ib 11 meiosis 2019 - science to a tee...variation in sexual reproduction how is variation in...
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Meiosis
Cells Division in Reproductive Cells
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Which traits do you have that are unique to you compared to your siblings?
Which traits do you have that are not in your parents?
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How Reproductive Cells are Produced ? Meiosis - process of making gametes (sex cells) - results in half the number of chromosomes in cells haploid = 1n
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How Reproductive Cells are Produced ?
Oogenesis - making of egg cells in female ovaries Spermatogenesis- making sperm in male testis
Oogenesis
Spermatogenesis
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Union of gametes returned cells to Diploid = 2n
**fertilization** 1/2 chromosomes
from father + 1/2 chromosomes from mother = diploid
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Are the following cells haploid (1N) or diploid (2N)? A liver cell? An unfertilized egg (ovum)? A zygote (fertilized egg)? A child? A brain cell? A sperm cell? A pollen grain? An apple seed? How many chromosomes are found in each? A liver cell? An unfertilized egg (ovum)? A zygote (fertilized egg)? A child?
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Stages of Meiosis
Occurs in two divisions (meiosis I & meiosis II) End result---> 4 haploid cells
2n
2n
2n
1n
1n
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Get a Comparison sheet….
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Stages of Meiosis
meiosis I = reduction stage since chromosomes are reduced
to 1n from 2n
Meiosis I2n
1n
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Stages of Meiosismeiosis I = reduction stage since chromosome bivalents split
and the ploidy is reduced to 1n from 2n
meiosis II - sister chromatids separate of chromosomes
separate
Anaphase I
Anaphase II
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What is the chromosome # of original cells of mitosis? What is the chromosome # of original cells of meiosis? What is the chromosome # of daughter cells of mitosis? What is the chromosome # of daughter cells of meiosis? What # of cells are produced in mitosis? What # of cells are produced in meiosis? What type(s) of cells involved in mitosis What type(s) of cells involved in meiosis? What is the ploidy of original cells in mitosis? What is the ploidy of original cells in meiosis? What is the ploidy of resulting cells in mitosis? What is the ploidy of resulting cells in meiosis?
A fruit fly has 8 chromosomes in G1 of Interphase
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Homologous Chromosomes Bivalents
• not genetically identical
• One originated from the father, one from the mother.
• they code for the same genes (hence the banding pattern is the same)
• during prophase I they come together and for BIVALENTS
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X
Y
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Why is Variation Important?• variation in genes allow organisms to survive
different circumstances • creates individuality • it allows the combining of traits from two into one,
possibly creating hybrid vigour • provide new combinations that might ensure future
success • essential for evolution - provides a route for change
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Variation in Sexual Reproduction
How is variation in chromosomes created during meiosis?
1. Crossing over
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Chiasma
Centromere of on homologue chromosome
in this case, crossing over can happen more than once
SYNAPSES of homologous chromosomes
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Variation in Sexual Reproduction
How is variation in chromosomes created during meiosis?
1. Crossing over • during Prophase I tetrads of homologous chromosomes
are formed (an event called Synapses) • non sister chromatids wind around each other forming
Chiasma or points where exchange of segments of chromosomes occurs
• resulting in new genetic combinations formed
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Variation in Sexual Reproduction
How is variation in chromosomes created during meiosis?
2. Independent assortment
or
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Variation in Sexual Reproduction
How is variation in chromosomes created during meiosis?
2. Independent assortment • during metaphase I when homologous chromosomes
line up at poles
or
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Variation in Sexual Reproduction
How is variation in chromosomes created during meiosis?
2. Independent assortment • during metaphase I when homologous chromosomes
line up at poles • homologous chromosomes line up independently ---> that is, ‘maternal’ chromosomes & ‘paternal’
chromosomes line up randomly
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Variation in Sexual Reproduction
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Errors can occur in Meiosis
• mutations can occur and are often detrimental
Nondisjunction - failure of chromosomes to separate properly - results in a missing or an extra chromosome in a gamete cell
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Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Nondisjunction of homologous chromosomes in meiosis I
Nondisjunction of sister chromatids in meiosis II
(a) (b)
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Karyotype Charts
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Karyotype Charts
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Errors can occur in Meiosis
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• Cat eye syndrome
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Homework
Page pg 181 #1-8
Read about Error in Chromosome Number, Monosomy and Trisomy on page 178-179