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CHAPTER 13
MEIOSIS AND SEXUAL LIFE
CYCLES
INTRODUCTION TO HEREDITY
HEREDITY- transmission of traits from one
generation to the next
GENETICS- the scientific study of heredity
and variation
Parents pass hereditary information to their
offspring in the form of GENES
- the tens of thousands of genes that we
inherit from our fathers and mothers
makes up our GENOME
- it is this genetic link that accounts for
family resemblance
Genes are segments of DNA:
- inherited information is passed on in the
form of each gene’s specific sequence of
nucleotides
- the cell translates these “genetic
sentences” into features
How is heredity possible?
- the transmission of hereditary traits
depends on the precise replication of DNA
- this produces copies of genes that
can be passed on from parents to
offspring
Every living species has a characteristic
number of chromosomes
- humans have 46 chromosomes in almost
all of their cells
- each chromosome contains thousands of
genes
- a gene’s specific location on a
chromosome is called its LOCUS
ASEXUAL VS. SEXUAL
REPRODUCTION
ASEXUAL REPRODUCTION
- a single individual is the SOLE parent
and passes all of its genes to its
offspring
- some single-celled eukaryotes can reproduce
asexually by mitotic cell division
- some multicellular organisms (such as
the Hydra) can reproduce asexually
- binary fission is a form of asexual
reproduction
- OFFSPRING ARE EXACT COPIES OF
THE PARENT
SEXUAL REPRODUCTION
- two parents give rise to offspring that have
unique combinations of genes inherited
from the two parents
- results in greater genetic variation
- “like begets like” only in the sense of
family resemblance
MEIOSIS AND SEXUAL LIFE
CYCLES
LIFE CYCLE- generation-to-generation
sequence of stages in the reproductive
history of an organism (from conception to
the production of its own offspring)
Human Life Cycle
SOMATIC CELL- any cell other than a sperm
or egg
- in humans, each somatic cell has 46
chromosomes
- a KARYOTYPE is a picture of an organism’s
chromosomes
- when looking at a human karyotype,
you can see that there are 2 of each
type of chromosome
- these are called HOMOLOGOUS
CHROMOSOMES  same length,
centromere position, and staining pattern
- the 2 chromosomes of each pair carry
genes controlling the same inherited
characteristics
X and Y chromosomes are an exception to the
rule of homologous chromosomes:
- human females are XX
- human males are XY
- only small parts of the X and Y are
homologous (the Y is much smaller)
- these are called SEX CHROMOSOMES
because they determine an individual’s
sex
- all other chromosomes are called AUTOSOMES
The 46 chromosomes in our somatic cells are a
result of the combination of chromosomes
from our father and mother
- we inherit 23 chromosomes from each parent
Sperm cells and ova are called GAMETES
- each of these cells has 22 autosomes plus 2
sex chromosomes
- these are called HAPLOID CELLS because
they have a single chromosome set
- abbreviated n (for humans, n = 23)
By means of sexual reproduction, a haploid
sperm cell from the father fuses with a
haploid ovum of the mother
- this is called FERTILIZATION or
SYNGAMY
- the result is the fertilized egg or ZYGOTE
- the zygote and all other cells having 2
sets of chromosomes are DIPLOID
(2n)
As a human develops from a zygote to a sexually
mature adult, genes are passed on to all
somatic cells of the body by the process of
mitosis
- the only cells NOT produced by mitosis are
the gametes, which develop in the gonads
- sexually reproducing organisms must carry out
a process that halves the chromosome number
in the gametes (compensating for fertilization)
MEIOSIS- form of cell division that occurs
only in the ovaries or testes
- mitosis conserves chromosome
number and meiosis reduces
chromosome number
OTHER SEXUAL LIFE CYCLES
There are 3 main types of life cycles:
1. Animals (including humans)
- gametes are the only haploid cells
- meiosis occurs during production of
gametes; no other cell division takes place
before fertilization
- diploid zygote divides by mitosis,
producing a diploid, multicellular
organism
2. Fungi and algae
- gametes fuse to form zygote, and then
meiosis occurs before the offspring
develop
- this produces haploid cells that divide by
mitosis to give rise to a haploid multicellular
adult organism
- the haploid organism produces gametes by
mitoisis
- only diploid stage is zygote
3. Plants and some algae
- called ALTERNATION OF GENERATIONS
- includes both haploid and diploid multicellular
stages
- multicellular diploid stage is called the
SPOROPHYTE
- meiosis produces haploid cells called SPORES
- a spore gives rise to a multicellular
individual without fusing with
another cell
- a spore divides mitotically to generate a
multicellular haploid stage called the
GAMETOPHYTE
- the gametophyte makes gametes by
mitosis
- fertilization produces a diploid zygote,
which becomes the next sporophyte
CLOSER LOOK AT MEIOSIS
Meiosis, like mitosis, is preceded by the
replication of chromosomes
