Meiosis

The creation of eukaryotic Eukaryotic Eukaryotes can be single-celled or multicellular organisms and include plants, animals, fungi, and protozoa. Eukaryotic cells contain a well-organized nucleus contained by a membrane, along with other membrane-bound organelles. Cell Types: Eukaryotic versus Prokaryotic gametes involves a DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication phase followed by 2 cellular division stages: meiosis I and meiosis II. Meiosis I separates homologous chromosomes into separate cells (1n, 2c), while meiosis II separates sister chromatids into gametes (1n, 1c). Unique combinations of gametes via sexual reproduction are a major driver of evolutionary fitness in complex organisms.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Overview of Meiosis

  • Meiosis: a cellular division process that creates aneuploid gametes in sexually reproducing species
  • Occurs in ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries and testes
  • Prior to mitosis, the cell undergoes a growth and DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication cycle called interphase.
  • 1 round of DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication (interphase), followed by 2 rounds of cellular division
  • 2 rounds of cell division (meiosis I and meiosis II) divided into phases:
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
    • Cytokinesis
  • The resulting daughter cells have ½ the number of chromosomes as the mother cells.
  • Nomenclature: n = number of homologous chromosomes, c = total number of chromosomes (homologous and sister)
Overview of meiosis i and ii

Overview of meiosis I and II, crossing over (homologous recombination), and independent assortment

Image by Lecturio.

Meiosis I

Following DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication, meiosis I creates 2 daughter cells containing half the genetic information of the mother cell (1n) but the same number of chromosomes (2c) by segregating sister chromatids into the same daughter cell.

  • Prophase I: longest phase of meiosis
    • Chromosomes start to condense.
    • Homologous chromosomes align and a synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission forms.
    • Crossing over (overlap and fusion of homologous areas of aligned chromosomes, causing an exchange of genetic material) occurs and is visible because of the chiasmata (point of crossing over).
    • Crossing over occurs between sister chromatid segments (homologous recombination).
    • Homologous chromosomes separate as the chromosomes become fully condensed.
  • Metaphase I
    • The nuclear membrane disassembles.
    • Chromosomes align themselves on the equatorial plane and attach to spindle fibers.
    • Spindle fibers push the chromosome pairs to stay in the middle of the cell.
    • Cohesins hold sister chromatids together from replication to anaphase.
  • Anaphase I
    • Spindle fibers shorten.
    • Separation of the chromosome pairs
    • The cell becomes oblong in preparation for division.
  • Telophase I and cytokinesis
    • Spindle fibers disassemble.
    • Chromosomes become less condensed, and the nuclear envelope may start to re-form.
    • Cytokinesis divides the cell into 2 daughter cells, each of which contains only 1 set of chromosomes and is considered haploid.
Telophase reappearance of the nuclear membrane and nucleolus

Reappearance of the nuclear membrane and nucleolus: the telophase

Image: “Reappearance of the nuclear membrane and nucleolus: the telophase” by Roy van Heesbeen. License: Public Domain

Meiosis II

Meiosis II is a cellular division event wherein the number of chromosomes in the daughter cells is halved from that of the mother cell.

  • Meiosis II: similar to meiosis I but not preceded by interphase ( DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication)
  • Stages of meiosis II:
    • Prophase II
      • The nuclear envelope disassembles.
      • Spindle fibers again form and chromosomes start to condense.
    • Metaphase II
      • Spindle fibers push the chromosomes to the middle of the cell.
      • The cell elongates.
    •  Anaphase II
      • Spindle fibers shorten.
      • Sister chromatids are separated.
    • Telophase II
      • Chromosomes become less condense.
      • Spindle fibers disassemble.
      • Cytokinesis: Cells divide into 2 daughter cells that are haploid.

Meiosis in Humans

The human karyotype normally contains 23 pairs of chromosomes.

  • 22 pairs of autosomes
  • 1 pair of allosomes (sex chromosomes)
  • 23 chromosomes from each parent
  • Total of 46 chromosomes

Meiosis is important in the production of haploid cells (gametes).

  • Spermatocytes and oocytes are the 2 types of haploid cells (gametes) produced during meiosis.
  • Sexual reproduction: 2 haploid cells combine to make 1 diploid cell.
  • 1 haploid from the mother, 1 haploid from the father
  • Spermatocytes are produced during spermatogenesis.
  • Oocytes are produced during oogenesis.

