Meiosis • The form of cell division by which _____________ with ________ the regular number of chromosomes, are produced. (complete set of chromosomes) (half the regular number of chromosomes) • Meiosis is needed for _______________________. • _______ divisions (____________ and __________).
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Meiosis
• The form of cell division by which _____________ with ________ the regular number of chromosomes, are produced.
(complete set of chromosomes) (half the regular number of chromosomes)
• Meiosis is needed for _______________________.
• _______ divisions (____________ and __________).
• Reproductive cells divide to produce ______________ (___________ or ________).
• Gametes have half the regular # of chromosomes.
• This process occurs only in gonads (__________ or ___________).
Male:
Female:
• Meiosis is similar to mitosis with some chromosomal differences.
Spermatogenesis
llll llll
human
sex cell
diploid (2n)
llll
llll
meiosis I
ll
ll
ll
ll
sperm
haploid (n)
meiosis II
haploid (n)
Interphase I
• Similar to mitotic interphase.
• __________________ replicate (S phase).
• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
• Centriole pairs also replicate (in animal cells).
Interphase I
• ______________ and _____________visible.
Meiosis I (four phases)
• Cell division reduces the _______________ number in daughter cells produced by ________________.
• four phases:
a.
b.
c.
d.
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• _____________ occurs: _________________________
come together to form a ________________.
• ___________ is a pair of ______________________ or four _________________ (sister and non sister
chromatids) that align in Prophase 1
Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatids Tetrad
Homologous Chromosomes
• Are a pair of chromosomes (____________and____________) that are similar in ________ and _______, and which carry _________controlling the same inherited ______________.
• Each __________ (position of a gene) is in the same position on homologues.
• Humans have _____ pairs of homologous chromosomes.
a. 22 pairs of _______________
b. 1 pair of ________________
Homologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus hair color
locus
Sex Chromosomes
XX
chromosome - female
XY
chromosome - male
Crossing Over
• Crossing over may occur between non sister chromatids at the ___________ during Prophase 1.
• During crossing over segments of _______________ _____________ overlap, break, and reattach to the other
• Crossing over increases ________________ in the gametes produced
• ____________ (_______________) are the sites of crossing over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over variation
Tetrad
Prophase I
centrioles spindle fiber
aster
fibers
Metaphase I • Shortest phase
• ________________ align on the metaphase plate.
• ________________________ occurs: Orientation of homologous pair to poles is random.
Metaphase I
metaphase plate
OR
metaphase plate
Anaphase I
• __________________________separate and move towards the poles.
• Sister chromatids remain attached at their centromeres.
Anaphase I
Telophase I
• Each pole now has ___________________ set of chromosomes.
• ___________________ occurs and _____ haploid daughter cells are formed.
Telophase I
Meiosis II
• There is no interphase II
(or, if it does occur, it is very short)
• There is no more _______________________
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• _____________ form.
• __________________ occurs.
• _________ haploid daughter cells (gametes) are produced.
gametes = ________ or ____________
Telophase II
Genetic Variation
• Important to population as the raw material for __________________ (____________).
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1.
2.
3.
Remember: variation is good!
Fertilization
• The fusion of a __________ and _______to form a ___________.
• A zygote is a fertilized __________
n=23
egg
sperm
n=23
2n=46
zygote
Question:
• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?