Remember! Homologous Chromosomes
• Paired chromosomes; both chromosomes in a pair
carry the same genes
– Control the same inherited characteristics
– Homologous – same information
Diploid (2n)
2n=4
1 copy from mom
& 1 copy from dad!
Human cells are
2n = 46!
(23 from mom &
23 from dad)
Cell Division & Asexual Reproduction
• Mitosis produces cells with the same information!
– Identical daughter cells
– Clones
– Same amount of DNA
Asexual Reproduction
• Binary Fission
– Prokaryotes (bacteria)
• Budding
– Mostly single-celled eukaryotes (yeasts, amoebas, etc.)
What about us?
• What if a complex, multicellular organism (like us) wants
to reproduce?
– Joining of egg & sperm
Are egg & sperm cells made by mitosis?
If they were, then…
46
46
Egg
Sperm
92
Zygote
It doesn’t work!
So, how do we make sperm & egg cells?
• Must reduce the number of chromosomes in the cell
from 46 chromosomes  23 chromosomes!
– Must reduce the number of chromosomes by half
Egg
Zygote
Sperm
Fertilization
Gametogenesis – Gamete Formation
• In male animals, the haploid gametes are called sperm.
• In the female animals, the haploid gametes are called
eggs.
• The process in which male & female gametes join is called
fertilization.
– This produces a fertilized egg called a zygote!
Meiosis & Sexual Reproduction
Meiosis
• The process that divides 1
diploid (2n) cell to form 4
haploid (n) cells
• How gametes (sex cells) are
formed
• Meiosis has 2 divisions:
Meiosis I and Meiosis II
Meiosis – Production of Gametes (Sperm & Egg)
• Chromosome number
must be reduced
– Diploid  Haploid
– 2n  n
– In humans, 46  23
• Fertilization restores the
chromosome number
– Haploid  Diploid
– n  2n
Sexual Reproduction Life Cycle
Fertilization
 2 copies
 Diploid (2n)
Meiosis
 1 copy
 Haploid (n)
Gametes
 1 copy
 Haploid (n)
Gametes
Meiosis
• Before meiosis begins, cells go through interphase!
• DNA Replication occurs in interphase, forming a
duplicate copy of each chromosome.
Meiosis
• After DNA Replication, each chromosome is made
of 2 sister chromatids!
Meiosis – 2 Cell Divisions
• Meiosis I
– Reduces Diploid (2n)  Haploid (n)
– Separates homologous pairs
• Meiosis II
– Sperm & egg formed
– Separates sister chromatids
Warning!
Meiosis evolved from mitosis, so
the stages & “machinery” are
similar, but the processes are
different! Do NOT confuse them!
Overview of Meiosis (I.P.M.A.T.P.M.A.T.)
2n = 4
1 copy of each
homologous pair
Overview
of Meiosis I
2n = 4
Now sister chromatids
Reduction
1n = 2
Homologous pairs
separated, sister
chromatids remain
Meiosis I – similar to mitosis!
Prophase I
1. Centrioles move to opposite sides of the nucleus
2. Nuclear envelope breaks down.
Prophase I
3. Homologous chromosomes pair up. This is different
than mitosis!
– Each pair of chromosomes align & a tetrad is formed.
Prophase I
– Chromosomes exchange portions of their chromatids in a
process called crossing over.
– Crossing over produces new combinations of DNA!
Here is a tetrad formed from 2 sets
of sister chromatids.
Gene A – Height (“A” is tall & “a” is
short)
Gene B – Hair Color (“B” is brown
hair & “b” is blonde)
The gametes produced can
carry several varieties of
genes; they’re not identical to
the parent cell!
This person can pass on tall/brown traits, tall/blonde traits,
short/brown traits, or short/blonde traits!
Metaphase I
• Spindle fibers attach to the chromosomes at the
centromere.
Anaphase I
• Spindle fibers pull the homologous chromosomes
toward opposite ends of the cell.
• Chromosomes move & separate into 2 groups near the
spindle.
• Anaphase ends when the chromosomes stop moving.
Telophase I
• Nuclear membranes form around chromosomes.
Remember! Cytokinesis is when the cytoplasm divides!
Meiosis I End Results
• Produces 2 haploid (n) daughter cells that have only
1 set of each chromosome
• Each set is made of 2 sister chromatids.
Meiosis I
Meiosis II
• After Meiosis I, cells
enter Meiosis II:
– No DNA Replication
occurs between Meiosis I
& Meiosis II.
– Meiosis II separates the
sister chromatids.
Prophase II
• Spindle fibers
appear.
Metaphase II
• Chromosomes align like
they do in mitosis & attach
to the spindle at the
centromere.
Anaphase II
• Sister chromatids
separate & move
towards opposite ends
of the cell.
Telophase II
• Nuclear membranes form
around chromosomes.
• Then, cytokinesis!
Meiosis II End Results
• Produces 4 haploid (n) daughter cells
What are the advantages to sexual reproduction?
• Increased diversity
– Independent assortment of chromosomes: random
alignment of homologous chromosomes in Metaphase 1
– Crossing over: mixing of alleles across homologous
chromosomes
– Random fertilization – which sperm fertilizes which egg?
• Driving evolution
– Providing variation for natural selection
Variation from Independent Assortment of Chromosomes
Random assortment in humans produces 223 (8,388,608)
different combinations in gametes!
Variation from Crossing Over
• Creates completely new combinations of traits on
each chromosome
Variation from Random Fertilization
• Sperm + Egg = ?
– Any 2 parents will produce a zygote with over 70 trillion
possible diploid combinations!
Sexual reproduction allows us to maintain both
genetic similarity & differences.
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