Spermatogenesis

  • Occurs in the seminiferous tubules of the testes
  • Spermatogonial stage
    • Mitotic clonal expansion occurs.
    • Spermatogonia = precursor cells for spermatocytes
    • Type A spermatogonia from the primordial germ cell divide to maintain the stem cell pool population.
    • Some of the type A spermatogonia return or stay at the resting pool while some proliferate and undergo differentiation.
    • Type A spermatogonia may be converted to type B spermatogonia.
    • Type B spermatogonia then enter the preleptotene stage and become primary spermatocytes.
  • Meiotic stage
    • Primary spermatocytes undergo 2 stages of meiosis.
    • Haploid daughter cells are called round spermatids.
  • Spermiogenesis stage
    • Round spermatids enter the spermatogenesis stage.
    • Spermatids undergo nuclear and cytoplasmic changes, leading to the formation of spermatozoa:
      • Formation of the acrosome (a modified lysosome)
      • The nucleus becomes condensed and is moved to the periphery of the cell.
      • Microtubules then form, producing flagella.
      • A large part of the cytoplasm is removed as a residual body.
      • Sertoli cells phagocytose these bodies.
    • Spermatozoa are stored to mature before being released into the epididymis.

Oogenesis

  • Birth to puberty Puberty Puberty is a complex series of physical, psychosocial, and cognitive transitions usually experienced by adolescents (11-19 years of age). Puberty is marked by a growth in stature and the development of secondary sexual characteristics, achievement of fertility, and changes in most body systems. Puberty
    • Before birth, oogonium, which also arise from germs cells, undergo mitosis to produce primary oocytes.
    • Undergoes meiosis I, but process is halted at the prophase I
  • After puberty Puberty Puberty is a complex series of physical, psychosocial, and cognitive transitions usually experienced by adolescents (11-19 years of age). Puberty is marked by a growth in stature and the development of secondary sexual characteristics, achievement of fertility, and changes in most body systems. Puberty
    • Primary oocytes complete meiosis I forming a secondary oocyte and the 1st polar body
    • The secondary oocytes then begin meiosis II and are ovulated.
    • The secondary oocyte are arrested in metaphase II.
  • Fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week 
    • A secondary oocyte and a sperm cell fuse.
    • Meiosis II proceeds and is completed when the sperm cell completely penetrates the secondary oocyte.
    • A 2nd polar body and the ovum are produced after meiosis II.
    • When the nuclei of the sperm cells and the ovum unite, a diploid zygote is produced.

Related videos

Meiosis vs. Mitosis

  • The major difference between these 2 processes is the number of steps needed to complete cell division.
  • Mitosis involves a single division producing 2 daughter cells similar to the parent cell.
  • Meiosis occurs as a 2-cell division process that produces 4 daughter cells that are completely distinct from both each other and the parent cells.
  • The final product of meiosis has half the number of chromosomes compared to the parent cell, while mitosis produces daughter cells with the same number of chromosomes.
  • Mitosis occurs in somatic cells and during early cell division in gamete formation, while meiosis occurs only at the final division of gamete maturation.
Comparing-mitosis-meisosis

Comparing mitosis and meiosis

Image by Lecturio.

