Asexual Reproduction
Asexual Reproduction
 Single parent – complete copy of DNA
 Offspring is identical to parent
 Prokaryotes

Advantages:

Disadvantages:
Types of Asexual Reproduction
 Binary fission –
 Fragmentation –
 Budding –
 Parthenogenesis -
Sexual Reproduction
Sexual Reproduction
 Eukaryotes
 Two parents give genetic material to produce offspring
that are genetically different from parents
 Gamete –
 Zygote –
 Germ cell –
 Somatic cell –
 Advantages /disadvantages of sexual reproduction
Chromosome Number
 Each chromosomes contains thousands of genes that
control development and function of organisms.
 Humans have _____ (diploid)
 Mom gives ____ and Dad gives ____ (haploid)
 Chromosome reduction is MEIOSIS!
 Meiosis means reduction division – halves the number
of chromosomes
 Diploid – 2n (human 2n=46)
 Haploid - n (human n=23)
Homologous Chromosomes
 Homologous chromosomes – similar in size, shape,
and genes. A diploid cell contains pairs of
chromosomes made up of two homologous
chromosomes. One set comes from mom, and one set
comes from dad.
Autosomes and Sex Chromosomes
 Autosomes – DO NOT determine sex of individual
 Sex Chromosomes – DETERMINE sex of individual
 MALE – XY (Mom donates the X, father donates the Y)
 FEMALE – XX (Mom donates the X, Father donates the
other X)
Sex Chromosomes
XX chromosome XYfemale
chromosome 9
Karyotype (Human)
An organized picture
of the chromosomes
of a human arranged
in pairs by size from
largest to smallest.
Pairs 1-22 called
AUTOSOMES
Last pair are SEX
CHROMOSOMES
Male - XY
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Human Chromosomes
Karyotype
Female - XX
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Gamete Production
Interphase I
 Similar to mitosis interphase.
 CHROMOSOMES (DNA) replicate in the S phase
 Each duplicated chromosome consist of two
identical SISTER CHROMATIDS attached at their
CENTROMERES.
 CENTRIOLE pairs also replicate.
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Interphase I
Nucleus and nucleolus visible.
chromatin
cell
membrane
Nucleus
nucleolus
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Meiosis
 Similar in many ways to mitosis
 Several differences however
 Involves 2 cell divisions
 Results in 4 cells with 1/2 the normal genetic information.
 Vocabulary:
 Diploid (2N) - Normal amount of genetic material
 Haploid (N) - 1/2 the genetic material.
 Meiosis results in the formation of haploid cells.
 In Humans, these are the Ova (egg) and sperm.
 Ova are produced in the ovaries in females
 Process is called oogenesis
 Sperm are produced in the testes of males.
 Process is called spermatogenesis
 Meiosis occurs in 2 phases: Meiosis 1 & Meiosis 2
Meiosis I (four phases)
Cell division that reduces the
chromosome number by one-half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
Prophase I
d. Telophase I
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Prophase I
Longest and most complex
phase (90%).
Chromosomes condense.
Synapsis occurs - Homologous
chromosomes come together
to form a tetrad.
Tetrad is two chromosomes
or four chromatids (sister and
non-sister chromatids).
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Non-Sister ChromatidsHOMOLOGS
•Homologs contain DNA that
codes for the same genes ,
but different versions of
those genes
•Genes occur at the same
loci
19
Meiosis I.
 Prior to division, amount of DNA doubles
Overview of Meiosis
Prophase 1
 During prophase 1
homologous
chromosomes lineup along the
metaphase plate
 Areas of
homologous
chromosomes
connect at areas
called chiasmata
Crossing Over
 Segments of
homologous
chromosomes break
and reform at similar
locations.
 Results in new
genetic combinations
of offspring.
 This is the main
advantage of sexual
reproduction
Non-Sister ChromatidsHOMOLOGS
•Homologs contain DNA that
codes for the same genes ,
but different versions of
those genes
•Genes occur at the same
loci
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Genetic Recombination
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
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Metaphase I
 Shortest phase
 Tetrads align on the equator.
 Independent assortment occurs – chromosomes
separate randomly causing GENETIC
RECOMBINATION
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Metaphase I
OR
Homologs line up at
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Question:
In terms of Independent
Assortment -how many
different combinations of
sperm could a human male
produce?
