Chapter 11:
Meiosis and sexual
reproduction
Sexual vs. Asexual Reproduction
Sexual:
•2 parents
•Offspring are
genetically different
from parent – mix of
traits
•Union of egg &
sperm
Asexual:
•1 parent
•Offspring are
identical to parent
•Binary fission in
bacteria
•Regeneration in
worms
•In sexual reproduction, each parent
produces a reproductive cell called a
gamete
•Male gamete – sperm
•Female gamete – egg
•Zygote – formed when sperm fertilizes
egg
•Fertilization – process in which sperm
and egg fuse together
Examples of asexual reproduction:
•Fragmentation – body breaks into several
pieces, each of which regrows into an adult
•Budding – new individuals split off from
existing ones
•Parthenogenesis – a female makes a viable
egg that grows into an adult without being
fertilized by a male
•Binary fission – a type of cell division used
by prokaryotic cells
Advantage of sexual reproduction:
•Produces offspring that are different from
their parents so they have a better chance of
surviving changes in the environment,
diseases, etc.
Advantages of asexual reproduction:
•Simple and efficient
•No need to find a mate
•May produce many offspring in a
relatively short period of time
Somatic cells – body cells such as
skin, hair, muscle, bones, or organs
that do no participate in sexual
reproduction
Germ cells – cells that are
specialized for sexual reproduction,
such as eggs or sperm
•Each species has a set number of
chromosomes:
•Humans – 46
•Crayfish – 200
•Corn – 20
•Adder’s tongue fern – 1262
•Chimpanzee - 48
•Sand dollar – 52
•Dog – 78
•Cat - 32
Viewing the Chromosomes
• Karyotype - a photograph of the
chromosomes in a dividing cell
• Shows the gender of the child
• Shows abnormalities in chromosome
number or structure.
• Humans have 23 pairs of homologous
chromosomes (chromosomes with the same
genes; similar in size and shape), or 46
chromosomes total.
• You get 1 of each chromosome from mom,
and 1 from dad.
•Of the 23 pairs of chromosomes:
•1 pair are sex chromosomes –
determine your gender; may determine
other traits as well
•Females are XX
•Males are XY
•22 pairs are autosomes – all other
pairs of chromosomes; do NOT
determine gender
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How is karyotyping done?
Cells are taken from the fetus of a
pregnant woman
Chemicals added to the cells make them
divide
Another chemical stops division at the
midpoint
The stained cells are photographed and
can be paired based on cross-bands, size,
and shape.
Human karyotype preparation
Normal male karyotype
Down syndrome karyotype
•Body cells contain the diploid (2n)
number of chromosomes – contain 2
chromosomes of each kind (1 from each
parent)
•Sex cells (eggs and sperm) contain only
1 chromosome of each kind – haploid
(1n) number of chromosomes
Meiosis – form of cell division tha produces
daughter cells with half the number of
chromosomes that are in the parent
• Produces sex cells (gametes)
• Female gamete – egg
• Male gamete - sperm
• Reduces the chromosome number
• Egg or sperm cells each have only one of
each kind of chromosome
• Diploid cells (2n)  haploid cells (1n)
Overview of meiosis
http://www.cellsalive.com/meiosis.htm
• Meiosis involves two cell divisions
(meiosis I and II)
• Produces four haploid cells.
Meiosis I
•Original cell produces 2 new cells
•Splits homologous chromosomes so that
each new cell has 1 chromosome from
each homologous pair
•Diploid cells (2n) create haploid cells
(1n)
•Crossing over (chromosomes exchange
genetic material) can occur
Meiosis II
•Each of the 2 cells made in meiosis I
splits
•2 haploid cells divide to make 4
haploid cells
•Chromatids of chromosomes are
separated
•Errors in meiosis can result in missing
or extra chromosomes
Changes in Chromosome Number
• Occurs when:
– In meiosis I, homologous pair both go into
the same daughter cell or
– In meiosis II, the sister chromatids both go
into the same gamete.
• The result is either:
– Trisomy (3 copies of a single chromosome)
– Monosomy (1 copy of a single
chromosome)
Nondisjunction in meiosis I
Nondisjunction in meiosis II
Types of chromosomal mutations:
•Deletion – a piece of a chromosome is lost
•Duplication – a chromosome with a
repeated section attached
•Inversion –a section of chromosome is
flipped in the reverse direction
•Translocation – a chromosome piece ends
up on another non-homologous chromosome
•Gene rearrangement – an entire gene moves
to a different location on the same
chromosome
• Ex. of deletion: Williams syndrome
• Chromosome 7 loses an end piece
• Children have a pixie look (turned-up
noses, wide mouth, small chin, large
ears)
• Poor academic skills, good verbal and
musical abilities
• Skin ages prematurely from lack of the
gene that controls the production of
elastin (also affects cardiovascular
health).
• Ex of duplication: An inverted
duplication in chromosome 15 causes
inv dup 15 syndrome
• Poor muscle tone, mental
retardation, seizures, curved spine,
and autistic characteristics
Duplication
• Ex of translocation: Alagille syndrome
results from a deletion of chromosome
20 or a translocation that disrupts an
allele on chromosome 20.
• The symptoms for Alagille syndrome
range from mild to severe, so people may
not be aware they have the syndrome.
• Distinctive face, abnormalities of eyes &
internal organs, and severe itching.
Translocation
Meiosis in males vs. females:
•Males:
•Occurs in testes
•Spermatogenesis
•Makes 4 haploid sperm
•Females:
•Occurs in ovaries
•Oogenesis
•Makes 1 large egg & 3 polar bodies
(that die) – cells divide unevenly
Spermatogenesis
Oogenesis
Meiosis vs. Mitosis
Mitosis
•Body cells
•Diploid cells make
diploid cells
•End result 2 cells
•No variation in
cells produced
Meiosis
•Sex cells
•Diploid cells make
haploid cells
•End result up to 4
cells
•Cells made are
different from parent
Meiosis compared to mitosis
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The Human Life Cycle
Requires both mitosis and meiosis.
In males, meiosis occurs as spermatogenesis and
produces 4 haploid sperm.
In females, meiosis occurs as oogenesis and
produces 1 egg cell.
Mitosis is involved in the growth of a child and
repair of tissues during life.
Cell differentiation occurs at many times during
development as cells take on specific roles
Life cycle of humans