Meiosis and Sexual Reproduction

• Meiosis and Sexual Reproduction

Basics

• Sexual reproduction requires the fusion of two specialized cells called gametes, each gamete contains one set of chromosomes.
• Gametes form a zygote with two sets of chromosomes upon uniting.
  • Cells having one chromosome set are "haploid."
  • Those containing two sets, "diploid."
• Sexual reproduction necessitates a nuclear division to halve chromosome sets, preventing the doubling of sets after fertilization.

Diploid Details

• Animals, plants, and unicellular creatures are diploid, having two chromosome sets.
• Somatic cells' nuclei in multicellular organisms include two copies of each chromosome called homologous chromosomes.
• Homologous chromosomes are matched pairs containing identical genes in identical locations along their lengths, inhereted one from each parent.

Meiosis Specifics

• Meiosis is a nuclear division that forms haploid cells from diploid ones, like mitosis with cellular mechanisms.
• Mitosis produces daughter cells with nuclei genetically identical to the parent.
• Mitosis maintains the same "ploidy level", in the case of most animals, diploid.
• Plants grow as sporophytes and produce gametes(eggs/sperm) as gametophytes through mitosis; therefore, mitosis is used for both haploid and diploid cells.
• In meiosis, the starting nucleus is always diploid, daughter nuclei haploid, and chromosome replication is followed by two nuclear divisions.
• Division stages are analogous to mitosis but designated with "I" or "II" due to two rounds of division. Meiosis I is first round.

Meiosis I Stages

• Meiosis I is preceded by an interphase with G₁, S, and G2 phases, similar to mitosis.
  • G₁ focuses on cell growth.
  • S phase replicates DNA chromosomes.
  • G2 prepares the cell for meiosis.
• In S phase each chromosome are replicated to produce sister chromatids held together by cohesin proteins at the centromere, it remains together until anaphase II.

Prophase I Facts

• In early prophase I homologous chromosomes attach by proteins to the nuclear envelope tips.
• Proteins bring pairs closer as the nuclear envelope degrades.
• Homologous chromosomes do not pair together in mitosis.
• The synaptonemal complex, a lattice of proteins, forms between homologous chromosomes starting at specific locations.
• Synapsis is the tight pairing of homologous chromosomes while genes on chromatids of homologous chromosomes align precisely.
• The synaptonemal complex supports chromosomal segment exchange between homologous nonsister chromatids called "crossing over".
• Crossing over can be visually observed as chiasmata.
• Humans possess a homology region on the X and Y chromosomes allowing pairing during prophase I and a partial synaptonemal complex develops between the regions of homology.

Additional Prophase I Details

• Recombination nodules, large protein assemblies, are located on intervals along the synaptonemal complex marking points of later chiasmata, mediating crossover.
• The double-stranded DNA of each chromatid is cleaved, modified, and reconnected with nonsister chromatids near the recombination nodule.
• The synaptonemal complex degrades as prophase I progresses, chromosomes condense, homologous chromosomes remain attached at the centromere and chiasmata with the latter remaining until anaphase I.
• The number of chiasmata varies by species and chromosome length, requiring at least one chiasma for proper separation during meiosis I.
• After crossover the synaptonemal complex breaks down and cohesin connection between homologous pairs is removed.
• Pairs are called tetrads (four sister chromatids) at the end of prophase I and the crossover events have been completed.

Genetic Variation Facts

• Crossover events create genetic variation in nuclei produced by meiosis.
• A single crossover event between homologous nonsister chromatids leads to DNA exchange between maternal and paternal chromosomes allowing a recombinant sister chromatid to carry a mix of parental genes not previously existing.
• Crossover can occur along the entire length of chromosomes, with different meiotic cells producing different recombinant chromatids and varying combinations of parental genes.

Prometaphase I

• Spindle fiber microtubules are attached to the kinetochore proteins at the centromeres.
• Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle.
• Microtubules grow from MTOCs,animal cells its located at centrosomes at opposite poles of the cell, and move toward the cell middle and attach to the fused homologous kinetochores.
• Each homologous pair member attaches to a microtubule from opposite poles so that the microtubules can pull the pair apart.
• A spindle fiber attached to a kinetochore is called a kinetochore microtubule.
• At the end of prometaphase I, each tetrad is attached to microtubules from both poles, each chromosome paired homologous facing each pole while chiasmata hold chromosomes together and the nuclear membrane has been broken down.

