1 Bacterial Cell Division Bacteria divide by binary fission – No sexual life cycle – Reproduction is clonal Single, circular bacterial chromosome is replicated

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Bacterial Cell Division

• Bacteria divide by binary fission– No sexual life cycle– Reproduction is clonal

• Single, circular bacterial chromosome is replicated• Replication begins at the origin of replication and

proceeds in two directions to site of termination• New chromosomes are partitioned to opposite ends

of the cell• Septum forms to divide the cell into 2 cells

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Prior to cell division,the bacterial DNAmolecule replicates.The replication of thedouble-stranded,Circular DNA mole-cule that constitutesthe genome of abacterium begins at aspecific site, calledthe origin of replica-tion (green area).

The replicationenzymes move outin both directionsfrom that site andmake copies of eachstrand in the DNAduplex. The enzymes continue until they meet at another specific site, the terminus of replication (red area).

As the DNA isreplicated, the cellelongates, and theDNA is partitioned in the cell such that the origins are at the ¼ and ¾ positions in the cell and the termini are oriented toward the middle of the cell.

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Bacterial cell

Bacterial chromosome:Double-stranded DNA

Origin ofreplication

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Septation then begins, in which new membrane and cell wall materialbegin to grow and form a septum at approximately the midpoint of the cell. A protein molecule called FtsZ (orange dots) facilitates this process.

When the septum is complete, the cell pinchesin two, and two daughter cells are formed,each containing a bacterial DNA molecule.

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Septum


Eukaryotic Chromosomes

• Every species has a different number of chromosomes

• Humans have 46 chromosomes in 23 nearly identical pairs– Additional/missing

chromosomes usually fatal

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950x


© Biophoto Associates/Photo Researchers, Inc.

Chromosomes• Composed of chromatin – complex of DNA and protein• DNA of a single chromosome is one long continuous

double-stranded fiber• RNA associated with chromosomes during RNA

synthesis• Typical human chromosome 140 million nucleotides

long• In the nondividing nucleus

– Heterochromatin – not expressed– Euchromatin – expressed

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Chromosome Structure

• Nucleosome– Complex of DNA and histone proteins– Promote and guide coiling of DNA– DNA duplex coiled around 8 histone proteins

every 200 nucleotides– Histones are positively charged and strongly

attracted to negatively charged phosphate groups of DNA

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Eukaryotic Cell Cycle

1. G1 (gap phase 1)– Primary growth phase, longest phase

2. S (synthesis)– Replication of DNA

3. G2 (gap phase 2)– Organelles replicate, microtubules

organize4. M (mitosis)– Subdivided into 5 phases

5. C (cytokinesis)– Separation of 2 new cells

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Interphase

Replication

• Prior to replication, each chromosome composed of a single DNA molecule

• After replication, each chromosome composed of 2 identical DNA molecules– Held together by cohesin proteins

• Visible as 2 strands held together as chromosome becomes more condensed– One chromosome composed of 2 sister

chromatids

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Homologous chromosomes Homologous chromosomes

Cohesinproteins

Kinetochores

Sister chromatids

Sister chromatids

Kinetochore

CentromereCentromere

Replication

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Duration

• Time it takes to complete a cell cycle varies greatly• Fruit fly embryos = 8 minutes• Mature cells take longer to grow– Typical mammalian cell takes 24 hours– Liver cell takes more than a year

• Growth occurs during G1, G2, and S phases– M phase takes only about an hour

• Most variation in length of G1

– Resting phase G0 – cells spend more or less time here

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M Phase

MetaphaseAnaphase

Telophase

Prometaphase

Prophase

S

G2

G1

Interphase

M Phase

G1

CytokinesisMitosis

SG2

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Interphase

• G1, S, and G2 phases– G1 – cells undergo major portion of growth– S – replicate DNA– G2 – chromosomes coil more tightly using motor proteins;

centrioles replicate; tubulin synthesis

• Centromere – point of constriction– Kinetochore – attachment site for microtubules– Each sister chromatid has a centromere– Chromatids stay attached at centromere by cohesin

