lecture 4

Question 1

What are microtubules?

Answer 1

• found in eukaryotic cells
• hollow rods of globular proteins and tubulins
• tubulin proteins consist of (alpha-tubulin; beta-tubulin)
• grow and shrink by adding/removing tubulin dimers (dynamic instability)
• ends differ (plus ends shrink/grow rapidly, minus end shrink/grow slowly)
• diameter 25nm with 15nm lumen

Question 2

what are centrosomes and centrioles?

Answer 2

• centrosomes act as microtubule organisation centre
• contains two centrioles at right angles to each other
• centrioles consist of nine sets of triplet arranged rings
• if centrosome removed/is not there they can still divide

Question 3

functions of centrosomes/centrioles?

Answer 3

• maintainence of cell shape
• cell motility
• transport
• chromosome movement

Question 4

how does the microdubules help in cell motility?

Answer 4

• cillia/flagella in eukaryotes
• microtubule containing extensions that project from some cells
• formed from speical arrangements of microtubules

Question 5

how do cilia/flagella help motility? describe motions and structure

Answer 5

• flagella have undulating motion
• cilia have alternating pattern (power stroke then recovery stroke)
  -nine doublets arranged in ring with two single microtubules in centre
• anchored by basal body (similar structure to centrioles)
• dyneins (motor proteins) attatched to each doublet
• doublets held together to central microtubules by cross-linking proteins
• dyneins walk along microtubule to next doublet along one side of cilia/flagella (causing movement)

Question 6

how is transport and chromosome movement influenced by microtubules?

Answer 6

• motor proteins attatched to certain oganelles
• motor protein walks along microtubule (carry organelle with them)
• eg.) movement of vesicle from ER to Golgi
• eg.) movement of neurotransmitters at nerve cells
• help seperate chromosomes during cell division
• microtubules attach to chromosomes then shorten - pulling them to opposite poles of cell

Question 7

what are microfilaments?

Answer 7

• in all eukaryotic cells
• thin solid rods of globular proteins
• known as actin filaments
• form a twisted double chain of actin subunits
• 7nm diameter
• grow/shrink rapidly

Question 8

what are the functions of microfilaments?

Answer 8

• maintenance of cell shape
• muscle contraction
• amoeboid movement
• cytoplasmic streaming
• cell division

Question 9

how do microfilaments help in muscle contraction and amoeboid movement?

Answer 9

• microfilaments and thicker filaments of motor protein myosin
• mysoin projections walk along microfilaments (moving filaments past one another)
• actin filaments approach one another, the muscle cell shortens causing muscle contractions
• occurs in amoeba/some white blood cells
• localised contractions of actin and myosin
• cell ‘crawls’ by extending pseudopodia (extensions of cell membrane supported by microfilaments)
• interactions between actin and myosin cause areas of cell to contract (pulling other end of cell forward)

Question 10

how do microfilaments help with cytoplasmic streaming and cell division?

Answer 10

• actin-protein interactions contribute to circular flow of cytoplasm in plants
• driven by myosin proteins attatched to some organelles that move along microfilament tracks
• causing layer of cytoplasm inside cell membrane to flow in circular motion
• cytoplasmic straming speeds up movement of other organelles/distribution of materials around cell)
• contractile ring of microfilaments associated with myosin
• interactions cause ring to contract (forming cleavage furrow)
• eventually leads to cell being pinched in two