Key for MCDB 100 Final – Part 2
(a), (b), and (d)
The rough ER consists of ER membranes and polyribosomes that are
in the process of translating and translocating proteins into the ER membrane and lumen.
Thus all proteins that end up in the lysosome, Golgi apparatus, or plasma membrane, or
are secreted, will be encoded by the RNAs associated with the rough ER. Mitochondrial
and ribosomal proteins are translated on free cytosolic ribosomes.
Choice (e) is the correct answer. Ions can pass either way through a channel; the
direction in which flow takes place depends on the concentration gradient and the
membrane potential. Some ion channels require a specific stimulus to open them;
others, such as K
leak channels, do not (choice (a)). Ion channels are passive
transporters and therefore require no energy source in order to function (choice
(b)). Ion channels are very fast relative to carrier proteins but are limited by the
rate at which ions can move through the channel (choice (c)). An ion channel
allows specific positive or negative ions to pass, but not both (choice (d)).
(c) One function of actin filaments, but not microtubules, is to provide a meshwork
beneath the plasma membrane that helps to form and strengthen this membrane.
Microtubules have all of the other functions that are described.
(c) Sarcomeres contain actin filaments and myosin filaments that slide past each
other during muscle contraction, leading to shortening of the sarcomere;
the actin filaments and myosin filaments do not change in length.
are microtubule-based structures that are not present on muscle cells.
At 37°C, the cells all have one genome worth of DNA, meaning that they
have not replicated their DNA and therefore have not entered S phase.
Hence they could either have a defect in initiation of DNA replication (a)
or a defect that causes inappropriate delay in G
Choices (c) and (e) are correct. The concentration of mitotic cyclin, which contributes
to the timing of mitosis (choice (e)), rises gradually during G
phase (thus (choice (a)) is
incorrect) and is independent of the relatively constant level of Cdk (choice (b) is
In late M phase, mitotic cyclin is ubiquitinated and destroyed (choice (c)),
thus falling to very low levels in G
phase (choice (d) is incorrect).
(c) The APC is active only in mid- to late-M phase.
All the other statements are true.
Cell size increases throughout interphase and not during M phase.
the other phenomena are observed in M phase.
A—16; B—16; C—16; D—2; E—4
sorry, I meant to modify this to a MC and
Choice (c) is the correct answer. When the attachment to one kinetochore is severed,
the whole chromosome moves to the opposite pole, showing that the kinetochore
microtubules are pulling on their attached chromatid, not pushing it. The jiggling