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Answer
Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations
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Anaphase 1
 During anaphase 1, each homologous chromosome is pulled to opposite
sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT
BREAK.
 Nuclei may or may not reform following division.
 Cytokenesis may or may not occur.
Telophase I
Each pole now has haploid (1n) set of
chromosomes.
Cytokinesis occurs and two haploid
daughter cells are formed.
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Telophase I
cytokinesis
32
Meiosis II
 DNA does not double
 Chromosomes randomly line-up along metaphase plate
like regular mitosis.
 During anaphase 2, CENTROMERES BREAK and each
chromosome is pulled to opposite sides of the cell.
 Nuclei reform and cytokenesis usually occurs (although it
is often unequal).
Telophase I
Each pole now has haploid (1n) set of
chromosomes.
Cytokinesis occurs and two haploid
daughter cells are formed.
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Telophase I
cytokinesis
35
Meiosis II
 No Interphase II or very short
 **No DNA Replication
Remember: Meiosis II is similar to
mitosis
36
Prophase II
Same as Prophase in mitosis
 Nucleus
& nucleolus disappear
 Chromosomes condense
 Spindle forms
37
Metaphase II
 Same as Metaphase in mitosis
Chromosomes (not homologs) line
38
Anaphase II
Same as Anaphase in mitosis
**SISTER CHROMATIDS
separate
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Telophase II
Same as Telophase in mitosis.
Nuclei and Nucleoli reform, spindle
disappears
CYTOKINESIS occurs.
Remember: FOUR HAPLOID
DAUGHTER cells are produced.
Called GAMETES (eggs and sperm)
1n Sperm
cell
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Telophase II
41
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Variation
Also known as GENETIC RECOMBINATION
Important to population as the raw
material for NATURAL SELECTION.
All organisms are NOT alike
Strongest “most fit” survive to
reproduce & pass on traits
43
Question:
What are the 3 sources of
genetic recombination or
variation?
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Answer:
1. CROSSING OVER (prophase I)
2. INDEPENDENT ASSORTMENT
(metaphase I)
3. RANDOM FERTILIZATION
45
Question:
A cell containing 20 chromosomes
(diploid) at the beginning of meiosis
would, at its completion, produce
cells containing how many
chromosomes?
46
Answer:
10 chromosomes (haploid or
1n)
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Diploid
Haploid
Meiosis and Gamete formation
 Oogenesis
 Ovum and 3 polar bodies
 Un-equal distribution of the cytoplasm
 Spermatogenesis
 4 sperm
Meiosis
Sex cells divide to produce GAMETES
(sperm or egg).
Gametes have HALF the # of
chromosomes.
Occurs only in GONADS (testes or
ovaries).
 Male: SPERMATOGENESIS -sperm
 Female: OOGENESIS - egg or ova
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Spermatogenesis
human
sex cell
n=23
n=23
n=23
sperm
2n=46
n=23
diploid (2n)
n=23
haploid (n
n=23
Meiosis IMeiosis II
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Oogenesis
human
sex cell
Haploid
egg (1n)
n=23
n=23
2n=46
Pol
ar
n=23
diploid (2n)
Bod
ies
(die
Meiosis IMeiosis II
52
Spermatogenesis & Oogenesis
4 sperm
Egg/Ova
 University of Arizona Biology Page
 http://www.biology.arizona.edu/cell_bio/tutorials/me
iosis/main.html
 Cell’s Alive
 http://www.cellsalive.com/meiosis.htm
Karyotype
Female - XX
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Fertilization
The fusion of a sperm and egg to
form a zygote.
A zygote is a FERTILIZED EGG
n=23
egg
sperm
n=23
2n=46
zygote
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Question:
A cell containing 40 chromatids at
the beginning of meiosis would, at
its completion, produce cells
containing how many
chromosomes?
(Tricky! Think!)
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Answer:
10 chromosomes
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Multicellular Life Cycles
LIFE CYCLE – events in the growth and development of
an organism until it reaches sexual maturity.
Most animals have a DIPLOID LIFE CYCLE
During meiosis, germ cells undergo meiosis to produce
haploid gametes.
Fertilization – joining of 2 haploid cells to form a zygote
which then undergoes mitosis to grow and develop
into a new organism.
Haploid Life Cycle
Alternation of Generations