BIo 161 - Mitosis: Cell Reproduction

somatic cells

all body cells forming an organism’s tissues and organs, excluding reproductive germ cells (sperm and eggs) (gametes = reproductive cells)

All cells in body have some rate of turnover

vary in specific genetically determined speeds/rates

Cell reproduction

cells must replicate (ie. DUPLICATE) themselves

• For growth, tissue turnover, repair, and replacement of cells.

Tissues (esp. with rapid turnover)

skin has fast turnover

1st Requirement for Cell Reproduction

DNA must be copied

2nd Requirement for Cell Reproduction

DNA must be separated

3rd Requirement for cell reproduction

Cytoplasm must be separated

Cell Reproduction Requirement 1: Why must DNA be copied before cell division?

• 1 cell to 2 cells, 2 copies of DNA needed

• Each daughter cell requires a complete copy of DNA for survival.

Cell Requirement 2: Why must DNA be separated?

daughter cell requires all DNA for survival

Cell Requirement 3: Why must cytoplasm be separated?

Physical separation required

How cell reproduction requirements fit into cell cycle

1) DNA copied in S phase (DNA synthesis)

2) DNA separated in M phase, Mitosis

3) Cytoplasm separated in Cytokinesis

Two Distinct Phases (parts) to the cell cycle

1) Interphase

2) Mitotic Phase

Which phase occupies most of the cell cycle?

Interphase

What phases make up interphase?

G1, S, and G2.

What is chromatin?

DNA associated with proteins in an uncondensed form.

What is a chromosome?

Condensed chromatin visible during cell division.

What are sister chromatids?

Two identical copies of DNA produced during DNA synthesis, CONDENSED & joined @ centromere

When are sister chromatids formed?

During S phase after DNA replication. (cohesins still intact, chromatids fused together, uncondensed)

When do sister chromatids separate?

During anaphase (in mitosis)

What is a centromere?

Region joining sister chromatids together (what is left of cohesin proteins that fused chromatin copies together after chromatin condenses)

What is a kinetochore?

Protein structure on the centromere where spindle microtubules attach. (protenacious attachment)

What allows chromosomes to stay attached while microtubules grow and shrink?

The kinetochore.

In the G1 phase the cell is

• Actively growing, synthesizing proteins, increasing volume, and preparing for DNA replication (S phase). It is part of the continuous cycle

In the G0 phase…

A quiescent stage existing outside the active cycle. Cells are differentiated and metabolically active but do not divide (& have no intention to go through cell division)

W/appropriate signals (accumulation of products + enzymes necessary for copying DNA)

cell will kick out of G0 phase into G1 phase to transition to DNA synthesis phase

Chromosomes in Cell Cycle: G1

Chromosomes exist as loose, decondensed chromatin to allow for gene expression.

• 4 chromosome example but Human cells have 46 single-stranded chromosomes (46 chromatids).

Chromosomes in Cell Cycle: S Phase

• DNA replicates. The chromosome number remains 4, but each chromosome is now duplicated

Chromosomes in Cell Cycle: G2 Phase (initially)

• (after DNA Synthesis) still 4 chromosomes —> 2 exact copies of chromatin

• cohesins (proteins) stitch copies of chromatin together! —> behave as 1 chromosome

Chromosomes in Cell Cycle: G2 Progression

• As cell progresses through end of G2, cohesins stay on

• still 4 chromosomes each consisting of 2 identical sister chromatids

  • (BUT STILL LONG/LOOSELY COILED & UNCONDENSED) (fully develop in prophase)

  • Prophase: most arm cohesins come off

Chromosomes in Cell Cycle: Brief Description Mitosis

• sister chromatids condense early on & then separate in mitosis

• POST mitotic stage = 2 cells EACH WITH 4 chromosomes

Prior to Mitosis: Interphase Cell

• nuclear membrane intact during interphase (G1, S, G2)

• chromatin uncondensed in nucleus

• cell in G2 (cell & nucleus prepare itself) (chromosomes in G2 = sister chromatids joined w/cohesins + the centromere, but they are still long and tangled)

• have Duplicate MTOC (centrosome in animals)

What moves chromosomes during mitosis?

