30/01/2013 § 2 Comments
This second blog of the unit will cover an overview of meiosis, its purposes, outcome, and different phases, as well as karyotypes and karyotyping.
There are three key things to know about meiosis:
- It involves two divisions, so one cell, or one nucleus, will end up becoming four cells (see image below!).
- The number of chromosomes is halved and so goes from a diploid state to a haploid state.
- The purpose of meiosis is to generate a massive amount of genetic variety in crossing-over and in the random orientation of bivalents.
Recall that a nucleus with two copies, or a pair of chromosomes is diploid while a nucleus with only one chromosome is haploid. In meiosis, the nucleus goes from diploid to haploid. Meiosis is also known as reduction division because it reduces the chromosomes from diploid to haploid. The chromosomes (each consisting of two identical chromatid) are replicated during interphase. The homologous chromosomes – homologous meaning chromosomes that are of the same type – partner up at the beginning of meiosis I. They then exchange genetic material by crossing over. By mitosis, the cell divides and results in two new haploid daughter cells. The cell divides again in meiosis II to produce a total of four haploid daughter cells.
Karyotypes are the number and appearance of chromosomes in an organism. Karyotypes are produced through banding patterns that are achieved when staining the chromosomes under a microscope. Karyotyping is used for multiple purposes but is most commonly known to deduce the gender of a fetus, therefore the most useful time for karyotyping is before birth. One of the two procedures to obtain fetal chromosomes is amniocentesis, which is an insertion of a needle through a mother’s abdomen and uterus to draw amniotic fluid that contains the fetus’s cells. The second procedure is chorionic villus sampling wherein cells from the placenta (the choiron) are sampled instead of from the amniotic flute. This is done by entering a catheter through the vagina and earlier than amniocentesis.
Karyotyping is also a way to determine chromosomal abnormalities, caused by non-separation of chromosomes during meiosis, or non-disjunction. When non-disjunction occurs, the gametes produced in meiosis have either one too few or too many chromosomes. These abnormalities can cause an organism (more specifically, humans) to have syndromes which are physical signs or symptoms. A very common syndrome is trisomy 21, also known as Down syndrome.
I mentioned above that meiosis is necessary for genetic variety. For humans, meiosis can produce over 8 million combinations among its haploids (2 to the 23rd power). Again, this variety is developed through crossing over, which occurs during prophase I. A synapsis causes bivalents (two homologous chromosomes) to exchange corresponding sections of DNA through breakage and rejoining of the chromatids.
- Define the terms gene and allele and explain how they differ. 4 marks
- Describe the consequences of a base substitution mutation with regards to sickle cell anemia. 7 marks
- Outline the formation of chiasmata during crossing over. 5 marks
- Explain how an error in meiosis can lead to Down syndrome. 8 marks
- Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. 5 marks
- Compare the processes of mitosis and meiosis. 6 marks
- Outline one example of inheritance involving multiple alleles. 5 marks
- Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father. 5 marks
- Outline sex linkage. 5 marks
- Explain, using a named example, why many sex-linked diseases occur more frequently in men than women. 9 marks
DATA BASED QUESTIONS
Page 134, life cycles
[Figure 2] shows the life cycle of humans and mosses, with n being used to represent the haploid number of chromosomes and 2n to represent the diploid number. Sporophytes of mosses grow on the main moss plant and consist of a stalk and a capsule in which spores are produced.
1. Outline five similarities between the life cycle of a moss and of a human.
Both life cycles go through mitosis. Both life cycles go through meiosis. Both cycles fertilize their eggs to form zygotes. Both cycles have diploid zygotes and haploid sex cells (sperm and egg). Both cycles use meiosis to decrease the ploidy from diploidy to haploidy. Both cycles have sperm and egg cells as their sex cells. Fertilization leads to mitosis in both cycles.
2. Distinguish between the life cycles of a moss and a human by giving five differences.
Humans do not have spores or sporophytes, only sperm or eggs. A human male would produce a sperm, a human female would produce eggs but all moss plants can produce both. The only haploidy state in the human cycle are the sex cells (sperm and egg) but in moss plants, the haploidy cells are the sex cells and the moss plant itself as well as the spores. Humans follow a strict mitosis, meiosis, fertilization pattern, but moss plants can do mitosis after meiosis. The zygote in moss plants only become sporophytes but the zygote in humans can become either a male or female.
Page 136, bog moss chromosomes
The DNA content of cells can be estimated using a stain that binds specifically to DNA. A narrow beam of light is then passed through a stained nucleus and the amount of light absorbed by the stain is measured, to give an estimate of the quantity of dNA. The results in Table 1 are for leaf cells in a species of bog moss (Sphagnum) from the Svalbard islands.
1. Compare the DNA content of the bog mosses.
The DNA content of the bog mosses ranges from 0.42 DNA/pg to 0.95 DNA/pg but doesn’t range in between 0.50 and 0.90. The bog moss is either between 0.42 – 0.48 or between 0.92 – 0.95, namely S. arcticum and S. olafii. It’s either one or the other, and nothing in between.
2. Suggest a reason for six of the species of bog moss on the Svalbard islands all having the same number of chromosomes in their nuclei.
The six species of bog moss on the Svalbard islands may have evolved from the same original moss, which is why they all have the same number of chromosomes in their nuclei.
3. S. arcticum and S. olafii probably arose as new species when meiosis failed to occur in one of their ancestors.
- a) Deduce the chromosome number of nuclei in their leaf cells. Give two reasons for your answer. The chromosome number [could be] is 38. This is because, if meiosis failed to split the cells from diploidy to haploidy, S. arcticum and olafii should have double the number of chromosomes than the rest of the bog moss’s nuclei. Also, maybe the leaves are just bigger and more complex than the other six species! (?)
