiple sitesCentromereCentromere – a place to attach the mitotic – a place to attach the mitotic

spindle for chromosome separationspindle for chromosome separationTelomereTelomere – at end of linear DNA to prevent – at end of linear DNA to prevent

the chromosome from getting shorter in the chromosome from getting shorter in every round of replicationevery round of replication

Important Parts During Cell CycleImportant Parts During Cell Cycle

TelomeresTelomeres

Telomerase is the enzyme responsible for Telomerase is the enzyme responsible for adding DNA sequence to the end of the adding DNA sequence to the end of the chromosome to act as a template to allow chromosome to act as a template to allow complete DNA replicationcomplete DNA replication

Also protects the DNA from degradationAlso protects the DNA from degradation

Chromatin vs ChromosomeChromatin vs Chromosome

Chromatin is a Chromatin is a complex of protein complex of protein and DNA – very and DNA – very disperseddispersed

Chromosome is the Chromosome is the compact form of compact form of DNA so that it is DNA so that it is protected during protected during mitosismitosis

NucleosomesNucleosomes

Nucleosome – “bead-on-a-string” look to DNA Nucleosome – “bead-on-a-string” look to DNA The string is the DNA and the bead is the The string is the DNA and the bead is the

nucleosome core particle that the DNA is nucleosome core particle that the DNA is wrapped around – the protein is wrapped around – the protein is histonehistone

50 nm

Histone OctmerHistone Octmer

Nucleosome is the Nucleosome is the histone proteins and the histone proteins and the adjacent adjacent linker DNAlinker DNA – – threads the histones threads the histones togethertogether

Histone is core around Histone is core around which the DNA coilswhich the DNA coils

Disc shaped with 2 Disc shaped with 2 copies of H2A, H2B, H3 copies of H2A, H2B, H3 and H4and H4

HistonesHistones Proteins with a large Proteins with a large proportion of positively proportion of positively charged amino acids (Lys charged amino acids (Lys and Arg)and Arg)

Helps to bind the Helps to bind the negatively charged DNAnegatively charged DNA

5 types in 2 main groups5 types in 2 main groups Nucleosomal histonesNucleosomal histones

Small and coil DNA into Small and coil DNA into nucleosomesnucleosomes

H2A, H2B, H3 and H4H2A, H2B, H3 and H4 H3 and H4 highly conservedH3 and H4 highly conserved

H1 histonesH1 histones Larger and less conservedLarger and less conserved

Interphase ChromosomesInterphase Chromosomes

While not as condensed as mitotic While not as condensed as mitotic chromosomes, there are areas more chromosomes, there are areas more tightly packed than otherstightly packed than others

Depends on the genes that are being Depends on the genes that are being expressedexpressed

Most highly condensed chromatin is called Most highly condensed chromatin is called heterochromatinheterochromatin

The more extended chromatin is called The more extended chromatin is called euchromatineuchromatin and is either being and is either being transcribed or easily available for transcribed or easily available for transcriptiontranscription

X ChromosomeX ChromosomeFemales have 2 X chromosomes and Females have 2 X chromosomes and

males have 1 X and 1 Y chromosomemales have 1 X and 1 Y chromosomeOne X becomes inactive and highly One X becomes inactive and highly

condensed – condensed – Barr bodyBarr bodySome cells have the paternal X off and Some cells have the paternal X off and

some have the maternal X offsome have the maternal X offThis happens during early development and This happens during early development and

once off, all cells that arise from that cell will once off, all cells that arise from that cell will have it offhave it off

This is then passed down (inherited) in all the This is then passed down (inherited) in all the cells that arise from that cellcells that arise from that cell

Barr Body FormationBarr Body Formation

Tortoise Shell and Calico CatsTortoise Shell and Calico Cats

Cytoskeleton’s RoleCytoskeleton’s Role

Mitotic spindleMitotic spindle – microtubules used to separate – microtubules used to separate the chromosomes – formation starts in late Gthe chromosomes – formation starts in late G22

Contractile ringContractile ring – myosin and actin filaments used – myosin and actin filaments used to separate the 2 daughter cells – formation starts to separate the 2 daughter cells – formation starts in M phasein M phase

Organelles FragmentOrganelles Fragment

Chromosomes divide as discussed in Chromosomes divide as discussed in Chapter 6Chapter 6

Mitochondria and chloroplasts will divide Mitochondria and chloroplasts will divide and double their numbersand double their numbers

