Cell Biology Exam 3 Prep

Phases of Interphase

G1, S, and G2 phases

M-phase DNA state

Highly condensed

Nucleosome

Eukaryotic DNA wrapped around histone proteins

Chromatin packaging sequence

Nucleosome, 30 nm chromatin, 300 nm chromatin, 700 nm fiber

Final degree of DNA condensation to 700 nm fiber

Because Cdk1 phosphorylates the histones

Lengthwise DNA condensation needed to fit in nucleus

~17,000x condensation

Homologous chromosomes

The same chromosome inherited from either the father or the mother

Sister chromatids

The two identical copies of a replicated chromosome

Centromere

The specific location on the chromosome where sister chromatids are held together

Kinetochore

The protein complex at the centromere that actively holds sister chromatids together

Early M-phase events

Centrosomes present, chromosomes condense, microtubules form spindle

Nuclear envelope breaks down and chromosomes align

Because they are triggered by Cdk1

Late M-phase events

Chromosomes align, sister chromatids segregate, cytokinesis occurs

Mitotic spindle definition

The array of microtubules responsible for chromosome segregation

Chromosomes fail to align or segregate

Because the microtubules have been destroyed

Early metaphase spindle formation

Microtubules polymerize randomly from the centrosome

Kinetochore Microtubules

Bind to chromosomes at (+) end and centrosome at (-) end

Astral Microtubules

Connect to cell membrane at (+) end and centrosome at (-) end

Polar Microtubules

Connect to opposite MTs via motor proteins at (+) end and centrosome at (-) end

Microtubules that polymerize at their (+) ends prior to metaphase

Kinetochore, polar, and astral MTs

Microtubules that depolymerize at their (+) ends during anaphase

Kinetochore and astral MTs

Taxol

A potent inhibitor of cell proliferation that causes metaphase arrest

Metaphase arrest occurs after Taxol treatment

Because Taxol explicitly inhibits microtubule depolymerization

Sister chromatids are pulled towards daughter centrosomes

Because Kinetochore MTs shorten and dynein motor proteins walk towards the centrosome

Centrosomes are pushed apart

Because Polar MTs lengthen and kinesin motor proteins walk towards the (+) ends

Centrosomes are pulled towards the cell membrane

Because Astral MTs shorten and dynein motor proteins walk towards the MT (-) ends

