Meiosis is a type of cell division which results in 4 cells that are genetically non-identical from one parent cell. In order for once cell to divide to result in 4 cells, how many divisions must take place?
Two. 1 cell becomes 2, then 2 become 4:
The first division is called meiosis I, and the second is called meiosis II.
…so far so easy? (it should be!)
Cells resulting from meiosis are gametes such as egg cells and sperm cells, hence meiosis only occurs in sexually reproducing organisms. There are 2 key points about this:
1. Gametes are genetically unlike one another – while cells in other tissues such as muscle or blood must be genetically identical to one another (clones), the very basis of sexual reproduction is genetic diversity. So somewhere in the process of division, something takes place which creates genetic diversity (we’ll come to that shortly).
2. If gametes are to fuse and result in a new organism, it is essential that the number of chromosomes should stay constant. Humans have 46 chromosomes in each cell (of course, apart from cells without DNA in them, and “spoiler alert!”, gametes) – if each gamete had 46 chromosomes, then fusing 2 together would result in a zygote with 92 chromosomes, whose offspring would have 184 chromosomes, and before you know it something terrible would have happened.
The above picture illustrates how the number of chromosomes is halved in the final 4 cells. The terms diploid and haploid refer to the number of sets of chromosomes. In humans, somatic cells (i.e. cells other than gametes) are diploid because there are two sets of chromosomes. Gametes are haploid because they have only one set of chromosomes.
A “set” is made up of all chromosomes which are unique, i.e. are not paired with any homologous chromosomes.
X x X X x X x x <———- haploid = 1 set
XX xx XX XX xx XX xx xx <———- diploid = 2 sets
In the first XX, X and X are homologous chromosomes because they occupy the same space and contain DNA with similar purpose/function. Essentially, they are more or less copies of each other. So when 2 gametes fuse, they form a diploid cell with the complete number of chromosomes.
Wikipedia does us the honour with this epic picture:
On to the very important bit now…
How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?
10 words: Independent Assortment of Homologous Chromosomes, &
Genetic Recombination by Crossing Over
What an unnecessary mouthful. You still have to learn them though.
Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X’s and which small x’s end up with each other in each cell is random. Pretty simple concept.
If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.
Genetic recombination by crossing over is a lot more interesting. It’s like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:
Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs. Also, the point where chromosomes cross over (in the diagram red “b” overlaps blue “B”) is called the chiasma. Chiasma formation enables the exchange of genetic material.
As a finishing touch, I read of this mnemonic to remember the purpose of meiosis.
It is so cringe-worthy, I would rather memorise meiosis off by heart.
In the UK, the antenatal (before birth) care programme provides people trying to conceive and those pregnant with advice, screenings and other help such as vouchers to optimise the health outcome of the pregnancy, through the NHS.
Prior to conception, pre-conceptual care is given through advice and monitoring of health to improve the probability of conceiving for people who find it challenging. Preparing for conception and pregnancy involves things such as improving one’s health, starting to take folic acid if aiming to become pregnant, improving diet and exercise to make conception more likely, learning how to track ovulation, and quitting noxious substances from drinking and smoking. These steps can increase the likelihood of conception through maximising sperm production and health, fertilisation and a good starting point for the beginning of pregnancy.
Diet improvements once pregnant (post-conceptual care) involve the supplementation of folic acid. This compound has been associated with a decreased probability of neural tube defects (that cause conditions such as spina bifida) in the developing foetus in the first 12 weeks of gestation, while the spinal cord is developing.
Other key nutrients include iron to prevent anaemia as the pregnancy increases the demand of respiratory substrates, calcium and vitamin D which together regulate bone formation, and vitamin C which is protective to cells. Some nutrients such as vitamin C can be obtained easily through an adequate diet, while others such as folic acid need to be supplemented.
Healthy Start vitamins is a voucher-based initiative that helps families with key nutrients. Types of items covered by the programme include vitamins, milk and vegetables.
Toxic substances such as those in alcoholic drinks and tobacco smoke are detrimental to both conception and pregnancy. Removing them from one’s diet is one of the pre-conceptual care steps, as well as indicated once pregnant. Foetal growth and development are affected negatively by these toxins.
Miscarriage, developmental issues such as Foetal Alcohol Syndrome and later issues such as learning difficulties are associated with drinking alcohol while pregnant. Severity of these problems increases with the amount of alcohol consumed. It’s advisable to not consume any alcohol at all, ideally.
