Meiosis

BIO 224

Intro to Molecular and Cell Biology

Meiosis

• Specialized cell cycle that reduces chromo-some number by half

• Two sequential cell divisions follow a single round of DNA replication

• Some unicellular eukaryotes use meiosis for cell division and reproduction

• Germ cells of multicellular eukaryotes undergo meiosis to produce haploid gametes for reproduction

Meiosis and Ploidy

• Chromosome number of an organism is its ploidy

• Meiosis produces haploid daughter cells• Haploid refers to a cell possessing half

the normal chromosome number: n– Gametes, spores

• Diploid refers to a cell possessing the normal chromosome number: 2n– Somatic cells

Meiosis

• Also called reduction-division due to reduction of chromosome number

• Some unicellular eukaryotes use both mitosis and meiosis

• Meiosis is restricted to germ cells for multicellular eukaryotes– Haploid gametes fuse at fertilization to begin

the creation of a diploid organism

Meiosis

• Occurs between interphase periods, like mitosis

• Stages are similar to mitosis, but chromo-some segregation occurs in a different way

• Differs from mitosis in that it results in 4 haploid daughter cells

• Divided into meiosis I and meiosis II

Meiosis I

• Homologous chromosomes pair with one another and members segregate to different daughter cells

• Sister chromatids remain together, resulting in two daughter cells with a member of each chromosome pair

• At the end of MI, cells are haploid with duplicated chromosomes

Prophase I

• An extended prophase, taking up to 90% of the meiotic cycle

• Divided into five stages based on chromo-some morphology– Leptotene– Zygotene– Pachytene– Diplotene– Diakinesis

Prophase I

• Leptotene: initial stage of prophase I– Homologous chromosomes pair up before

condensing– Centromeres are barely visible

• Zygotene: stage where homologous chromosomes become closely associated at synapsis via the synaptomenal complex– Chromosomes remain closely aligned through

following stages

Prophase I

• Pachytene: stage where recombination occurs– Chromosomes continue to shorten and

condense– May last for several days– Synaptomenal complex keeps chromsomes

closely associated through the end of this stage

The synaptonemal complex

Prophase I

• Diplotene: stage where homologous chromosomes begin to separate– Chromosomes remain associated at

chiasmata– Each pair is called a bivalent

• Diakinesis: the final stage where chromo-somes fully condense in preparation for metaphase

• Nucleoli disappear, nuclear membrane degrades, spindle begins to form

A bivalent chromosome at the diplotene stage

Prophase I

• Recombination is finished by the end of the pachytene stage, where chromsomes remain linked at the site of crossing over (chiasmata)– Linkage at the chiasmata allows them to align

properly during metaphase• Nucleoli disappear, nuclear membrane

degrades, spindle begins to form as in prophase of mitosis

Stages of the prophase of meiosis I

Metaphase I

• Bivalent chromosomes align on the spindle• Kinetochores of sister chromatids are

adjacent and oriented in the same direction • Kinetochores of homologous chromosomes

are pointed toward opposite spindle poles• Spindle tubules attach to kinetochores and

separate members of homologous pairs– Chromosomes are still linked at chiasmata– Disruption of chiasmata initiates anaphase

Meiosis, metaphase I

Anaphase I

• Homologous chromosomes are separated into daughter cells

• Duplicated sister chromatids remain associated at their centromeres

• Chromosomes migrate to poles of daughter cells, which are now haploid– Major difference in anaphase of mitosis and

anaphase of meiosis

Meiosis, early anaphase I

Chromosome segregation in meiosis I

Telophase I

• Chromosomes reach cell poles• Nuclear membrane begins to reform• Chromosomes may begin to decondense • Daughter cells separate by cytokinesis• No interphase follows this phase• Meiosis II begins after the end of MI

Meiosis, telophase I

Meiosis II

• Resembles stages of mitosis• Prophase II: chromatids condense, nuclear

membrane disappears, spindle forms• Metaphase II: chromosomes align along the

spindle and microtubules from opposite poles attach to kinetochores of sister chromatids

• Anaphase II: sister chromatids separate• Telophase II: chromatids migrate to poles

– Cytokinesis follows, allowing formation of 4 total haploid daughter cells

Meiosis, prophase II

Meiosis, metaphase II

Meiosis, anaphase II

Meiosis, telophase II

Comparison of meiosis and mitosis

Regulation of Oocyte Meiosis

• Meiosis of vertebrate oocytes is regulated at two points in the cell cycle

• The diplotene stage of MI has the first regulatory point– Oocytes may remain arrested in this stage for

many years, up to 50 in humans• Chromosomes decondense and undergo active

transcription• During this period oocytes grow immensely, and

accumulate proteins and RNA for support of embryonic development

Regulation of Oocyte Meiosis

• Resuming of oocyte meiosis as well as when fertilization occurs varies in different species

• Some animals’ oocytes remain in the diplotene stage until fertilization, then finish meiosis

• Most vertebrates’ oocytes resume meiosis in response to hormonal stimulation and complete meiosis I prior to fertilization

• Meiosis I results in uneven cell division creating one normal ooctye and a small nonfunctional polar body

• The oocyte enters meiosis II without reforming a nucleus or decondensing chromosomes, arresting at metaphase II until fertilization

Meiosis of vertebrate oocytes

Regulation of Oocyte Meiosis

• Cdk1/cyclin B complexes regulate meiosis, just like in M phase of somatic cells

• Cdk1 is also responsible for unique events, regulating MI to MII progression and arresting cells in metaphase II

• Cdk1 is activated by hormones to trigger the resumption of meiosis, then to keep cells in M phase

Activity of Cdk1/cyclin B during oocyte meiosis

Fertilization

• The sperm binds to a receptor on the egg’s surface and fuses with the plasma membrane of the egg– Initiates development of a new diploid organism that

contains genetic information from both parents• Fertilization activates changes in the cytoplasm of

the egg necessary for further development– Changes activate the egg to complete meiosis and initiate

mitotic cell cycles of the early embryo– Levels of intracellular Ca2+ increase in eggs due to binding

of sperm preventing other sperm from entering the egg• This ensures the formation of a normal diploid embryo

– Increased Ca2+ levels signal the end of meiosis

Fertilization

• Following the end of meiosis, the zygote contains two haploid pronuclei, one from each parent

• In mammals, the pronuclei enter S phase and migrate toward each other, then meet as the zygote enters M phase of the first mitotic division

• Nuclear envelopes break down and the chromosomes align on a common spindle before mitosis completes with the production of two diploid cells that will divide further to create a new organism

Fertilization

Fertilization and completion of meiosis

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