Mitosis

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Questions and Answers

What is essential for proper chromosome segregation during cell division?

Chromosomes must be bioriented to segregate properly. (correct)

Microtubules must be static throughout the process.

Chromosomes must align at the cell center before division.

Cohesin must form rings around chromatids.

What role do outer kinetochore proteins play in the attachment of microtubules?

They bind to the very ends of microtubules.

They completely prevent microtubule polymerization.

They form extended structures that bind to the sides of microtubules. (correct)

They stabilize the microtubule structure permanently.

Which process is driven by the depolymerization of microtubule plus ends at the kinetochore?

Anaphase A (correct)

Anaphase B

Kinetochore assembly

Chromosome alignment

What happens to sister chromatids after cohesin is removed?

They are pulled toward opposite spindle poles. Signup and view all the answers

How does dynein contribute to the separation of spindle poles during Anaphase B?

It generates a pulling force towards the plasma membrane. Signup and view all the answers

During which phase does the spindle elongate, further separating chromosomes?

Anaphase B Signup and view all the answers

What is the primary reason chromosomes can stay attached even as microtubules grow and shrink?

Kinetochore proteins allow dynamic interaction with microtubules. Signup and view all the answers

What initiates the separation of sister chromatids during cell division?

Cleavage of cohesin rings by separase. Signup and view all the answers

Which statement accurately describes the behavior of kinetochores during chromosome oscillation?

Chromosomes oscillate while remaining attached to dynamic kinetochores. Signup and view all the answers

What happens during the S phase of the cell cycle?

DNA duplication Signup and view all the answers

What is the role of the cohesin complex during mitosis?

To link sister chromatids until anaphase Signup and view all the answers

What event occurs during prophase of mitosis?

Centrosomes move apart and chromosomes condense Signup and view all the answers

During which phase do chromosomes align at the metaphase plate?

Metaphase Signup and view all the answers

Which statement correctly describes spindle assembly?

Microtubules capture chromosomes during prometaphase. Signup and view all the answers

What is the significance of chromosome condensation during mitosis?

To ensure chromosomes fit within the cell Signup and view all the answers

What characterizes the state of sister chromatids after S phase?

They are linked together by the cohesin complex. Signup and view all the answers

Which structure forms at the center of the cell during telophase?

Contractile ring Signup and view all the answers

Which of the following structures is involved in the dynamic movement of chromosomes?

Microtubule-based spindle Signup and view all the answers

What occurs during cytokinesis?

Cell pinches into two Signup and view all the answers

How many stages comprise the process of mitosis?

Six Signup and view all the answers

In which phase is the nuclear envelope reformed?

Telophase Signup and view all the answers

What happens to chromosomes during anaphase?

Sister chromatids separate and move to opposite poles. Signup and view all the answers

What is the primary purpose of mitotic spindle in cell division?

To align and segregate chromosomes Signup and view all the answers

What role do condensins play in chromosome organization during mitosis?

They form rings that compact DNA into loops for full condensation. Signup and view all the answers

What is the primary function of kinesin-5 during spindle assembly?

To push apart overlapping interpolar microtubules. Signup and view all the answers

How does the dynamic instability of microtubules in mitosis contribute to chromosome alignment?

It enables microtubules to grow towards kinetochores effectively. Signup and view all the answers

Which component organizes microtubules into a bipolar spindle during mitosis?

Centrosomes Signup and view all the answers

What is the role of kinetochores during the 'search and capture' process?

To form attachments with dynamic microtubules. Signup and view all the answers

What is the effect of increased catastrophe frequency of microtubules during mitosis?

It enables efficient chromosome biorientation and alignment. Signup and view all the answers

Which structural component is crucial for the formation of stable kinetochore-microtubule attachments?

Outer kinetochore proteins Signup and view all the answers

Which statement correctly describes the positioning of sister chromatids during metaphase?

Each chromatid is attached to opposite spindle poles. Signup and view all the answers

What happens to microtubules in the mitotic spindle as the cell enters anaphase?

They disassemble to release tension on sister chromatids. Signup and view all the answers

What is primarily responsible for the movement of chromosomes during mitosis?

Dynamic reorganization of microtubules. Signup and view all the answers

Which of the following statements is true about sister chromatids?

They are identical copies formed during DNA replication. Signup and view all the answers

During which phase do chromosomes first become distinctly visualized?

M phase Signup and view all the answers

What is an essential characteristic of microtubules during the M phase?

They become stabilized through cross-linking proteins. Signup and view all the answers

How does the breakdown of the nuclear envelope influence microtubule function?

