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MEIOSIS Ch. 8
CELLS FOR SEXUAL REPRODUCTION
Meiosis for Sexual Reproduction
Sexual Reproduction - two parents
a. Offspring are genetic mix of both parents
b. Have a NEW combination of genes
Advantage – genetic variation in offspring
a. Some may have traits that favor survival
b. Can pass these traits on to offspring
c. Darwin’s theory - “ survival of the fittest”
d. Variation in individuals allows a species to evolve
Sexual Reproduction in bacteria and protists
CONJUGATION
a. Recipient cell gets new genes
b. Bacteria and protists
Complex organisms – make special cells
a. gametes – sperm and egg
b. Gametes combine in fertilization
- make a zygote  new organism
Chromosome Number: Diploid and Haploid
Homologous chromosomes
a. matched chromosome pairs
b. one member of pair from each parent
c. carry genes for the same traits
d. 22 autosome pairs; one pair sex chromosomes X, Y
Gene for one trait
Cells with paired chromosomes are diploid
a. 2n (n = number)
b. Human: 2n = 46 (23 pairs)
b. Somatic (body) cells are diploid
2 sets of chromosomes
- 2 of every gene
Locus – location of
gene on a
chromosome
Fruit fly 2n = 8
Haploid Cells (n)
• Gametes – sperm and egg only
• one set of chromosomes, one from each pair
•
•
somatic cell
sex cell
Meiosis is “Reduction Division”
REDUCES chromosome number by half
Haploid (n) - one set of chromosomes
-human: 2n = 46
n = 23
Meiosis Reduces the Chromosome Number
2n parent cell
DNA replicates in interphase
First division – pairs separate
Second division – sister
chromatids separate
 4 haploid daughter cells
Homologous pairs separate in MEIOSIS
TWO cell divisions
Diploid cell - Has pairs
- Daughter cells have ½ parent
(2n = 2)
chromosome number
Meiosis I - Pairs separate
(n = 1)
Meiosis II - copies separate
(n = 1)
Haploid cells - (n = 1)
Crossing over – only in meiosis
a. In Meiosis I
b. Homologous chromatids trade pieces
c. Increases genetic variation
Meiosis I: homologous pairs separate
- makes two daughter cells, but sister chromatids are still attached
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Sites of crossing over
Spindle
Chromatin
2n parent cell
Sister
chromatids
synapsis
Tetrad
Microtubules
attached to
kinetochore
ANAPHASE I
Metaphase
plate
Centromere
(with kinetochore)
pairs line up
Sister chromatids
remain attached
Homologous
chromosomes separate
pairs separate
disjunction
Meiosis II: sister chromatids separate  4 haploid cells
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
2n  n
two daughter cells
one chromosome set each
two copies (sisters)
chromatids
separate
disjunction
Haploid daughter cells
forming
4 daughters
one set
single copies
8.15 Review: Comparing mitosis and meiosis
Mitosis
Meiosis
2n
copies
Parent cell
(before chromosome replication)
Prophase I
Prophase
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
replication
Chromosome
replication
Duplicated
chromosome
(two sister chromatids)
2n
Meiosis i
2n = 4
Metaphase
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Metaphase I
2n
copies
2n
copies
copies
2n
single
2n
Daughter cells
of mitosis
2n
No further
chromosomal
replication; sister
chromatids
separate
during
anaphase II
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
n
n
n
Daughter cells of meiosis II
copies
n
single
Meiosis ii
n
n
Sources of genetic variation
1. Homologous pairs have different genes
•same traits, but may be different forms
2. Crossing over – homologs trade pieces before separating
 new gene combinations
3. Pairs position in Metaphase I – independent of each other
•n pairs  2n possible combinations
4. Random fertilization of eggs by sperm
•Any egg or sperm is equally likely to be used
5. Gene or chromosome mutation
- Error in replication or cell division
8.16 Chromosomes line up randomly in meiosis
– “Independent Assortment”
– Many different gene combinations in haploid gametes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Figure 8.16
Making sperm and egg
Sperm:
2n parent cell
 4 haploid sperm
Ovum:
2n parent cell
 1 haploid egg
+ haploid polar bodies
Ovum needs all the cytoplasm
Ovum and polar body (0.1mm)
Sperm needs only DNA
- and flagellum
- and mitochondria for power
- and acrosome to penetrate ovum
Fertilization restores diploid
What happens after fertilization?
Cleavage: mitotic divisions in early embryo
reduce cell size, until normal
Cells differentiate  new organism
When meiosis goes wrong
Nondisjunction
- do not separate correctly
In mitosis  defective nucleus, cell usually dies
In meiosis  defective gamete
 wrong number of chromosomes in zygote
8.21 Accidents during meiosis can alter chromosome number
Nondisjunction
in meiosis I
Normal
meiosis I
Normal
meiosis
II
Nondisjunctio
n in meiosis II
Gametes
Gametes
n1
n1
n1
n 1
n 1
n 1
n
Number of chromosomes
Number of chromosomes
Nondisjunction in meiosis I
All gametes abnormal
Nondisjunction in meiosis II
Some gametes normal
n
Wrong chromosome number in zygote
 wrong number in every cell in organism
• Fertilization after nondisjunction  trisomy in zygote
Trisomy = 3
Egg cell
n+1
Zygote
2n + 1
Sperm cell
n (normal)
Abnormal chromosome number = aneuploidy
KARYOTYPE
picture of a person’s chromosomes
Photographed during mitosis
- sorted into homologous pairs
- largest-to-smallest
- sex chromosomes last
Abnormalities visible:
- missing or extra
- pieces broken or moved
- pieces added or lost
autosomes
sex chrom.
Trisomy 21
Normal male
karyotype
Normal female
karyotype
Down Syndrome
Trisomy chromosome # 21
8.22 Abnormal number of sex chromosomes
usually do not affect survival in humans
• Nondisjunction of large chromosomes is usually lethal
• Down Syndrome - # 21 is very small, carries few genes
• In sex chromosomes, leads to varying degrees of
malfunction, but usually not lethal
Turner Syndrome XO
Characteristic facial
features
Web of
skin
Constriction
of aorta
Poor breast
development
Under developed
ovaries
Figure 8.22B
Turner Syndrome
Klinefelter Syndrome XXY
Klinefelter Syndrome
Abnormalities of Sex Chromosomes in Humans
Other chromosome changes can cause
birth defects or cancer
Chromosomes break – pieces lost or rearranged
- in somatic cells  increases cancer risk
- in gametes  genetic disorders