Topic 2.5 Cell Division

Question 1

2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

Answer 1

Interphase

largest part of cell cycle in most cells; longest and most variable of the cell cycle phases

G1 — growth of cells, DNA transcription, protein synthesis and ATP production; at beginning of G1, cell is the smallest it will ever be

S — main activity is replication of the DNA of the cell (chromosomes)

G2 — second growth phase, preparation for mitosis; organelles may increase in number, DNA starts to condense from chromatin to chromosomes, microtubules may start forming

Mitosis (M phase)

4 phases: prophase, metaphase, anaphase, telophase

process in which replicated chromosomes separate and move to opposite poles of cell, forming 2 genetically identical nuclei for daughter cells

Cytokinesis

cytoplasm divides to create 2 daughter cells

in animal cell — inward pinching of fluid plasma membrane to form cleavage furrows

in plant cell — forms a cell plate midway between 2 poles of cell and moves outward towards sides of cell from a central region

Question 2

2.5.2 What are tumours (cancers) the result of? Where do they occur?

Answer 2

Uncontrolled cell division; can occur in any organ or tissue.

Question 3

2.5.3 What happens during interphase?

Answer 3

Interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

Question 4

2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).

Answer 4

Prophase

• chromatin fibres become more tightly coiled to form chromosomes
• nuclear envelope disintegrates and nucleoli disappear
• mitotic spindle begins to form and is complete at the end of prophase
• centromere of each chromosome has a region called the kinetochore that attaches to the spindle
• centrosomes move toward opposite poles of cell due to lengthening microtubules

Metaphase

• chromosomes are moved to middle or equator of the cell; referred to as metaphase plate
• chromosome’s centromeres lie on the plate
• movement of chromosomes is due to the action of the spindle which is made of microtubules
• centrosomes are now at the opposite poles

Anaphase

• usually shortest phase of mitosis; begins when 2 sister chromatids of each chromosome are split
• these chromatids, now chromosomes, move toward the opposite poles of the cell
• chromatid movement is due to shortening of the microtubules of the spindle
• centromeres move toward the poles first because they are attached to microtubules
• at end of anaphase, each pole of the cell has complete, identical set of chromosomes

Telophase

• chromosomes are at each pole
• nuclear membrane (envelope) begins to reform around each set of chromosomes
• chromosome start to elongate form chromatin
• nucleoli reappear
• spindle apparatus disappears
• cell is elongated and ready for cytokinesis

Question 5

2.5.5 Explain how mitosis produces two genetically identical nuclei.

Answer 5

During interphase (S phase) the DNA is replicated to produce two copies of genetic material (sister chromatids) which are held together by a single centromere. During the events of mitosis, the sister chromatids are separated and drawn to opposite poles of the cell. When the cell divides (cytokinesis), the two resulting nuclei will each contain one of each chromatid pair and thus be genetically identical.

Question 6

2.5.6 What do growth, embryonic development, tissue repair and asexual reproduction have in common?

Answer 6

They involve mitosis.