Mitosis

Question 1

What are the Stages of mitosis? What happens at each stage?

Answer 1

• Interphase

• Prophase:
o Chromosomes condense (visible).
o Centromeres are formed & microtubules start to grow.
o Nuclear membrane dissolves.

• Metaphase:
o Spindles attach with kinetochores (proteins associated with centromere).
o Paired chromosomes line up at the metaphase plate.

• Anaphase:
o Cohesins are broken.
o Paired chromosomes move apart.

• Telophase:
o Chromosomes clump together at the poles.
o The nuclear membrane reforms.

Question 2

What are Cohesins? What is their structure? What is their function? When are they broken?

Answer 2

• Proteins that stick chromosomes together.
• 4 subunits that form a ring.
• Keeps products of replication together.
• Promotes spindle attachment.
• Promotes repair of recombination damage.
• Broken during anaphase checkpoint.

Question 3

What diseases are associated with damage regulation?

Answer 3

• Diseases associated with damaged regulation:
o Roberts syndrome – increased mitotic failure  apoptosis.
o Cornelia De Lange syndrome – multispectrum birth defects.

Question 4

What is a Centromere? How does it vary between yeast and humans? What is the assembly site?

Answer 4

• Segment of chromosome essential for pairing & separation.
• Transcriptionally inert – no resulting gene (heterochromatin).
• Sequence varies from specific binding sites (yeast) to extended repeated sequences (human).
• Assembly site for kinetochore.

Question 5

What is a Kinetochore? What does it do? What is it made up of?

Answer 5

• Protein complex on centromere.
• Links chromosome to microtubules of mitotic spindle.
• Contains multiple protein species (45+)
• Provides directionality.
• Proteins:
o Inner plate: CENP-A a specialised histone.
o Outer plate: attachment site for microtubules – fibrous corona kinesin CENP-E = a + end motor couples chromosome and tubule with depolymerising activity.
o Motor proteins: kinesin- moves to + end, dynein – moves to – end.

Question 6

What are Microtubules (MTs)? What are the ends called and what are they near? What is its relationship to kinetochore? What does it do in mitosis?

Answer 6

• Assembled from alpha and beta tubulin.
• Tubulin assembly nucleated by centrosome.
• Asymmetric extremes: -ve end near centrosome & +ve end that grows towards cell centre.
• Attachment to kinetochore stabilises MT – turns other kinetochore towards opposite pole.
• Action in mitosis:
o Early anaphase  anaphase = kinetochore microtubules depolymerise at kinetochore ends and kinetochores move towards poles.

Question 7

What is a Centrosome? Where is it found? When does it replicate and how many times per cycle? What does it do in prophase?

Answer 7

• Organelle found in animals (other eukaryotes have equivalent).
• Main microtubule organising centre (MTOC)
• Protein body containing microtubule initiation and anchoring proteins.
• Replicates once per cell cycle in S phase.
• Associates with nuclear membrane in prophase.