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Meiosis
Gametogenesis &
Genetic VAriation
Meiosis
Meiosis is a type of cell division in which the number of chromosomes is reduced by half
Cells have half the amount of DNA as cells after mitosis
4 cells produced
2 divisions
-meiosis 1
-meiosis 2
Meiosis
Phases of Meiosis
occurs after DNa replication in the cell cycle in replace of mitosis
4 stages of meiosis 1 and meiosis 2
-prophase
-metaphase
-anaphase
-telophase
Meiosis 1
prophase 1
metaphase 1
anaphase 1
telophase 1
Prophase 1
6 parts
- nuclear envelope breaks down
-chromosomes condense
-centrioles move to opposite ends of the cell
-spindle starts to form
-homologus chromosomes pair up
-crossing over occurs
Metaphase 1
2 parts:
-spindle fibers attach to homogous chromosomes
-homologous chromosomes(tetrad) line up at the center of the cell
Anaphase 1
2 parts
- spindle fibers shorten
- homologous chromosomes separate and move to opposite ends of the cell
Anaphase 1
Telophase and Cytokinesis
3 parts:
- spindle breaks down
- nuclear membrane forms
-cytoplasm divides
Telophase and Cytokinesis
Meiosis 2
prophase II
Metaphse II
Anaphase II
Telophase II
Meiosis 2
Prophase II
What happens
-nuclear envelope breaks down
-spindles form
-centrioles move to opposite sides of the cell
Prophase II
Metaphase II
Spindles line up chromosomes at the equator
Metaphase II
Anaphase II
Sister chromatids separate and move apart
Anaphase II
Telophase II and Cytokinesis
What happens:
-spindles break down
-neculear envelope reforms
- cytoplasm separates
Telophase II and Cytokinesis
Gametogenesis
The development of haploid cells into gametes
male gamete: sperm
Female gamete: egg
Spermatogenesis
Spermatogenesis
-primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes.
-Secondary spermatocytes then quickly complete the meiotic division to become spermatids
- The four haploid cells produced from meiosis develop a flagellum tail and compact head piece to become mature sperm cells
Oogensis
Oogensis
Creation of the egg
only 1 viable cell is created
besides DNA the egg also contributes organelles, building blocks for compounds such as proteins and nucleic acids, and other necessary materials
polar body: cells created during oogensis that only contain DNA and cannot survive
The unequal distribution of the cytoplasm during oogenesis is necessary as the zygote that results from fertilization receives all of its cytoplasm from the egg.
Genetic Variation
Crossing-over is the exchange of genetic material between homologous chromosomes.
occurs during prophase I
independent assortment is hen homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other
Random fertalization: the idea that there are millions of possible combinations of egg and sperm and it is only a matter of chance of which two meet.
Crossing Over
Crossing Over
tetrad: forming a configuration with four chromatids
-during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length
-the chromatids are very close to each other and some material from two chromatids switch chromosomes
-the material breaks off and reattaches at the same position on the homologous chromosome