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Ch. 10: Meiosis
MEIOSIS
Purpose: To reduce the total number of chromosomes from diploid to haploid
MOST ANIMALS
Majority of life is spent as a diploid organism
Only specialized cells go through meiosis to produce haploid gametes

Gametes are not free-living, and must fuse to form a diploid zygote, which will
immediately go through mitosis to form the new diploid organism
Chromosome = the organizational unit of heredity material in the nucleus of eukaryotic
organisms

DNA (deoxyribonucleic acid) = polymers of four different kinds of nucleotides

Genes = units of heredity information made of DNA
o have specific nucleotide sequences

Allele = alternate forms of a gene
Homologues (or “homologous chromosomes”) = a pair of chromosomes that have the
same genes, size, centromere position, and staining pattern

Humans have 46 chromosomes in their somatic (body) cells

The sex chromosomes may be different (XX v. XY)
Gametes (1n) are produced through MEIOSIS

when fused during fertilization, a zygote (2n) is formed
STEPS of MEIOSIS
Interphase

Chromosomes replicate

Each chromosome consists of two identical sister chromatids attached at a
centromere
MEIOSIS I
Prophase I

Nuclear envelope and nucleoli disappear

Spindle starts to form (and centrioles start to move apart)

Chromosomes condense

SYNAPSIS occurs
o Homologous chromosomes come together as pairs
o Form a tetrad (four chromosomes together)
o Nonsister chromatids are linked by X-shaped Chiasmata
o Crossing over may occur here = exchange of genetic material between
strands

Good because it alters the genetic makeup of the chromosome,
which means it increases genetic variation!
Metaphase I

Tetrads line up on the metaphase plate (instead of each c’some)

Spindle is complete
Anaphase I

Homologues separate and are dragged to opposite poles

Sister chromatids remain attached and move as a unit toward the same pole

AT THIS POINT, THE CELLS HAVE BECOME HAPLOID!!!
Telophase I and Cytokinesis

separate homologous c’some pairs reach the poles

each pole now has a haploid set of c’somes that are still made of two sister
chromatids

usually cytokinesis occurs simultaneously with telophase I
MEIOSIS II
Prophase II

new spindle forms
Metaphase II

chromosomes align in single file on the metaphase plate
Anaphase II

Sister chromatids are pulled apart and dragged to opposite poles in both cells
Telophase II and Cytokinesis

Nuclei form at opposite poles of cell

Cytokinesis occurs, producing 4 haploid daughter cells
Genetic Variation
Sexual life cycles produce genetic variation among offspring by:
1) Crossing Over = exchange of genetic material between homologues during
prophase I of meiosis.

Occurs when homologous portions of sister chromatids trade places

Form recombinants as a result

Produce chromosomes that have genes from both parents, resulting in an
all new combination!
2) Law of Independent Assortment = each homologous pair of chromosomes
separates independently of the other pairs around it during meiosis I.

Each pair consists of one paternal and one maternal chromosome
(doubled)

Therefore the possible combinations = 2n (where n = haploid #)
Ex. In humans: 2n = 46 so n = 23
Possible combinations for EACH sperm and EACH egg:
223 = 8,388,608 !!!
3) Random Fertilization
Ex. In humans: Which lucky egg meets which lucky sperm?
Asexual v. Sexual Reproduction
Asexual:
(1) is efficient in terms of time and resources
(2) produces genetically identical offspring
Sexual:
(1) is inefficient in terms of time and resources
(2) allows for great genetic variability
Human Meiosis:
Spermatogenesis = sperm formation

The process is the same as above, except after completing meiosis, a special
additional step occurs called flagellation = the addition of a tail to the sperm to
help it swim

Occurs in the testes
Oogenesis = egg (ovum) formation

the steps are the same, but the cytokinetic divisions are uneven.
o Meiosis I: occurs during embryonic development and produces 1 large
cell and one smaller one  “polar body”  discarded
o Meiosis II: This cell begins to divide, but stops at metaphase II unless
sperm enters the cell

produces one large cell and another polar body, which is also
discarded
o Result: 2 divisions results in one large haploid egg cell
o Advantage: Large cell will contain as much cytoplasmic material as
possible to give the zygote a good head start.
Other Life Cycles
PLANTS: Alternation of Generations = alternate between multicelluar haploid and
diploid generations

