Reproduction Ch.7
Asexual Reproduction
• One parent
• The prokaryotic chromosome is a ring of DNA
• Binary fission-bacteria
– Cell splits in two
– Produces two genetically identical daughter cells, exactly
like the original
– Identical offspring (clones)
• Different kinds
– Binary fission - bacteria
– Budding – yeast, sponges
– Fragmentation – sea stars
Binary Fission
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chromosome
1. Attachment of chromosome to
a special plasma membrane
site indicates that this
bacterium is about to divide.
cell wall
plasma
membrane
cytoplasm
2. The cell is preparing for binary
fission by enlarging its cell wall,
plasma membrane, and overall
volume.
3. DNA replication has produced
two identical chromosomes.
Cell wall and plasma membrane
begin to grow inward.
4. As the cell elongates, the
chromosomes are pulled apart.
Cytoplasm is being distributed
evenly.
5. New cell wall and plasma
membrane has divided the
daughter cells.
© Dennis Kunkel Microscopy, Inc./Visuals Unlimited
SEM 2,345X
Sexual Reproduction
• Two parents
• Sexual Reproduction
– Cell splits twice
– Produces four daughter cells,
genetically different from the
parent and each other –
meiosis
• Increases genetic variation
Haploid vs. Diploid
HAPLOID
• 1n
DIPLOID
• 2n
• One copy of each
chromosome
• Two copies of each
chromosome
• Homologous
chromosomes-similar in
size, shape and genetic
content
Homologous Chromosomes
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•Homologous
chromosomes-similar
in size, shape and
genetic content
Sister chromatids
a.
duplication
nonsister
chromatids
duplication
kinetochore
centromere
chromosome
homologous pair
paternal chromosome
b.
chromosome
maternal chromosome
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a: © L. Willatt/Photo Researchers, Inc.
How does Meiosis work?
• Two stages, Meiosis I and Meiosis II
• Meiosis I: Homologous chromosomes are separated
from each other
– Chromosomes replicate first, so each chromosome has a
“twin”
– Example: this is where the X and Y chromosomes separate
from each other and go into different cells
• X chromosome is a double X, Y chromosome is a double Y
– Two new cells have 23 duplicated chromosomes
• Crossing over-during Prophase I
– Homologous chromosomes exchange genes while
they’re lined up next to each other
– You don’t inherit entire chromosomes from your
parents—only pieces of them
– Creates more variation and diversity within a
species
– Important for natural selection and evolution
2. Crossing Over
• Homologous chromosomes
swap genes
• Happens during Prophase I
• Increases genetic variation
even more
• Meiosis II
– Just like mitosis, but with 23 chromosomes
instead of 46
• Remember these are 23 duplicated chromosomes
– Starts with 2 cells, which divide into 4
– In males: all 4 cells become sperm
– In females: one cell becomes an egg, the other 3
die
What’s a gamete?
• Female gamete = egg (ovum); produced in
ovaries; oogenesis
• Male gamete = sperm; produced in testes;
spermatogenesis
Overview of Meiosis
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• Diploid parent cell
2n = 4
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Overview of Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
centrioles
nucleolus
centromere
chromosome
duplication
2n = 4
2n = 4
MEIOSISI
Homologous pairs
synapse and then separate.
15
Overview of Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
First division
centrioles
nucleolus
centromere
Second division
Four haploid
daughter cells
sister chromatids
synapsis
chromosome
duplication
2n = 4
2n = 4
MEIOSISI
Homologous pairs
synapse and then separate.
n=2
n=2
MEIOSISII
Sister chromatids separate,
becoming daughter chromosomes.
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Comparison of Mitosis & Meiosis
Mitosis
Results in 2 daughter
cells
Daughter cells are
diploid
Daughter cells are
identical to each
other and to parent
Meiosis
Results in 4
daughter cells
Daughter cells are
haploid
Daughter cells are
different from
each other and
from parent
Mitosis v. Meiosis
Mechanisms for Increasing Genetic
Variation
1. Independent Assortment
Each of the 23 pair of chromosomes separates into
gametes independently (223different gametes can
form=8 million possible gamete combinations)
2. Crossing Over
• Homologous chromosomes
swap genes
• Happens during Prophase I
• Increases genetic variation
even more
3. Random
Fertilization
For humans - 23 pairs of chromosomes:
Fertilization of an egg by a sperm is
random
# possible outcomes 223 (egg) X
223(sperm)= 64 trillion
Determination of Sex
•Females: two X chromosomes (XX)
•Males: XY
•All offspring will receive an X chromosome
from the mother and either an X or a Y from
the father
•The X chromosome is large and contains
over 1,000 genes
•The Y chromosome is tiny and contains
less than 200 genes