Meiosis

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Meiotic division

Meiosis is the process by which gametos are produced in the reproductive organs

It involves the reduction divison of a diploid germline cell into four genetically distinct haploid nuclei

The process involves two cellular divisions

The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)

The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)

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Sister chromatids

Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies.

Two indenetical DNA molecules are identified as sister chromatids, and are held together by a single centromere

The single chromatids are separated during meiosis II , following the separations of homologous chromosomes in meiosis I

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Stages of meiosis

Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)

Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids.

A second growth phase known as interkinesis may occur between meiosis I and meiosis II, however no DNA replication takes place at this stage.

Meiosis I

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

Prophase I : chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs

Metaphase I: Spindle fibres from opposing cetrosomes connect to bivalents and align them along the centre of the cell

Anaphase I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

Telophase I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

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Meiosis II

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

Prophase II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

Metaphase II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

Anaphase II: Spindle fibres contract and separate the sister chromatids. Chromatids (now called chromosomes) move to opposite poles.

Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells.

The final outcome of meiosis is the production of four haploid daughter cells

These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

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Crossing over:

In prophase I, homologous chromosomes undergo a process my named synapsis, where they pair up to become bivalent.

The homologous chromosomes are held together at points called chiasmata

Crossing over of genetic material between non-sister chromatids can occur at the chiasmata

As a result Of the exchange of genetic material, new gene combinations are formed on chromatids (recombination) .

Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis

If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)

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Random Assortment.

During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:

Maternal copy left / paternal copy right or paternal copy left / maternal copy right

This orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes

The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I

As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs

Gamete combinations = 2n (where n represents the haploid number)

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Sexual life cycle

Most sexually reproducing organisms are diploid, meaning they have two copies of every chromosome (maternal / paternal copy)

In order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome)

Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis

If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)

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Genetic variation

The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring

The three main sources of genetic variation arising from sexual reproduction are:

Crossing over (in prophase I)

Random assortment of chromosomes (in metaphase I)

Random fusion of gametes from different parents

Random Fertilisation

The fusion of two haploid gametes results in the formation of a diploid zygote

This zygote can then divide by mitosis and differentiate to form a developing embryo

As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses

Random Orientation

When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random

The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I

The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes

Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation

If crossing over also occurs, the number of different gamete combinations becomes immeasurable

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Non-Disjunction

Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

The failure of chromosomes to separate may occur via:

Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)

Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)

If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body

Conditions that arise from non-disjunction events include:

Patau’s Syndrome (trisomy 13)

Edwards Syndrome (trisomy 18)

Down Syndrome (trisomy 21)

Klinefelter Syndrome (XXY)

Turner’s Syndrome / Fragile X (monosomy X)

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)

One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction

The other parental gamete was normal and had a single copy of chromosome 21

When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21

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Karyotyping

Karyotyping is the process by which chromosomes are organised and visualised for inspection.

Karyotyping is normally used to determine the gender of an unborn child and test for chromosomal abnormalities.

Cells are collected from the foetus and chemically induced to undertake cell division ( in order for chromosomes to be visible)

The stage in which mitosis is arrested will determine whether sister chromatids are visible

Finally, chromosomes are stained and photographed, before being organised according to structure

The visual profile generated is called a karyogram

Chronic villi

Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix

It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)

Amniocentesis

Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle

It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)