• Replaced by condensin in metazoans

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M phase

Mitosis is divided into 5 phases:1. Prophase2. Prometaphase3. Metaphase4. Anaphase5. Telophase

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Prophase

• Individual condensed chromosomes first become visible with the light microscope– Condensation continues throughout prophase

• Spindle apparatus assembles– 2 centrioles move to opposite poles forming spindle

apparatus (no centrioles in plants)– Asters – radial array of microtubules in animals (not plants)

• Nuclear envelope breaks down

Metaphase

• Alignment of chromosomes along metaphase plate– Not an actual

structure– Future axis of cell

division

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Polarmicrotubule

Centrioles

Metaphaseplate

Aster

Kinetochoremicrotubule

Sister chromatids

57µm


© Andrew S. Bajer, University of Oregon

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• All chromosomes are aligned at equator of the cell, called the metaphase plate• Chromosomes are attached to opposite poles and are under tension

Polar microtubule

Chromosomesaligned on

metaphase plateKinetochoremicrotubule


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Anaphase

• Begins when centromeres split• Key event is removal of cohesin proteins from

all chromosomes• Sister chromatids pulled to opposite poles• 2 forms of movements– Anaphase A – kinetochores pulled toward poles– Anaphase B – poles move apart

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Chromosomes

• Proteins holding centromeres of sister chromatids are degraded, freeing individual chromosomes• Chromosomes are pulled to opposite poles (anaphase A)• Spindle poles move apart (anaphase B)


Polarmicrotubule


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Telophase

• Spindle apparatus disassembles• Nuclear envelope forms around each set of

sister chromatids– Now called chromosomes

• Chromosomes begin to uncoil• Nucleolus reappears in each new nucleus

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Polar microtubule

Nucleus reforming

• Chromosomes are clustered at opposite poles and decondense• Nuclear envelopes re-form around chromosomes• Golgi complex and ER re-form



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Cytokinesis

• Cleavage of the cell into equal halves• Animal cells – constriction of actin filaments

produces a cleavage furrow• Plant cells – cell plate forms between the

nuclei• Fungi and some protists – nuclear membrane

does not dissolve; mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis

Sexual life cycle

• Made up of meiosis and fertilization• Diploid cells– Somatic cells of adults have 2 sets of

chromosomes• Haploid cells– Gametes have only 1 set of chromosomes

• Offspring inherit genetic material from 2 parents

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Haploid sperm

Haploid egg

Diploid zygote

Fertilization

Paternalhomologue

Maternalhomologue

The Process of Meiosis• Meiosis I– Prophase I– Metaphase I– Anaphase I– Telophase I

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase II

• Meiotic cells have an interphase period that is similar to mitosis with G1, S, and G2 phases

• After interphase, germ-line cells enter meiosis I

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Kinetochore

Sister chromatids

Homologues

a.

Centromere

Synaptonemalcomplex


Prophase I

• Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms

• Each chromosome composed of 2 sister chromatids• Synapsis– Homologues become closely associated– Crossing over occurs between nonsister chromatids– Remain attached at chiasmata• Chiasmata move to the end of the chromosome arm

before metaphase I

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Crossing over

• Genetic recombination between nonsister chromatids

• Allows the homologues to exchange chromosomal material

• Alleles of genes that were formerly on separate homologues can now be found on the same homologue

• Chiasmata – site of crossing over– Contact maintained until anaphase I

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Meiosis II

• Resembles a mitotic division• Prophase II: nuclear envelopes dissolve and

new spindle apparatus forms• Metaphase II: chromosomes align on

metaphase plate• Anaphase II: sister chromatids are separated

from each other• Telophase II: nuclear envelope re-forms around

4 sets of daughter chromosomes; cytokinesis follows

Final result

• Four cells containing haploid sets of chromosomes

• In animals, develop directly into gametes• In plants, fungi, and many protists, divide

mitotically– Produce greater number of gametes– Adults with varying numbers of gametes

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Meiosis vs. Mitosis

Meiosis is characterized by 4 features:1. Synapsis and crossing over2. Sister chromatids remain joined at their

centromeres throughout meiosis I3. Kinetochores of sister chromatids attach to

the same pole in meiosis I4. DNA replication is suppressed between

meiosis I and meiosis II

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Meiosis I Mitosis

Metaphase I

Anaphase I

Metaphase

Anaphase

Crossovers andsister chromatidcohesion lockhomologuestogether.Microtubulesconnect to thekinetochores ofsister chromatids sothat homologues arepulled towardopposite poles.

