Title: Meiosis
1Meiosis
2Life Cycles are a Diagrammatic Representation of
the Events in the Organisms Development and
Reproduction
3The Problem
- Mitosis produces two cells with the same number
of chromosomes as the parent cell. Mitosis of a
diploid cell (2n) produces two diploid daughter
cells. If two diploid cells went on to
participate in sexual reproduction, their fusion
would produce a tetraploid (4n) zygote.
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7Ploidy
- Diploid
- - Two sets of chromosomes
- - 2n
- Haploid
- - 1 set of chromosomes
- - 1n
- Polyploid
- - More than two sets of chromosomes
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9Homologous Chromosomes carry the same
genes Alleles are a type of gene that carry
traits
10Gametes are Sex Cells Sperm (male) Egg (female)
11Gametes each contain one set of genetic
information (Haploid) Gametes unite to form a
zygote (Diploid) The zygote then divides by
mitosis to form an organism
12It is essential for organisms that reproduce
sexually to half their number of chromosomes,
otherwise, with each successive generation, the
number of chromosomes would double.
13Plant Life Cycles (Alternation of Generations)
- Sporophyte Phase
- 1. Diploid Phase
- 2. Meiosis Occurs Here
- Gametophyte Phase
- 1. Haploid Phase
- 2. Gametes are Produced by Mitosis
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15Meiosis
- The process in which haploid gametes (egg and
sperm) are formed. - Consists of Two Phases
- Meiosis I
- Meiosis II
16Interphase
Identical to Interphase in Mitosis. Start out
with a diploid cell.
17Prophase I
Prophase I is virtually identical to prophase in
mitosis. Chromosomes and spindle fibers
appear. Nuclear membrane and nucleolus
disappears. Homologous chromosomes pair up.
18Metaphase I
Chromosomes line up in homologous pairs across
equator of cell forming tetrads. Spindle fibers
attach to chromosomes. Crossing-over may occur.
19Anaphase I
Spindle fibers contract, separating the
homologous pairs of chromosomes.
20Telephase I
Two daughter cells form each with a single set of
chromosomes, half the total number in the
original cell. Nuclear membrane and nucleolus may
reappear. Cell is now haploid.
21Prophase II
Nuclear membrane and nucleolus disappear. Centriol
es migrate toward poles. A new set of spindle
fibers form and chromosomes begin to move toward
the equator of the cell.
22Metaphase II
Chromosomes line up across the equator of the
cell.
23Anaphase II
The spindle fibers shorten, drawing the
chromosomes toward each pole of the cell.
24Teleophase II
Formation of new nuclear membrane. Fading of
chromosomes. Cytokinesis occurs resulting in four
haploid daughter cells.
25Comparison of Mitosis and Meiosis
- Mitosis
- Results in two genetically identical daughter
cells. - Diploid cell ? 1 cell division ? 2 diploid
daughter cells - Involves 1 single nuclear division after DNA
replication.
- Meiosis
- Results in four genetically different daughter
cells - Diploid cell ? 2 cell divisions ? 4 haploid
daughter cells - Involves 2 nuclear divisions with no DNA
replication between them.
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30This Part Reduces Chromosome Number
This Part is Simply Mitosis All Over Again to
Duplicate Cells
31Gender Based
- Spermatogenesis
- Meiosis and gamete formation in males.
- 1 diploid cell ? 4 mature sperm
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- Oogenesis
- Meiosis and gamete formation in females.
- 1 diploid cell ? 1 mature ovum 3 polar bodies.