- b) Suggest a disadvantage to S. arcticum and S. olafii of having more DNA than other bog mosses. There is a higher chance (proportionally) of mutations and genetic diseases for S. arcticum and S. olafii.
4. It is unusual for plants and animals to have an odd number of chromosomes in their nuclei. Explain how mosses can have odd numbers of chromosomes in their leaf cells.
Okay, so when plants do meiosis to reproduce, the double chromosomes (with sister chromatids) are supposed to be reduced from diploidy to haploidy but sometimes, like with trisomy 21, meiosis fails the chromosomes don’t completely separate. As a result, one of the resulting daughter cells are left with an odd number chromosomes, either 1 chromosome or 3 chromosomes. This could cause problems in the organism later, like with children who have Down syndrome (trisomy 21).
Page 137, risk of chromosomal abnormalities with advancing age of the parent
The data presented in Figure 5 shows the relationship between maternal age and the incidence of trisomy 21 and of other chromosomal abnormalities.
1. Outline the relationship between maternal age and the incidence of chromosomal abnormalities in live births.
As the maternal age increases from age 20, the incidence of chromosomal abnormalities in live births increases. The increase is at first slow but after age 40, the increase is steep and rapid.
- a) For mothers 40 years of age, determine the probability that they will give birth to a child with trisomy 21. —> 16/1000 or 2/125 births
- b) Using the data in Figure 5, calculate the probability that a mother of 40 years of age will give birth to a child with a chromosomal abnormality other than trisomy 21. —> 30/1000 or 15/500 births
3. Only a small number of possible chromosomal abnormalities are ever found among live births, and trisomy 21 is much the commonest. Suggest reasons for these trends.
The chromosomal abnormalities found among live births are found on chromosomes 13, 19 and 21 – not the larger and first chromosomes. These abnormalities can be found because the later chromosomes (13, 19, 21) don’t hold too many genes that would render the zygote too abnormal and unhealthy to be born.
4. Discuss the risks parents face when choosing to postpone having children.
When parents postpone having children, they probably think about waiting to be more prepared but as they grow older – especially the mother, the parents risk a higher chance in conceiving a child with a chromosomal abnormality, shown in the figure 5.
Page 138, a human karyotype
1. a) In figure 9, distinguish between:
- i) chromosome 5 and chromosome 6 – Chromosome 5 is slightly longer than chromosome 6, notably because chromosome 5’s stains reach the yellow colors while chromosome 6’s stains stop at about the green stains.
- ii) chromosome 17 and chromosome 18 – Chromosome 18 is slightly longer than chromosome 17 but not by much. This just means it holds more alleles.
- iii) the X and Y chromosome – There is no Y-chromosome in this human karyotype. The x-chromosome should be long, like one of the last two chromatid shown in the figure, but a y-chromosome is much smaller.
- a) State the gender of the subject of the human karyotype in Figure 9. It is female.
- b) State whether the karyotype shows any abnormalities. It does not.
Chapter 11 Questions
1. For each set of four words, identify the term that does not belong and explain why:
- non-disjunction, crossing over, synapsis, tetrad: non-disjunction happens later in meiosis while the other three happen a little before
- independent assortment, metaphase I, random orientation, segregation: segregation because the other three happen during metaphase I and before segregation, which happens during anaphase I
- length, banding pattern, centromere position, chemical composition: chemical composition has nothing to do with karyotyping
- sickle-cell anemia, Turner’s syndrome, trisomy 21, Klinefelter’s syndrome: sickle-cell anemia because the other three are genetic diseases caused by non-disjunction but sickle-cell anemia is not (a non-disjunction-caused disease)
2. Where are samples drawn from for:
- a) amniocentesis —> IV
- b) chorionic villus sampling? —> I
3. Human somatic cells have 46 chromosomes, while our closest primate relatives, the chimpanzee, the gorilla and the orangutan, all have 48 chromosomes. One hypothesis is that the human chromosome number 2 was formed from the fusion of two chromosomes in a primate ancestor. The image below shows human chromosome 2 compared to chromosome 12 and 13 from the chimpanzee.
- a) Compare the human chromosome 2 with the two chimpanzee chromosome. We can see that the first half and the second half are incredibly similar, except for the portion of genes in the middle. That portion is the only different arrangement between the two chromosomes. Chimpanzee chromosomes are also about half the size of human chromosomes, it seems.
- b) The ends of chromosomes, called telomeres, have many repeats of the same short DNA sequence. If the fusion hypothesis were true, predict what would be found in the region of the chromosome where fusion is hypothesized to have occurred. At that region, there would be genes that would code for fur, ability to climb trees exceedingly well, and other monkey traits. When the genes of these traits fused, they rearranged (hypothetically) themselves to become something that evolved into us humans. Also, okay: near the centromere on the long arm of the human chromosome, you would find a number of repeats that were moor characteristic of telomeres than sequences normally found near the centromere.
4. Construct a Venn diagram which compares mitosis and meiosis.
It’s kind of inefficient if I tried to construct a Venn diagram, but the comparisons are (this is vital for the essay questions):
- Unique to meiosis – used for sexual reproduction, reduces chromosome number from diploid to haploid, involves two divisions, and four cells are produced
- Shared features – both are used for reproduction, both involve nuclear division!
- Unique to mitosis – mainly for growth maintenance, repair, and asexual reproduction, maintains chromosome number (diploid –> diploid / haploid –> haploid)
5. Identify the stages of meiosis shown in Figures 17 and 18.
Figure 17: Anaphase I, Figure 18: Telophase II.