It permits microtubules to capture chromosomes. Signup and view all the answers

Study Notes

Chromosome Segregation

Chromosomes must be bioriented to segregate properly
Biorientation creates tension on the kinetochores, allowing the cell to monitor attachments
Microtubules remain dynamic and chromosomes oscillate even after biorientation
Outer kinetochore proteins bind to the sides of microtubules, allowing microtubules to dynamically grow and shrink without detaching from kinetochore

Cohesin Removal and Separase

For sister chromatids to seperate, cohesin must be removed
Separase is the protein that cleaves cohesin rings

Anaphase

Two mechanisms drive chromosome segregation during anaphase: Anaphase A and Anaphase B
Anaphase A: chromosomes move to spindle poles driven by microtubule depolymerization at kinetochore
Anaphase B: spindle poles separate driven by kinesin-5 and dynein

Dynein Activity in Anaphase B

Astral microtubules extend towards the cell cortex
Dynein at the cell cortex walks towards the minus ends of astral microtubules
This generates a pulling force that reels in the astral microtubule and pulls the centrosome closer to the plasma membrane

Interphase vs. Mitosis Chromosomes

Individual chromosomes are visible in mitosis due to increased condensation.
It is harder to distinguish individual chromosomes in interphase because they are less condensed.

Condensins in Chromosome Condensation

Condensins are protein complexes that form rings that function to condense DNA in mitosis.
They help organize the DNA into loops which results in full condensation of mitotic chromosomes.

Spindle Assembly

Centrosomes nucleate microtubules
Centrosomes duplicate during S phase
Centrosomes separate along the outside of the nuclear envelope
A bipolar spindle forms with a centrosome at each pole nucleating microtubules
The nuclear envelope breaks down to allow microtubules to capture chromosomes

Microtubule Dynamic Instability

Microtubules undergo phases of growth and shrinkage by addition/loss of tubulin subunits
Dynamic instability increases in mitosis compared to interphase
This increased dynamics facilitates breakdown and rearrangement of the interphase microtubule array and attachment of chromosomes to the spindle

Microtubule Dynamics in the Mitotic Spindle

While microtubules are dynamic in mitosis, specific populations are stabilized.
Microtubules growing out of the centrosome are highly dynamic and form asters; these are astral microtubules.
Some astral microtubules become stabilized by cross-linking proteins and become interpolar microtubules.
Interpolar microtubules help form the football shape of the spindle
The overlap zone between interpolar microtubules is where kinesin-5 walks to generate outward force.

Kinesin-5 in Spindle Assembly

Kinesin-5 is a tetrameric kinesin with two heads on each side.
Both sets of heads can walk to MT plus ends.
Each set of heads associates with a different microtubule.

Kinesin-5 Function in Spindle Assembly

Kinesin-5 localizes to the overlap zone of interpolar microtubules.
It walks towards the plus end of each antiparallel microtubule.
This outward force pushes the poles apart, promoting centrosome separation during spindle assembly.

Chromosome Capture and Alignment

After spindle formation, chromosomes begin attaching to microtubules.
After attachment, captured chromosomes are moved to the middle of the cell, called chromosome congression

Kinetochore Organization

Kinetochores are protein structures that mediate attachment of microtubules to the centromere.
Multiple microtubules attach to each kinetochore.

Kinetochore Structure

Core kinetochore proteins form a platform on the centromere region.
Outer kinetochore proteins extend and bind to the microtubules.

Search and Capture Process

Microtubules emanating from centrosomes grow and shrink.
When a microtubule hits a kinetochore it forms an attachment, known as microtubule capture.
Eventually, chromosomes become bioriented, with one sister attached to each spindle pole.

Biorientation of Sister Chromatids

Both sister chromatids must be attached to opposite spindle poles.

M Phase Events

Chromosome duplication and condensation
Spindle assembly
Chromosome capture and alignment
Chromosome segregation
Cytokinesis

Chromosome Duplication and Condensation

DNA duplicates prior to mitosis, resulting in sister chromatids.
Chromosomes condense to fit the long DNA molecules into the nucleus.

DNA Replication and Sister Chromatids

Chromosomes duplicate during S phase.
Sister chromatids are connected by the cohesin complex.

Cohesin Complex

Cohesin forms a ring around duplicated strands during S phase.
Responsible for keeping sister chromatids attached until anaphase.

Stages of Mitosis

Prophase: chromosomes condense, centrosomes move apart.
Prometaphase: nuclear envelope breaks down, spindle forms, chromosomes captured, chromosome congression.
Metaphase: chromosomes line up at the metaphase plate.
Anaphase: sister chromatids move to opposite spindle poles.
Telophase: contractile ring forms, nuclear envelope reforms.
Cytokinesis: contractile ring constricts, cell pinches in two, chromosomes decondense.

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