1 11–4 meiosis the making of sex cells!. 2 making sex cells meiosis is the process of making sex...
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11–4 Meiosis
The making of sex cells!
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Making Sex Cells Meiosis is the
process of making sex cells (gametes)
Gametes are sex cells Ex: sperm, egg,
pollen, etc…
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Female Gametes - Eggs
Human Eggs
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Male Gametes - Sperm
Human Sperm
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Chromosomes A body cell in an
adult fruit fly has 8 chromosomes, as shown in the drawing on the right.
Four of the chromosomes came from the fruit fly's male parent, and 4 came from its female parent.
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Homologous Chromosomes
chromosomes that each have a corresponding chromosome from the opposite-sex parent Example: Fruit-Fly (Drosophila) Chromosomes - 8 chromosomes (4 from mom and 4 from dad)
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Different Types of Cells
A cell that contains both sets of homologous chromosomes is said to be diploid (“two sets”)
a diploid cell is sometimes represented by the symbol 2N for Drosophila, the diploid number is
8, which can be written 2N = 8 Diploid cells = body cells (somatic
cells)
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Haploid Cells = a cell that contains only a single set of chromosomes Therefore only a single set of genes
a haploid cell is sometimes represented by the symbol N for Drosophila, the haploid number is
4, which can be written N = 4 Haploid cells = sex cells (gametes)
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How are haploid (N) gamete cells produced from diploid (2N) cells?
Meiosis
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Meiosisa process of reduction
division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
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Phases of Meiosistwo distinct divisions, called
meiosis I and meiosis II
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
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What is the diploid number for these cells?
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What is the haploid number for these cells?
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Meiosis I Before meiosis I, each chromosome is
replicated. Division looks similar to mitosis Prophase I, Metaphase I, Anaphase I,
Telophase I prophase of meiosis I - each
chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad
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Tetrads
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Crossing Over As homologous chromosomes pair up
and form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over
results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles
Alleles = alternative forms of the same gene (ex: blue eyes vs. brown eyes)
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After Crossing Over…
homologous chromosomes separate two new cells are formed New cells:
each pair of homologous chromosomes was separated
neither of the daughter cells has the two complete sets of chromosomes (they have been shuffled and sorted)
The new cells are DIFFERENT from each other
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Meiosis II two cells produced by meiosis I
now enter a second meiotic division NO DNA replication before Meiosis II Prophase II, Metaphase II, Anaphase
II, Telophase IIpaired chromatids separate Produces: haploid cells (N)
Haploid cells are DIFFERENT from each other
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Gamete Formation
Males gametes = sperm (pollen in plants) All sperm the same size at the end of Meiosis 4 sperm produced for each round of meiosis
Female gametes = eggs One egg produced and 3 polar bodies (egg is
MUCH larger in size) The one egg receives the most cytoplasm One egg produced for each round of meiosis
(and 3 polar bodies which can’t be fertilized)
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Fertilization of a human egg by sperm.
Notice the HUGE size difference.
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Comparing Mitosis and Meiosis
Mitosis and Meiosis sound alike but are VERY different!
Mitosis produces two genetically identical diploid somatic cells
Meiosis produces four genetically different haploid sex cells
Animated comparison
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Mitosis Makes body cells (somatic cells) diploid cell two diploid (2N)
daughter cells. Daughter cells are identical to each
other & the original parent cell Purpose: Allows an organism's body
to grow and replace cells. In eukaryotic unicellular organisms, it is a form of reproduction.
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Meiosis Makes sex cells (gametes) a diploid cell four haploid (N)
cells These cells are genetically different
from the diploid cell and from one another.
Purpose: Sexual reproduction.