Download Meiosis Meiosis Stages of Meiosis Meiosis I Meiosis II Meiosis

Meiosis
What is it?
• cell division that occurs in organisms that reproduce sexually
• called Reduction Division b/c it reduces the # chromosomes in the cell to ½ Why does it happen?
Meiosis
• for production of gametes (AKA sex cells ­ egg and sperm in us)
• for production of haploid spores in some organisms (ex. fungi)
• to prevent doubling of chromosomes from generation to generation
Result...
Cells that have half the number of chromosomes so they can join to form an embryo with the proper # chromosomes (a full set).
Meiosis
Stages of Meiosis
Some Important Definitions
Diploid ­ number of chromosomes in normal body cells (complete set, 2N)
Haploid ­ number of chromosomes in reproductive cells (half, designated N)
Homologous pair ­ pair of chrs that contain same order of genes and look
similar, but may have different alleles or different versions of some of the genes (1 from mother, 1 from father = homologous pair)
Think about it...
ex. An animal has 16 chromosomes in one of its liver cells.
sex cells?
How many chromosomes does it have in diploid cells?
How many maternal chromosomes does it have?
How many homologous pairs of chromosomes does it have?
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
• first division, separates homologous pairs
• reduces # chromosomes to half (haploid)
• called "Reduction Division" as a result
• results in 2 daughter cells • cells are haploid with 2 copies of each chr (copies still together as sister chromatids)
• second division, separates sister chromatids • results in total of 4 daughter cells • cells are haploid with single copy of each chr
Meiosis
INTERPHASE
● Gap1 (G1) ­ growth, duplication of organelles, etc.
● Synthesis (S) ­ DNA replication (copy chromosomes)
● Gap2 (G2) ­ new proteins for membranes, enzymes
● DNA not visible thru phase (chromatin)
MEIOSIS I ­ Reduction Division
1) PROPHASE I
● nuclear membrane starts to dissolve
● chromosomes become visible as sister chromatids
● pairs of centrioles appear + move twd opposite poles
● spindle fibers begin to form between centrioles ● homologous chrs pair up (synapsis) to form tetrads
2) METAPHASE I
● centrioles reached opposite poles
● spindle complete, stretching from pole to pole
● homologous pairs line up at equator
● spindle fibers attach to tetrads at centromeres
Meiosis
Meiosis
INTERPHASE
● short­lived stage if any
● no DNA replication
● cells enter prophase II
3) ANAPHASE I
● spindle fibers pull homologous pairs apart
● homologous pairs move twd opposite poles
● sister chromatids still attached by centromere
Meiosis II
1) PROPHASE II
● nuclear membrane starts to dissolve
● chromosomes become visible as sister chromatids
● centrioles appear + move to poles at 90 from 1 st div
● spindle fibers begin to form between centrioles 4) TELOPHASE I
● homologous pairs at opposite poles
● nucleus forms in each cell, nucleolus appears
● furrow begins to develop at equator
2) METAPHASE II
● centrioles reached opposite poles
● spindle complete, stretching from pole to pole
● chromatids line up at equator
● spindle fibers attach to chromatids at centromeres
CYTOKINESIS
● cytoplasm divides to create two daughter cells
● each cell has its own nucleus with ½ # chrs
Meiosis
Meiosis Animation
3) ANAPHASE II
● spindle fibers pull sister chromatids pairs apart
● chromatids “u­shaped” as move twd opposite poles
4) TELOPHASE II
● sister chromatids at opposite poles
● nucleus forms in each cell, nucleolus appears
● furrow begins to develop at equator
CYTOKINESIS
● cytoplasm divides to create two daughter cells
● total of 4 cells produced (2 from Meiosis I ­> 4 now)
● each cell has its own nucleus, nucleolus
● each cell is haploid (½ # chromosomes)
● chromosomes of different cells are NOT IDENTICAL
Mitosis vs Meiosis
Comparison Animation
Sources of Genetic Variation
1) Random Assortment
• homologous chromosomes line up randomly during Metaphase I • maternal / paternal chromosomes are pulled to opposite poles
• random arrangement of maternal + paternal chrs in daughter cells
Random Assortment
2) Crossing Over
• regions of like chrs are exchanged during Prophase I while homologous pairs are synapsed or in tetrad form
• results in more random distribution of genes and greater assortment of traits
Crossing Over
