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Meiosis
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Meiosis:
Halves the Chromosome Number
• Special type of cell division
• Used only for sexual reproduction
• Halves the chromosome number prior to
fertilization
– Parents diploid
– Meiosis produces haploid gametes
– Gametes fuse in fertilization to form diploid zygote
– Becomes the next diploid generation
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One process, three aspects
• Meiosis
• Sexual reproduction
• Heredity
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Heredity
• DNA
• Genes = specific sequences of nucleotides
• Inheritance possible because
– a. DNA replication is an exact process
– b. Sperm and ova are combined in nucleus of fertilized
egg
• Transmission of genes depends on behavior of
chromosomes
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Chromosome
• Single long DNA molecule; highly folded and
coiled with proteins
• Contains genetic information arranged in linear
fashion
• Contains 100s or 1000s of genes, each in a specific
region of DNA; locus
• Each species has a specific number of
chromosomes
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Homologous Pairs of Chromosomes
• In diploid body cells chromosomes occur in pairs
• Humans have 23 different types of chromosomes
• Diploid cells have two of each type
• Chromosomes of the same type are said to be homologous
– They have the same length
– Their centromeres are positioned in the same place
– One came from the father (the paternal homolog) the other from
the mother (the maternal homolog)
– When stained, they show similar banding patterns
– Because they have genes controlling the same traits at the same
positions
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Homologous Chromosomes
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Homologous Pairs of Chromosomes
• Homologous chromosomes have genes controlling the
same trait at the same position
– Each gene occurs in duplicate
– A maternal copy from the mother
– A paternal copy from the father
• Many genes exist in several variant forms in a large
population
• Homologous copies of a gene may encode identical or
differing genetic information
• The variants that exist for a gene are called alleles
• An individual may have:
– Identical alleles for a specific gene on both homologs
(homozygous for the trait), or
– A maternal allele that differs from the corresponding paternal
allele (heterozygous for the2011-12
trait)
Asexual vs. sexual reproduction
Asexual Reproduction
Sexual Reproduction
Single individual is sole parent. Two parents give rise to offspring.
Single parent passes on all its
genes to its offspring.
Each parent passes on half its
genes to its offspring.
Offspring are genetically
identical to the parent.
Offspring have a unique
combination of genes inherited
from both parents
Results in a clone. Rarely
differences occur and as a
result of mutation.
Results in greater genetic
variation; offspring vary
genetically from their siblings
and parents.
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Sexual Life Cycles
Alternation of generations
The only haploid cells
spores
The only diploid stage
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Terms to review
• Life cycle
• Somatic cell
• Homologous chromosome
pairs (homologues)
– Except for X/Y they carry
same genetic loci (locus)
• Karyotype
– Use human lymphocytes
– Can find chromosomal
abnormalities
• Diploid (2n)
• Haploid (n); gametes
– Sperm cells/ova (23
chromosomes)
– Only cells in body not
produced by mitosis
• Fertilization
• Zygote
• Autosomes (22 pairs)
• Sex chromosomes (1 pair)2011-12
Overview of Meiosis
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Phases of Meiosis I:
Prophase I & Metaphase I
• Meiosis I (reductional division):
–Prophase I
• Each chromosome internally duplicated (consists of two
identical sister chromatids)
• Homologous chromosomes pair up – synapsis
• Physically align themselves against each other end to end
• End view would show four chromatids – Tetrad
–Metaphase I
• Homologous pairs arranged onto the metaphase plate
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Phases of Meiosis I:
Anaphase I & Telophase I
• Meiosis I (cont.):
–Anaphase I
• Synapsis breaks up
• Homologous chromosomes separate from one another
• Homologues move towards opposite poles
• Each is still an internally duplicate chromosome with two
chromatids
–Telophase I
• Daughter cells have one internally duplicate chromosome from
each homologous pair
• One (internally duplicate) chromosome of each type (1n,
haploid)
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Phases of Meiosis I:
Cytokinesis I & Interkinesis
• Meiosis I (cont.):
–Cytokinesis I
• Two daughter cells
• Both with one internally duplicate chromosome of each type
• Haploid
• Meiosis I is reductional (halves chromosome number)
• Interkinesis
–Similar to mitotic interphase
–Usually shorter
–No replication of DNA
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Genetic variations
• Sexual life cycles --> variations
– Independent assortment of chromosomes
– Crossing over during prophase I
– Random fusion of gametes during fertilization
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Genetic Variation:
Crossing Over
• Meiosis brings about genetic variation in two key ways:
–Crossing-over between homologous chromosomes, and
–Independent assortment of homologous chromosomes
• 1. Crossing Over:
–Exchange of genetic material between nonsister chromatids during
meiosis I
–At synapsis, a nucleoprotein lattice (called the synaptonemal
complex) appears between homologues
• Holds homologues together
• Aligns DNA of nonsister chromatids
• Allows crossing-over to occur
–Then homologues separate and are distributed to different daughter
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cells
Crossing Over
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Crossing over
Protein strands
hold homologues
in register with
each other
Protein cross
bridges
DNA loops
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Crossing over (2)
Harlequin chromosomes
demonstrate reciprocal
exchange between
sister chromatids
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Genetic Variation:
Independent Assortment
• 2. Independent assortment:
–When homologues align at the metaphase plate:
• They separate in a random manner
• The maternal or paternal homologue may be oriented
toward either pole of mother cell
–Causes random mixing of blocks of alleles into
gametes
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Independent Assortment
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Independent assortment
Homologous pairs
behave independently
of each other
Consider the possible combinations of 23 pairs !
