Post on 29-Dec-2015
transcript
Word(s) of the Day
• Diploid : Is a Cell that contains two identical sets of DNA
• Haploid: Cell that contains Half the total number of chromosomes
Goals for the day !
Identify the order in which different cellular organelles produce haploid cells
Use new vocabulary such as haploid and diploid to describe meiosis
Compare and Contrast Mitosis and Meiosis
Meiosis
• The form of cell divisioncell division by which gametes, gametes, with halfhalf the number of chromosomes,chromosomes, are produced.
• Diploid (2n) Diploid (2n) haploid (n)haploid (n)
• MeiosisMeiosis is sexual reproduction.sexual reproduction.
• Two divisionsTwo divisions (meiosis I meiosis I and meiosis IImeiosis II).
MeiosisMeiosis
• Sex cells Sex cells divide to produce gametesgametes (sperm (sperm or egg)or egg).
• GametesGametes have halfhalf the # of chromosomeschromosomes.
• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female: oogenesisFemale: oogenesis
• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.
SpermatogenesisSpermatogenesis
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
“Sperm” “Formation”
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
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
Interphase IInterphase I
• Similar to mitosismitosis interphase.
• ChromosomesChromosomes replicate (S phase).(S phase).
• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.
Meiosis I (four phases)Meiosis I (four phases)
• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.
• four phasesfour phases:
a.a. prophase Iprophase I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
Prophase IProphase I
• Longest and most complex phase (90%).Longest and most complex 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).
Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
Homologous ChromosomesHomologous Chromosomes
• 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.
• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.
Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
Crossing OverCrossing Over• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids
• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing crossing overover.
Crossing Over - variation Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
Sex Chromosomes Sex Chromosomes DO NOT DO NOT crossover !crossover !
XX chromosome - female XY chromosome - male
Metaphase IMetaphase I• Shortest phaseShortest phase
• TetradsTetrads align on the metaphase platemetaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example:Example: 2n = 42n = 4then then n = 2 n = 2
thusthus 2 22 2 = 4 combinations= 4 combinations
AnswerAnswer
• Formula: 2Formula: 2nn
• Human chromosomes:Human chromosomes: 2n = 462n = 46
n = 23n = 23
• 222323 = ~8 million combinations = ~8 million combinations
Anaphase IAnaphase I
• Homologous chromosomes Homologous chromosomes separate and move towards the poles.
• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
Telophase ITelophase I
• Each pole now has set of chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.
Meiosis IIMeiosis II
• No interphase II No interphase II
(or very short - no more DNA replicationDNA replication)
• Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis
Metaphase IIMetaphase II
• same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate
Anaphase IIAnaphase II
• same as anaphaseanaphase in mitosismitosis
• sister chromatids separatesister chromatids separate
Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis.
• Nucleus reforms .
• CytokinesisCytokinesis occurs.
• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.
gametes = sperm or egggametes = sperm or egg
Question:Question:
• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?