Meiosis
Mitosis
Meiosis
Mitosis
• occurs only in sex organs to produce sperm/egg cells
• occurs only in sex organs to produce sperm/egg cells • results in daughter cells that are all unique (genetically)
• involved in sexual reproduction • results in 2 diploid cells
• no synapsis
• involved in asexual reproduction • results in 4 haploid cells
• synapsis occurs during prophase I
• homologous pairs are separated during cell division
results in daughter cells that are genetically identical •
• homologous pairs are never separated during cell division
• occurs in somatic cells for growth, maintenance and repair to the parent cell
• occurs in somatic cells for growth, maintenance and repair
• involves only 1 division
• involves 2 divisions
• involved in asexual reproduction
• involved in sexual reproduction
• results in 2 diploid cells
• results in 4 haploid cells
• results in daughter cells that are genetically identical to the parent cell
• results in daughter cells that are all unique (all genetically different)
• no synapsis
• synapsis occurs during prophase I
• no tetrads
• homologous chromosomes form tetrads
• homologous pairs are never separated during cell division
• homologous pairs are separated during cell division
• homologous chromosomes form tetrads • no tetrads • involves 2 divisions • involves only 1 division
Meiotic Nondisjunction
Nondisjunction Animation
• when chromosomes "fail to come apart" during meiosis
• results in monosomy and trisomy conditions
(1 copy of chr) (3 copies of chr) Syndromes due to Nondisjunction
Down Syndrome
• trisomy 21
• mental impairment
• thick tongue
• speech difficulties
• heart defects
• respiratory probs
• 1/800 to 1/1000
Klinefelter Syndrome
• XXY
• immature sex organs
• sterile
• breast dev
• long legs
• no facial hair
• learning difficulties?
• 1/500 boys
Turner Syndrome
• monosomy X
• shorter stature
• no ovaries
• no menstruation
• infertile
• 1/2500
Triple X Syndrome
• trisomy X
• appear normal
• may be sterile
• 1/1000
Jacob's Syndrome
• XYY
• appear normal
• low mental ability
• 1/1000 boys
Karyotype
Which karyotype goes with:
• an illustration or photograph that shows the chromosomes from the nucleus of a somatic cell in an organism
• Homologous pairs are usually lined up in order.
• allows for easy detection of nondisjunction and some chromosome mutations Turner Syndrome
Klinefelter Syndrome
Down Syndrome
Spermatogenesis & Oogenesis
Spermatogenesis
• production of sperm in testes of male
• meiosis results in 4 haploid spermatids that develop into sperm
Spermatogenesis
• occurs year round in humans, but ONLY at select times in other species
• germ cells continue undergoing mitosis for life, so males can reproduce longtime!
• all 4 gametes have the potential to become fully functioning sperm cells
• 4 spermatids produced in testes must mature in epididymis
(lose the cytoplasm, form a head, grow a long flagellum for locomotion)
• human males produce approx 1 billion sperm per day
Oogenesis
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all germ cells stop undergoing mitosis in ovary before birth (fixed # eggs dev)
in meiosis I, cytoplasm is not evenly distributed so 1 oocyte + 1 polar body
polar bodies provide nourishment to growing oocyte (later egg)
only 1 out of 4 possible germ cells becomes a viable sex cell (3 polar bodies)
Meiosis II does not finish until fertilization of the egg takes place
Oogenesis • production of egg cells in ovaries of female
• meiosis results in 1 haploid ootid + 3 polar bodies that nourish the egg
Spermatogenesis
Oogenesis
• both meiotic divisions happen every time to produce reproductive cells
• both meiotic divisions only go to completion if fertilization takes place
• germ cells stop undergoing mitosis before birth • results in 4 haploid spermatids
(fixed # cells can undergo meiosis) • equal division of cytoplasm
• results in 1 haploid egg cell and 3 polar bodies
• 1 cell is released from the • 1 billion reproductive cells are • unequal division of cytoplasm
reproductive organ each month produced each day
• Oocytes are "stuck" at Prophase I from birth til puberty
• Each month, one proceeds to Metaphase II and is released (ovulation)
• Only completes Meiosis II if fertilization occurs
• germ cells can undergo mitosis until death
(keep producing cells that can undergo meiosis)