Module 1 Unit 3

Question 1

What is the difference between chromosomes and chromatin?

Answer 1

• • DNA molecules are packaged into structures called chromosomes
• • Together, the entire complex of DNA and associated proteins is the building material of chromosomes is referred to as chromatin
• • chromatin is the less condensed version of chromosome, which is highly condensed

Question 2

What are sister chromatids?

Answer 2

• • Each duplicated chromosome consists of two sister chromatids, which are joined copies of the original chromosome
• • are initially attached all along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion
• • Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell. Once the sister chromatids separate, they are no longer called sister chromatids but are considered individual chromosomes

Question 3

What is a centromere?

Answer 3

• • a region made up of repetitive sequences in the chromosomal DNA where the chromatid is attached most closely to its sister chromatid
• • This attachment is mediated by proteins bound to the centromeric DNA; this close attachment causes a constriction in the condensed chromosome
• • The portion of a chromatid to either side of the centromere is referred to as an arm of the chromatid

Question 4

What is binary fission?

Answer 4

• • Prokaryotes (bacteria and archaea) can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells
• • Bacteria only have one circular chromosome (tightly coiled around proteins)
• • cell division is initiated when the DNA of the bacterial chromosome begins to replicate in both directions at a specific place on the chromosome called the origin of replication, producing two origins that each move to opposite ends of the cell
• • When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent bacterium into two daughter cells
• • bacteria don’t have visible mitotic spindles or even microtubules; how bacterial chromosomes move and how their specific location is established and maintained are active areas of research
• • Mitosis has evolved from binary fission

Question 5

What is mitosis and cytokinesis?

Answer 5

• • Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm
• • mitosis begins when chromosome condensation reaches a point where the individual chromosomes become visible in the microscope

Question 6

What is the mitotic (M) phase?

Answer 6

• • includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle
• • cytokinesis completes the mitotic phase

Question 7

What is interphase?

Answer 7

• • accounts for about 90% of the cycle
• • Interphase can be divided into subphases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”)
• • During all three subphases of interphase, a cell grows by producing proteins and cytoplasmic organelles and chromosomes are relatively loose, but organized, in nucleus.
• • G1 phase: growth phase of the cell cycle. Most cells spend virtually all their time in G1 (or G0)
• • S phase: synthesis phase during which DNA is replicated
• • G2 phase: the second growth phase after DNA synthesis. After S phase, the DNA in all of the chromosomes has been replicated, and each chromosome contains 2 sister chromatids which are attached at the centromere. Chromosomes condense, and the cell begins to assemble the machinery that will pull sister chromatids apart. G2 cell has 2 centrosomes

Question 8

What are mitotic spindle?

Answer 8

• • The components of the spindle are the centrosomes, fibres and asters (radial array of short microtubules that anchor spindle poles to the membrane)
• • elongate (polymerize) by incorporating more subunits of the protein tubulin from the disassembly (depolymerization) of microtubules in the cytoskeleton
• • The number of kinetochore microtubules attached varies according to species

Question 9

What are centrioles?

Answer 9

• • composed of nine sets of triplet microtubules arranged in a ring
• • they are not essential for cell division: If the centrioles are destroyed with a laser microbeam, a spindle nevertheless forms during mitosis
• • In fact, centrioles are not even present in plant cells, which do form mitotic spindles.

Question 10

What is the centrosome?

Answer 10

• • functions as a microtubule organizing centre and is important during cell division
• • generates mitotic spindle
• • a centrosome has two centrioles

Question 11

What are kinetochores?

Answer 11

• • a structure made up of proteins that have assembled on specific sections of DNA at each centromere. The chromosome’s two kinetochores face in opposite directions
• • the are the sites at which microtubules attach to chromosomes

Question 12

What are the stages of mitosis?

Answer 12

• • prophase
• • prometaphase
• • metaphase
• • anaphase
• • telophase

Question 13

What is prophase?

Answer 13

– Chromosomes have condensed into threads
– The centrosome begins generating the spindle;
fibres lengthen and centrosomes start moving to opposite poles
– Nucleolus disappears

Question 14

What is prometaphase?

Answer 14

• • Nuclear envelope fragments
• • Microtubules from each pole interact with kinetochores at the centromere of each chromosome (called kinetochore microtubules)
• • Non-kinetochore microtubules interact at spindle midpoint; microtubules from one pole overlap with those from opposite pole

Question 15

What is metaphase?

Answer 15

• • By metaphase, the microtubules of the asters have also grown and are in contact with the plasma membrane.
• • Kinetochores of sister chromatids are attached to microtubules from opposite poles
• • What happens next is like a tug-of-war that ends in a draw
• • Chromosomes are pulled in opposite directions by kinetochore microtubules and finally assemble at the metaphase plate (an imaginary plane between the spindles two poles)

Question 16

What is anaphase?