Fertilization = (223)2 = 64 x 1012 variations
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2n = 223 = 8 x 106 (1 parent)
Recombination
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Genetic Variation:
Fertilization
• When gametes fuse at fertilization:
–Chromosomes donated by the parents are combined
–In humans, (223)2 = 70,368,744,000,000 chromosomally
different zygotes are possible
• If crossing-over occurs only once
–(423)2, or 4,951,760,200,000,000,000,000,000,000 genetically
different zygotes are possible
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Genetic Variation:
Significance
• Asexual reproduction produces genetically identical
clones
• Sexual reproduction cause novel genetic
recombinations
• Asexual reproduction is advantageous when
environment is stable
• However, if environment changes, genetic
variability introduced by sexual reproduction may
be advantageous
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Phases of Meiosis II:
Similar to Mitosis
• Metaphase II
–Overview
•Unremarkable
•Virtually indistinguishable from mitosis of two
haploid cells
–Prophase II – Chromosomes condense
–Metaphase II – chromosomes align at metaphase
plate
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– Anaphase II
• Centromere dissolves
• Sister chromatids separate and become
daughter chromosomes
– Telophase II and cytokinesis II
• Four haploid cells
• All genetically unique
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Meiosis I & II in Plant Cells
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Meiosis versus Mitosis
•Mitosis
•Meiosis
– Requires one nuclear division
– Requires two nuclear divisions
– Chromosomes do not synapse nor
cross over
– Chromosomes synapse and cross
over
– Centromeres survive Anaphase I
– Centromeres dissolve in mitotic
anaphase
– Halves chromosome number
– Preserves chromosome number
– Produces four daughter nuclei
– Produces two daughter nuclei
– Produces daughter cells genetically
different from parent and each other
– Produces daughter cells genetically
identical to parent and to each other
– Used only for sexual reproduction
– Used for asexual reproduction and
growth
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Meiosis Compared to
Mitosis
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Meiosis I Compared to
Mitosis
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Meiosis II Compared to
Mitosis
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Life Cycle Basics:Plants
• Haploid multicellular “individuals”
alternate with diploid multicellular
“individuals”
• The haploid individual:
–Known as the gametophyte
–May be larger or smaller than the diploid individual
• The diploid individual:
–Known as the sporophyte
–May be larger or smaller than the haploid individual
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Life Cycle Basics:Plants
• Mosses are haploid most of their life cycle
• Ferns & higher plants have mostly diploid
life cycles
• In fungi and most algae, only the zygote is
diploid
• In plants, algae, & fungi, gametes produced
by haploid individuals
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Life Cycle Basics: Animals
• In familiar animals:
–“Individuals” are diploid; produce haploid
gametes
–Only haploid part of life cycle is the
gametes
–The products of meiosis are always gametes
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Life Cycle Basics: Animals
– Meiosis occurs only during gametogenesis
• Production of sperm
–Spermatogenesis
–All four cells become sperm
• Production of eggs
–Oogenesis
–Only one of four nuclei get cytoplasm
»Becomes the egg or ovum
»Others wither away as polar bodies
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The Human Life Cycle
• Sperm and egg are produced by meiosis
• A sperm and egg fuse at fertilization
• Results in a zygote
–The one-celled stage of an individual of the
next generation
–Undergoes mitosis
• Results in multicellular embryo that gradually
takes on features determined when zygote was
formed
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The Human Life Cycle
• All growth occurs as mitotic division
• As a result of mitosis, each somatic cell
in body
–Has same number of chromosomes as
zygote
–Has genetic makeup determined when
zygote was formed
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The Human Life Cycle
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Gametogenesis in Mammals
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Oogenesis
Long interkinesis
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Spermatogenesis
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Meiosis
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Mitosis
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