Table listing the terms used to describe different states of polyploidy

In diploid organisms, one set of chromosomes comes from each parent. When the chromosomes of a diploid cell are arranged in terms of their appearance (called a karyotype), it is evident that chromosomes can be paired based on their physical characteristics, as shown in the following figure.

This figure shows a human karyotype. There are 23 pairs of chromosomes (total = 46). Chromosome pairs 1-23 are autosomes, while chromosome pair 23 are the sex chromosomes (XX = female; XY = male). Chromosome 1 is the largest, while chromosome 21 is the smallest - the sizes of the chromosomes decrease from chromosome 1 to chromosome Y. Image credit: NGHRI.

Table listing the lengths of human chromosomes. Chromosome lengths are estimated by multiplying the number of base pairs by 0.34 nanometers (the distance between base pairs in DNA); Credit: Wikipedia.

Thus a pair of chromosomes with similar physical characteristics is referred to as homologous chromosomes (recall that each chromosome of a homologous pair comes from a biological parent). The shared characteristics between a homologous pair of chromosomes include:

• similar lengths

• similar gene positions

• similar locations of the centromeres

The following figure illustrates the shared characteristics of human homologous chromosomes.

Figure showing the human karyotype, with homologous chromosome pairs showing similar lengths, G-banding patterns and centromere location. Image credit: By Courtesy: National Human Genome Research Institute - Modified from Human Genome ProjectFrom en: with same file name, contributor: en:User:TedE, Public Domain.

Figure showing the arrangement of genes in a pair of homologous chromosomes. Note that the arrangement of genes in the two chlorosomes is the same, but there may be allelic differences. For example, in this image, both chromosomes contain the same r, P, A and b genes, but have c and C alleles for the C gene. Image credit: Biology Stackexchange.

An overview of meiosis

Cells enter meiosis after duplicating their genome

An outline of meiosis: 1-2: Interphase, including DNA replication. 3: Homologous recombination in meiosis I; 4: Completion of meiosis I; 5: meiosis II. Image credit: Peter Coxhead. CC 1.0.

Meiosis I

Meiosis I differs from mitosis I in several significant ways.

Prophase I

As in mitosis, chromosome condensation, centrosome duplication, spindle fibre formation, and nuclear envelope breakdown occur. However, the chromosomal events in prophase I are different to mitotic prophase.

Leptotene (“thin threads”)

Leptotene is the phase during which chromosomal condensation occurs

Zygotene (“paired threads”)

Homologous chromosomes are lined up next to each other, in close proximity (~100 nm). This pairing of homologous chromosomes is also called synapsis. The homologous chromosome pairs are held together by a protein complex called the synaptonemal complex.

Pachytene (“thick threads”)

At various points along their lengths, non-sister chromatids undergo a process known as recombination (also referred to as crossing over). During recombination, the DNAs of the sister chromatids go through double-stranded breaks and recombine. Thus, segments of DNA are exchanged between the non-sister chromatids. Those break (recombination) points are called chiasma (singular, chiasmata).

Diplotene ("two threads")

The synaptonemal complex holding the homologous chromosome pairs breakdown, and the chromosome pairs move slightly apart. However, the non-sister chromatids are still joined at the chiasma.

Diakinesis

Chromosomal condensation is complete. Visually, the chromosomes appear as tetrads and are attached to spindle fibres. The diplotene stage signals the end of prophase I.

Formation of the synaptonemal complex between homologous chromosome pairs. Image credit: Bioninja

Recombination (crossing over) and chiasma formation in a homologous chromosome pair. The chiasma are important for the segregation of homologous chromosomes during Anaphase I. Since there are no crossing-over events in mitosis, homologous chromosome pairs behave independently. However, because of chiasma formation in meiosis I, the homologous chromosome pairs remain together during Prophase I and Metaphase I. It is only in Anaphase I that the homologous chromosome pairs separate and move into different daughter cells. Image credit: Biology Stackexchange

Double-stranded breaks (DSB) form between non-sister chromatids during recombination. At the DSB (white arrows), protein “bridges” form between the non-sister chromatids. Some of these DSBs will undergo recombination, resulting in the exchange of DNA between those non-sister chromatids. As a result, the recombination points become chiasma. Image credit: Gerton, J. L. & Hawley, R. S. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6, 481 (2005). 

