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)