1.6 Cell Division

Question 1

Define mitosis.

Answer 1

The division of the nucleus into two genetically identical daughter nuclei.

Question 2

Of the following, which are duplicated: chromosomes, sister chromatids, centromeres.

Answer 2

Chromosomes and centromeres.

Question 3

What are some instances when you expect to see cells in mitosis?

Answer 3

Embryonic development, growth, tissue repair, asexual reproduction

Question 4

What two processes only occur during interphase?

Answer 4

-Each chromosome is converted from a single DNA molecule into two identical DNA molecules, called chromatids. -Protein synthesis

Question 5

Label and annotate the cell cycle. What follows the A, B, C?

Answer 5

A. G1: Cyclin D triggers cells to move from G0 to G1. In G1 cellular contents, apart from chromosomes, are duplicated. Cyclin E prepares the cell for DNA replication in S phase.

B. S: Cyclin D trigges cells to move from G1 to S. Cyclin E trigers Cyclin A to activate DNA replication inside the nucleus in S phase. Each of the chromosomes is then duplicated.

C. G2: Rapid cell growth and protein synthesis occur. Cyclin B promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.

After A, B, and C, mitosis and then cytokinesis occur.

Question 6

How do the chromosomes fit into the nucleus?

Answer 6

• Chromosomes packaged into tiny structures by “condensation,” which occurs during prophase.
• In condensation, DNA molecule is continuously coiled to make the chromosome shorter and wider, called supercoiling. Largely done by proteins called histones and some enzymes.

Question 7

What is the purpose of histones?

Answer 7

To supercoil chromosomes during prophase.

Question 8

Identify and outline this phase.

Answer 8

Prophase.

• Chromosomes become shorter and fatter by coiling.
• Nucleolus breaks down.
• Microtubules grow from structures called microtubule organizing centres [MTOC] to form a spindle-shaped array that links the poles of the cell.
• Nuclear membrane breaks down.

Question 9

Identify and outline this phase.

Answer 9

Metaphase.

-Microtubules attach to the ceontromeres of chromosomes. Centromeres attached to both poles. Put under tension to ensure attachment is correct. If correct, chromosomes stay on plate equator.

Question 10

Identify and outline this phase.

Answer 10

Anaphase

• Each centromere divides, allowing sister chromatids to separate. Spindle microtubules pull them towards poles.
• Sister chromatids guaranteed to move to opposite sides of the cell, ensuring two identical sister nuclei.

Question 11

Identify and outline this phase.

Answer 11

Telophase

• Chromatids have reached poles and are now called chromosomes.
• Chromosomes pulled into tighter group near MTOC and a nuclear membrane reforms around them.
• Chromosomes uncoil and a nucleolus is formed.
• Cell begins dividing, forming a cleavage furrow.
• Cells reenter interphase.

Question 12

What is the formula for mitotic index?

Answer 12

(# cells in mitosis)/(total # cells)

Question 13

Define cytokinesis.

Answer 13

The process of dividing the cytoplasm of the cell.

Question 14

Outline cytokinesis in animals.

Answer 14

Plasma membrane puled inwards around the equator of the cell. A ring of contractile proteins at the equator do this, comprised of actin and myosin proteins. This causes the split.

Question 15

Outline step 1 of cytokinesis in plants.

Answer 15

Vesicles moved to equator where they fuse to form tubular structures across equator. With more vesicles, these structures merge to form two layers of membranes across the whole of the equator, which develop into the plasma membranes of the two daughter cells and are connected to the existing membranes at the sides of the cell, completing division.

Question 16

Outline step 2 of cytokinesis in plants.

Answer 16

Pectin and other substances are brought in by vesicles and deposited by exocytosis by the two new membranes. This forms the middle lamella that will link the new cell walls. Both daughter cells then bring cellulose to the equator and deposit it by exocytosis adjacent to the middle lamela. As a result, each cell builds its own cell wall adjacent to the equator.

Question 17

What kind of macromolecule are cyclins?

Answer 17

Cyclins are a family of proteins formed by a long chain of amino acid residues.

Question 18

How are cyclins involved in the control of the cell cycle? How do they work?

Answer 18

• Cyclins ensure tasks are performed at the correct time and that the cell only moves on to the next stage of the cycle when appropriate.
• Cyclins bind to enzymes called cyclin-dependent kinases (cdk). These kinases then become active and attach phosphate groups to other proteins to become active and carry out tasks specific to one of the phases of the cycle.
• Unless these cyclins reach a threshhold concentration, the cell does not progress to the next stage of the cell cyclin. Cyclins therefore control the cell cycle and ensure that cells divide when new cells are needed, but not at other times.

Question 19

Define mutagen, oncogene, metastasis.

Answer 19

Mutagens: Agents that cause gene mutations which may cause cancer.

Oncogenes: The few genes that can become cancer-causing after mutating. In a normal cell, oncogenes are involved in the control of the cell cycle and cell division. This is why mutations in them can result in uncontrolled cell division and therefore tumor formation.

Metastasis: The movement of cells from a primary tumor to set up a secondary tumor in other parts of the body