Mitotic spindle fibers = microtubules!

Microtubules are made up. by

alpha & beta tubulin (protein) subunits

—> form cylindrical hollow tubules

What motor proteins move along microtubules?

Kinesin and dynein motors

—> motors carry components (organelles, structures) along microtubules, but can aslo take 2 microtubules, connect them w/moto protein so walking makes them slide past each other!

What properties of microtubules are important during mitosis?

• grow & shrink

• slide past each other (connected w/motoprotein so walking makes them slide past each other)

• serve as tracks for kinesin & dynein motors

Microtubules have..

• a positive and negative end!

Where are the positive end of microtubules?

extending outward toward the cell periphery (the cell membrane or edges)

Where are the negative end of microtubules?

embedded into microtubule organizing center

Microtubules are organized by..

• centrosome/spindle pole (Microtubule organizing center)

Zooming into MTOC…What are centrioles?

• center of centrosome from which all of microtubule fibers/spindles are going to derive from

What are centrioles in the MTOC made up of?

made up of triplets of microtubules organized in a nonomer (9) pattern

First Stages of Mitosis: Prophase

• “early phase”

• Each MTOC separate itself & nevigate toward seperate poles of cells

• chromatin condense into sister chromatid + long, loose sister chromatids condense to become short, thick, and moveable (X shape)

What happens to chromatin during prophase?

it condenses into visible sister chromosomes (X shape)

First Stages of Mitosis: Prometaphase

• “Early Middle” phase

• Microtubules from MTOC reach out & pair up w/sister chromatids —> Attach to kinetochores

• full condensation of chromosomes

• degradation of nuclear membrane

What important event allows spindle fibers to access chromosomes?

Breakdown of the nuclear envelope.

What phase of mitosis does the breakdown of nuclear envelope occur?

Prometaphase

First Stages of Mitosis: Metaphase

• each sister chromatid tethered to MTOC via microtubule attachment @ kinetochore of centromere

• MOTC fully positioned on opposite poles of cell

• alignment of all chromosomes @ metaphase plate

What is the metaphase plate?

Imaginary plane where chromosomes line up during metaphase.

Last Stages of Mitosis: Anaphase

• '“Against” Phase

• Last of cohesins degraded by enzymes & sister chromatids can move against each other & pull away towards independent poles of parent cell

• DOUBLE number of chromosomes

What triggers chromosome movement during anaphase?

Microtubule shortening and motor proteins.

What are the two types of movement during anaphase?

sister chromatids move toward poles and spindle poles move farther apart.

Last Stages of Mitosis: Telophase

• End phase

• nuclear membranes reform around decondensing chromosomes (decondense back into chromatin)

• degradation of microtubules

Microtubule Degradation: Power to pull chromosomes & move towards independent poles comes from….

comes from hydrolysis of alpha & beta tubulin subunits (microtubule degradation)

Microtubule degradation

• Kinetochore fibers & ring (attached to centromere) pulls chromosome while degrading microtubules

Last Stages of Mitosis: Cytokinesis

• cell division

• Division of the cytoplasm into two daughter cells.

What proteins are involved in cytokinesis in animal cells?

Actin and myosin

• think “cinching belt”

• motor force drives cinching/closing of actin filaments driven through myosin protein

Division of Cell: As myosin walks along actin filaments…

actin filaments pulled closer together, shrink ring & create cleaver furrow —> breakage between cells & detaches cells apart

Chromosomes condense and spindle begins forming.

Prophase

Nuclear envelope breaks down and spindle attaches to chromosomes.

Prometaphase

Chromosomes aligned at cell center.

Metaphase

Sister chromatids separate.

Anaphase

Nuclear Envelope reform

Telophase

Cytoplasm divides

cytokinesis

Why is cell cycle regulation important?

Controls timing/order of division and prevents damaged cells from dividing.

• some cells rapidly divide (RBC’s 2-3 million/second)

• some cells rarely divide (neurons seldomly divide, usually in G0)

Some regulation cell has to keep in mind

• Rate of cell division

• Maintaining order of stages

• Stop if problems

What regulates progression through the cell cycle?

