Meiosis
Heredity and Variation
• Heredity– Is the transmission of traits from one generation to
the next• Variation
– Shows that offspring differ somewhat in appearance and other traits from parents and siblings
• Genetics– Is the scientific study of heredity and hereditary
variation
Breeding in English Shepherds
FatherMother
Breeding in English Shepherds - Offspring
Genes – A simple definition
• Genes
– Are the units of heredity
– Are segments of DNA
• Each gene in an organism’s DNA has a specific locus on a certain chromosome
• Sexual organisms inherit one set of chromosomes from the mother and one set from the father
The Shallow End of the Gene Pool
Asexual Reproduction
• For example – budding in Hydra is asexual reproduction that forms clones
Asexual Reproduction
• Strawberry reproducing by putting out asexual runners – also forming clones
Key
Haploid (n)Diploid (2n)
Egg (n)
Haploid gametes (n 23)
Sperm (n)
Ovary Testis
Mitosis anddevelopment
Diploidzygote(2n 46)
Multicellular diploidadults (2n 46)
MEIOSIS FERTILIZATION
Chromosomes• Homologous chromosomes
– Are the two chromosomes composing a pair– Have the same characteristics– Autosomes are the non-sex determining chromosomes
• Sex chromosomes– Are distinct from each other in their characteristics– Are represented as X and Y in mammals– Determine the sex of the individual, XX being female,
XY being male; X and X are homologous in female• A diploid cell
– Has two sets of each of its chromosomes– In a human has 46 chromosomes (2n = 46)
Pair of homologousduplicated chromosomes
Centromere
Sisterchromatids
Metaphasechromosome
5 m
Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes
Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes
Hairy Ears – a sex linked trait
• Square symbol = male
• Round symbol = female
• Dark square = hairy ear trait
Chromosome number in humans if there was no meiosis prior to reproduction
• First Generation - Sperm and Egg with 46 chromosomes each
• Second generation 92 chromosomes
• Third generation 184 chromosomes
• Fourth generation 368 chromosomes
• Etc.
Sexual life cyclesKey
Haploid (n)
Diploid (2n)
Gametes
MEIOSIS FERTILIZATION
Zygote
MitosisDiploidmulticellularorganism
(a) Animals
n
n
n
2n 2n 2n2n
2n
n n
n
n n
nn
n
n
nMEIOSIS
MEIOSIS
FERTILIZATION
FERTILIZATION
Mitosis Mitosis
Mitosis
Mitosis Mitosis
GametesSpores
Gametes
Zygote
Zygote
Haploid multi-cellular organism(gametophyte)
Diploidmulticellularorganism(sporophyte)
Haploid unicellular ormulticellular organism
(b) Plants and some algae (c) Most fungi and some protists
Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids Diploid cell with
duplicatedchromosomes
Interphase
Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids Diploid cell with
duplicatedchromosomes
Homologouschromosomes separate
Haploid cells withduplicated chromosomes
Meiosis I
1
Interphase
Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids Diploid cell with
duplicatedchromosomes
Homologouschromosomes separate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
Haploid cells with unduplicated chromosomes
Interphase
Meiosis I
Meiosis II
2
1
MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I andCytokinesis
Centrosome(with centriole pair)
Sisterchromatids
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs.
Sister chromatidsremain attached
Homologouschromosomesseparate
Each pair of homologous chromosomes separates.
Cleavagefurrow
Two haploid cellsform; each chromosomestill consists of twosister chromatids.
MEIOSIS I: Separates sister chromatids
Prophase II Metaphase II Anaphase IITelophase II and
Cytokinesis
Sister chromatidsseparate
Haploid daughtercells forming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
Prophase I Metaphase I Anaphase I Telophase I andCytokinesis
Centrosome(with centriole pair)
Sisterchromatids
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs.
Sister chromatidsremain attached
Homologouschromosomesseparate
Each pair of homologous chromosomes separates.
Cleavagefurrow
Two haploid cells form; each chromosomestill consists of two sister chromatids.
Prophase II Metaphase II Anaphase II Telophase II andCytokinesis
Sister chromatidsseparate
Haploid daughtercells forming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
Prophase
Duplicatedchromosome
MITOSIS
Chromosomeduplication
Parent cell
2n 6
Metaphase
AnaphaseTelophase
2n 2n
Daughter cellsof mitosis
MEIOSIS
MEIOSIS I
MEIOSIS II
Prophase I
Metaphase I
Anaphase ITelophase I
Haploidn 3
Chiasma
Chromosomeduplication Homologous
chromosome pair
Daughter cells of
meiosis I
Daughter cells of meiosis II
n n n n
SUMMARY
Property Mitosis Meiosis
DNAreplication
Number ofdivisions
Synapsis ofhomologouschromosomes
Number of daughter cellsand geneticcomposition
Role in the animal body
Occurs during interphase beforemitosis begins
One, including prophase, metaphase,anaphase, and telophase
Does not occur
Two, each diploid (2n) and geneticallyidentical to the parent cell
Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase,and telophase
Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as manychromosomes as the parent cell; genetically differentfrom the parent cell and from each other
Produces gametes; reduces number of chromosomesby half and introduces genetic variability among the gametes
Independent Assortment
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
Independent Assortment
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
Metaphase II
Independent Assortment
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
• Mitosis and meiosis have several key differences.– The chromosome number is reduced by half in
meiosis, but not in mitosis.– Mitosis produces daughter cells that are genetically
identical to the parent and to each other.– Meiosis produces cells that differ from the parent
and each other.
• Three events, unique to meiosis, occur during the first division cycle.
1. During prophase I, homologous chromosomes pair up in a process called synapsis.– A protein zipper, the synaptonemal complex, holds
homologous chromosomes together tightly.– Later in prophase I, the joined homologous
chromosomes are visible as a tetrad.– At X-shaped regions called chiasmata, sections of
nonsister chromatids are exchanged.– Chiasmata is the physical manifestation of crossing
over, a form of genetic rearrangement.– Prophase I is longest and most important phase
2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.
• In humans, you would see 23 tetrads.3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.– Sister chromatids remain attached at the centromere
until anaphase II.• The processes during the second meiotic division are
virtually identical to those of mitosis.
The synaptonemal complex binding together four homologous chromosomes
Chiasmata and crossing over
Prophase Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Prophase Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Prophase Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Prophase Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Prophase Iof meiosis
Nonsister chromatidsheld togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughtercells
Recombinant chromosomes
Human Female vs Male Meiotic Timelines
Sexual reproduction leads to genetic variation via:
• Independent assortment during meiosis
• Crossing over during meiosis
• Random mixing of gametes (sperm and egg)