Sporophyte = multicellular diploid stage that produces 1n spores through
meiosis

Gametophyte = multicellular haploid stage that produces 1n gametes through
mitosis
o These fuse to form a new diploid zygote
FUNGI & SOME PROTISTS

Only diploid stage is the zygote which immediately goes through meiosis to
create a haploid individual
o This means they spend the majority of their lives as a haploid

Produces gametes by mitosis, which fuse to form the new zygote
Mitosis v. Meiosis (WILL DO IN CLASS TOGETHER)
MITOSIS
MEIOSIS
Original cell chromosome number
Number of Divisions
Number of Nuclei/New Cells Formed
New cell chromosome number
Synapsis of Chromosomes?
New cell compared to original cell
CHANGES IN CHROMOSOME NUMBER
Chromosomes can be viewed on a karyotype = a picture of an individual’s metaphase
chromosomes arranged in standard sequence (largest to smallest)
Nondisjunction = meiotic or mitotic error during which certain homologous
chromosomes or sister chromatids fail to separate

Results in one gamete receiving two of the same type of chromosome and another gamete
receiving no copy
Aneuploidy = condition of having an abnormal number of certain chromosomes

May result if a normal gamete unites with and abnormal one produced with
nondisjunction
o
If the aneuploid cell has a 3 copies of one chromosome = trisomy
o
If the aneuploid cell has a missing chromosome = monosomic for that
chromosome

o
In humans, monosomy in autosomes (non-sex c’somes) is lethal
Ex. Trisomy 21 (a.k.a. Down Syndrome)

Affects 1 in 700 U.S. children

Characteristic facial features, short stature, heart defects, mental
retardation, susceptibility to respiratory infections, and a proneness to
leukemia and Alzheimer’s disease. Shorter than average life spans.

Effects due to extra chromosome contribute larger doses of the proteins it
carries (150%)

Correlates with maternal age

may be due to the long time between the first meiotic division
and the completion of meiosis at fertilization

21% of cases are traced to father’s sperm
Barr Body = an inactivated X-chromosome found in female mammalian cells

Can result in patchy expression of genes
o Ex. Tortoiseshell cats
Aneuploidy of Sex Chromosomes

A single Y chromosome is needed to produce maleness.
o

It is really the presence of the SRY gene that matters (on Y chromosome)
Turner Syndrome: XO (the only known viable human monosomy)
o
Short stature; at puberty, secondary sex characteristics fail to develop; internal
sex organs do not mature; sterile; normal intelligence

Klinefelter Syndrome: XXY (but can be XXYY, XXXY, XXXXY, and XXXXXY)
o
Incidence: 1 in 500 to 1 in 1000 males
o
Have large hands and very long arms and legs; male sex organs with abnormally
small testes; sterile; may have feminine body contours and breast enlargement;
and normal intelligence


Triple X Syndrome: XXX (a.k.a. poly-X syndrome)
o
1 in 1500
o
Fertile; tall, thin, normal female phenotype
Jacobs Syndrome: XYY
o
1 in 1000
o
Normal male; usually taller than average; persistent acne; speech and reading
problems; and normal fertility
Alterations of Chromosome Structure

Chromosome breakage can occur as a result of radiation, certain organic
chemicals, or even viruses

Chromosome breakage can alter chromosome structure in four ways:
1. Deletion = a chromosome that has lost a fragment

These broken off fragments may be lost when the cell divides, or they may:
2. Join to a homologous chromosome = Duplication
3. Join to a nonhomologous chromosome = Translocation

These are often reciprocal, due to synapsis and crossing over with a
nonhomologous chromosome.
4. Reattach to the original chromosome in reverse order = Inversion
Ex. Williams Syndrome = deletion on chromosome 7

Pixie features: turned-up noses, wide mouths, small chin and ears; academic
learning difficulties, but excellent verbal and musical abilities; skin ages
prematurely and have cardiovascular problems; very social
Ex. Cri du Chat = deletion of chromosome 5

Mental retardation, small head with unusual facial features, and a cry that sounds like a
mewing cat
Translocation Syndromes
Ex. Alagille Syndrome = translocation between chromosomes 2 and 20

Distinctive face, abnormalities in eyes and internal organs, and severe itching
Ex. Chronic Myelogenous Leukemia = translocation between chromosomes 22 and 9
All of these conditions can be diagnosed before birth by fetal testing, but at this time they
CANNOT be treated.