Upload
chester-lee
View
237
Download
0
Tags:
Embed Size (px)
Citation preview
1
Production of Gametes
MEIOSISMEIOSIS
2
Meiosis Meiosis FactsFacts
3
MeiosisMeiosis• The form of cell divisioncell division by which
gametesgametes, , with halfhalf the number of chromosomeschromosomes, , are produced
• DiploidDiploid (2n) (2n) haploidhaploid (n) (n)• Called a Called a reduction division reduction division
(because # of chromosomes is (because # of chromosomes is reduced)reduced)
• MeiosisMeiosis is sexualsexual reproduction reproduction• Two divisionsTwo divisions (meiosis I meiosis I and
meiosis IImeiosis II)
4
MeiosisMeiosis•Germ Line cells (aka sex
cells) divide to produce gametes (sperm or egg)
•Gametes have half the number of chromosomes
•Meiosis is similar to mitosis with some chromosomal differences
5
MeiosisMeiosis
•Occurs only in gonads (testes or ovaries)
•Male: Spermatogenesis
•Female: Oogenesis
6
SpermatogenesiSpermatogenesiss
2n=46
Humansex cell
Diploid (2n)
n=23
n=23
Meiosis I
n=23
n=23
n=23
n=23
Sperm
Haploid (n)
Meiosis II
7
Interphase IInterphase I
• Similar to mitosismitosis interphase• ChromosomesChromosomes replicate (S (S
phase)phase)• Each duplicated
chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres
• CentrioleCentriole pairs also replicate.
8
Interphase IInterphase I
• NucleusNucleus and nucleolusnucleolus visible.
Nuclear membrane
Nucleolus
Cell membrane
Chromatin
9
Meiosis IMeiosis I
10
Stages of Meiosis IStages of Meiosis I
• Cell division Cell division that reduces the chromosomechromosome number by one-halfone-half
• FourFour phases phases:
a.a. Prophase IProphase I
b.b. Metaphase IMetaphase I
c.c. Anaphase IAnaphase I
d.d. Telophase ITelophase I
11
Prophase IProphase I
• Longest and most complexLongest and most complex phase (90%)phase (90%)
• ChromosomesChromosomes condense• SynapsisSynapsis occurs:
homologous chromosomes homologous chromosomes come together to form a tetradtetrad
• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids)
12
Prophase IProphase I - -
SynapsisSynapsis
Homologous chromosomes
Sister chromatids Sister chromatidsTetrad
13
Homologous Homologous ChromosomesChromosomes
• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size
• Homologous pairs (tetrads)(tetrads) carry genes controlling the same inherited traits
• Each locuslocus (position of a gene)(position of a gene) is in the same position on homologues
14
Homologous Homologous ChromosomesChromosomes
• Humans have 23 pairs of homologous chromosomeshomologous chromosomes
• First 22 pairs of chromosomes called autosomesautosomes
• Last pair called sex sex chromosomeschromosomes
• XX female or XY maleXX female or XY male
15
Homologous Homologous ChromosomesChromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
Gene Loci
16
17
Crossing OverCrossing Over
• Crossing overCrossing over (variation) (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata
• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid
• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing overcrossing over
18
Crossing Over - Crossing Over - VariationVariation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
19
Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
20
Prophase IProphase I
CentriolesSpindle fiber
Asterfibers
21
Metaphase IMetaphase I
• ShortestShortest phase phase• TetradsTetrads align on the metaphase metaphase
plateplate• Independent assortment occursIndependent assortment occurs• Chromosomes separate Chromosomes separate
randomlyrandomly to the poles of the to the poles of the cellscells
22
Metaphase IMetaphase I• Independent assortment causes
variation in the forming cells• Orientation of homologous pair
to poles is random• Formula for determining
variation: Formula: 2n
ExampleExample: 2n = 4: 2n = 4 then n = 2then n = 2
thus 2thus 22 2 = 4 = 4 combinationscombinations
23
Metaphase IMetaphase I
Metaphase plate
OR
Metaphase plate
24
Question:Question:
• In terms of In terms of Independent Independent Assortment, Assortment, how many how many different combinations different combinations of sperm could a of sperm could a human male produce? human male produce?