Microtubules pullthe homologouschromosomesapart, but sisterchromatids areheld together atthe centromere.

Homologues donot pair;kinetochores ofsister chromatidsremain separate;microtubulesattach to bothkinetochores onopposite sides ofthe centromere.

Microtubulespull sisterchromatidsapart.

Vocabulary• Genetics: The scientific study of heredity• Character: heritable feature• Trait: each variant for a character• True-breeding: plants that self-pollinate all

offspring are the same variety• Allele: alternate version of a gene• Dominate allele: An allele which is expressed (masks

the other) in the heterozygote & homozygote• Recessive allele: An allele which is present but

remains unexpressed (masked) in the heterozygote

Vocabulary (continued)• Homozygote – pair of identical alleles for a character– Homozygous dominant- BB– Homozygous recessive - bb

• Heterozygote – two different alleles for a character (Bb)

• Genotype – genetic makeup; combination of alleles an organism has

• Phenotype – appearance of an organism; the characteristics determined by the genotype and environmental influences

Punnett Squares

• Genetic problems can be easily solved using a tool called a Punnett square.– Tool for calculating genetic probabilities

A Punnett square

Monohybrid cross (cross with only 1 trait)

• Problem:• Using this is a several step process, look at the

following example– Tallness (T) is dominant over shortness (t) in pea plants.

A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the genotypic makeup of the offspring? The phenotypic makeup ?

Punnet process

1. Determine alleles of each parent, these are given as TT, and tt respectively.

2. Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.

Punnett process continued

• Lastly, write the letter for each allele across each column or down each row.

• The resultant mix is the genotype for the offspring.

• In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.


• Lets take this a step further and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get.

• Since each parent can contribute a T and a t to the offspring, the punnett square should look like this…


• Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.

Dihybrid Crosses

• Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed.

• Process is very similar to monohybrid crosses.• Example:– 2 traits are being analyzed– Plant height (Tt) with tall being dominant to short,– Flower color (Ww) with Purple flowers being dominant to

white.

Dihybrid Cross Example• The cross with a pure-breeding (homozygous)

Tall,Purple plant with a pure-breeding Short, white plant should look like this.

F1 generation

Dihybrid Cross Example continued

• Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here)

F2 Generation

Laws of Inheretance

• Law of Segregation: When gametes (sperm, egg, etc…) are formed each gamete will receive one allele or the other.

• Law of Independent Assortment: Two or more alleles will separate independently of each other when gametes are formed

Polygenetic Inheritance• Qualitative variation

usually indicates polygenic inheritance. This occurs when there is an additive effect from two or more genes. Pigmentation in humans is controlled by at least three (3) separately inherited genes.

Sex Linkage

• All chromosomes are homologous except on sex chromosomes.

• Sex chromosomes are either X or Y.• If an organism is XX, it is a female, if XY it is male.• If a recessive allele exists on the X chromosome. It

will not have a corresponding allele on the Y chromosome, and will therefore always be expressed

Other Factors: Incomplete Dominance

• Some alleles for a gene are not completely dominant over the others. This results in partially masked phenotypes which are intermediate to the two extremes.

Incomplete Dominance

Codominance

• Two alleles affect the phenotype in separate and distinguishable ways.

• Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form.

• Example: red flowers that are crossed with white flowers that yield red and white flowers.

• 1) In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle?

• A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same

percentage of roan.

Documents

1 Bacterial Cell Division Bacteria divide by binary fission – No sexual life cycle – Reproduction is clonal Single, circular bacterial chromosome is replicated