- this SINGLE replication is followed by two
consecutive cell divisions called MEIOSIS I
and MEIOSIS II
- these divisions result in 4 haploid
daughter cells (half as many
chromosomes as parent cell)
INTERPHASE
- chromosomes replicate
- for each chromosome, the result is 2
genetically identical sister chromatids
attached at their centromeres
- centrosomes also replicate
PROPHASE I
- longer and more complex than prophase in
mitosis
- chromosomes begin to condense, and
homologous chromosomes pair up 
called SYNAPSIS
- in synapsis, a protein structure attaches
the homologous chromosomes tightly
together all along their lengths
- the pair of homologous chromosomes is
known as a TETRAD- a cluster of 4
chromatids
- at various places along their length,
chromatids are crisscrossed  called
CHIASMATA
- the chromosomes can trade segments at
the chiasmata
- centrosomes move away from each other
and spindle microtubules form between
them
- nucleoli and nuclear envelope
disappear
- spindle microtubules capture the
kinetochores and chromosomes begin
moving toward metaphase plate
- Prophase I takes up about 90% of
the time required for meiosis
METAPHASE I
- chromosomes are arranged on the
metaphase plate, STILL IN HOMOLOGOUS
PAIRS
ANAPHASE I
- spindle apparatus guides the movement of
the chromosomes toward the poles
- sister chromatids remain attached at
their centromeres
- homologous chromosomes move toward
opposite poles of the cell
TELOPHASE I AND CYTOKINESIS
- each pole of the cell has a haploid
chromosome set, but each chromosome
still has 2 sister chromatids
- cleavage furrows form in animal cells
and cell plates appear in plant cells
PROPHASE II
- a spindle apparatus forms and
chromosomes progress toward the
metaphase plate
METAPHASE II
- chromosomes are positioned on
metaphase plate (now very similar to
mitosis)
ANAPHASE II
- centromeres of sister chromatids
finally separate
- sister chromatids of each pair (now
individual chromosomes) move toward
opposite poles of the cell
TELOPHASE II AND CYTOKINESIS
- nuclei form at opposite poles of the cell
and cytokinesis occurs
- there are now 4 HAPLOID DAUGHTER
CELLS
MITOSIS VS. MEIOSIS
MITOSIS
- DNA replication occurs during
interphase before nuclear division
- there is 1 division, including prophase,
metaphase, anaphase, and telophase
- synapsis does NOT occur
- 2 diploid daughter cells are produced
that are GENETICALLY IDENTICAL to
parent
- role in the body: enables multicellular
adult to arise from zygote; produces cells
for growth and repair
MEIOSIS
- DNA replication occurs only once,
during interphase before meiosis I
- there are 2 divisions, EACH including
prophase, metaphase, anaphase, and
telophase
- synapsis does occur; crossing over is
associated with synapsis
- 4 haploid cells are formed, each having
half as many chromsomes as the parent
cell
- role in body: produces gametes; reduces
chromosome number by half and
introduces genetic variability
GENETIC VARIATION
The following are responsible for most
genetic variation in SEXUALLY
REPRODUCING organisms:
Independent assortment of
chromosomes
Crossing over
Random fertilization
INDEPENDENT ASSORTMENT
During metaphase, the orientations of
homologous pairs of chromosomes relative
to the poles of the cell are random
- each gamete represents one outcome
of all possible combinations of
maternal and paternal chromosomes
- the number of combinations possible for
gametes formed by meiosis starting with 2
homologous pairs of chromosomes is 4
In general, the number of combinations
possible when chromosomes assort
independently into gametes is 2n
- n is the haploid number of the organism
- in humans, n = 23 so 223 = about 8
million
- the number of possible combinations of
maternal and paternal chromosomes in
the resulting gametes is about 8 million
- each gamete that a human produces
contains about one of 8 million possible
assortments of chromosomes inherited
from the father and mother
CROSSING OVER
Crossing over produces RECOMBINANT
CHROMOSOMES, which combine genes
inherited from our 2 parents
- crossing over begins very early in
prophase I
- homologous portions of 2 nonsister
chromatids trade places
- at metaphase II, chromosomes containing
recombinant chromatids can be oriented in
2 alternative ways
- the independent assortment of these
nonidentical sister chromatids increases
the genetic variability in gametes
RANDOM FERTILIZATION
A human ovum representing one of about 8
million possible chromosome combinations
is fertilized by a single sperm cell
representing one of 8 million possible
combinations
- even without crossing over, a zygote
is produced with any of 64 trillion
combinations
EVOLUTION AND VARIATION
Darwin recognized the importance of
genetic variation in natural selection:
- individuals best suited to their environment
leave the most offspring, passing on their
genes to them
- this natural selection results in
adaptation, the accumulation of
favorable genetic variations
- different genetic variations may work
better in old environments than new