Smoking harms the foetus and is associated with lower birth weights, cot death (sudden infant death) and other later issues including asthma.
Assessing foetal development
Ultrasound scans can reveal information about the developing foetus. Various measurements, such as the biparietal diameter (BPD) of the cranium and crown-rump length of the back can be used to determine the age of a pregnancy (gestational age) and assess whether the foetus is developing as expected in terms of size.
BPD is measured as the maximum width of the head (temple to temple) as seen from the top, while the crown-rump length is the length of the whole foetus in the curled “C” position.
These measurements alongside others such as thigh length can be used to estimate the foetus weight (estimated foetal weight, EFW). The proportion of different weights over the time of gestation provides information on how close the foetus is to the expected size. Charts like this can be used to check where a particular weight falls in this range.
At 32 weeks for example, an estimated weight of 1600 grams would match the 10th percentile, meaning that this places it closer to the underweight side. Half of foetuses are around 1900 grams at 32 weeks (50th percentile), while the heaviest are around 2300 grams (90th percentile).
Diagnostic procedures regarding foetuses include ultrasonography, amniocentesis and chorionic villus sampling.
Ultrasonography is not invasive and hence a very practical option for things such as sex determination from around 12 weeks. By visualising various features of the foetus, diagnostics can be run to check for conditions including Down’s syndrome. A nuchal scan can reveal various conditions by measuring the thickness of the translucent portion beneath the foetus neck. This is not conclusive, and must be taken alongside other data such as the age of the person pregnant with the foetus.
Amniocentesis is carried out invasively and carries an up to 1% risk of miscarriage. It involves extracting a sample of amniotic fluid from near the foetus, and analysing the DNA in its residual foetal tissue for sex determination, checking for infections and chromosomal conditions. It is performed between 14 and 16 weeks of gestation.
Chorionic villus sampling (CVS) can be performed earlier than amniocentesis, at 10 – 12 weeks. It also uses DNA to check for chromosomal and genetic conditions. The data is based on a placental sample rather than amniotic fluid (the chorionic villi are part of the placenta). Carrying out chorionic villus sampling follows findings from ultrasonography, such as high nuchal transluscence. The procedure’s miscarriage risk is up to 2%, in addition to a risk of infection and leakage of amniotic fluid.
Overall, amniocentesis and chorionic villus sampling provide conclusive results as they analyse DNA itself, while posing slight risks to the pregnancy. Ultrasonography is not as conclusive, but is safer. CVS can be carried out earlier than amniocentesis.
Karyotyping is the determination of an individual’s chromosomes. It serves to indicate chromosomal conditions such as Down’s syndrome (caused by chromosomal mutation) as well as sex determination.
Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome
During meiosis, different chromosome distribution in the gametes can occur which can leave them without the expected number of chromosomes. If there are more chromosomes than expected (2 of each in humans), this is termed polysomy and can result in conditions such as Down’s syndrome. If there are fewer chromosomes than expected, it is monosomy and can result in conditions such as Turner’s syndrome.
Down’s syndrome involves an extra chromosome 21, and expresses itself in terms of many different features, some of which are detrimental to health. Common outcomes include unique facial features, slower overall development, higher incidence of congenital heart abnormalities, decreased or absent fertility and overall lower life expectancy.
Turner’s syndrome is in a way the “reverse” of Down’s syndrome as it presents one fewer chromosome rather than one extra. Specifically, it is a diminished or absent X chromosome. Since XY embryos missing their only X chromosome would not be viable, this syndrome only presents itself in births of would-be XX babies who end up having just one X chromosome, or one X and a partial X.
As many as 99% of Turner’s syndrome cases are thought to terminate via miscarriage or stillbirth. For those who survive and are born alive, common features include a webbed neck, low-set ears, short stature, lack of puberty without hormonal treatment and heart defects. Their overall life expectancy is shorter due to the development of heart disease, diabetes, thyroid problems and others, and constant health monitoring is required.
Klinefelter’s syndrome is a relatively common chromosomal variation (up to 1 in 500 male assigned births) where an XY individual has an extra X chromosome, hence being an XXY individual. Life expectancy is comparable to XY individuals.
Symptoms of the condition include reduced fertility or infertility, lower levels of testosterone, increased height, reduced muscle strength and coordination, breast growth, and learning and speech difficulties. Interventions include speech therapy, testosterone replacement therapy, breast surgery and assisted reproductive technology.