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Life Cycles and Meiosis
The life cycle of individuals begins at fertilization.
Fertilization is the union of two haploid cells each with one complete set of chromosomes (half as many as body cells).
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Meiosis precedes fertilization.
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Reading Assignment
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Chapter 12, study the figures and understand the color coding.
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Focus on the KEY CONCEPTS, the important thing about meiosis is segregation of the homologs and crossing-over, not the names of the stages.
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Stages of Drosophila Life Cycle
Larva (or maggot)
Pupa
Adult (male & female)
Egg
From egg to egg can be as
little as 14 days at the optimal
temperature.
Adults have to produce gametes to initiate a new individual.
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The sequence of changes can be described as STAGES or by AGE. Sexual maturity is limited to adult stage.
Dog lineage as a cycle
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Diploid
Diploid
Diploid
Gametes = sex cells
Zygote
Time
Gametes = sperm or eggs
Zygote
Zygote
Eukaryotic Life Cycle as a lineage
LINE is short for a “line of descent through time.”
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Cycles emphasize the fact that events are repeated in a regular sequence, but the reality is that time is ‘going by’ and things are not exactly repeated.
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Fertilization
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For multicellular organisms fertilization (=sex) is union of two cells called gametes.
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All the chromosomes from both gametes remain in the zygote.
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The gametes have only half as many chromosomes as are in body cells.
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The cell divisions halving the number of chromosomes is called meiosis.
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Body cells have chromosomes in pairs
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Meiosis
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There is a special type of cell division that occurs in diploid cells called meiosis.
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Meiosis reduces the two complete sets of information in the diploid cell into cells that have only one complete set.
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Both mitosis and meiosis occur only after replication of DNA.
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Meiotic divisions lead to a population of cells that are all different. They become all different because crossing over can occur at any place along the chromosomes.
S phase, Diploid cell, n = 1
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Meiosis proceeds Gamete Formation
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Male ♂ gametes are sperm in animals and pollen in plants.
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Small
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Motile, must find female
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Female ♀ gametes are eggs in animals and ovules in plants.
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Large, contain nutrients
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Each gamete is genetically unique if the diploid parent cell is heterozygous at many loci.
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Individuals from Fertilization
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Each individual is a clone (population) of diploid cells tracing back to zygote.
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Individuals begin at fertilization, but life does not. Life can be traced back to the beginning of life.
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Sexes,
Animals vs. Plants
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Most animal species have individuals divided into two sexes (genders) that determine many aspects of individual and social behavior.
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Most flowering plants have both anthers (male) and ovules (female) in each flower. Individual plants do not have a ‘gender’.
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Crossing over during Meiosis
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Note that each cell has one big and one small chromosome.
Many chromosomes in gamete have some DNA from maternal (red) and some from paternal (blue) chromosome.
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Results of Meiosis
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Each of the four cells resulting from the two divisions of meiosis has one of each type of chromosome.
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After meiosis each chromosome is usually a mixture of pieces from the maternal chromosome and from the paternal chromosome thru breakage and reunion called crossing over.
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Sex and Reproduction
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Individually and collectively humans have a high level interest in sexuality.
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Suppression of exposure of juveniles to sexually explicit material is a common, if not universal, pattern of human culture.
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Sexual behavior and sex ‘appropriate’ appearance is something most individuals feel strongly about.
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Asexual Reproduction
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Reproducing (make more individuals) without involving meiosis and union of gametes.
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Cloning (forming a new individual from a small piece of another one) has been done in plants for a long time.
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Cloning does not result in a genetically new individual (fertilization does).
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Uniparental Reproduction
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Uniparental means a single individual can produce progeny without a partner.
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If via meiosis, process is called selfing.
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The individual is source of both the male and female gametes used in fertilization.
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Many crop plants reproduce by selfing. Peas and corn both normally have most ovules fertilized by pollen from the same plant.
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Uniparental means the individual has only one parent.
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The Garden Pea
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Variety A
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Annual
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Propagated as a true-breeding line by selfing.
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GREEN seeds
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WRINKLED seeds
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Variety B
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Annual
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Propagated as a true-breeding line by selfing.
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YELLOW seeds
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ROUND seeds
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VOCABULARY
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Clonal life cycle
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Crossing over
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Diploid
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Fertilization
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Gamete(s)
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Haploid
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Haploid number
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Homologs
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Meiosis
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Selfing
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Sex
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Sexes
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