Cell Division

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Mastering Concepts

8.1

1. Explain the roles of mitotic cell division, meiosis, and fertilization in the human life cycle.

Mitosis allows for growth and repair from the fertilized egg to the adult. In reproductively mature individuals, meiosis creates gametes with half the genetic material. The male gamete then fertilizes the female gamete during sexual reproduction, forming a zygote with a full set of genetic material.

2. Why are both cell division and apoptosis necessary for the development of an organism?

Cell division is needed to produce the large number of cells necessary to form the individual. Combined with apoptosis, the structures in the organism can be carved out and constantly recycled.

8.2

1. Why does DNA replicate?

DNA replicates before a cell divides to give a complete set of genetic instructions to each daughter cell.

2. What is semiconservative replication?

Semiconservative replication means that only one strand of a DNA molecule is newly formed. The other strand is original DNA from the parent cell.

3. What are the steps of DNA replication?

The helicase enzyme unwinds and separates a double-stranded DNA molecule; the primase enzyme forms a short stretch of complementary RNA on each DNA template; starting at the end of the RNA primer, DNA polymerase adds DNA nucleotides that are complementary to the template strand, proofreading and correcting errors as it goes; RNA primers are removed; on the lagging strand, the enzyme ligase joins fragments of DNA.

4. What is the role of RNA primers in DNA replication?

The RNA primer is added to the start of the DNA segment being replicated and acts to attract the DNA polymerase, which can only add nucleotides to an existing strand.

5. What happens if DNA polymerase fails to correct an error?

A mutation occurs.

8.3

1. What is the relationship between chromosomes and chromatin?

Chromosomes are chromatin coiled tightly on itself.

2. How does DNA interact with histones?

A stretch of DNA wraps around a cluster of eight histones to form a nucleosome.

3. What are the main parts of a chromosome?

The main parts include two identical sister chromatids and the centromere where they are attached.

8.4

1. What are the three main events of the cell cycle?

The three main events are interphase, during which the cell grows and replicates its DNA, mitosis, during which the DNA divides, and cytokinesis, during which the cytoplasm divides.

2. What happens during interphase?

During interphase, a cell grows and produces proteins so that its normal biochemical functions proceed. DNA replicates during interphase as a cell prepares to divide.

3. How does the mitotic spindle form, and what is its function?

The mitotic spindle is composed of microtubules associated with cytoskeleton proteins. The spindle originates from centrosomes that are at opposite ends of the cell. Spindle fibers grow across to join at the midline of the cell. The function of the mitotic spindle is to form “trackways” for the movements of chromosomes as cells divide.

4. What happens during each phase of mitosis?

The events of mitosis include:
• Prophase: chromosomes condense and become visible; mitotic spindle forms • Prometaphase: nuclear envelope breaks up; spindle fibers attach to kinetochores on each chromosome • Metaphase: chromosomes line up on equator of cell

• Anaphase: sister chromatids separate and move to opposite poles of the cell • Telophase: nuclear membranes reassemble around the daughter nuclei; chromosomes decondense; spindle disappears

5. Distinguish between mitosis and cytokinesis.

Mitosis is the division of duplicated chromosomes into new daughter nuclei. Cytokinesis is the division of cytoplasm and organelles into two new daughter cells and the separation of these cells.

8.5

1. What prevents normal cells from dividing when they are not supposed to?

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