5. Cell Division +

Question 1

Cell Division

Answer 1

• nuclear division (karyokinesis) followed by cytokinesis.

Question 2

Diploid Cells

Answer 2

• two copies of every chromosome, forming a pair (homologous chromosomes)
• humans have 46 chromosomes, 23 homologous pair, a total of 92 chromatids (depending on stage of division)

Question 3

Microtubule Organizing Centers (MTOCS)

Answer 3

• also known as centrosomes
• pair of these lay outside nucleus
• in animal cells, each MTOC contains a pair of centrioles
• plants do have MTOC’s called centrosomes, but they aren’t composed of centrioles

Question 4

Prophase (Mitosis)

Answer 4

• nucleus disassembles: nucleolus disappear, chromatin condenses into chromosomes, and nuclear envelope breaks down.
• mitotic spindle is formed and microtubules (composed of tubulin) begin connecting kinetochores.

Question 5

Metaphase (Mitosis)

Answer 5

• chromosomes line up single file at center
• each chromatid is complete w/ a centromere and a kinetochore. Once separated, it is a chromosome (to keep track of chromosomes, count centromeres).
• once separated -> that’s the end of metaphase, so to be precise, the chromosome # doubles at anaphase.
• Karyotyping performed here.

Question 6

Anaphase (Mitosis)

Answer 6

• microtubules shorten, each chromosome is pulled apart into two chromatids (once separated, it is a chromosome; chromosome # doubles), pulls chromosomes to opposite poles (disjunction).
• at end of this phase, each pole has a complete set of chromosomes, same as original cell before replication.

Question 7

Telophase (Mitosis)

Answer 7

• concludes nuclear divion
• nuclear envelope develops
• chromosmes -> chromatin
• nucleoli reappear

Question 8

Cytokinesis (Mitosis)

Answer 8

• division of cytoplasm to form two cells.

Question 9

Cell Cycle

Answer 9

• Interphase + Mitosis + Cytokinesis
• G1 -> S -> G2 -> M
• Interphase (G1, S, G2)
• M phase (Mitosis + Cytokinesis

Question 10

Cytokinesis in Plants vs. Animals

Answer 10

• Plants: CELL PLATE, vesicles originating from golgi bodies migrate to the plane between the two newly forming nuclei. the vesicles form a cell plate, which subsequently becomes the plasma membranes for the two daughter cells. cell walls develop between the membranes.
• Animals. CLEAVAGE FURROW. microfilaments form a ring inside the plasma mebrane between the two newly forming nuclei. As the microfilaments shorten, they act like purse strings to pull the plasma membrane into the center, dividing the two daughter cells. The groove that forms as the strings are tightened is called a cleavage furrow.

Question 11

Growth Period of Cell Cycle:

Answer 11

• G1, S, and G2
• In addition to DNA replication, S phase also includes growth.
• During G2 period of growth, materials for the next mitotic division are prepared.

Question 12

Meiosis I

Answer 12

• homologous chromosomes pair at metaphase plate, and then the homologues migrate to opposite poles
  1. Prophase I: nucleolus disappears, chromatin condenses into chromosomes, and nuclear envelope breaks down, and the spindle apparatus develops. Homologous chromosomes pair, a process called synapsis. The pairs are referred to as tetrads or bivalents. Corresponding regions along nonsister chromatids form close associations called chiasmata. Chiasmata are sites of crossing over. Tetrad + chiasmata = synaptonemal complex.
  2. Metaphase I: homologous pairs are spread across metaphase plate. Microtubules extend and attach to kinetochore of each member of homologous pair.
  3. Anaphase I: homologues within tetrads uncouple and are pulled to opposite poles.
  4. Telophase I: nuclear membrane develops around chromosomes. Each pole will form a new nucleus w/ half number of chromosomes, but each chromosome will contain two chromatids.
• beginning in telophase I, cells of many species begin cytokinesis and form cleavage furrows or cell plates. In other species, cytokinesis is delayed until after meiosis II. A short interphase II may begin but no replication of DNA occurs.

Question 13

Meiosis II

Answer 13

• Prophase II: nuclear envelope disappears and spindle develops. No chiasmata and no crossing over.
• Metaphase II: chromosomes align singly on metaphase plate (not tetrads as in metaphase I)
• Anaphase II: each chromosome is pulled apart into two chromatids by the microtubules of the spindle apparatus.
• Telophase II: nuclear envelope reappears at each pole and cytokinesis occurs.
• End result is four haploid cells (n). Each cell contains half the number of chromosomes, and each chromosome consists of only one chromatid. Later in interphase, a second chromatid in each chromosome is replicated.

