Meiosis | Cell division | Biology (article)

Meiosis


Whereas mitosis occurs in somatic tissue and results in tow identical daughter cells, meiosis occurs in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes). Meiosis shares some likenesses with mitosis. In both processes, for example, the genetic material must be duplicated, chromatin is condensed to form chromosomes and microtubules emanating from centrioles are involved in dividing genetic material. However, the MCAT tends to ask regarding the differences between these two processes.

In contrast to mitosis, which consists of one round of replication and division, meiosis consists of one round of replication pursued by two rounds of division, as shown in Figure 2.5. Meiosis I consequences in homologous chromosomes being isolated, generating haploid daughter cells; this is studied/known as a reductional division. Meiosis II similar to mitosis, in that it results in the segregation of sister chromatids and is known as equational division.

Meiosis I

The human genome is composed of 23 homologous couples of chromosomes (homologues), each of which contains one chromosome inherited from each parent. This brings up an important note about terminology: whereas homologous pairs are considered separate chromosomes (such as maternal chromosome 15 and paternal chromosome 15), sister chromatids are duplicate strands of DNA connected at the centromere. Hence, after S phase, there are 92 chromatids arranged into 46 chromosomes, that are organized into 23 homologous pairs.

Prophase I

During prophase I, the chromatin precipitates into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear. The first vital difference between meiosis and mitosis occurs at this point: homologous chromosomes come together and intertwine in a process called synapsis. At this point, every chromosome consists of two sister chromatids, so every synaptic pair contains four chromatids and is referred to as a tetrad. Chromatids of homologous chromosomes may break at the point of synapsis, called the chiasma (plural: chiasmata) and exchange equal parts of DNA, as shown in Figure 2.6 This process is called crossing over. 

Note that crossing over occurs amidst homologous chromosomes and not between sister chromatids of the same chromosome⎯the latter are identical, so crossing over would not produce any change. Those chromatids included are left with a changed but structurally complete set of genes. Such genetic recombination can unlink joined genes, thereby increasing the variety of genetic combinations that can be produced via gametogenesis. 

Linkage refers to the trend for genes to be hereditary together; genes that are found further from each other physically are fewer likely to be inherited together, and more likely to undergo crossing over relative to each other. Thus, as opposed to asexual reproduction, which produces identical offspring, sexual reproduction provides the advantage of great genetic diversity, which is believed to increase the mastery of a family to evolve and adapt to a changing environment.



REAL WORLD

The rate of gene unlinking is used to map differences between two genes on the same chromosome. The farther apart two genes are, the more probably they are to become unlinked during crossing over. These statistics can then be practiced to prevail the distance between genes on the chromosome, measured in units called centimorgans.

Because of crossing over, each daughter cell will have a unique pool of alleles (genes coding for different forms of a given trait) from a random mixture of maternal and paternal origin. In classical genetics, crossing over explains Mendel’s second law (of independent assortment), which states that the inheritance of one allele has no impact on the likelihood of inheriting specific alleles for other genes.

Metaphase I

During metaphase I, homologous pairs (tetrads) align at the metaphase plate, and every pair attaches to a separate spindle fiber by its kinetochore. Note the difference from mitosis: in mitosis, every chromosome is lined up on the metaphase plate by two spindle fibers (one from each pole); in meiosis, homologous chromosomes are lined up crosswise from each other at the metaphase plate and are held by one spindle fiber.

Anaphase I 

During anaphase I, homologous pairs divide and are pulled to opposite poles of the cell. This process is known as disjunction, and it accounts for Mendel’s first law (of segregation). During disjunction, every chromosome of paternal origin separates (or disjoins) from its homologue of maternal origin, and either chromosome can end up in either daughter cell. Consequently, the division of homologous chromosomes to the two intermediate daughter cells is random with respect to the parental origin. This dividing of the two homologous chromosomes is referred to as segregation.

KEY CONCEPT

It is significant to understand how meiosis I is varying from mitosis. The chromosome numeral is halved (reductional division) in meiosis I, and the daughter cells have the haploid number of chromosomes (23 in humans). Meiosis II is similar to mitosis in that sister chromatids are disjunct from each other; therefore, no alter in ploidy is watched.

Telophase I

During telophase I, a nuclear membrane forms around every new nucleus. At this point, every chromosome still consists of two sister chromatids joined at the centromere. The cells are now haploid; ance homologous chromosomes separate, only n chromosomes are found in each daughter cell (23 in humans). The cell isolates into two daughter cells by cytokinesis. Between cell separations, there may be a short rest period, or interkinesis, during which the chromosomes partially uncoil.

REAL WORLD

If during anaphase I or II of meiosis, homologous chromosomes (anaphase I) or sister chromatids (anaphase II) fail to diverge, one of the resulting gametes will have two copies of a particular chromosome and the other gamete will have none. Afterward, during fertilization, the resulting zygote may have too many or too few copies of that chromosome. Nondisjunction can impact both autosomal chromosomes (like trisomy 21, resulting in Down syndrome) and the sex chromosomes (like Klinefelter’s and Turner syndromes).

MEIOSIS II

Meiosis II is very similar to mitosis in that sister chromatids instead than homologues are separated from each other.

Prophase II 

During prophase II, the nuclear envelope disintegrates, nucleoli disappear, the centrioles migrate to opposite poles, and the spindle apparatus begins to form.

Metaphase II

During metaphase II, the chromosomes queue on the metaphase plate.

Anaphase II

During anaphase II, the centromeres separate, isolating the chromosomes into sister chromatids. These chromatids are dragged to opposite poles by spindle fibers.

Telophase II

During telophase II, a nuclear membrane forms around every new nucleus. Cytokinesis follows, and two daughter cells are established/formed. Thus, by the completion of meiosis II, up to four haploid daughter cells are produced per gametocyte. We use the phrase up to on account of oogenesis discussed later in this chapter, which may result in fewer than four cells if an egg remains unfertilized after ovulation.

KEY CONCEPT

Mitosis

  1. 2n → 2n
  2. Occurs in all separating cells
  3. Homologous chromosomes do not pair
  4. No crossing over

Meiosis

  1. 2n → n
  2. Occurs in sex cells only
  3. Homologous chromosomes align on the opposite side of the metaphase plate
  4. Crossing over can occur

MCAT Concept Check 2.2:

Before you go forward, estimate/assess your understanding of the material with these important questions.

1. What is the ploidy of the daughter cells generated from meiosis I? From meiosis II?

2. What is the difference between homologous chromosomes and sister chromatids?

3. For every phase of meiosis I recorded below, what are the differences from the analogous phase of mitosis?

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