TA Section 6 (3/21)

Across different myosin proteins, the ___ is conserved and the ___ varies.

• head, tail

• tail, head

• plus end, minus end

• minus end, plus end

Answer explanation

The head of myosin is the domain that binds actin and uses ATP to move along the actin filament, so this is conserved.

The tail domain binds vesicles, and this varies across myosin molecules.

Which is FALSE about myosin in muscles?

• the tail domains allow for dimerization

• they move toward the plus ends of actin filaments

• it is myosin-IV that interacts with actin filaments in muscle

• it makes up part of the sarcomere

Answer explanation

Myosin-II is the protein responsible for interacting with actin filaments for muscle contraction.

The sarcomere is the basic contractile unit of muscle contraction, composed of actin and myosin flanked by Z-discs.

During muscle contraction, ____ molecules shorten/change shape.

• actin

• myosin

• titin

• all of the above

Answer explanation

Titin is the ONLY thing that shortens during muscle contraction, acting as a molecular spring.

Overall, the sarcomere shortens due to actin and myosin sliding past each other.

True or False: muscle contraction is caused by an influx of sodium ions from the sarcoplasmic reticulum into the cytosol.

• TRUE

• FALSE

Answer explanation

It is calcium that is released by the SR for muscle contraction.

What does tropomyosin do in resting muscle?

• Blocks calcium from interacting with myosin

• Blocks myosin dimerization

• Controls levels of troponin in the cytosol

• Blocks the myosin binding site on actin

On microtubules, kinesin walks toward the ___ end and dynein walks toward the ___ end.

• + , -

• - , +

In early embryos, the cells replicate DNA and divide into two daughter cells that are half the size of the original cell. Why is this?

• The mitosis is not complete

• There are no Gap phases

• There is no S phase

• The cells are quiescent

Answer explanation

During gap phases (G1 and G2), the cells grow to 2x the original size so each daughter cell can be the original size.

Early embyronic cleavages occur without these gaps, so the number of cells grow but the total size of the embyro does not. This allows the cells to divide faster.

___ are present throughout the cell cycle, but are only active when bound to ___ (which vary in expression throughout the cell cycle).

• Cdk's, cyclins

• Cyclins, Cdk's

How are cyclin protein levels controlled throughout the cell cycle?

• Phosphorylation and dephosphorylation

• Binding of an inhibitory protein

• ubiquitination by APC

• exocytosis

___ promote(s) cell cycle entry by activating synthesis of G1 and G2/S cyclins.

• mitogens

• retinoblastoma protein

• p53

• Cdc6