25
AnswerAnswer
• Formula: 2Formula: 2nn
• Human chromosomes:Human chromosomes:
2n = 462n = 46
n = 23n = 23
222323 = ~8 million = ~8 million combinationscombinations
26
Anaphase IAnaphase I
• Homologous Homologous chromosomes chromosomes separate and move towards the poles
• Sister chromatids Sister chromatids remain attached at their centromerescentromeres
27
Anaphase IAnaphase I
28
Telophase ITelophase I
•Each pole now has haploidhaploid set of chromosomeschromosomes
• 1n = 23 (human)1n = 23 (human)• CytokinesisCytokinesis occurs and
two haploid daughter cells are formed
29
Telophase ITelophase I
30
31
Meiosis IIMeiosis II
32
Meiosis IIMeiosis II• Short or NoShort or No interphase II interphase II • DNA NOT replicatedDNA NOT replicated
againagain• Remember: Meiosis II Remember: Meiosis II is
similar to mitosismitosis• Prophase I, Metaphase II, Prophase I, Metaphase II,
Anaphase II, and Anaphase II, and Telophase IITelophase II
33
Prophase IIProphase II• Same as prophaseprophase in
mitosismitosis• Nuclear envelope breaks Nuclear envelope breaks
down and the spindle down and the spindle apparatus formsapparatus forms
34
Metaphase IIMetaphase II• Same as metaphasemetaphase in
mitosismitosis• Chromatids lined up at Chromatids lined up at
equatorequator
Metaphase plateMetaphase plate
35
Anaphase IIAnaphase II
• Same as anaphaseanaphase in mitosismitosis
• Sister chromatids Sister chromatids separateseparate
36
Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis
• Nuclei reform• CytokinesisCytokinesis occurs• Remember: Remember: Four haploid Four haploid
daughter cellsdaughter cells produced produced• GametesGametes = sperm or egg = sperm or egg
37
Telophase IITelophase II
38
MeiosisMeiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
Meiosis I
n=2
n=2
n=2
n=2
Sperm
Haploid (n)
Meiosis II
39
40
Genetic Genetic VariationVariation
41
VariationVariation
Important to the Important to the survival of populationssurvival of populations
Aids in natural Aids in natural selectionselection
Strongest individuals Strongest individuals are able to survive are able to survive and reproduceand reproduce
42
QuestionQuestion
•What are the three What are the three sources of genetic sources of genetic variation in sexual variation in sexual reproduction?reproduction?
43
***Answer:***Answer:
1.1. Crossing OverCrossing Over (Prophase I)(Prophase I)
2. 2. Independent Independent AssortmentAssortment (Metaphase (Metaphase I)I)
3. 3. Random FertilizationRandom Fertilization(sperm joins with egg)(sperm joins with egg)
44
Remember: Variation is
helpful to the survival of a
species!
45
Question:Question:
•A diploid cell containing 20 chromosomes20 chromosomes (2n = (2n = 20)20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
46
Answer:Answer:
•10 chromosomes 10 chromosomes (haploid)(haploid)
•1n = 101n = 10
47
KaryotypeKaryotype• A method of organizing the chromosomes
of a cell in relation to number, size, and type.
48
Trisomy• Genetic disorders such as
Down’s Syndrome (Trisomy 21) are the result of nondisjunction
49
50
FertilizationFertilization• The fusion of a spermsperm and eggegg to
form a zygotezygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
51
Question:Question:
•A cell containing 40 40 chromatidschromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
52
Answer:Answer:
•Four cells with 10 Four cells with 10 chromosomes chromosomes eacheach