Mitosis

Question 1

What are the different phases of the cell cycle before mitosis

Answer 1

G1: Metabolic changes, cell prepares for cell division

S: DNA replication, 2 sister chromatids

G2: Assembles for cytoplasmic materials necessary for mitosis and cytokinesis

M: nuclear division followed by cell division ( cytokinesis)

G1 is transformed into G0, if the cell has
no stimulus for division

Question 2

What is the evolution the mitosis

Answer 2

Nuclear envelope may or may not breakdown
Some unicellular eukaryotes, spindle acts in nucleus = CLOSED MITOSIS
Animals and plants= OPEN MITOSIS

Question 3

What are the features of mitotic phases

Answer 3

PROPHASE: chromosomes condense and visible
nuclear envelope breaks down
Spindle fibres emerge from centrosomes

PROMETAPHASE: kinetochores appear at centromere
spindle fibres attach to kinetochores

METAPHASE: Chromosomes lines at equator
Each sister chromatid attached to spindle fibre

ANAPHASE: Spindle pulls and centromeres split
sister chromatids (now called chromosomes)
pulled to opposite poles
Some spindle elongate cell

TELOPHASE: chromosomes decondense
Nuclear envelopes form
Spindle breaks down

CYTOKINESIS: Animal- cleavage furrow due to contractile actin
ring splits daughter cells
Plant- cell plate, precursor to new cell
wall separates daughter cells

Question 4

What are the 3 types of spindle microtubules

Answer 4

astral, kinetochore and overlap

Question 5

What are the functions of each spindle fibres

Answer 5

Overlap(polar): keeps chromosome in limited zone and supports spindle

Kinetochore: holds chromosomes

Astral: defines position of spindle in cell

Question 6

difference between centromere and kinetochore

Answer 6

Centromere = region of chromosome that holds sister chromatids together ( constricted due to cohesisns and condesins)

Kinetochore= disc shaped protein complex allows spindle fibres to attach during cell division

Question 7

How does microtubules become shorter or longer

Answer 7

MOTOR PROTEINS adds/removes tubulin subunits (elongation or shortening)

Question 8

How do chromosomes move during anaphase

Answer 8

shortening of kinetochore fibres + elongation of polar (overlap) fibres)

Question 9

What are the motor proteins for spindle assembly

Answer 9

Microtubule bound motors: promotes bipolar spindle
formation

Chromosome associated motors: allows proper kineto
orientation and chromosome movement to equator

Question 10

What are some chromosome associated motors

Answer 10

Kinesisin 5= bipolarity established. Slides antiparallel microtubules apart with their minus ends so plus ends directed to spindle equator

Dyenin= moves microtubules to pole with “-“ ends. K fibres incorporates into spindle and spindle poles form

Kinetochore associated dyenin= transpoets chromosomes along astral microtubules toward spindle poles

Kinesisin 4+10= eject chromosome arms outward

Kinesisn 7= transports kinetchores toward ewquator along spindle microtubyukles

Question 11

How is cytokinesis in animal cells

Answer 11

Contractile ring (actin and myosin filaments) enter cleavage furrow midway between 2 spindle poles

splits plams membrane of developing daughter cells

Question 12

How is nuclear envelope broken down and reassembled

Answer 12

BREAKDOWN: End of G2, cyclin dependent kinases triggers start. Nuclear lamins phosphorylated so nuclear envelope breaksdown

REASSEMBLY: late anaphase, proteins ( SUN1 and LAP2) concentrated around chromosome.During telophase
they direct NE reassembly.

Question 13

What are some strange mitosis in intracellular organisms

Answer 13

Endodyogeny: 2 daughter cells form while still in mother cell
TOXOPLASMA GONDII

Endopolyogeny: Many daughter cells form while in mother
PLASMODIUM

Question 14

What are the main cell cycle checkpoints

Answer 14

G1- called restriction point, checks if cell big enough, has nutirents and growth factors

G2- checks if DNA is copied properly or damaged DNA

M- checks at metaphase before anaphase, checks if all chromosomes arranged properly at metaphase plate

Question 15

How is mitosis controlled and what are exceptions

Answer 15

mediated by hormones/ growth factors

exceptions are early embryonic cells and cancer cells dont need permission

Question 16

What are condesins and cohesins

Answer 16

Condesins: compact chromosomes during mitosis, produces loops

Cohesins: hold together 2 sister chromatids together. Produces loops, which organize genome during interphase

(cohesin mediated loops=TADS: organize DNA into compartments)

Question 17

What is the action of cohesin

Answer 17

• embraces DNA within its coiled arms
• linkage between sister chromatids established during replication-forms loops during interphase
• During anaphase, cleavage of 1 subunit opens ring + release chromatids for separation

Question 18

What is the action of condensins

Answer 18

*Create loops at each chromatid

• Gradual release of cohesisn + cooperative action of condensins lead to assembly of metaphase chromosome with 2 visible sister chromatids- condensation at metaphase

Question 19

What is a cyclosome

Answer 19

Anaphase promoting complex

Activated if chromosomes well arranged at equator

They trigger transition from metaphase to anaphase by tagging specific proteins for degradation by UBIQUITIN

Question 20

What enzyme does cyclosome release

Answer 20

SEPARASE

Breaks down last cohesins holding together sister chromatids

After cohesin breakdown, the kinetochore microtubules pull the chromosomes apart, to the opposite spindle poles.
Sister chromatids become separated chromosomes.

Question 21

What are the proteins that direct the cell cycle
( G1, S, G2, M )

Answer 21

Cyclin dependent kinases and cyclins

Question 22

What do the cyclin dependant kinases do for mitosis

Answer 22

Controls the stages of mitosis.
G1/s phase transition, DNa replication, chromosome condensation..

Question 23

What is importance of satellites in chromosomes

Answer 23

(segment separated from rest of chrom by secondary constriction) helps formation of chromosome structures + heterochromatin

Question 24

Open vs closed mitosis

Answer 24

open - segregation of chromosomes occurs after nuclear envelope breaks down
closed - segregation occurs with nuclear envelope still present