Answer 16

• • Begins when cohesin complex is cleaved (cut) by an enzyme called Separase
• • At this point each sister chromatid is a chromosome
• • Kinetochore fibres shorten and pull sister chromatid to each pole (motor proteins on the kinetochores “walk” the chromosomes along the microtubules, which depolymerize at their kinetochore ends after the motor proteins have passed)
• • Non-kinetochore microtubules move against each other and begin to elongate the cell (motor proteins attached to the microtubules walk them away from one another, using energy from ATP. As the microtubules push apart from each other, their spindle poles are pushed apart, elongating the cell. At the same time, the microtubules lengthen somewhat by the addition of tubulin subunits to their overlapping end)
• • At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell

Question 17

What is telophase?

Answer 17

• • Non-kinetochore microtubules continue to elongate the cell
• • Nuclear envelope starts to reform (nuclear division is complete)
• • Chromosome starts to decondense and genes become active (especially rRNA)
• • Spindle apparatus is dismantled and the microtubules are used to reassemble the cytoskeleton

Question 18

What is cytokinesis?

Answer 18

• • Cytokinesis generally begins during anaphase or telophase
• • Cytoplasm begins to divide in two (cytokinesis) and forms a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate
• • on the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract; The contraction of the dividing cell’s ring of microfilaments is like the pulling of a drawstring. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells

Question 19

How is cytokinesis different in plant cells?

Answer 19

• • There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate
• • The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. This forms the new cell wall and ultimately results in two daughter cells.

Question 20

What is a checkpoint?

Answer 20

– a control point in the cell cycle where stop and go-ahead signals can regulate the cycle. Three important checkpoints are found in the G1, G2, and M phases

Question 21

What are cyclins and cyclin-dependant kinases (Cdk)

Answer 21

• • Protein kinases are enzymes that activate or inactivate other proteins by phosphorylating them
• • Many of the kinases that drive the cell cycle are actually present at a constant concentration in the growing cell, but much of the time they are in an inactive form. To be active, such a kinase must be attached to a cyclin; Because of this requirement, these kinases are called cyclin-dependent kinases (Cdks)

Question 22

What are maturation promoting factor (MPF)?

Answer 22

• • cyclin-Cdk complex
• • peaks of MPF activity correspond to the peaks of cyclin concentration. The cyclin level rises during the S and G2 phases and then falls abruptly during M phase
• • can think of MPF as “M-phase-promoting factor” because it triggers the cell’s passage past the G2 checkpoint into M phase
• • when cyclins that accumulate during G2 associate with Cdk molecules, the resulting MPF complex phosphorylates a variety of proteins, initiating mitosis. MPF acts both directly as a kinase and indirectly by activating other kinases. For example, MPF causes phosphorylation of various proteins of the nuclear lamina, which promotes fragmentation of the nuclear envelope
• • During anaphase, MPF helps switch itself off by initiating a process that leads to the destruction of its own cyclin

Question 23

What are the cell cycle checkpoints?

Answer 23

• • the G1 checkpoint, dubbed the “restriction point” in mammalian cells, seems to be the most important. If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2, and M phases and divide. If it does not receive a go-ahead signal at that point, it may exit the cycle, switching into a nondividing state called the G0 phase
• • The M checkpoint appears to depend upon the attachment of microtubules to the kinetochores; Anaphase does not begin until all the chromosomes are properly attached to the spindle at the metaphase plate; Only when the kinetochores of all the chromosomes are properly attached to the spindle does the appropriate regulatory protein complex become activated which then activates the enzyme separase, which cleaves the cohesin
• • a checkpoint in S phase stops cells with DNA damage from proceeding in the cell cycle
• • The source of the signals that trigger or halt progression through checkpoints may be internal, or external (chemical/physical); growth factors, density-dependent inhibition (when crowded cells stop dividing), anchorage dependance (must be attached to substratum to divide)

Question 24

What happens in cancer cells?

Answer 24

– Cancer cells escape controls on the cell cycle; divide rapidly, often in the absence of growth factors
and grow without being inhibited by other cells
– If and when they stop dividing, cancer cells do so at random points in the cycle, rather than at the normal checkpoints
– spread to other tissues through the circulatory system
– cancer cells evade the normal controls that trigger a cell to undergo apoptosis when something is wrong

Question 25

What are HeLa cells?

Answer 25

• • a cell line that has been reproducing in culture since 1951. Cells of this line are called HeLa cells because their original source was a tumour removed from a woman named Henrietta Lacks
• • Cells in culture that acquire the ability to divide indefinitely are said to have undergone transformation, the process that causes them to behave like cancer cells.

Question 26

What is a benign and a malignant tumour?

Answer 26

• • benign: the abnormal cells may remain at the original site if they have too few genetic and cellular changes to survive at another site
• • a malignant tumour includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs; these cells are also considered transformed cells (This spread of cancer cells to locations distant from their original site is called metastasis)

Question 27

What is heterochromatin and euchromatin?

Answer 27

• • heterochromatin: eukaryotic chromatin that remains highly compacted during interphase and is generally not transcribed
• • euchromatin: the less compacted, more dispersed euchromatin (“true chromatin”)
• • Because of its compaction, heterochromatic DNA is largely inaccessible to the machinery in the cell responsible for transcribing the genetic information coded in the DNA, a crucial early step in gene expression. In contrast, the looser packing of euchromatin makes its DNA accessible to this machinery, so the genes present in euchromatin can be transcribed