PowerPoint PresentationCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
From PowerPoint® Lectures for Biology: Concepts & Connections
CHAPTER 8
Modules 8.12 – 8.18
8.12 Chromosomes are matched in homologous pairs
Somatic cells of each species contain a specific number of chromosomes
Human cells have 46, making up 23 pairs of homologous chromosomes
MEIOSIS AND CROSSING OVER
8.13 Gametes have a single set of chromosomes
Cells with two sets of chromosomes are said to be diploid
Gametes are haploid, with only one set of chromosomes
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At fertilization, a sperm fuses with an egg, forming a diploid zygote
Repeated mitotic divisions lead to the development of a mature adult
The adult makes haploid gametes by meiosis
All of these processes make up the sexual life cycle of organisms
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The human life cycle
8.14 Meiosis reduces the chromosome number from diploid to haploid
Meiosis, like mitosis, is preceded by chromosome duplication
However, in meiosis the cell divides twice to form four daughter cells
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In the first division, meiosis I, homologous chromosomes are paired
While they are paired, they cross over and exchange genetic information
The homologous pairs are then separated, and two daughter cells are produced
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.14, part 1
INTERPHASE
Meiosis II is essentially the same as mitosis
The sister chromatids of each chromosome separate
The result is four haploid daughter cells
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.14, part 2
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
8.15 Review: A comparison of mitosis and meiosis
For both processes, chromosomes replicate only once, during interphase
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Figure 8.15
2n
2n
MEIOSIS II
8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
Each chromosome of a homologous pair comes from a different parent
Each chromosome thus differs at many points from the other member of the pair
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The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes
Random fertilization also increases variation in offspring
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.16
POSSIBILITY 1
POSSIBILITY 2
Metaphase II
8.17 Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci
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Figure 8.17A, B
8.18 Crossing over further increases genetic variability
Crossing over is the exchange of corresponding segments between two homologous chromosomes
Genetic recombination results from crossing
over during prophase I of meiosis
This increases variation further
Figure 8.18A
How crossing over leads to genetic recombination
Figure 8.18B
Parental type of chromosome
1
2
3
4
Coat-color
genes
Eye-color
genes