Question 14

Mitosis Vs. Meiosis

Answer 14

• Mitosis ends with two diploid daughter cells, each with a complete set of chromosomes. Each chromosome is composed of only one chromatid. The second chromatid is synthesized in S phase of interphase. Mitosis occurs during growth and development of multicellular organisms and for repair of existing cells and asexual reproduction. Occurs in somatic cells, all cells except those that produce eggs and sperm
• Meiosis ends with four haploid daughter cells, each with half the number of chromosomes. Produces gametes, egg and sperm. The fusion of an egg and sperm (fertilization) (or syngamy) gives rise to a diploid cell, the zygote. The single-celled zygote then divides by mitosis to produce a multicellular organism.

Question 15

Alternation of Generations

Answer 15

• Both gametophyte and sporophyte stages are multicellular. The fern illustrates this type of reproductive cycle.

Sporophyte (2n) (multicellular)——>sporangia——–meiosis—–> spores—–growth(mitosis)—–> gametophyte (n) (multicellular)—-> archegonium becomes egg(n) by mitosis, antheridium becomes sperm (n) by mitosis ——fertilization—-> zygote (2n)—–growth(mitosis)—->sporophyte (2n) (multicellular) (fern)

Question 16

Genetic Variation

Answer 16

Genetic recombination originates from three events in the reproductive cycle:

1. Crossing over: during prophase I, nonsister chromatids of hmologous chromosomes exchange pieces of genetic material—> each chromosome no longer entirely from a single parent.
2. Independent assortment of homologues: during metaphase I, tetrads of homologous chromsomes separate independetly of each other.
3. Random joining of gametes: random joining of sperm and egg. In some cases however, may be affected by genetic composition of a gamete. For example, some sperm may be faster swimmers.

Question 17

Regulation of the Cell Cycle

Answer 17

1. Surface-to-volume ratio (S/V). When a cell grows, the volume of a cell increases faster than the surface area of the plasma membrane enclosing it. When S/V is large -> cell can efficiently react w/ outside environment, when S/V is small–> surface area is small compared to volume and cell can’t efficiently react with outside environment —> cell growth stops or cell division begins.
2. Genome-to-volume ratio (G/V). As the cell grows, its volume increases, but its genome size remains constant. Ag G/V decreases, the cell’s size exceeds the ability of its genome to produce sufficient amounts of materials for regulating cellular activities. Some large cells (paramecium, human skeletal muscle) are multinucleated to deal with this.

Question 18

Cell-specific factors that influence onset of cell division:

Answer 18

1. Checkpoints:
   - G1 checkpoint: near end of G1 phase. If conditions are not appropriate or cell is programmed not to divide, the cell proceeds no further in cell cycle, remaining in an extended G1 phase (G0 phase). Nerve or muscle cells, for example, remain in G0 phase, rarely dividing after they have matured.
   - G2 checkpoint: at end of G2 phase of cell cycle. Evaluates accuracy of DNA replication and signals whether or not to begin mitosis.
   - M checkpoint: during metaphase, ensures microtubules are properly attached to all kinetochores at the metaphase plate before division continues with anaphase.
2. Cyclin-dependent kinases (Cdk’s): cdk enzymes activate proteins that regulate the cell cycle by attaching a phosphate group to them (phosphorylation). Cdk’s are activated by protein cyclin.
3. Growth Factors: plasma membranes of cells have receptors for external molecules, or growth factors that stimulate a cell to divide. One such factor is produced by damaged cells, stimulating other cells to divide.
4. Density-dependent inhibition. Many cells stop dividing when the surrounding cell density reaches a certain maximum.
5. Anchorage dependence. Most cells only divide when they are attached to an external surface (side of culture dish or flat surface of neighboring cells).
   - Cancer is characterized by uncontrolled cell growth and division. Transformed cells —> cells that have become cancerous grow without checkpoints or any regulation discussed. Thus, cancer is a disease of the cell cycle.

Question 19

Do plants have centrioles?

Answer 19

• No, most plants don’t have centrioles.

Question 20

More time spent in interphase or mitosis?

Answer 20

• interphase (90%0

- growth occurs in G1, S, and G2

Question 21

Chiasmata

Answer 21

• region where crossing over occurs of non-sister chromatids.

Question 22

Synaptonemal Complex

Answer 22

• protein structure that temporarily forms between homologous chromsomes: gives rise to the tetrad w/ chiasmata and crossing over.

Question 23

of chromosomes at anaphase of mitosis?

Answer 23

• 92 chromosomes (92 chromatids) if a cell has 46 chromosomes.
• count centromeres

Question 24

of chromosomes at anaphase I of meiosis?

Answer 24

• 46 chromosomes if a cell has 46 chromosomes because no chromatids are separated at anaphase I.