Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Nature of science: Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells.
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 nondisjunction. • 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.
Theory of knowledge: • In 1922 the number of chromosomes counted in a human cell was 48. This remained the established number for 30 years, even though a review of photographic evidence from the time clearly showed that there were 46. For what reasons do existing beliefs carry a certain inertia?
Utilization: • An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling.
Aims: • Aim 8: Pre-natal screening for chromosome abnormalities gives an indication of the sex of the fetus and raises ethical issues over selective abortion of female fetuses in some countries.
Understanding: One diploid nucleus divides by meiosis to produce four haploid nuclei.
Meiosis is the process in which the diploid (2n) nucleus divides to form four haploid (n) nuclei
Meiosis has two divisions called Meiosis I and Meiosis II
In the first division the diploid nucleus 2n, which consists of homologous pairs of chromosomes (half maternal and half paternal chromosomes), divides to form two haploid cells (n). These cells after the first division are considered haploid because the homologous pairs of the nucleus are separated into the two new cells.
In meiosis II, the haploid chromosomes in the two cells (each have 2 chromatids because replication occurs before meiosis takes place) divide to form four haploid cells each with one set of chromosomes
This is called reduction division because the chromosome number is halved
Understanding: The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
During sexual reproduction there is the fusion of two gametes to form a new cell with double the number of chromosomes. The fusion of the gametes takes place during fertilization.
If an organism did not reduce or half the number of chromosomes during meiosis before fertilization took place, the new cell would contain double the number of chromosomes in comparison to the original cell.
This means there would be a doubling of chromosomes with each new generation or sexual life cycle.
This is why reduction division during meiosis is essential for the sexual life cycle to occur in eukaryotes.
This also creates genetic diversity as the alleles on the chromosomes from each parent might be different.
In prokaryotes asexual division occurs given rise to offspring that are genetically identical to their parents Understanding: DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
Chromosomes are replicated in the synthesis (S) phase during interphase
This means that each chromosome will have an attached identical copy before meiosis occurs
These are called sister chromatids
Understanding: The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
At the start of meiosis (prophase I), the replicated chromosomes begin to condense and become visible.
Homologous chromosomes synapse (pair up) to form bivalents or tetrads.
Crossing over occurs between non-sister chromatids. Crossing over occurs when two of the non-sister chromatids exchange a segment of their chromosome with each other. Since the genes between the two chromosomes are the same, but the alleles may differ between the maternal and paternal chromosome, a new combination of alleles will be present when the chromosomes separate.
These crossover points are random and lead to genetic variation in the gametes
Understanding: Orientation of pairs of homologous chromosomes prior to separation is random.
When homologues line up along the equatorial plate in metaphase I, the orientation of each pair is random; meaning the maternal or paternal homologue can orient towards either pole.
The two homologous chromosomes in each bivalent is attached to a different spindle fiber, randomly attaching them to either pole
The orientation of how one set of chromosomes lines up has no effect on the other bivalents (i.e. The bivalent formed for chromosome 1, does not affect how the bivalent for chromosome 2 will orient)
This means the number of combinations that can occur in the gamete is 2n (n=number of chromosome pairs).
Therefore, in a female or male gamete there can be 223 or 8,388,608 different possible combinations.
Now when you consider there is the same number of possible combinations in the other gamete that it will combine with to form a zygote (random fertilization); the genetic possibilities are staggering.
If one takes into consideration crossing over, which was explained above, the genetic variation possibilities in the offspring is immeasurable
Understanding: Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
In meiosis I, homologous chromosomes split, but the centromeres do not divide since the sister chromatids do not separate
One chromosome from each pair separate and migrate towards separate poles. This separation is called a disjunction.
This halves the chromosome number of each cell and is therefore called reduction division. The two new cells formed after the first division are haploid (n)
Understanding: Crossing over and random orientation promotes genetic variation.
Meiosis is the formation of gametes that produce offspring that are genetically different than their parents.
The two main ways variation is created in the offspring is through crossing-over and through random orientation of the chromosomes.
Crossing over
Occurs in prophase I of meiosis.
Crossing over occurs between non-sister chromatids of a particular chromosome.
Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis.
Chromosomes intertwine and break at the exact same positions in non-sister chromatids.