Membrane bound organelles fragment to Membrane bound organelles fragment to increase the likelihood to be divided increase the likelihood to be divided evenly between 2 daughter cellsevenly between 2 daughter cells

Cell CycleCell Cycle

The orderly sequence of events which a The orderly sequence of events which a cell duplicates its contents (DNA and cell duplicates its contents (DNA and organelles) and then divides in 2organelles) and then divides in 2

Fundamental task is to copy and pass on Fundamental task is to copy and pass on its genetic information to daughter cellsits genetic information to daughter cells

Cell CycleCell Cycle

New cells need to get New cells need to get a copy of the entire a copy of the entire genome in the genome in the process – most cells process – most cells double their mass as double their mass as wellwell

4 Phases of 4 Phases of the Cell Cyclethe Cell Cycle

MitosisMitosis or M phase or M phase has 2 stepshas 2 steps Nucleus divides – Nucleus divides –

mitosismitosis Cell divides in 2 - Cell divides in 2 -

cytokinesiscytokinesis

4 Phases of the Cell Cycle4 Phases of the Cell Cycle

Period between mitosis Period between mitosis is called is called interphaseinterphase – – has 3 phaseshas 3 phases Synthesis (S) phase is Synthesis (S) phase is

when DNA is replicatedwhen DNA is replicated GapGap11 (G (G11) phase is phase ) phase is phase

between M and S phasebetween M and S phase GapGap22 (G (G22) phase is phase ) phase is phase

between S and M phasebetween S and M phase

GG11 and G and G22 Phases Phases

Portion of the cell cycle that allows for the Portion of the cell cycle that allows for the synthesis of proteins, lipids and other molecules synthesis of proteins, lipids and other molecules needed for cell divisionneeded for cell division

Duplicates the organelles and keeps the cells Duplicates the organelles and keeps the cells from getting smaller on each divisionfrom getting smaller on each division

At end of GAt end of G22, the DNA condenses into , the DNA condenses into

chromosomes from its normal chromatin chromosomes from its normal chromatin structurestructure

5 Stages of Mitosis 5 Stages of Mitosis

Continuous event but divided into 5 stagesContinuous event but divided into 5 stages Prophase – chromosomes condenses and mitotic Prophase – chromosomes condenses and mitotic

spindle forms outside the nucleusspindle forms outside the nucleus Prometaphase – nuclear membrane breaks down, Prometaphase – nuclear membrane breaks down,

attach mitotic spindle to chromosomesattach mitotic spindle to chromosomes Metaphase – mitotic spindle lines up chromosomes Metaphase – mitotic spindle lines up chromosomes

along the equator of the cellalong the equator of the cell Anaphase – sister chromatids separate and move to Anaphase – sister chromatids separate and move to

opposite endsopposite ends Telophase – nuclear envelope reassembles and cell Telophase – nuclear envelope reassembles and cell

is ready for cytokinesisis ready for cytokinesis

ProphaseProphase

Centrosome is duplicated in late S Centrosome is duplicated in late S phase and as prophase starts they phase and as prophase starts they separate and move to opposite poles separate and move to opposite poles of the cell using motor proteins and of the cell using motor proteins and ATP hydrolysisATP hydrolysis

Each centrosome organizes its Each centrosome organizes its microtubules which then interact to microtubules which then interact to form the mitotic spindle which form the mitotic spindle which undergoes dynamic instabilityundergoes dynamic instability

Spindle PolesSpindle Poles

During prophase some of During prophase some of the microtubules become the microtubules become stabilized and form the stabilized and form the mitotic spindlemitotic spindle

Some microtubules Some microtubules interact and are called interact and are called polar microtubules and the polar microtubules and the centrosome is called the centrosome is called the spindle polespindle pole

PrometaphasPrometaphasee

Starts with the break up of Starts with the break up of the nuclear envelope the nuclear envelope because of the because of the disassembly of the disassembly of the intermediate filaments in intermediate filaments in the nuclear laminathe nuclear lamina

Spindle microtubules will Spindle microtubules will bind to the chromosomes bind to the chromosomes at a structure called the at a structure called the kinetochorekinetochore which form on which form on chromosomes during late chromosomes during late prophase on the prophase on the centromerecentromere

KinetochoreKinetochore Kinetochore protein Kinetochore protein

assembles on centromereassembles on centromere Microtubules probe into Microtubules probe into

the area and when the area and when encounters a kinetochore encounters a kinetochore will attach – now called the will attach – now called the kinetochore microtubulekinetochore microtubule

