Meiosis and Sexual Life Cycles

Fred and George Weasley… Identical Twins

Heredity

Heredity: the transmission of traits from one generation to the next

Asexual reproduction: clones

Sexual reproduction: variation

A budding hydra

Karyotyping Chromosomes are

photographed, paired, and arranged into a karyotype

Homologous chromosomes- same length, centromere position, and staining pattern– Carry same types of genes

Arranged from longest to shortest– 1-22 = autosomes– #23 = sex chromosomes (X

and Y)

Human Life Cycle 23 pairs of homologous

chromosomes (46) 1 pair of sex and 22 pairs of

autosomes Gametes are haploid (1N); all

other cells are diploid (2N) Fertilization results in a

zygote Meiosis: cell division to

produce haploid gametes

Alternate Life Cycles

Plants and Some Algae

Alternation of generations:

2N sporophyte produces 1N spores by meiosis

Spore can divide by mitosis to create a 1N gametophyte

Gametophyte makes gametes by mitosis which fertilize to become a 2N sporophyte

Most fungi and some protists Meiosis produces 1N

cells that divide by mitosis to produce 1N adults

Adults undergo mitosis to produce gametes

Fertilized gametes become a 2N zygote

Meiosis

After chromosome replication

2 cell divisions (Meiosis I & Meiosis II)

Produces 4 daughter cells – 1/2 chromosome number

(1N)– Variation in gametes that

are produced

Meiosis I Interphase

– Chromosomes and centrosomes replicate

Prophase I – Chromosomes condense– Homologous chromosomes pair

up and crossing over occurs– Synaptonemal complex (protein)

forms between homologues, forming a tetrad

– Centrosomes move to opposite ends of the cell

– Spindle fibers attach to kinetochores and begin moving tetrads to the metaphase plate

Metaphase I– Tetrads line up along the

metaphase plate– Both sister chromatids of a

chromosome are controlled by a single kinetochore

Anaphase I– Homologous chromosomes

are pulled apart– Sister chromatids remain

intact Telophase I and Cytokinesis

– Haploid set of chromosomes– Clevage furrow/Cell plate

forms– In some species,

chromosomes uncoil and nuclear envelope forms

Meiosis II

Prophase II– Mitotic spindle

forms again– Spindle fibers

attach to kinetochores and chromosomes begin moving toward the metaphase plate

MetaphaseII– Chromosomes line

up on metaphase plate as in mitosis

Anaphase II– Chromosomes

separate at the centromere

Telophase II and Cytokinesis– Nuclei form– Chromosomes

decondense– Each of the 4

daughtercells are genetically distinct from the parent cell

Meiosis vs. Mitosis Synapsis/tetrad/chiasmata

(prophase I) Homologous vs.

individual chromosomes (metaphase I)

Sister chromatids do not separate (anaphase I)

Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

Genetic Variation Independent assortment:

– homologous chromosomes position and orient randomly (metaphase I)

– In humans, about 8 million different combinations

Recombinant chromosomes (from crossing over)– chromatids are no longer

identical– More possibilities for

genetic variation

Crossing Over Synapsis- homologous

chromosomes come together – Synaptonemal complex holds

them together Form a tetrad Arms of homologues

overlap and form a chiasma Arms of homologues

exchange at gene lines May occur multiple times Produces recombinant

chromosomes

No wonder siblings can be so different!

Random fertilization: 1 sperm (1 of 8 million

possible combinations) x 1 ovum (1 of 8 million possibilities) = 64 trillion diploid combinations!

With the variation from crossing over, the number of possibilities is astronomical

George W and Jeb Bush

Prince Harry and Prince William

Venus and Serena Williams

The Jonas Brothers