AB5: Cytoskeleton, Cell Movement and Cell Division

Question 1

What are the 3 major components of the cytoskeleton?

Answer 1

Microfilaments, Microtubules and intermediate filaments

Question 2

Describe the microfilaments in the cytoskeleton

Answer 2

Comprise of filaments of F-actin polymers made up of globular G-actin subunits
Actin microfilaments can be pulled together by the heads of motor proteins called myosins which hydrolyse ATP and use energy derived from this reaction to move away from the so called minus end (pointed end) of the actin filament
The plus end is the end at which the actin monomer units are being added
This basic movement is the basis of muscle movement- also important in movement of single celled organisms like amoebae and fibroblasts in mammals and phagocytes

Question 3

What is the function of the microfilaments in the cytoskeleton ?

Answer 3

Functions include resisting tension, transport of membrane-packaged material across the plasma membrane (endo/exocytosis) and localisation contraction of cells- cleavage furrows and amoeboid movements

Question 4

Describe the microtubules in the cytoskeleton

Answer 4

Hollow tubes made up of alpha and beta tubulin
Are moved by kineson and dynein motor proteins- similar to myosin on F-Actin
Can be easily made and taken apart
The plus end of microtubules is the growing end to which new tubulin sub-units are being added- vesicles carried towards this end, away from the nucleus and toward the plasma membrane- kinesins often move to the plus end
Dynein’s often move towards the minus end

Question 5

What are the functions of the microtubules in the cytoskeleton?

Answer 5

Functions include compression resistance (supports cell structure), movement of vesicles and cell division

Question 6

Describe the intermediate filaments in the cytoskeleton

Answer 6

Comprise of keratin proteins assembled into fibrous subunits, then super-coiled together to form cage-like networks throughout the cell- do not readily disassemble
Mechanical toughness is apparent in the extracellular forms of keratin in skin, hair and nails
There are related structures that perform similar function in bacteria and archaea

Question 7

What are the functions of the intermediate filaments in the cytoskeleton?

Answer 7

Functions include tension resistance, maintain cell shape, fix the position of nucleus and organelles + maintain their shape and from nuclear lamellae (inner layer of nuclear envelope) out of lamin proteins

Question 8

What are differences between flagella and cilia

Answer 8

Flagella are larger, beat in a different way and are limited to one per cell unlike cilia, are the driving movement of animal sperm, cilia are responsible for fluid circulation (moving mucus in mammalian respiratory tract)

Question 9

What is binary fission? How is it carried out?

Answer 9

Splitting of a single-cell organism into 2 daughter organisms- division in half
Firstly, single bacterial chromosome is replicated from the origin and the origins move to opposite poles of the cell- 2 chromosomes appear in 1 cell
The cell elongates as replication continues, the cell then undergoes cytokinesis- ring of cytoskeleton proteins thought to be involved- similar to cleavage furrow in eukaryotes
There is no nucleus or nuclear envelope

Question 10

What is interphase split up into?

Answer 10

Interphase can be split into G1, which is protein synthesis and organelle duplication, S which is for DNA synthesis and replication, and G2 which is more growth of the cell
G1 takes 5-6 hours, S phases takes 10-12 hours, G2 phase takes 4-6 hours

Question 11

What is mitosis?

Answer 11

Asexual reproduction- occurs by mitosis- splitting of a single-celled organism into 2 daughter organisms in single celled eukaryotes - takes 1 hour

Question 12

What are centrosomes, centrioles and centromeres?

Answer 12

Centosome: structure in the cytoplasm that is involved in organising the mitotic spindle
Centriole: pair of structures in each centrosome comprising of specialist microtubules and a complex system of associated proteins- not in plants
Centromere: point at which 2 sister chromatids of a chromosome are connected

Question 13

What is the kinetochore?

Answer 13

a protein structure that forms at the centromere during mitosis- between 1-40 microtubules attached to each kinetochore- during anaphase, motor proteins pull the separated sister chromatids along the microtubule- dismantled behind the moving chromosome

Question 14

What occurs during prophase?

Answer 14

Chromatin condenses into mitotic chromosomes, centrioles move apart and microtubules from cytoskeleton disassemble and from spindle and aster (smaller)

Question 15

What occurs during prometaphase?

Answer 15

• The spindle microtubules are in 2 populations, 1) kinetochore microtubules- attached to kinetochore of the chromosome 2) non-kinetochore microtubules
• The nuclear envelope has fragmented

Question 16

What occurs during metaphase?

Answer 16

• Spindle is complete and asters extend to plasma membrane
• Centrosomes at opposite poles of the cell
• Kinetochore microtubules have all attached to chromosomes
• The resulting tension in the spindle makes the chromosomes line up along the middle of the cell in a plane known as the metaphase plate

Question 17

What occurs during anaphase?

Answer 17

• Kinetochores are cleaved by separase enzymes
• Chromatids move to poles of cell by a combination of kinesin motor proteins moving towards the poles of the cell and microtubules being pulled in by motor proteins at poles
• Non-kinetochore microtubules are pulled apart by kinesin motor proteins, causing the cell to elongate by the addition of extra tubulin subunits so there is continued overlap

Question 18

What occurs during telophase and cytokinesis

Answer 18

• Nuclear envelope reforms and chromosomes become less condensed and nucleolus reappears- 2 separate nuclei start to form
• Remaining spindle disassembles and the interphase cytoskeleton reforms
• Cytokinesis occurs by ATP powered myosin causing contraction of actin microfilaments in cortex of cell causing cleavage furrow to form
• Cytokinesis in plant cells involves vesicles containing cell wall material travel along microtubules to middle of cell, vesicles then fuse to form growing cell plate (cell wall section surrounded by plasma membrane)- cell wall and plasma membrane of parent cell fuse with cell plate