Understandings:
- One diploid nucleus divides by meiosis to produce four haploid nuclei.
- The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
- DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
- The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
- Orientation of pairs of homologous chromosomes prior to separation is random.
- Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
- Crossing over and random orientation promotes genetic variation.
- Fusion of gametes from different parents promotes genetic variation.
Applications and skills: - Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
- Application: Studies showing age of parents influences chances of non-disjunction.
- Application: Description of methods used to obtain cells for karyotype analysis, e.g. chorionic villus sampling and amniocentesis, and the associated risks.
- Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Guidance
● Preparation of microscope slides showing meiosis is challenging and permanent slides should be available in case no cells in meiosis are visible in temporary mounts.
● Drawings of the stages of meiosis do not need to include chiasmata. ● The process of chiasmata formation need not be explained
PRODUCING FOUR HAPLOID NUCLEI
The vast majority of cells in a person’s body each contains 46 chromosomes. Gametes (sperm cells and egg cells) cannot contain 46 chromosomes for the simple reason that, if they did, when they fused together during fertilization, the baby that would be formed would have a total of 92 chromosomes, and each new generation would double its chromosome number, making an impossibly large amount of DNA to deal with. To avoid this problem of accumulating too many chromosomes, humans and other animals produce egg cells and sperm cells in such a way that the number of chromosomes in their nuclei is halved. Hence, sperms and eggs only contain 23 chromosomes, one from each pair, rather than complete pairs. In order to make such special cells with half the chromosomes, a special type of cell division is needed: meiosis. Such a splitting is called a reduction division.
HALVING THE CHROMOSOME NUMBER
Whereas mitosis produces diploid (2n) nuclei containing 46 chromosomes (organized into 23 pairs), meiosis produces haploid (n) nuclei that contain 23 chromosomes, each representing half of one pair. In the figure above, from a single cell on the left, four cells were produced on the right. Also, the number of chromosomes in the example is 4 in the parent cell at the start (so 2n=4), because there are 2 in each pair. In contrast, the number of chromosomes at the end is only 2 (n=2), because each 'pair' is not a pair anymore but rather a single representative from each pair. In the testes and oversee, respectively, meiosis produces haploid sperms and eggs, so that, when fertilization occurs, the zygote will receive 23+23=46 chromosomes; half from the mother and half from the father. This is how the problem of changing chromosome number is avoided. As a result, the human number of 46 is preserved by the sexual life cycle.
DNA IS REPLICATED BEFORE MEIOSIS
The reason why chromosomes are represented as having the shape reminiscent of the letter X or H, is because at this stage in chromosome's existence, the DNA has been replicated so that a full copy of the original DNA has been produced.
As a result, the single chromosome comprises two sister chromatids side-by-side and joined in the middle at the centromere
In reality, before the chromosomes start preparing for the cell to divide, they are all uncoiled and are not visible in the nucleus. This is one of the reasons why, when looking at cells under a microscope, it is not usually possible to see chromosomes all coiled up. It is only in the early stages of the preparation for cell division that condensation happens and the chromosomes coil up into the shapes you are being shown in this chapter.
PAIRING OF HOMOLOGOUS CHROMOSOMES AND CROSSING OVER
Meiosis is a step-by-step process by which a diploid parent cell produces four haploid daughter cells. Before the steps begin, DNA replication allows the cell to make a complete copy of its genetic information during interphase. This results in each chromatid having an identical copy, or sister chromatid, attached to it at the centromere.
In order to produce a total of four cells, the parent cell must divide twice: the first meiotic division makes two cells, and then each of these divides during the second meiotic division to make a total of four cells.
One of the characteristics that distinguishes meiosis from mitosis is that, during the first step, called prophase I, there is an exchange of genetic material between non-sister chromatids in a process called crossing over. This trading of segments of genes happens when sections of two homologous chromatids break at the same point, twist around each other, and then each connects to the other’s initial position.
Whereas mitosis produces diploid (2n) nuclei containing 46 chromosomes (organized into 23 pairs), meiosis produces haploid (n) nuclei that contain 23 chromosomes, each representing half of one pair. In the figure above, from a single cell on the left, four cells were produced on the right. Also, the number of chromosomes in the example is 4 in the parent cell at the start (so 2n=4), because there are 2 in each pair. In contrast, the number of chromosomes at the end is only 2 (n=2), because each 'pair' is not a pair anymore but rather a single representative from each pair. In the testes and oversee, respectively, meiosis produces haploid sperms and eggs, so that, when fertilization occurs, the zygote will receive 23+23=46 chromosomes; half from the mother and half from the father. This is how the problem of changing chromosome number is avoided. As a result, the human number of 46 is preserved by the sexual life cycle.