Segments of the adjacent homologues are exchanged during crossing over, therefore the two sister chromatids are no longer identical.
Crossing over creates new combinations of linked genes (genes on the same chromosome) from the mother and the father.
When the chromatids are separated into different gametes after anaphase II, the gametes produced will not contain the same combination of alleles as the parental chromosomes.
This creates variation in the offspring regardless of random orientation.
Random Variation
Occurs in metaphase I of meiosis.
When homologues line up along the equatorial plate in metaphase I, the orientation of each pair is random; meaning the maternal or paternal homologue can orient toward either pole.
This means the number of combinations that can occur in the gamete is 2n (n=number of chromosome pairs).
Therefore, in a female or male gamete there can be 223 or 8,388,608 different possible combinations.
Now when you consider there is the same number of possible combinations in the other gamete that it will combine with to form a zygote (random fertilization); the genetic possibilities are staggering.
If one takes into consideration crossing over, which was explained above, the genetic variation possibilities in the offspring is immeasurable.
Understanding: Fusion of gametes from different parents promotes genetic variation.
The fusion of two gametes to form a zygote is the start of a new organism and new life
It combines genetic information from two different individuals
As explained above in the section on random variation. When considering the different combinations that could exist in each gamete, the number of male gametes released at one time (millions), and because of crossing over, the possible different combinations of alleles in the zygote is immeasurable
Fusion of gametes from different parents therefore, promotes genetic variation
Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
A non-disjunction is an error in meiosis, where the chromosome pairs fail to split during cell division.
Non-disjunction can occur in anaphase I where the homologous pairs fail to split, or it can occur in anaphase II, where the sister chromatids fail to split.
The result of this error is too many chromosomes in a gamete cell or too few chromosomes in the final gamete cell.
One of the gamete cellscould have 22 chromosomes and one could have 24 chromosomes. The resulting zygote will therefore have 47 or 45 chromosomes.
An example of a non-disjunction is Down syndrome.
Down syndrome occurs when chromosome 21 fails to separate, and one of the gametes ends up with an extra chromosome21. Therefore, a child that receives that gamete with an extra chromosome 21 will have 47 chromosomes in every cell.
Down syndrome is also called Trisomy 21.
Some Down syndrome symptoms include impairment in cognitive ability and physical growth, hearing loss, oversized tongue, shorter limbs and social difficulties.
Other types of non-disjunctions are trisomy 18 (Edwards Syndrome - many of these fetuses die before birth), trisomy 13 (Patau’s syndrome – causes multiple and complex organ defects and highly effects normal development).
Application: Studies showing age of parents influences chances of nondisjunction.
Studies showing the how the age of parents affects the chances of a non-disjunction occurring
The study of Yoon and colleagues (1996) concluded that 86% of the trisomy 21 cases from 1989-1993 in Atlanta were maternal in origin, 9% were paternal in origin, and 5% occurred during the mitotic divisions of the embryo. They also showed that 75% of the maternally originated Down syndrome cases arose from non-disjunction during the first meiotic division, and 25% originated in the second meiotic division.
Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling andamniocentesis and the associated risks.
Karyotyping is performed by collecting cells using one of two methods; chorionic villus sampling or amniocentesis.
Karyotyping is used for pre-natal diagnosis of chromosome abnormalities such as Down syndrome (Trisomy 21), Turner syndrome (XO), and Klinefelter syndrome (XXY).
The cells obtained by chorionic villus sampling and amniocentesis come from the embryoand not the mother, allowing doctors to analyze the DNA genome of the embryo.
Amniocentesis procedure involves the extraction of a small amount of amniotic fluid (contains fetal tissues) with a needle, from the amnion or amniotic sac surrounding a developing fetus. The fetal DNA is examined for genetic abnormalities through karyotyping.
Chorionic villus sampling involves removing a sample of the chorionic villus (placental tissue) to test for genetic abnormalities through karyotyping. CVS can be carried out 8-12 weeks into the pregnancy.
Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Meiosis is the process of reduction division in which the number of chromosomes per cell is halved.
Meiosis results in the production of haploid gametes and is essential for reproduction.
Meiosis occurs in two stages; meiosis I and meiosis II.