MEIOSIS
Chapter 8
Chromosome Pairs

Human somatic (body) cells consist
of 23 homologous chromosome
pairs

Identical length, centromere position,
and loci



Alleles are different versions
First 22 are autosomes
Pair 23 are sex chromosomes
Few homologous parts
 XX = female and XY = male


Gametes (eggs or sperm) consist of
23 unpaired chromosomes
Karyotypes

Size ordered chart of all
chromosomes in a cell
 Size
 Banding
patterns
 Centromere location

Visualizes chromosomal
makeup
 Matched
up homologs
Cell Types


n = number of chromosomes in a single set
Diploid cell has 2 homologous sets of chromosomes
 One
 If
from each parent
synthesis has occurred, each appears as sister chromatids
 Total
number of chromosomes is the diploid number (2n)
 Humans

have 23 sets so diplod nubmer is (2 * 23 = 46)
Haploid cell has 1 set of chromosomes
 Called
gametes
 Total number of chromosomes is the haploid number (n)
 22

autosomes and 1 sex chromosome (X or Y)
Egg always X and sperm is X or Y
Fertilization
Sperm (male)
+
n
Haploid
+
+

egg (female)
n
haploid
= zygote
= 2n
= diploid
Meiosis makes gametes
 Only
in reproductive areas
 Reduce chromosome number by half
 Haploid
cells keep number of chromosomes from doubling
with each generation

Mitotic phase ensures that chromosome number is
maintained
Meiosis (Overview)


Creates haploid gametes (n)
in diploid (2n) organisms
2 phases
 Meiosis
I (homologs separate)
 Meiosis II (sister chromatids
separate)


Produces 4 daughter cells with
½ number of chromosomes
Only time homologs
associated with each other
Meiosis I

Prophase I
 90%
of meiotic time
 Synapsis is when the chromosomes pair up
2
homologous duplicated chromatids = tetrad
 Crossing over exchanges genetic info
 Similar

to other processes in mitotic prophase
Metaphase I
 Tetrad
arranged in middle of cell
 Spindle microtubules attach to the centromere
of 1 chromosome (2 sister chromatids)
Meiosis I (cont.)

Anaphase I
 Chromosomes
move to respective
poles, splitting the tetrad

Telophase I
 Chromosomes
 Haploid

reach poles
sets at each
Cytokinesis
 Occurs
with telophase 1
 Creates 2 haploid daughter cells

Interphase may or may not occur
 No
duplication occurs regardless
Meiosis II


Like mitosis except it starts with haploid cells
Prophase II


Metaphase II


Sister chromatids split and move to poles
Telophase II



Chromosomes align
Anaphase II


Spindles form and move chromosomes to middle
Nuclei form at poles
Cytokinesis occurs
4 daughter cells (haploid)
Comparing Mitosis and Meiosis

Mitosis




Meiosis





For growth, repair, and asexual reproduction
Daughter cells are genetically identical
1 nuclear division = 2 daughter cells same # of chromosomes
Produces gametes for sexual reproduction
Haploid daughter cells with a half of each homologous pair
2 nuclear division = 4 haploid cells
Meiosis I has all unique stages
Both


Chromosomes duplicate only once
Meiosis II and mitosis events
Sources of Genetic Variability

Independent orientation at metaphase I (meiosis I)

Random fertilization

Crossing over in prophase I

Mutation is the ultimate source
Independent Orientation and Random Fertilization


Genetic diversity from
reshuffling of genes during
sexual reproduction
Occurs in metaphase I


1 chromosome from mom
and 1 from dad
Tetrads randomly line up on
either side


About equal probability for
all cases
Total arrangements = 2n
Humans: 223 = 8 million
 2 different gametes =
8 million2 = 64 trillion

Crossing Over

Exchange corresponding
segments between homologs
 Occur

at chiasma
Results in genetic
recombination
 Varies
from original
chromosome

Happens average of 1 – 3
times in human meiosis
Errors in Meiosis

Abnormal chromosome numbers
or size

Nondisjunction, pairs of
chromosomes don’t separate
Meiosis I = 4 abnormal cells
 Meiosis II = 2 abnormal and 2
normal




Unknown what causes
Body generally miscarries when
this occurs
Often leads to disorders or
syndromes
Down Syndrome




Trisomy 21
Most common
chromosome abnormality
and most serious birth
defect in US
Incidence rate increases
with age of mother
Generally don’t
reproduce
 Sterile
or chances of
passing to offspring
Sex Chromosome Disorders
Sex Chromosome Abnormalities
Genotype
Gender
Syndrome
Physical Traits
XXY, XXYY, XXXY
Male
Klinefelter syndrome
Sterile, small testicles, breast
enlargement
XYY
Male
Jacob’s syndrome
Normal male traits,
previously thought ‘supermales’, karyotype ID only
XO
Female
Turner syndrome
sex organs don't mature,
sterile, short stature
XXX
female
Trisomy X
Karyotype only identifier
Structural Abnormalities

Duplication
 Huntington’s

disease
Deletion
 Cri
du chat: developmental &
neurological problems

Inversion
 Less
likely to be harmful to affected
 Increase miscarriage

Translocation
 Not
between homologs
 Can cause cancers