What is Meiosis?
o Meiosis is double nuclear division which produces four haploid gametes (also called germ cells).
o In humans, only the spermatogonium and the oogonium (both diploid) undergo meiosis.
o All other cells are somatic and undergo mitosis.
o The gametes are haploid.
o After replication occurs in S phase of interphase, the cell is called a primary spermatocyte or primary oocyte (both diploid).
o In the human female, replication takes place before birth, and the life cycle of all germ cells are arrested at the primary oocyte stage until puberty.
o Arrested in prophase 1 Just before ovulation, a primary oocyte undergoes the first meiotic division to become a secondary oocyte (haploid).
o The secondary oocyte is released upon ovulation, and the penetration of the secondary oocyte by the sperm stimulates anaphase II of the second meiotic division in the oocyte.
o Meiosis is two rounds of division called meiosis I and meiosis II.
o Meiosis I proceeds similarly to mitosis with the following differences.
Diagram of Meiosis Phases
Stages of Meiosis
Prophase I
In prophase I homologous chromosomes line up along side each other, matching there genes exactly.
o At this time, they may exchange sequences of DNA nucleotides in a process called crossing over (synapsis).
o Genetic recombination in eukaryotes occurs during crossing over.
o You can also have Double crossovers:
Scenario 1: results in no genetic recombination. The chromatids involved in this double crossover exchange alleles at first, but then it exchanges them back, resulting in no net recombination. This is called the 2-strand double crossover. Results in 0/4 recombinants.
o Scenario 2: results in genetic recombination. The chromatids exchange alleles during a crossover. Then, one of the crossover chromatid exchanges with a different chromatid. This is called the 3-strand double crossover. Results in 2/4 recombinants.
o Scenario 3: results in genetic recombination. The chromatids exchange, then 2 totally different chromatids on the same chromosome exchange. This is called the 4-strand double crossover. Results in 4/4 recombinants.
o Since each duplicated chromosome in prophase I appear as an ‘x’, the side by side homologues exhibit a total of four chromatids, and are called tetrads.
o If crossing over does occur, the two chromosomes are “zipped” along each other where nucleotides are exchanged, and form what is called the synaptonemal complex.
o A chiasma (plural: chiasmata) is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis.
o Sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells.
o Genes located close together on a chromosome are more likely to cross over together, and are said to be linked!!!
Metaphase I
o In metaphase I the homologues remained attached, and move to the metaphase plate.
o Rather than single chromosomes aligned along the plate as in mitosis, tetrads align in meiosis.
Anaphase I
o Anaphase I separates the homologues from their partners, the centromeres stay together (which is different from anaphase in mitosis).
Telophase I
o In telophase I, a nuclear membrane may or may not reform, and cytokinesis may or may not occur. In humans the nuclear membrane does reform and cytokinesis does occur.
o If cytokinesis occurs, the new cells are haploid with 23 replicated chromosomes, and are called secondary spermatocytes and secondary oocytes.
o In the case of a female, one of the oocytes, called the first polar body, is much smaller and degenerates (it may or may not go through meiosis II).
o This occurs in order to conserve cytoplasm, which is contributed only by the ovum.
"These four phases together are called meiosis I. Meiosis I is reduction division"
o Meiosis II proceeds with prophase II, metaphase II, anaphase II, and telophase II.
o The final products are haploid gametes each with 23 chromosomes.
o In the case of the spermatocytes, four sperm cells are formed. In the case of the oocyte, a single ovum is formed.
o In the female, telophase II produces one gamete and a second polar body.
o If during anaphase I or II the centromere of any chromosome doesn’t split, this is called nondisjunction (it can also happen in mitosis but the ramifications are less severe).
o As a result of primary nondisjunction (nondisjunction in anaphase I), one of the cells with end up with two extra chromatids
o Complete extra chromosome) and the other will be missing a chromosome.
o If nondisjuction occurs in anaphase II that will result in one cell having an extra chromatid and one lacking a chromatid.
o The number of different possible gametes that can be formed by diploid organisms as a result of independent assortment of chromosomes during meiosis can be calculated by using the formula 2^n where n is the number of heterozygous genes.
o Ex. AaBbCc ⇒ this can produce 2^3 number of different haploid cells.
o Parthenogenesis (means the growth and development of an embryo or seed without fertilization by a male.
o Parthenogenesis occurs naturally in some lower plants, invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even turkeys).
o Parthenogenetic populations are typically all-female.
o As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis.
o Spermatogonium (diploid) ⇒ primary spermatocyte (diploid) ⇒ 2ndary spermatocyte (haploid) ⇒ spermatid (haploid) ⇒ spermatozoa (haploid)
o Oogonium (diploid) ⇒ primary oocyte (diploid) ⇒ secondary oocyte (haploid) ⇒ zygote (diploid)
o A Barr body is a permanently inactivated X chromosome that forms a dense stainable nuclear mass.
o A normal female with XX inactivates one of her X’s while expressing the other.
o She therefore has 1 Barr body.
o A normal male, who is XY, does not inactivate his only X chromosome, and therefore has no Barr bodies.
o The rule is that the number of X chromosomes is always 1 MORE than the number of Barr bodies.
o Ex. A person with 2 Barr bodies has to have 3 X chromosomes.
Meiosis Citations
- Elevated Mutagenicity in Meiosis and Its Mechanism. Bioessays . 2019 Apr;41(4):e1800235.
- Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol . 2015 May 18;7(6):a016626.
- Meiosis: the chromosomal foundation of reproduction. Biol Reprod . 2018 Jul 1;99(1):112-126.
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