Meosis and Sexual Life Cycles

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Single celled and many multicellular eukaryotic organisms are capable of asexual reproduction by a variety of processes such as budding and fragmentation. Asexual reproduction is efficient but lacks the ability to promote adaptation, isnce the offspring are clones of the parent.

Budding yeast (candida) Budding animal (hydra)

Clonal propagation of beech trees via underground shoots

Ploidy: Diploid vs. haploid Ploidy: number of chromosome (genome) sets Haploid: organism with a single chromosome set Diploid: organism with two chromosome sets

HAPLOID

DIPLOID

FERTILIZATION

FUSION

MEIOSIS

The diploid/haploid transition is the heart of sexual reproduction. We think of the diploid form as the ‘normal’ adult form, but many organisms (e.g., fungi such as sordadia) are haploid except for short-lived reproductive stage. Other organsism (e.g., ferns) have both haploid and diploid adult phases which can look completely

different.

or

Most animals

Most fungi, some protists

Plants and some algae

Meiosis is similar to mitosis but is somewhat more complex. It differs from mitosis in that the ploidy (chromosome number) is reduced by the separation of the homologous pairs of chromosomes. The critical first step is the formation of tetrads of homologous chromosomes by a unique process called synapsis during Prophase I.

Interphase

Prophase I

Metaphase I

Anaphase I

Telophase I

and Cytokinesis

Prophase IICell is haploid but sister chromatids haven’t separated yet- meiosis II is like mitosis

Metaphase IILook at the paired sister chromatids lined up on the metaphase equatorial plane- just as in mitosis, they will be split by forces exerted through microtubules attached at the kinetochores

Anaphase II

Telophase II& Cytokinesis

Independent assortment of chromosomes is the primary agent of genetic variation. A parent with 2 pairs of chomosomes could produce 4 different gametes. A fruit fly, with 4 pairs, could produce 16 different gametes. A human has 23 pairs of chromosomes, allowing ~ 107 different gametes. This

is greatly increased by crossing over.

Crossing over allows the separation of alleles of genes on the same chromosome and increases genetic variation in a species. The frequency of crossing over depends on how far apart genes are on a chomosome. In pre-genomic systems this was used to map the location of genes.