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Unit 3: Cell Division
Left Side Pg Right Side Pg
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
Meiosis
Unit 3: Cell DivisionChapter 11-
4
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.
MeiosisMeiosis
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
Homologous Chromosomes
Each cell has 22 sets of chromosomes Homologous ChromosomesHomologous Chromosomes =
a pair of chromosomes that have the same genes at the same locations
1 came from mom 1 came from dad
From mom
From dad
Homologous chromosomes
Chromosome NumberDiploid (2n)Diploid (2n): cells that have 2 sets of homologous chromosomes (2n)
Human body cells are diploid (4646 chromosomes or 23 pairs)
Haploid (n): cells that have 1 set of chromosomes (n)
sex cells (gametes) are haploidhuman haploid cells (egg & sperm) have 23 chromosomes total
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)?
meiosismeiosis
Haploid gamete
Diploid
Haploid gamete
2n
n n
Human Female Karyotype
Human Male Karyotype
Down Syndrome Female
Trisomy
Meiosis OverviewA 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 cellEach new cell is genetically different
Steps of Meiosis
InterphaseDNA is copied before the start of Meiosis I.This makes 2 identical sister chromatids They have the
same genes at the same locations.
Homologous chromosomes vs. sister chromatids
From mom
From dad
Meiosis 1
Four steps: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1
Prophase 1
The homologous pairs of chromosomes come together and line up side by side. (tetrad = 4)
Crossing-overHomologous chromosomes can Cross-over and exchange a portion of the chromosome (DNA) at random
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.
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 hairFreckles
Chromosome 16
After crossing-over, the homologous chromosomes separate & the cells divide
Meiosis II
Four steps: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
Sister chromatids separate & the cells divide.Result is 4 genetically different haploid daughter cells
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.
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
Gamete FormationGametes = Sex Cells (eggs & sperm)
Spermatogenesis: In males meiosis occurs in the testes produces 4 sperm
23
23
23
23
Spermatogenesis
Oogenisis:In females meiosis occurs in the ovaries
produces only 1 egg and 3 polar bodies
Egg23
Polar Bodies
Oogenesis
Fertilization: the fusion of haploid sex cells
Zygote
46
2323
First cell of a new organism
Only one sperm and one egg come togetherThe first sperm to reach the egg will be the one to fertilize it.
Comparing Mitosis & Meiosis
Mitosis results in the production of 2 genetically identical diploid cells.
Meiosis produces 4 genetically different haploid cells.
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.