A microtubule from each A microtubule from each pole will bind to the pole will bind to the chromosome chromosome Humans bind 20 to 40 Humans bind 20 to 40

microtubules per microtubules per kinetochorekinetochore

3 Classes of Microtubules3 Classes of Microtubules

MetaphaseMetaphase

Chromosomes line up Chromosomes line up at the metaphase plate at the metaphase plate that is halfway between that is halfway between spindle polesspindle poles

Chromosomes are Chromosomes are constantly under constantly under tension from both tension from both kinetochore kinetochore microtubulesmicrotubules

AnaphaseAnaphase

The connection between chromatids is cut by The connection between chromatids is cut by proteolytic enzymes – now called daughter proteolytic enzymes – now called daughter chromosome – pulled to the spindle polechromosome – pulled to the spindle pole

Process of Process of chromosome segregationchromosome segregation

Anaphase A and BAnaphase A and B

Anaphase A – separate the chromosomes Anaphase A – separate the chromosomes to the spindle poleto the spindle pole

Anaphase B – the spindle poles separate Anaphase B – the spindle poles separate further separating the chromosomesfurther separating the chromosomes

TelophaseTelophase

Nuclear envelope reforms around each group Nuclear envelope reforms around each group of chromosomesof chromosomes

Nuclear lamina reforms Nuclear lamina reforms Chromosomes de-condense – return to Chromosomes de-condense – return to

chromatinchromatin

CytokinesisCytokinesis

Separation of the new nuclei and Separation of the new nuclei and cytoplasmic organelles between 2 new cytoplasmic organelles between 2 new daughter cellsdaughter cells

Starts in anaphase but is not complete Starts in anaphase but is not complete until after 2 new nuclei are formeduntil after 2 new nuclei are formed

Caused by the contractile ring of actin Caused by the contractile ring of actin and myosinand myosin

Cleavage FurrowCleavage Furrow

Evidence of cytokinesis Evidence of cytokinesis is by the presence of the is by the presence of the cleavage furrowcleavage furrow

Dependent on the Dependent on the location of the mitotic location of the mitotic spindlespindle

Usually forms around Usually forms around the equator of the cell - the equator of the cell - exception is during early exception is during early development development

Contractile RingContractile Ring

Array of actin and myosin filaments that assembles Array of actin and myosin filaments that assembles during anaphaseduring anaphase

Force to divide cells comes from the actin moving over Force to divide cells comes from the actin moving over the myosin getting continuously smallerthe myosin getting continuously smaller

Disassembles when cell is divided into 2Disassembles when cell is divided into 2

MeiosisMeiosis

Process of getting a cell with half the number of Process of getting a cell with half the number of chromosomes – cell would be chromosomes – cell would be haploidhaploid

‘‘Normal’ cells would have a full set of Normal’ cells would have a full set of chromosomes from the mom and the dad and chromosomes from the mom and the dad and are called are called diploiddiploid

Fertilization will return the haploid sex cell to a Fertilization will return the haploid sex cell to a diploid cell by uniting sperm and eggdiploid cell by uniting sperm and egg

MeiosisMeiosis Germ cells or gamates Germ cells or gamates

(in plants they may be (in plants they may be spores) contain one set spores) contain one set of chromosomesof chromosomes

Egg is large and non-Egg is large and non-motilemotile

Sperm is small and Sperm is small and motilemotile

Diploid cells formed by Diploid cells formed by fertilizationfertilization

MeiosisMeiosis

Involves the duplication of the Involves the duplication of the chromosomes and then 2 successive cell chromosomes and then 2 successive cell divisions to yield the haploid cellsdivisions to yield the haploid cells

Can get genetic recombination between Can get genetic recombination between the chromosomes prior to the first cell the chromosomes prior to the first cell division since they are in such close division since they are in such close proximity – allows for new phenotypes proximity – allows for new phenotypes

Mitosis vs MeiosisMitosis vs Meiosis

In mitosis all 46 chromosomes line up along the In mitosis all 46 chromosomes line up along the metaphase plate and 1 copy of each gets taken metaphase plate and 1 copy of each gets taken to the new daughter cell to the new daughter cell Each cell gets both ♀ and ♂ (46 chromosomes)Each cell gets both ♀ and ♂ (46 chromosomes)

In meiosis the chromosomes find their In meiosis the chromosomes find their homologous pair and these line up at the homologous pair and these line up at the metaphase plate – get parental chromosome metaphase plate – get parental chromosome shuffling during the divisionshuffling during the division First division the cell gets either the ♀ or ♂ First division the cell gets either the ♀ or ♂

chromosome but not both (23 sister chromatids)chromosome but not both (23 sister chromatids) Second division the cell gets a single copy of the Second division the cell gets a single copy of the

chromosome (23 chromosomes)chromosome (23 chromosomes)

