Genetics II:

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Genetics II The Cell Cycle/ Mitosis
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• You began life as a single cell, but there are
  now more cells in your body than stars in the
  Milky Way

• Just in the past second, millions of your cells
  have divided in two

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• Cell division plays a role in

• The replacement of lost or damaged cells
• Cell reproduction and growth

• Before a cell divides, it must make a complete
  copy of the DNA for each daughter cell

• All of its chromosomes are duplicated, resulting
  in 2 copies called sister chromatids

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Chromosome in unduplicated state
Chromosome in duplicated state containing 2
chromatids
Sister chromatids are connected at the
centromere
chromatid
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• When the cell divides, the sister chromatids
  separate from each other
• This ensures each daughter cell gets a complete
  copy of the DNA

Chromosome duplication
Sister chromatids
Chromosome distribution to daughter cells
Figure 8.5
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The Cell Cycle

• Eukaryotic cells that divide undergo an orderly
  sequence of events called the cell cycle

S phase (DNA synthesis chromosome duplication)

• consists of two distinct phases

Interphase (90 of time)
G1
G2
A. Interphase B. Mitotic (M) phase
Mitotic phase (M) (10 of time)
Mitosis
Cytokinesis
Figure 8.6
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A. Interphase 1. G1 gap- growth 2.
S synthesis 3. G2 gap- growth
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B. M Phase 1. Mitosis Prophase Metaphase
Anaphase Telophase 2. Cytokinesis
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III. Once the DNA has been replicated, how are
chromosomes distributed to other cells?
Mitosis

• Nuclear division,
• or the division of the chromosomes

• produces Somatic (non-sex) cells

• the 2 new daughter cells will have same number
  and
• types of chromosomes that the parent cell had

• Nuclear division involves 4 stages
• 1. Prophase
• 2. Metaphase
• 3. Anaphase
• 4. Telophase

• Chromosomes move around the cell with the help
• of a spindle apparatus

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Mitosis is preceded by Interphase
Inside the nucleus...
Original parent cell has 4 chromosomes (2
maternal, 2 paternal)
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Stages of Mitosis
Pro means first
1. Prophase

• Chromosomes condense
• Nucleoli disappear
• Spindle apparatus
• forms
• Nuclear membrane begins
• to disappear

4 chromosomes in duplicated state
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2. Metaphase
Meta means Middle

• Nuclear membrane is absent
• Spindle fibers pull chromosomes
• in opposite directions
• Chromosomes line up at the
• equator (metaphase plate)

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3. Anaphase
Ana means split

• Sister chromatids separate and
• move along microtubules to
• opposite poles
• Some microtubules lengthen
• and push poles apart

Connection between chromatids broken at
centromere
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4. Telophase and Cytokinesis

• Chromosomes are at the poles
• Chromosomes decondense
• Chromosomes detach from
• spindle fibers
• Nucleoli reappear
• Nuclear membranes re-form
• Cytokinesis begins during telophase

New cells each have 4 chromosomes
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Cytokinesis
the division of the cytoplasm
Plant Cells
Animal Cells
Pinching in of the cytoplasm in animal cells
forms a cleavage furrow
Plant cells begin to lay down a cell plate cell
wall-like material in between the two nuclei
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Study Objectives 1. List the major stages of
the cell cycle. 2. Describe the structure of a
chromosome in its replicated state. Include the
terms chromatid and centromere. 3. Define
mitosis and explain what happens during the 4
stages of mitosis. 4. Explain the function of
the spindle apparatus. 5. What is the genetic
result of mitosis? 6. Describe the process of
cytokinesis in both plant and animal cells.
Include an explanation of the cleavage
furrow and cell plate.