Chapter 12: The Cell Cycle

Question 1

Chromosomes

Answer 1

Structures of DNA molecules and associated proteins that consist of a single long strand of DNA

Question 2

Chromatin

Answer 2

Complex of DNA and proteins that compose the chromosome

Question 3

Somatic cells

Answer 3

All cells except reporductive cells

Question 4

Gametes

Answer 4

Sperm and egg cells; have half as many chromosomes as somatic cells

Question 5

Sister chromatids

Answer 5

Identical copies of chromosomes which are joined together along their lengths by protein complexes called cohesions

A chromatid is one of the two identical halves of a chromosome

Sister chromosomes are duplicaed during the S phase of mitosis and condense during prophase and prometaphase of mitosis

Question 6

Centromere

Answer 6

Narrow central region where sister chromatids are attached most closely that gives them a narrow “waist”

Question 7

Cytokinesis

Answer 7

The division of the cytoplasm that follows mitosis and completes the mitotic phase

Usually is well under way by late telophase

Question 8

Phases of the cell cycle

Answer 8

Interphase- accounts for 90% of the cell cycle; cell grows by producing proteins and cytoplasmic organelles

• G1 (first gap) phase
• S (synthesis) phase- duplication of the chromosomes occurs
• G2 (second gap) phase

Mitotic Phase- shortest part of the cell cycle

• Mitosis
• Cytokinesis

Question 9

Mitotic spindle

Answer 9

Forms in the cytoplasm during prophase; consists of fibers composed of microtubules and associated proteins, the centrosomes, and the asters

Spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten (depolymerize) by losing subunits

Question 10

Aster

Answer 10

A radial array of short microtubules that extend from each centrosome

Question 11

Kinetochore

Answer 11

Structure made up of proteins that have assembled on specific section fo DNA at each centromere

During prometaphase some spindle microtubules attach to the kinetochores and are called kinetochore microtubules

Question 12

Cleavage

Answer 12

Process by which cytokinesis occurs in animal cells

First sign of cleavage is the appearance of a cleavage furrow- a shallow groove in the cell surface near the old metaphase plate

A contractile ring of actin microfilaments on the cutoplasmic side of the furrow contracts to complete cytokenisis

Question 13

Cell plate

Answer 13

Process by which cytokenisis occurs in plant cells

Vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell where they coalesce

Cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell

Question 14

Prokaryotic reporduction

Answer 14

Prokaryotes (bacteria annd archea) and single-celled eukaryotes (amoeba) undergo binary fission in which cells grows to roughly double in size and then divides to form two identical cells

The plasma membrane pinches inward, dividing the cell into two

Question 15

Checkpoints

Answer 15

A control point in the cell cycle where “stop” and “proceed” signals can regulate the cycle

Three checkpoints:
1. G₁ checkpoint
2. G₂ checkpoint
3. M checkpoint

If a cell receives a go-ahead signal at the G₁ checkpoint, it will usually complete the S, G₂, and M phases and divide

If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G₀ phase

Question 16

Cell cycle clock

Answer 16

Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the sequential events of the cell cycle

Regulator molecules are two types of proteins:
1. Protein kinases- enzymes that activate or inactivate other proteins by phosphorylating them; cyclin dependent kinases (Cdks)
2. Cyclins- a protein that gets its name from its cyclically fluctuating concentrations

Together, Cdks and cyclin form a complex called maturation promoting factors (MPF)

Question 17

Maturation promoting factors (MPF)

Answer 17

Complexes formed by Cdk and their cyclin partners

Act as M-phase promoting factors that trigger a cell’s passage into M phase past the G₂ checkpoint

MPF activity peaks in conjunction with cyclin concentration; cyclin levels rise during the S and G₂ phases and then fall abruptly during the M phase

Question 18

G2 of interphase

Answer 18

Question 19

Prophase

Answer 19

Question 20

Prometaphase

Answer 20

Question 21

Metaphase

Answer 21

Duplicated chromosomes line up along the metaphase plate

In humans all 46 chromosomes line up along the metaphase plate

Question 22

Anaphase

Answer 22

Cohesins are cleaved by an enzyme called separase

Sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell

Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell

Question 23

Telophase and cytokenesis

Answer 23

Genetically identical daughter nuclei form at opposite ends of the cell

Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles

Question 24

Density-dependent inhibition

Answer 24

An external physical factor that limits cell division; a phenomenon in which a cell in a crowded environment stops growing

The binding of cell-surface proteins to their counterparts on adjoining cells sends a division-inhibiting signal which limits further growth even in the presence of growth factors

Question 25

Anchorage dependence

Answer 25

A cell must be attached to a substratum such as the extracellular matrix of a tissue in order to divide

Anchorage is signaled to the cell cycle control system via pathways involving plasma membrane proteins and elements of the cytoskeleton

Question 26

Transformation

Regarding normal vs. cancerous cells

Answer 26

A process that imparts the ability of cells to be able to divide indefinitely and behave like cancer cells