Difference #1Difference #1

Mitosis – chromosomes Mitosis – chromosomes line up in the metaphase line up in the metaphase plateplate

Meiosis – homologous Meiosis – homologous pairs line up togetherpairs line up together

ReassortmentReassortment

Genetic RecombinationGenetic Recombination

DNA can undergo rearrangements, DNA can undergo rearrangements, caused by genetic recombinationcaused by genetic recombination

General recombinationGeneral recombination – between any – between any pair of homologous DNA sequencespair of homologous DNA sequences2 copies of same chromosome2 copies of same chromosomeCrossing over of chromosomes during Crossing over of chromosomes during

meiosismeiosisNo nucleotides get altered – no gain or No nucleotides get altered – no gain or

loss of nucleotides at the cross over pointloss of nucleotides at the cross over point

Genetic RecombinationGenetic Recombination

Holliday JunctionHolliday Junction

The Holliday junction is the area of cross-over The Holliday junction is the area of cross-over that occurs during recombinationthat occurs during recombination

Difference #2Difference #2

Mitosis – has only one divisionMitosis – has only one division Meiosis – has 2 divisions Meiosis – has 2 divisions

First division is to separate homologous pairsFirst division is to separate homologous pairs Second division is to separate sister chromatidsSecond division is to separate sister chromatids

⋇

Bivalent ChromosomesBivalent Chromosomes

Homologous pairs form a structure called bivalent – Homologous pairs form a structure called bivalent – consists of 4 chromatidsconsists of 4 chromatids

Genetic recombination can occur at this time – mixing of Genetic recombination can occur at this time – mixing of the the ♀ and ♂ genes by crossing over – one arm or ♀ and ♂ genes by crossing over – one arm or portion of arm can be exchanged due to close proximityportion of arm can be exchanged due to close proximity

Allows for offspring having a novel assortment of genesAllows for offspring having a novel assortment of genes

Remainder of Meiosis IRemainder of Meiosis I

Very similar to mitosis from this point onVery similar to mitosis from this point onNuclear membrane disassemblesNuclear membrane disassemblesMitotic spindle attaches to the bivalentMitotic spindle attaches to the bivalentLine up on metaphase plateLine up on metaphase plateHomologs separateHomologs separateNuclear membrane reforms and cells divideNuclear membrane reforms and cells divide

Daughter Cells from Meiosis IDaughter Cells from Meiosis I

Differ from diploid cells Differ from diploid cells They have 46 chromosomes but both They have 46 chromosomes but both

copies come from the same parent with copies come from the same parent with the exception of whether there was any the exception of whether there was any crossing overcrossing over

Inherited as if a single chromosomeInherited as if a single chromosome

Meiosis IIMeiosis II

No DNA replication in this step and no No DNA replication in this step and no significant interphasesignificant interphase

Follows like mitosis but the new daughter Follows like mitosis but the new daughter cell have only 1 copy of each chromosome cell have only 1 copy of each chromosome – total of 23– total of 23

MeiosisMeiosis

NondisjunctionNondisjunction

Occasionally homologs do NOT separate Occasionally homologs do NOT separate during meiosisduring meiosis

This causes one of the haploid cells to have This causes one of the haploid cells to have an extra copy of the chromosome and the an extra copy of the chromosome and the other cell to have no copyother cell to have no copy

Upon fertilization, one embryo will have 3 Upon fertilization, one embryo will have 3 copies and the other would have 1 copy (from copies and the other would have 1 copy (from the ‘normal’ parent)the ‘normal’ parent)

Cause of Down’s syndrome – they have 3 Cause of Down’s syndrome – they have 3 copies of chromosome 21copies of chromosome 21

Gene Gene ExpressionExpression

Hard to believe that a Hard to believe that a lymphocyte and a lymphocyte and a neuron contain the neuron contain the same DNAsame DNA

All cells contain the All cells contain the entire genomeentire genome

Differences in cells Differences in cells arises from what genes arises from what genes are expressed whereare expressed where

Gene RegulationGene Regulation

Differentiation occurs Differentiation occurs because various cell types because various cell types have different genes being have different genes being expressedexpressed

In the frog, you can take the In the frog, you can take the nucleus from an adult cell nucleus from an adult cell and put it in an egg and still and put it in an egg and still get a tadpoleget a tadpole