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Unit 3: Cell Division Left Side Pg Right Side Unit Page 48
Table of Contents 49 Flow Map – Cell Cycle 50 C.N. – The Cell Cycle 51 Brace Map – Meiosis 52 C.N. – Meiosis 53
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Unit 3: Cell Division Chapter 11-4
Meiosis Unit 3: Cell Division Chapter 11-4
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Learning Goals 1. Describe the overall process of meiosis.
2. Define homologous chromosomes and explain diploid and haploid. 3. Explain crossing over, its importance, and during which step it occurs. 4. Describe the two types of gamete formation and how fertilization occurs.
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Meiosis A process of cell division that creates gamete cells, or non-identical cells with half the number of chromosomes The gametes from a male and a female combine in sexual reproduction to make a new individual
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Homologous Chromosomes
Each cell has 2 sets of chromosomes Homologous Chromosomes = a pair of chromosomes that have the same genes at the same locations 1 came from mom 1 came from dad
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Homologous chromosomes
From mom From dad
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Chromosome Number Diploid (2n): cells that have 2 sets of homologous chromosomes (2n) Human body cells are diploid (46 chromosomes or 23 pairs) Haploid (n): cells that have 1 set of chromosomes (n) sex cells (gametes) are haploid human haploid cells (egg & sperm) have 23 chromosomes total
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1) A bug has a haploid number n=5. What is the diploid number (2n)?
2) A crocodile has a diploid number 2n=50. What is the haploid number (n)? meiosis Haploid gamete Diploid 2n n n
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Human Female Karyotype
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Human Male Karyotype
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Down Syndrome Female Trisomy
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Meiosis Overview A type of cell division specific to sexual reproduction (two parent cells) Cells (diploid) divide twice resulting in 4 daughter cells (haploid) Each cell has half the number of chromosomes as the original cell Each new cell is genetically different
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Steps of Meiosis
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Interphase DNA is copied before the start of Meiosis I.
This makes 2 identical sister chromatids They have the same genes at the same locations.
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Homologous chromosomes vs. sister chromatids
From mom From dad
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Meiosis 1 Four steps: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1
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Prophase 1 The homologous pairs of chromosomes come together and line up side by side. (tetrad = 4)
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Crossing-over Homologous chromosomes can Cross-over and exchange a portion of the chromosome (DNA) at random
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Crossing-over is very important in meiosis because it creates genetic variation (new combinations of genes), which makes us all different. This is why you do not look exactly like your siblings.
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Crossing over happens randomly.
However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together. Red hair Freckles Chromosome 16
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After crossing-over, the homologous chromosomes separate & the cells divide
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Meiosis II Four steps: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
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Sister chromatids separate & the cells divide.
Result is 4 genetically different haploid daughter cells
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Principal of Independent Assortment
Genes for different traits can segregate (separate) independently during the formation of gametes. Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms.
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Assume there was only one cross-over in a man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children
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Gamete Formation Gametes = Sex Cells (eggs & sperm) Spermatogenesis:
In males meiosis occurs in the testes produces 4 sperm 23 23 23 23
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Spermatogenesis
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Oogenisis: In females meiosis occurs in the ovaries
produces only 1 egg and 3 polar bodies Egg 23 Polar Bodies
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Oogenesis
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First cell of a new organism
Fertilization: the fusion of haploid sex cells Zygote 46 23 First cell of a new organism
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Only one sperm and one egg come together
The first sperm to reach the egg will be the one to fertilize it.
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Comparing Mitosis & Meiosis
Mitosis results in the production of 2 genetically identical diploid cells. Meiosis produces 4 genetically different haploid cells.
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Learning Goals 1. Describe the overall process of meiosis.
2. Define homologous chromosomes and explain diploid and haploid. 3. Explain crossing over, its importance, and during which step it occurs. 4. Describe the two types of gamete formation and how fertilization occurs.
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