Reproduction Ch.7
Jan 05, 2016
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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1. Attachment of chromosome toa special plasma membranesite indicates that thisbacterium is about to divide.
2. The cell is preparing for binaryfission by enlarging its cell wall,plasma membrane, and overallvolume.
3. DNA replication has producedtwo identical chromosomes.Cell wall and plasma membrane begin to grow inward.
4. As the cell elongates, thechromosomes are pulled apart.Cytoplasm is being distributedevenly.
5. New cell wall and plasmamembrane has divided thedaughter cells.
chromosome
cell wall
plasmamembrane
cytoplasm
SEM 2,345X© Dennis Kunkel Microscopy, Inc./Visuals Unlimited
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
• One copy of each chromosome
DIPLOID• 2n
• Two copies of each chromosome
• Homologous chromosomes-similar in size, shape and genetic content
Homologous Chromosomes
6
a. Sister chromatids
duplicationduplication
chromosomematernal chromosome
b.
paternal chromosomehomologous pair
nonsisterchromatids
centromere
kinetochore
chromosome
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a: © L. Willatt/Photo Researchers, Inc.
• Homologous chromosomes-similar in size, shape and genetic content
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
14
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2n = 4
• Diploid parent cell
Overview of Meiosis
15
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centrioles nucleoluscentromere
MEIOSISIHomologous pairs
synapse and then separate.
2n = 4 2n = 4
chromosomeduplication
Overview of Meiosis
16
centrioles nucleoluscentromere
sister chromatids
MEIOSISIHomologous pairs
synapse and then separate.
MEIOSISIISister chromatids separate,
becoming daughter chromosomes.
n = 2
Four haploiddaughter cells
Second divisionFirst division
synapsis
2n = 4 2n = 4
chromosomeduplication
n = 2
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Comparison of Mitosis & Meiosis
MitosisResults in 2 daughter
cellsDaughter cells are
diploidDaughter cells are
identical to each other and to parent
MeiosisResults in 4
daughter cellsDaughter cells are
haploidDaughter cells are
different from each other and from parent
Mitosis v. Meiosis
Mechanisms for Increasing Genetic Variation
1. Independent AssortmentEach 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