Sperm & Eggs & Variation..OH MY!
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What
if a new
individual was formed
through mitosis?
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 allele
 gamete
 amniocentesis
 genome
 asexual
 haploid
reproduction
 autosome
 binary fission
 chorionic villi
sampling
 crossing over
 diploid
 exon
 fertilization
 homologues
 intron
 karyotype
 nondisjunction
 placenta
 reduction
division
 sexual reproduction
 zygote
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 Genome:
Complete complement of an
organism’s DNA.

includes genes (control traits) and non-coding DNA
organized in chromosomes
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 Eukaryotic DNA is organized in chromosomes
 genes have specific places on chromosomes
 exon: portion of a gene that is translated into

protein
intron: non-coding segment of DNA, often found
within an exon; removed before transcription
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 Heredity
– way of transferring genetic
information to offspring
 Chromosome theory of heredity:
chromosomes carry genes
 Gene – “unit of heredity”
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
Asexual



splitting
budding
parthenogenesis (egg develops w/o fertilization)

occurs naturally in some invertebrate animal species (e.g., water
fleas, aphids, nematodes, some bees, some scorpion species,
and parasitic wasps) and a few vertebrates (e.g.,
some fish, amphibians, reptiles, and very rarely birds)
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 Fusion
of two gametes to produce a single
zygote.
 Introduces greater genetic variation, allows
genetic recombination.
 With exception of self-fertilizing organisms
(e.g. some plants), zygote has gametes from
two different parents.
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 Karyotype:



ordered display of an individual’s chromosomes
collection of chromosomes from mitotic cells
staining can reveal visible band patterns, gross
anomalies
 Make
a Karyotype
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 Obtain
some cells from the individual
 Culture them in a test tube with nutrients
 Treat cells w/chemical that stops them
exactly midway through cell division
(chromosomes are coiled thickly and more
visible than usual)
 Cells are placed on a microscope slide and a
stain is added that binds to the
chromosomes, making them visible
 Chromosomes are arranged by size and shape
and displayed on a monitor or in a
photograph
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 Chromosomes
exist in homologous pairs in
diploid (2n) cells.
 One chromosome of each homologous pair
comes from the mother (called a maternal
chromosome) and one comes from the
father (paternal chromsosome).
 Homologous chromosomes are similar but
not identical. Each carries the same genes
in the same order, but
the alleles (alternative form of a gene)
for each trait may not be the same.
 Exception: sex chromosomes (X, Y)
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 23
chromosomes donated by each parent
(total = 46 or 23 pairs).
 Gametes (sperm/ova):


contain 22 autosomes and 1 sex chromosome
haploid (haploid number “n” = 23 in humans)
 Fertilization
results in zygote with 2 sets
of chromosomes - now diploid (2n).
 Most cells in the body produced by
mitosis.
 Only gametes are produced by meiosis.
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All are even
numbers –
diploid (2n) sets
of homologous
chromosomes.
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



Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
 synapsis


homologous chromosomes pair up
chiasmata form (crossing over of non-sister chromatids)
metaphase I: homologous pairs line up at metaphase
plate
 anaphase I: sister chromatids do NOT separate
 overall, separation of homologous pairs of
chromosomes, rather than sister chromatids of
individual chromosome

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 Prophase
1
each chromosome duplicates and remains
closely associated (sister chromatids)
 crossing-over can occur during the latter part
of this stage

 Metaphase

1
homologous chromosomes align at the
equatorial plate
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 Anaphase

homologous pairs separate with sister
chromatids remaining together
 Telophase

1
1
two daughter cells are formed with each
daughter containing only one chromosome of
the homologous pair
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Second division of meiosis: Gamete formation
 Prophase 2

DNA does not replicate
 Metaphase

2
chromosomes align at the equatorial plate
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 Anaphase
2
centromeres divide
 sister chromatids migrate separately to each
pole

 Telophase
2
cell division is complete
 4 haploid daughter cells

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 During
normal cell growth, mitosis
produces daughter cells identical to
parent cell (2n to 2n)
 Meiosis results in genetic variation by
shuffling of maternal and paternal
chromosomes and crossing over
no daughter cells formed during meiosis are
genetically identical to either mother or
father
 during sexual reproduction, fusion of the
unique haploid gametes produces truly unique
offspring

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Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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Chiasmata – sites of crossing over, occur
in synapsis. Exchange of genetic
material between non-sister chromatids.
Crossing over produces recombinant
chromosomes.
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•Females don’t have a
Y chromosome in any
of their cells, yet they
are able to develop
and live normal,
healthy lives.
•For this reason, we
know that nothing on
the Y chromosome is
absolutely necessary.
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At least 8 million combinations from Mom, and
another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!
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 Life
cycle = sequence of stages in
organisms reproductive history;
conception to reproduction
 Somatic cells = any cell other
than gametes, most of the cells in
the body
 Gametes produced by meiosis
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Generalized animal life cycle
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Unequal distribution of
chromosomes during
meiosis
Resulting gametes zero or
two copies of a
chromosome instead of a
single copy
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 Large
amounts of energy required to find a
mate and do the mating: specialized
structures and behavior required
 Intimate contact provides route for
infection by parasites (AIDS, syphilis, etc.)
 Genetic costs: in sex, we pass on only half
of genes to offspring.
 Males are an expensive luxury - in most
species they contribute little to rearing
offspring.
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More
genetic diversity: more
potential for survival of species
when environmental conditions
change.
 shuffling
of genes in meiosis
 crossing-over in meiosis
 fertilization: combines genes from 2
separate individuals
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 In
what cellular processes is mitosis
involved? In what cellular processes is
meiosis involved?
 In what type of cells does mitosis occur? In
what type of cells does meiosis occur?
 How many times does DNA replicate in
mitosis? How many times does DNA
replicate in meiosis?
 How many cellular divisions occur in
mitosis? How many cellular divisions occur
in meiosis?
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How many daughter cells are formed by mitosis?
How many daughter cells are formed by meiosis?
 What is the chromosome number in daughter
cells formed by mitosis from diploid parent cells?
What is the chromosome number in daughter
cells formed by meiosis from diploid parent
cells?
 In mitosis, are daughter cells identical to or
different from parent cells? In meiosis, are
daughter cells identical or different from parent
cells?
 In mitosis, when do synapsis and crossing over
occur? In meiosis, when do synapsis and crossing
over occur?

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 cchs.churchill.k12.nv.us/marshk/Notes/meio
sis.ppt
 Genetic Science Learning Center, University
of Utah, http://learn.genetics.utah.edu
 http://www.phschool.com/science/biology_
place/labbench/lab3/concepts2.html
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