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Chapter 4 Cell Division
Review:
What is a cell?
Animal /plant cell
Some definitions:
• Nucleus – the large organelle in all cells containing the genetic
material, or chromosomes
• Chromosomes – extremely large strands of DNA
• Genes – sections of chromosomes which constitute the ‘plans’
for a specific characteristic of the organism.
• Diploid – having two complete sets of chromosomes (2n)
• Haploid – having one complete set of chromosomes (1n)
Two major types of cell division:
Mitosis • Involving: diploid body cells
• Producing: identical diploid daughter cells
• Result: growth or replacement of tissues
Duplicate, split and grow → Duplicate, split and grow → Duplicate, …
2n→4n→2n
→
2n→4n→2n
→
2n→4n→2n …
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The mechanism:
1. Interphase – a period between cell divisions characterized by
increase in size and accumulation of raw materials needed for
the division process
2. Prophase – threadlike chromosomes (chromatin) shorten and
thicken; centrioles separate and move to opposite ends of the
cell; nuclear membrane disappears. (***each chromosome is
made up of two sister chromatids attached by a centromere)
3. Metaphase – chromosomes migrate to equatorial plate; spindle
fibers attach centrioles and centromeres.
4. Anaphase – centromeres divide and chromatids move toward
opposite centrioles.
5. Telophase – chromosomes lengthen and unwind; nuclear
membrane reappears
6. Cytokinesis – cytoplasm/cell membrane split
Handout #65
Nelson – Blackline Master
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Meiosis –
• Involving : diploid sex cells
• Producing: non-identical gametes (sperm and egg cells) which
are haploid
• Result: new combinations of genes to be passed to offspring.
Duplicate, split and split
♀2n→4n→2n→n
2n
♂2n→4n→2n→n
The mechanism:
Meiosis I
1. Prophase I. Nuclear membrane dissolves; centriole splits and
moves to opposite ends of cell; spindle fibers form between
centioles and centromeres; chromosomes come together in
homologous (same type) pairs. Chromatids intertwine
(synapsis) and sometimes exchange pieces (***crossing over).
2. Metaphase I. Homologous chromosomes attach to spindle
fibres and line up along equatorial plate.
3. Anaphase I. Homologous chromosomes move toward opposite
poles (segregation)
4. Telophase I. Nuclear membrane reappears.
5. Cytokinesis
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Meiosis II
1. Prophase II. Nuclear membrane dissolves; centriole splits and
moves to opposite ends of cell; spindle fibers form between
centioles and centromeres;
2. Metaphase II. Chromosomes attach to spindle fibres and line
up along equatorial plate.
3. Anaphase II. Centromeres split and chromatids move toward
opposite poles; nuclear membrane begins to reform.
4. Telophase II. Cytokinesis
Handout #66
Nelson – Blackline Master
The significance of crossing over and random
assortment in light of evolutionary thought is that it
allows for expression of recessive genes (that is,
characteristics which had not been previously seen).
(Figure 4.14, 4.15, 4.16)
Assignment 1: DNA/Meiosis Model
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