Why Meiosis?
Meiosis is a special type of cell division that is used in sexual reproduction. The cells that result from meiosis, such as sperm and eggs (gametes, or sex cells), have half the number of chromosomes as the parent. The sperm and egg from two different individuals can fuse to make a new, genetically different individual.
Take a look at the picture below that shows this process, called fertilization:
Ok, so that’s pretty cool – it explains why you share characteristics with each of your parents. But how does this happen? How can a cell divide and reduce the number of chromosomes? Why is it a good thing for offspring to be different from their parents? We’ll take a closer look at these questions throughout this lesson.
Meiosis
Watch the Ameoba Sisters’ video on Meiosis to preview the process of meiosis. Consider the following questions while you watch the video:
- Why is meiosis called “reduction division?”
- How many times does interphase occur?
- What are the stages of meiosis?
- What is crossing over?
- How many divisions happen in meiosis?
- Are the gametes (sperm/egg) produced by meiosis identical?
- What is non-disjunction?
Meiosis is “reduction division” whereby the cell divides twice to reduce the chromosome number by half. It results in four cells. It involves two divisions called Meiosis I and Meiosis II. Interphase occurs once, only before Meiosis 1.
Before reviewing the stages of meiosis in detail, you need to remind yourself about:
- Homologous Chromosomes – The 46 human chromosomes are arranged in 23 homologous pairs so that there are two of each type of chromosome. Each chromosome of the pair is called a homolog, or homologous chromosome. Each homolog carries genes for a particular trait in the same place on its own chromosome. For this reason, every individual has two alternate forms of each gene that are referred to as alleles. Half of an individuals genetic information comes from the mother and the other half comes from the father. (source)
- The terms diploid and haploid – Diploid Cells have two of every kind of chromosome, one set that came from each parent – Haploid Cells: have one of each kind chromosome; also known as sex cells or gametes
Watch the presentation on the stages/events of meiosis. Remember your note pages!
Why Meiosis … Again?
Meiosis does reduce chromosome number so that sexual reproduction can occur. But why is this beneficial? The answer is that you are able to produce offspring that are different from their parents. There are three ways that meiosis introduces genetic variation:
- crossing over
- random arrangement of tetrads in metaphase I
- random combinations of sperm and eggs
Gametogenesis: Making Sperm or Eggs
Gametogenesis is a fancy term that means to make sperm or eggs by meiosis. There are two types of gametogenesis:
- Oogenesis: meiosis that produces four cells, 1 large ovum and 3 tiny polar bodies. Only the ovum will survive to have a chance at being fertilized. The tiny polar bodies have the correct number of chromosomes, but they are not viable and dissolve.
- Spermatogenesis: meiosis that produces sperm; all four are tiny and all four are capable of fertilizing an egg (ova); process is similar to oogenesis
Problems with Meiosis
Sometimes meiosis does not proceed perfectly. When sister chromatids do not separate correctly, and the resulting cells do not receive the correct number of chromosomes, we call this nondisjunction.
If a cells has one too many chromosomes (3 copies of the chromosome) the condition is called trisomy. If a cell is lacking a chromosome (it only contains one of the two expected) it is called monosomy.
There are a number of genetic conditions that result from nondisjunction, including:
- Trisomy 21 or Down Syndrome
- Trisomy 23 or Klinefelter Syndrome: XXY
- Monosomy 23 or Turner Syndrome: XO
Notice that we name the condition based on how many chromosomes (trisomy, monosomy) and which chromosome is lacking/extra (21, 23).
Doctors can often determine these genetic conditions before a baby is born by examining its karyotype. Recall from a previous lesson that a karyotype is a picture of an individual’s chromosomes. To obtain a karyotype from a child in the womb, doctors may use:
- Amniocentesis: long needle withdraws fluid around fetus; some of the embryo’s cells that have sloughed off into the fluid will be collected and examined
- Chorionic Villi Testing: scraping a few cells from villi of the placenta which connect fetus to mother in the uterus
Instead of the invasive procedures above, ultrasound pictures can be taken to determine physical abnormalities that may indicate a genetic condition is possible.
Chromosome Number: Diploid or Haploid?
In humans, all of the body cells, or somatic cells (ex: skin cells, liver cells, bone cells, nerve cells, etc.) have 46 chromosomes. Each somatic cell has 23 homologous pairs consisting of 46 homologs.
Only the sex cells, called gametes (sperm and egg), of a human do not have all 46 chromosomes. These cells have half of the total chromosome number, in this case, 23. Gametes have one homolog of each chromosome pair in the nucleus of its cells. When a sperm and egg unite in fertilization, the total number of chromosomes is restored to 46.
When a body cell of an organism has the full amount of chromosomes, with all of its homologous pairs, such as a human cell containing 46 chromosomes, it is said to have the diploid number of chromosomes. This is because di- means two and these cells have two of each type of chromosome present. The human diploid number, then, is 46.
When a gamete of an organism has half of the full amount of chromosomes, such as a human sperm cell containing 23 chromosomes, it is said to have the haploid numberof chromosomes. Just think of haploid-half. The human haploid number, then, is 23.
How can body end up with the diploid number of chromosomes (the full amount) and other body cells, specifically the sex cells, end up with the haploid number of chromosomes (half of the full amount)? The answer is that there are two types of cell division: mitosis and meiosis.
- In mitosis, that the body cells (skin, nerve, liver, bone, etc.) are produced by a type of cell division that begins with mitosis. The end result of mitosis, followed by cytokinesis, is two identical diploid somatic cells.
- Gametes (egg and sperm) are produced in the ovaries and testes. They are produced by a type of cell division called meiosis. The end result of meiosis is the production of 4 haploid gametes.
(source)
Mitosis vs. Meiosis
Although mitosis and meiosis are both types of cell division and therefore have many things in common, they are also different in significant ways. Each plays a role in life on Earth that is significant.
Mitosis, if you recall, allows for eukaryotic organisms to grow (in size), repair tissue and even reproduce. Meiosis allows eukaryotic organisms to produce gametes and introduces variation among organisms.
Sexual vs. Asexual Reproduction
Organisms that produce gametes (sperm, egg, pollen or spores) reproduce sexually. On the other hand, organisms that reproduce by dividing, budding or fragmentation reproduce asexually. Some species are able to utilize both types of reproduction.
Advantages and disadvantages of sexual reproduction compared to asexual reproduction:
| Sexually Reproducing Organisms | Asexually Reproducing Organisms | |
|---|---|---|
| Number of Offspring | Usually limited to small sets, 1 to 10 at a time, but can be hundreds in the case of some arthropods | Generally only one new organism per division |
| Number of Parents Required to Reproduce | At least 2; some species will not reproduce if the population is small | Only one, since it simply divides itself |
| Genetic Variation | Yes, through crossing over, independent assortment of chromosomes and recombination of genomes | None; each offspring is an exact copy of its parent |
| Gestation or Care of Young | Yes, some internally reproducing organisms have protective structures for embryos and may care for their young | None; no responsibility for parent |
| Adaptability to Changing Environmental Conditions | Good since traits are randomly shuffled and passed on to offspring | Poor since the only opportunity for variation comes from mutations |
| Lifespan | Lifespan can be very short or hundreds of years | Lifespan is usually short since each generation “disappears” with each division |
| Reproductive Season | Limited; can depend upon age of individuals or specific times of fertility | Unlimited; can reproduce as frequently as every 30 minutes, indefinitely |
(source)