Bacterial cell division process

Fission

Steps of bacterial fission

DNA Replication, Cell lengthening, DNA Segregation, Cytokinesis

Daughter cells in fission are genetically equivalent

Because they are identical except for mutations

Primary differences between Eukaryotic Cell Cycle and Fission

Eukaryotic cycle is strictly regulated and involves mitosis

Interphase

G1, S, and G2 phases combined

The cell appears inactive during Interphase

Because DNA is not condensed and not visible

G1 Phase events

DNA check, Growth, Signal to proliferate

S Phase event

DNA Replication

G2 Phase events

Growth, DNA check

M Phase events

Chromosome condensation, spindle assembly, segregation, cytokinesis

Frog oocytes are arrested here before hormone signaling

At the end of G2 phase

Progesterone

The hormone that signals frog oocytes to mature and complete meiosis

Untreated oocyte undergoes meiosis when injected with cytoplasm

Because a cytoplasmic factor called MPF triggers M-phase

MPF

Maturation Promoting Factor

Key properties of MPF activity

Found in ALL dividing cells, universally transplantable

MPF composition

Cyclin B + Cdk1

Cdk1 becomes active

Because it undergoes allosteric activation by Cyclin B

Cdk1 becomes inactive after mitosis

Because Cyclin B is degraded

Progression from G1 to S phase

Driven by Cyclin E & Cdk2

Progression from G2 to M phase

Driven by Cyclin B & Cdk1

Chromosome condensation occurs

Because active Cdk1 phosphorylates Histone proteins

The Nuclear Envelope breaks down

Because active Cdk1 phosphorylates Lamin A

The Mitotic Spindle forms

Because active Cdk1 phosphorylates Microtubule associated proteins

Cytokinesis definition

Cell division; occurs during telophase

Karyotype

The number and types of chromosomes in a cell

Nondisjunction

Abnormal chromosome segregation during mitosis or meiosis

Aneuploidy

Abnormal number of chromosomes

Aneuploidy occurs

Because the metaphase-to-anaphase transition happens prematurely

Animal Cell Cytokinesis

A contractile ring forms and tightens

Cytokinesis happens in cells treated with colchicine

Because microtubules are not needed for cytokinesis

Animal cell cytokinesis requires specific proteins

Actin and Myosin II

Plant Cell Cytokinesis

A cell plate forms instead of a contractile ring

Plant cell plates form

Because cellulose-containing vesicles are transported along leftover microtubules to the equator

Cohesin

Protein at centromeres holding sister chromatids together

Separase

Protease enzyme that degrades cohesin at anaphase transition

Cohesin degradation occurs

Because there is physical tension on the kinetochore

Tension is achieved and anaphase begins

Because microtubules from BOTH poles have bound to all kinetochores

The G1/S Checkpoint

The restriction point; the point of no return for proliferation

Tumor Suppressor Genes

Genes whose normal proteins prevent cell cycle progression at checkpoints

Proto-oncogene

Gene whose normal protein activates the cell cycle appropriately

Oncogene

Mutated gene whose protein activates the cell cycle inappropriately

Radiation therapy works

Because it induces DNA damage to persistently activate p53, leading to apoptosis

Taxol works

Because it arrests cells at metaphase

5-fluorouracil works

Because it competitively inhibits DNA polymerase and prevents replication

Herceptin works

Because it inhibits the RTK growth factor receptor HER2

Necrosis

Passive cell death causing bursting and inflammation

Apoptosis

Active, programmed cell suicide causing shrinking

DNA Laddering occurs during apoptosis

Because an apoptosis signal activates an Endonuclease to digest DNA

Loss of membrane asymmetry occurs during apoptosis

Because a protease degrades flippase, moving Phosphatidylserine to the outer membrane

Rb prevents cell cycle progression

Because it binds E2F, preventing S-phase gene transcription

Growth factors cause the cell to bypass Rb

Because they activate Ras, producing G1/S Cyclin to activate G1/S Cdk, which phosphorylates Rb so it falls off E2F

Mutated Rb causes constitutive DNA replication

Because it cannot bind E2F, leaving E2F always active

HPV E7 causes constitutive DNA replication

Because E7 binds Rb, forcing it off E2F

Cells arrest at G1/S or G2/M to repair DNA

Because DNA damage activates p53, which activates p21 to inhibit Cdk, so Rb inhibits E2F

Unrepairable DNA damage leads to apoptosis

Because long-term p53 activation triggers cell suicide genes

Normal Ras turns off

Because its inherent GTPase activity converts GTP to GDP

Mutated Ras causes continuous division

Because it loses GTPase activity and gets stuck in the "on" position

Asexual reproduction

Duplication of one genome where offspring are genetically identical to the parent

Sexual reproduction

Combines genetic material from two parents; offspring are distinct

Appropriate chromosome numbers are maintained

Because genomes are halved during meiosis before fertilization

Ploidy of a Meiotic Parent Cell in G1

Diploid (2N, 1X)

Ploidy of a Meiotic Cell entering Meiosis I

2N, 2X

Prophase I event (Chiasma)

Homologous chromosomes exchange DNA via crossing over

Metaphase I event

Homologous chromosomes align adjacent to each other

Anaphase I event

Homologous chromosomes segregate, sister chromatids stay together

Result of Meiosis I

Separation of homologous chromosomes (1N, 2X)

Metaphase II event

Chromosomes align on metaphase plate

Anaphase II event

Sister chromatids separate

Result of Meiosis II

Separation of sister chromatids into haploid gametes (1N, 1X)

Formula for possible gamete combinations

Number of homologs ^ number of chromosomes

Possible combinations in human gamete

2^23 (8,388,608)

Aneuploidy occurs

Because of Nondisjunction during Meiosis I or II

Trisomy 21 phenotypic cause

Because of excess production of protein for genes on Chromosome 21

Klinefelter syndrome

47, XXY karyotype; biologically male, sterile