Clinical Relevance

  • Down syndrome Down syndrome Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down Syndrome ((DS) trisomy 21): the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Down syndrome Down syndrome Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down Syndrome results from a failure of meiosis. Characteristic traits include upslanting, almond-shaped eyes with skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin covering the inner aspects, a broad flattened nasal bridge, small rounded ears, and a small mouth with a large tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue
  • Patau’s syndrome ( trisomy 13 Trisomy 13 Trisomy 13, or Patau syndrome, is a genetic syndrome caused by the presence of 3 copies of chromosome 13. As the 3rd most common trisomy, Patau syndrome has an incidence of 1 in 10,000 live births. Most cases of Patau syndrome are diagnosed prenatally by maternal screening and ultrasound. More than half of the pregnancies result in spontaneous abortions. Patau Syndrome (Trisomy 13)): a genetic syndrome caused by the presence of 3 copies of chromosome 13. Findings include craniofacial and cardiac malformations, severe intellectual disability, and greatly reduced life expectancy. Most infants do not survive past 3 months. 
  • Edwards syndrome Edwards syndrome Edwards syndrome, or trisomy 18, is a genetic syndrome caused by the presence of an extra chromosome 18. The extra chromosome is either from 3 full copies of chromosome 18 or an additional segment of chromosome 18. As the 2nd most common trisomy, Edwards syndrome is seen in 1 out of every 5,500 live births. Edwards Syndrome (Trisomy 18) ( trisomy 18 Trisomy 18 Edwards syndrome, or trisomy 18, is a genetic syndrome caused by the presence of an extra chromosome 18. The extra chromosome is either from 3 full copies of chromosome 18 or an additional segment of chromosome 18. As the 2nd most common trisomy, Edwards syndrome is seen in 1 out of every 5,500 live births. Edwards Syndrome (Trisomy 18)): a genetic syndrome caused by the presence of an extra chromosome 18, which can be from 3 full copies of chromosome 18 or an additional segment of chromosome 18. Noted abnormalities include intrauterine growth retardation, overlapping fingers, typical craniofacial features, rocker bottom feet, and congenital heart defects.
  • Klinefelter syndrome Klinefelter syndrome Klinefelter syndrome is a chromosomal aneuploidy characterized by the presence of 1 or more extra X chromosomes in a male karyotype, most commonly leading to karyotype 47,XXY. Klinefelter syndrome is associated with decreased levels of testosterone and is the most common cause of congenital hypogonadism. Klinefelter Syndrome: a chromosomal aneuploidy characterized by the presence of 1 or more extra X chromosomes in a male karyotype, most commonly leading to the karyotype 47,XXY. Klinefelter syndrome Klinefelter syndrome Klinefelter syndrome is a chromosomal aneuploidy characterized by the presence of 1 or more extra X chromosomes in a male karyotype, most commonly leading to karyotype 47,XXY. Klinefelter syndrome is associated with decreased levels of testosterone and is the most common cause of congenital hypogonadism. Klinefelter Syndrome is associated with decreased levels of testosterone and is the most common cause of congenital hypogonadism Hypogonadism Hypogonadism is a condition characterized by reduced or no sex hormone production by the testes or ovaries. Hypogonadism can result from primary (hypergonadotropic) or secondary (hypogonadotropic) failure. Symptoms include infertility, increased risk of osteoporosis, erectile dysfunction, decreased libido, and regression (or absence) of secondary sexual characteristics. Hypogonadism. Individuals with the syndrome tend to present as tall, phenotypic men with small testes, decreased body hair, gynecomastia Gynecomastia Gynecomastia is a benign proliferation of male breast glandular ductal tissue, usually bilateral, caused by increased estrogen activity, decreased testosterone activity, or medications. The condition is common and physiological in neonates, adolescent boys, and elderly men. Gynecomastia, and infertility Infertility Infertility is the inability to conceive in the context of regular intercourse. The most common causes of infertility in women are related to ovulatory dysfunction or tubal obstruction, whereas, in men, abnormal sperm is a common cause. Infertility.
  • Turner syndrome Turner syndrome Turner syndrome is a genetic condition affecting women, in which 1 X chromosome is partly or completely missing. The classic result is the karyotype 45,XO with a female phenotype. Turner syndrome is associated with decreased sex hormone levels and is the most common cause of primary amenorrhea. Turner Syndrome: a genetic condition affecting women in whom 1 X chromosome is partly or completely missing. The classic result is the karyotype 45,X0 with a female phenotype. The characteristic appearance is that of a female person with short stature, a webbed neck, a broad chest with widely spaced nipples, a low posterior hairline, and peripheral edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema of the hands and feet.

References

  1. Clark, MA, Douglas, M, & Choi, J. (2018.) The chemistry of life: Introduction. Biology 2e. (2021). OpenStax. Retrieved August 15, 2021, from https://openstax.org/books/biology-2e/pages/1-introduction
  2. Jones, KL. Down syndrome. In: Smith’s recognizable patterns of human malformation, 6th ed, Elsevier Saunders, Philadelphia 2006. p.7.
  3. Gottlieb, S, Gulani, A, & Tegay, D. (2021). Genetics, Meiosis. Retrieved August 15, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK482462/
  4. O’Donnell, L, Nicholls, PK, O’Bryan, MK, McLachlan, RI, & Stanton PG. Spermiation: The process of sperm release. Spermatogenesis. 2011;1(1):14–35. https://pubmed.ncbi.nlm.nih.gov/21866274/
  5. Wattendorf, DJ, & Muenke, M. (2005). Klinefelter syndrome. Am Fam Physician.72(11):2259–2262.
  6. Morgan, T. (2007). Turner syndrome: Diagnosis and management. Am Fam Physician. 76(3):405–417.

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