Cyclins and cyclin-dependent kinases (Cdks) (MPF)

• kinases = proteins that phosphorylate

What is MPF?

• Mitotic Promoting Factor

• Mitotic cyclin/Cdk complex that promotes entry into mitosis.

Regulatory Subunit of MPF

Cyclin (does not act alone)

MPF CDK has an allosteric site (on top)

• determine whether it is activated or inactivated

• allosteric site dephosphorylated to kickstart mitosis & cyclin dependent —→ linase is active

MPF CDK is active when

it has 1 phosphate

MPF CDK is inactive when

it has 2 phosphates

MPF Cyclin increases during…

interphase

• cyclin-dependent Kinase inactive (double phosphorylated)

Basic Cell Regulator: CDK + cyclin

• Mitotic CDK & cyclin to push cell into Mitosis

• BUT CDK & cyclin also present to push cell into S-phase (checkpoints scattered throughout cell cycle)

G1 Checkpoint

• checks Cell size (after last mitotic event, cell needs to grow to appropriate size before it can transition to new cell cycle)

• check nutrients (measuring available energy, need high ATP/ADP ratio)

• growth signals —> social signals (E2F)

• & check DNA condition (making sure it is undamaged)

G2 Checkpoint

check if replication is complete

check DNA condition again (is it undamaged?, was S phase carried out properly?)

M Phase Checkpoint has 2 components: What does the first check?

1) Checking that all chromosomes attached to spindle (& aligned in middle, metaphase)

M Phase Checkpoint has 2 components: What does the second check?

Whether all chromosomes properly segregated

• anaphase —> sister chromatids broken apart & go to opposite poles

Cancer: issue with cell cycle regulation

= unchecked cell replication

→ leads to overgrowth of cells in unregulated manner

Mitosis 1 brief

Interphase (G1, S, G2) (brief)

Mitosis 2 brief

Prophase (brief)

Mitosis 3 (brief)

Prometaphase (brief)

Mitosis 4 (brief)

Metaphase (brief)

Mitosis 5 (brief)

Anaphase (brief)

Mitosis 6 (brief)

Telophase (brief)

Mitosis 7 (brief)

cytokinesis: cell division (brief)

What is the difference between a chromatid and a chromosome?

• A chromatid is one half of a replicated chromosome

• whereas a chromosome consists of DNA wrapped around proteins in a highly organized manner.

A cell in G₁ phase has 6 unreplicated chromosomes. How many DNA double helices does the cell have after chromosome replication?

12

Scientists isolate cells in various phases of the cell cycle. They find a group of cells that have

1 & one half times more DNA than G₁ phase cells. The cells of this group are ________.

in the S phase of the cell cycle (In G2 have 2x amount of DNA)

A cell has 6 unreplicated chromosomes. How many replicated chromosomes does the cell have after chromosome replication?

6

Which of the following phases are characterized by having cells with chromosomes consisting of two sister chromatids?

G_2, prophase and metaphase

After the G1 phase, what makes up a single chromosome?

a double stranded DNA molecule

For eukaryotic cells in the G₂ phase, each chromosome is composed of ________ joined at the ________.

two sister chromatids; centromere

Progression through the cell cycle is regulated by oscillations in the concentration of which type of molecule?

cyclins

Identify the events in the cell cycle that must be completed successfully for daughter cells to share an identical complement of chromosomes.

• All the chromosomes must be replicated during the S phase.

• The spindle apparatus must connect with the kinetochores of each sister chromatid in prometaphase.

• The sister chromatids of each replicated chromosome must be partitioned in anaphase and fully separated into daughter cells by cytokinesis.

Which of the following would be used to differentiate between benign and malignant tumors?

The ability to invade other tissues

DNA & Chromosome count: Original cell/Parent cell

• same number of chromosomes

• original amount of DNA

DNA & Chromosome count: After DNA replication (S Phase) in Parent cell

• same number of chromosomes

• double the DNA

DNA & Chromosome Count: After Mitosis (In Daughter cells)

• same number of chromosomes as parent cell

• same amount of DNA as original amount of DNA in parent cell