DNA IS REPLICATED BEFORE MEIOSIS
The reason why chromosomes are represented as having the shape reminiscent of the letter X or H, is because at this stage in chromosome's existence, the DNA has been replicated so that a full copy of the original DNA has been produced.
As a result, the single chromosome comprises two sister chromatids side-by-side and joined in the middle at the centromere
In reality, before the chromosomes start preparing for the cell to divide, they are all uncoiled and are not visible in the nucleus. This is one of the reasons why, when looking at cells under a microscope, it is not usually possible to see chromosomes all coiled up. It is only in the early stages of the preparation for cell division that condensation happens and the chromosomes coil up into the shapes you are being shown in this chapter.
PAIRING OF HOMOLOGOUS CHROMOSOMES AND CROSSING OVER
Meiosis is a step-by-step process by which a diploid parent cell produces four haploid daughter cells. Before the steps begin, DNA replication allows the cell to make a complete copy of its genetic information during interphase. This results in each chromatid having an identical copy, or sister chromatid, attached to it at the centromere.
In order to produce a total of four cells, the parent cell must divide twice: the first meiotic division makes two cells, and then each of these divides during the second meiotic division to make a total of four cells.
One of the characteristics that distinguishes meiosis from mitosis is that, during the first step, called prophase I, there is an exchange of genetic material between non-sister chromatids in a process called crossing over. This trading of segments of genes happens when sections of two homologous chromatids break at the same point, twist around each other, and then each connects to the other’s initial position.
Crossing over allows DNA from a person’s maternal chromosomes to mix with DNA from the paternal chromosomes. In this way, the recombinant chromatids that end up in the sperm or the egg cells are a mosaic of the two parent cells’ original chromatids. This helps increase the variety among offspring from the same two parents, and so increases the chances of survival of some offspring if one combination of alleles is more favourable for survival than others.
Meiosis I takes place in order to produce two cells, each with a single set of chromosomes
RANDOM ORIENTATION
during metaphase I, the homolgous pairs of chromosomes line up along the centre of the cell. The way that they happen to line up is by chance, and that is why it is called random orientation. As seen with crossing over, this is another adaptation that increases variety in the offspring. The result of random orientation is that a male will only very rarely produce two sperm cells that are identical. Likewise, for a female, it is highly likely that she will never produce the same egg twice in her lifetime. These are among the reasons why a couple will never have the same offspring twice. The only way that a male and a female can naturally have the same offspring twice is by producing identical twins, but, in this case, it is two children from the same egg cell and the same sperm cell.
HALVING THE CHROMOSOME NUMBER
Prophase I
Spindle fibres from the poles attach to chromosomes and pull them to opposite poles of the cell.
Telophase I
Now meiosis II takes place in order to separate the sister chromatids (see Figure 3.18).
Prophase II
FERTILIZATION AND VARIATION
As can be seen with siblings from the same mother and father who are not identical twins, crossing over during prophase I and random orientation during metaphase I allow variation in the offspring. There is one other way that genetic variation is also promoted: fertilization. When the egg and sperm cells meet, there is a great deal of chance involved. For example, a man can produce millions of different sperm cells, each with a unique combination of half his DNA.
How is this calculated? If only the number of chromosomes in each haploid cell (n) is considered, the calculation is 2n because there are two possible chromosomes in each pair (maternal and paternal) and there are n chromosomes in all. For humans, the number is 223 because there are 23 chromosomes in each gamete. So the probability that a woman could produce the same egg twice is 1 in 223 or 1 in 8 388 608. Even this calculation is an oversimplification, however, because it does not take into consideration the additional variety that results from crossing over.
In addition, the calculation 2n only considers one gamete. To produce offspring, two gametes are needed, and the chances that both parents produce two identical offspring (apart from identical twins) is infinitesimal.