Chromosomal events in Prophase I. Image credit: Bioninja

Metaphase I

The tetrads line up along the metaphase plate.

Anaphase I

The spindle fibres contract, causing the homologous chromosome pairs to separate.

Telophase I

Two nuclei form.

Cytokinesis

Two daughter cells form as the cytoplasm divides between the newly-formed nuclei. Each daughter cell has a haploid complement of chromosomes.

Meiosis II

Meiosis II follows Meiosis I. The phases of Meiosis II are identical to those of mitosis. After the completion of meiosis II and cytokinesis, four haploid daughter cells are formed.

The phases of meiosis. Image credit: Ali Zifan - Own work; Used information from Campbell Biology (10th Edition) by: Jane B. Reece & Steven A. Wasserman., CC BY-SA 4.0

Genetic variation caused by meiosis

In sexually reproducing species, meiosis causes genetic variation between generations. The three main processes responsible for this are:

1. Homologous recombination in Prophase I. Complementary regions of non-sister chromatids undergo double-stranded DNA breaks and are swapped. This results in maternal and paternal alleles being exchanged. One analogy to describe homologous recombination is the shuffling of two decks of playing cards: if two piles of cards are made (e.g. red and black), then the shuffling and combining of these piles brings the red and black cards for each suit together.

2. Random segregation of homologous chromosomes in Anaphase I: In Metaphase I, homologous chromosome pairs line up along the metaphase (equatorial) plate. However, for each homologous pair, the maternal and paternal chromosomes line up randomly on either side of the metaphase plate. Thus, during Anaphase I, there is a randomised collection of maternal and paternal homologs moving towards each pole. Consequently, the two haploid daughter cells that form after meiosis I will contain different maternal and paternal chromosome combinations. The random segregation of homologous chromosome pairs is also referred to as the Principle of Independent Assortment. In human cells, there are 223 (~ 8 million) possible combinations of maternal and paternal chromosomes at each meiotic division.

3. Random fertilisation of gametes: The gametes produced as a result of meiosis are genetically diverse. During sexual reproduction, one male gamete will fertilise a single female gamete. This process is random; thus, the zygote is genetically distinct from its parents.

Homologous recombination between non-sister chromatids. Image credit: Ellen Sidransky: Homologous recombination. The image is in the public domain. Courtesy: National Human Genome Research Institute

Independent assortment of homologous chromosomes. In Metaphase I, there are two possible maternal and paternal chromosome arrangements. Each possibility produces daughter cells with different complements of maternal and paternal chromosomes. Image credit: Bioninja

Sexual life cycles

Based on the preceding discussion, it is evident that most sexually reproducing have distinct haploid and diploid phases in their life cycle. In organisms such as humans, both the diploid and haploid phases occur within each individual person. However, in other organisms such as fungi and ferns, those phases occupy distinct stages of their life cycles.

In animals such as humans, haploid gametes form in diploid adults. Fusion of the gametes gives rise to a fertilised egg cell or zygote. In humans, fertilisation is internal, while in some other animals, fertilisation is external. The zygote will divide mitotically to produce multicellular offspring. Embryonic development may be internal (e.g. humans) or external (e.g. amphibians, birds, reptiles). In humans, the diploid stage is dominant. Image credit: LabXchange

In many fungi, the haploid stage is dominant. The diploid zygospore divides meiotically and produces haploid spores. Germination of the haploid spores gives rise to haploid hyphae. These hyphae can produce haploid spores through mitosis (the asexual life cycle). When conditions become harsh, haploid hyphae from different mating types can fuse to produce diploid organisms. Image credit: LabXchange (Zygomycota micrograph: modification of work by “Fanaberka”/Wikimedia Commons).

Plants, such as ferns, also demonstrate alternating generations (equal parts of their life cycles are in diploid and haploid phases). The diploid sporophyte produces haploid spores through meiosis. The haploid spores germinate to produce haploid gametophyte. Notice that haploid gametophytes and diploid sporophytes are structurally different. The haploid sporophytes produce haploid gametes (eggs and sperm cells), which fertilise to produce the diploid sporophyte. Image credit: LabXchange (“fern”: modification of work by Cory Zanker; “sporangia”: modification of work by “Obsidian Soul”/Wikimedia Commons; “gametophyte and sporophyte”: modification of work by “Vlmastra”/Wikimedia Commons).