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Chapter 10 Meiosis and Sexual Life Cycles
 Heredity: the transmission of traits from one generation to the next  Variation: any difference
between cells, individual organisms, or groups of organisms of any species caused by either
genotypic variation or phenotypic variation  Genetics: the scientific study of heredity and
hereditary variation 
10
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Sexual Reproduction combines genes from two
parents, leading to genetically diverse offspring 
Asexual Reproduction: a form of reproduction that
does not involve meiosis, ploidy reduction, or
fertilization, and the offspring is an organism – no
exchange of genetic material  Clone: a group of genetically identical individuals  Sexual
Reproduction: two parents give rise to offspring that have unique combinations of genes inherited
from the two parents 
10
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Sets of Chromosomes in Human Cells
 Normal human somatic cells are diploid
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Human diploid cells have 22 homologous pairs
of autosomes and one pair of sex chromosomes; the latter determines whether the person is female
(XX) or male (XY)
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During fertilization, an egg and a sperm unite,
forming a diploid (2n=46) single-celled zygote, which develops into a multicellular organism by
mitosis  Fertilization: the union of gametes, culminating in fusion of their nuclei  Zygote:
fertilized egg  meiosis: gamete formation, cell division 
The Variety of Sexual Life Cycles
 Sexual life cycles differ in the timing of meiosis relative to fertilization and in the point(s)
of the cycle at which a multicellular organism is produced by mitosis  Alternation of generations:
alternation of a sexual phase (gametophyte) and a nonsexual phase (sporophyte) in the life cycle of
an organism 

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The number of chromosomes
sets is reduced from two diploid to one haploid
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Cohesions are cleaved along the
arms at anaphase I, allowing homologs to separate and at the centromeres in anaphase II, releasing sister
chromatids
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Meiosis I  separates homologous chromosomes
a
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Pro-metaphase I: the nuclear membrane breaks down  centrosome formation
 spindle formation 
c
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Anaphase I: breakdown of cohesion between sister’s chromatids  homologs
move toward opposite poles, guided by a spindle  sister chromatid cohesion
continues at the centromere, causing chromatids to move as a unit toward the same
pole
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Telophase I and Cytokinesis: division of cytoplasm  cleavage furrow forms 
2
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Prophase II: spindles form  chromosomes begin lining up on metaphase plate

b
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Anaphase II: breakdown of proteins holding sister chromatids together 
chromatids move to opposite poles 
d
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A comparison of Mitosis and Meiosis

10
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During crossing over, DNA of non-sister chromatids in
homologous pair is broken and rejoined
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Mutations are the original source of this variation; recombination of variant genes
generate additional diversity  recombinant chromosomes: individual chromos that carry genes
(DNA) derived from two different parents

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