Meiosis I takes place in order to produce two cells, each with a single set of chromosomes
RANDOM ORIENTATION
during metaphase I, the homolgous pairs of chromosomes line up along the centre of the cell. The way that they happen to line up is by chance, and that is why it is called random orientation. As seen with crossing over, this is another adaptation that increases variety in the offspring. The result of random orientation is that a male will only very rarely produce two sperm cells that are identical. Likewise, for a female, it is highly likely that she will never produce the same egg twice in her lifetime. These are among the reasons why a couple will never have the same offspring twice. The only way that a male and a female can naturally have the same offspring twice is by producing identical twins, but, in this case, it is two children from the same egg cell and the same sperm cell.
HALVING THE CHROMOSOME NUMBER
Prophase I
- Chromosomes become visible as the DNA becomes more compact.
- Homologous chromosomes, also called homologues, are attracted to each
other and pair up: one is from the individual’s father, the other from the
mother. - Crossing over occurs.
- Spindle fibres made from microtubules form.
- The homologous chromosomes line up across the cell’s equator by random orientation.
- The nuclear membrane disintegrates.
Spindle fibres from the poles attach to chromosomes and pull them to opposite poles of the cell.
Telophase I
- Spindles and spindle fibres disintegrate.
- Usually, the chromosomes uncoil and new nuclear membranes form.
- Many plants do not have a telophase I stage.
Now meiosis II takes place in order to separate the sister chromatids (see Figure 3.18).
Prophase II
- DNA condenses into visible chromosomes again.
- New meiotic spindle fibres are produced.
- Nuclear membranes disintegrate
- The individual chromosomes line up along the equator of each cell in no special order; this is called random orientation.
- Spindle fibers from opposite poles attach to each of the sister chromatids at the centromeres.
- Centromeres of each chromosome split, releasing each sister chromatid as an individual chromosome.
- The spindle fibers pull individual chromatids to opposite ends of the cell.
- Because of random orientation, the chromatids could be pulled towards either of the newly forming daughter cells.
- In animal cells, cell membrane pinch off in the middle, whereas in plant cells new cell plates form to demarcate the four cells.
- Chromosomes unwind their strands of DNA
- Nuclear envelopes form around each of the four haploid cells, preparing them for cytokinesis.
FERTILIZATION AND VARIATION
As can be seen with siblings from the same mother and father who are not identical twins, crossing over during prophase I and random orientation during metaphase I allow variation in the offspring. There is one other way that genetic variation is also promoted: fertilization. When the egg and sperm cells meet, there is a great deal of chance involved. For example, a man can produce millions of different sperm cells, each with a unique combination of half his DNA.
How is this calculated? If only the number of chromosomes in each haploid cell (n) is considered, the calculation is 2n because there are two possible chromosomes in each pair (maternal and paternal) and there are n chromosomes in all. For humans, the number is 223 because there are 23 chromosomes in each gamete. So the probability that a woman could produce the same egg twice is 1 in 223 or 1 in 8 388 608. Even this calculation is an oversimplification, however, because it does not take into consideration the additional variety that results from crossing over.
In addition, the calculation 2n only considers one gamete. To produce offspring, two gametes are needed, and the chances that both parents produce two identical offspring (apart from identical twins) is infinitesimal.
Extra or missing chromosomes
Sometimes errors occur during meiosis and a child can receive an atypical number
of chromosomes, such as 47 instead of 46. One such anomaly is called Down syndrome, and it happens when there is an extra chromosome in the 21st pair. The extra chromosome results from a phenomenon called non-disjunction, which can happen at different times but most often occurs when the 21st pair of homologous chromosomes fails to separate during anaphase I. Hence, the egg the woman produces has two 21st chromosomes instead of one. And when a sperm cell fertilizes the egg, the total number of 21st chromosomes is three.
Obtaining cells for karyotyping
An unborn baby’s cells can be extracted in one of two ways: either by a process called amniocentesis or by removing cells from the chorionic villus. Amniocentesis involves using a hypodermic needle to extract some of the amniotic fluid around the developing baby. Inside the liquid, some of the baby’s cells can be found and used for the preparation of a karyotype. For the second method, cells are obtained by chorionic villus sampling, which involves obtaining a tissue sample from the placenta’s finger-like projections into the uterus wall.
In either case, among the cells collected are foetal cells that are then grown in the laboratory. The preparation of a karyotype is an expensive and invasive procedure. It is usually used for seeing whether an unborn baby has any chromosomal anomalies, e.g. 45 or 47 chromosomes instead of 46. If the parents or doctors are concerned about the chromosomal integrity of an unborn child (for example, if an expectant mother is over the age of 35), a karyotype is recommended.