Metaphase I

• In metaphase I, homologous chromosomes align at the metaphase plate with kinetochores facing opposite poles.
• Homologous pairs orient randomly at the equator.
• If chromosome 1 members are labeled a and b, the chromosomes could line up a-b or b-a.
• This is important for determining the genes carried since each gamete will only receive one of the chromosomes, with each gamete having a unique genetic makeup from the slight differences after the crossover.
• The randomness in chromosome alignment and crossing over causes substantial variation in offspring contributing to different possibilities for orientation at the metaphase plate.
• Possibility number equals 2n in diploid cell, where n is number of chromosomes per halpoid set.

Chromosome Arrangement

• Humans have 23 chromosome pairs, resulting in over eight million different gamete combinations from the random alignment of chromosomes at the metaphase plate, not including crossing over.
• It is unlikely that any two resulting from meiosis will have the same genetic composition.
• During prophase I, the crossover that takes place between nonsister chromatids happens across each homologous pair of chromosomes.
• Random assortment of tetrads on the metaphase plate also generate recombinant chromatids.

Anaphase I

• Microtubules pull linked chromosomes apart, chiasmata break, and sister chromatids remain tightly bound together at the centromere.

Telophase I & Cytokinesis

• Separated chromosomes arrive at opposite poles.
• Typical telophase events of chromosomes "decondensing" and nuclear forming around separated sets may or may not occur based on species.
• Cytokinesis, separation of cytoplasmic components into daughter cells, occurs without nuclei reformation.
• Animals and some fungi separate cell contents via furrow. Plants form cell plates from fused Golgi vesicles at the metaphase plate.

Haploid Product

• Division of a diploid cell in the first meiotic cycle results in two haploid cells.
• Cells are haploid because sets of the homologous chromosomes exist on each pole.
• Chromosome set consists of two sister chromatids, duplicates of one of homologous chromosomes (except where crossing over causes a change).
• Meiosis II will separate the two sister chromatids, creating four haploid daughter cells.

Meiosis II

• Some cells enter a brief interphase, or interkinesis, but chromosomes are not duplicated because interkinesis does not lack an S phase.
• The two meiosis I cells go through meiosis II synchronously, the sister chromatids separate to form new haploid gametes.
• Mechanics are similar to mitosis, but divides with only one set of homologous chromosomes.
• Each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis, thus cells are similar to haploid cells in G2, preparing to undergo mitosis.
  • If the chromosomes decondensed in telophase I, they condense again.
  • MTOCs duplicate during interkinesis and move away from each other toward opposite poles, new spindles form.
  • Nuclear envelopes are broken down entirely and spindles are fully formed, with each sister chromatid forming an individual kinetochore attaching to microtubules from opposite poles.

Metaphase II & Anaphase II

• Sister chromatids are maximally condensed and aligned at the equator of the cell (metaphase II)
• Sister chromatids are pulled apart by kinetochore microtubules and move toward opposite poles, nonkinetochore microtubules elongate the cell (anaphase II).

Telophase II & Cytokinesis

• Chromosomes arrive at opposite poles and begin to decondense; nuclear envelopes form around chromosomes.
• Meiosis, a diploid cell, separates into four unique haploid cells due to random assortment of maternal/paternal homologs and the set genes being recombined via crossover.

Meiosis vs Mitosis

• Mitosis and meiosis are forms of division of the nucleus in eukaryotic cells.
• Mitosis is a single nuclear division results in two nuclei partitioned into new cells, and contain the same number of sets of chromosomes.
• Meiosis has two nuclear divisions resulting in four genetically distinct nuclei containing one chromosome set
• Division differences: homologous chromosomes pair crossover with non-sister chromatids.

Meiosis I: A Review

• Very different nuclear division as homologous chromosome pairs physically meet and bind with the synaptonemal complex, develop chiasmata, and crossover between nonsister chromatids.
• Chromosomes line up along the metaphase plate as tetrads, with kinetochore fibers from opposite spindle poles attached to each homolog's kinetochore, but all events occur only in meiosis I.
• When chiasmata resolve and homologs move to separate poles reduces the number of chromosomes in each future nucleus. Meiosis I is a reductional division.

Meiosis II: A Review

• Meiosis II is analogous to mitotic division.
• Duplicated chromosomes line up on the metaphase plate with divided kinetochores attached to fibers from poles.
• As in mitotic anaphase kinetochores' divide and sister chromatid— now a chromosome — is pulled to a pole.
• With crossover, products from two meiosis II divisions would be genetically identical. Meiosis II is not a reduction division.