4.2 Meiosis. Meiosis is a reduction division. Assessment statements. The process of meiosis

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1 4.2 Meiosis Assessment statements State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. Define homologous chromosomes. Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells. Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down s syndrome (trisomy 21). State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure. State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for prenatal diagnosis of chromosome abnormalities. Analyse a human karyotype to determine gender and whether non-disjunction has occurred. Meiosis is a reduction division Meiosis is a type of cell division that produces gametes (sex cells). In any organism, each cell that is produced as a result of meiosis has half the number of chromosomes of other cells in the body. Eukaryotic body cells have a diploid nucleus, which contains two copies of each chromosome, in homologous pairs. For example, humans have a diploid number of 46 chromosomes in 23 pairs, mangos and soybean both have 40 chromosomes in 20 pairs, and the camel has 70 chromosomes in 35 pairs. During sexual reproduction, two gametes fuse together so, in order to keep the chromosome number correct in the offspring that are produced, each gamete must contain only one of each chromosome pair. That is, it must contain half the diploid number of chromosomes, which is called the haploid number. During gamete formation, meiosis reduces the diploid number to the haploid number. At the moment of fertilisation, the normal diploid number is restored as the gametes fuse. So, in the camel, the haploid sperm (35 chromosomes) and haploid egg (35 chromosomes) fuse at fertilisation to form the diploid zygote, with 70 chromosomes. Homologous chromosomes a pair of chromosomes with the same genes but not necessarily the same alleles of those genes There are a few organisms that are haploid, for example the male honey bee, Apis mellifera. This means it produces gametes through mitosis. The process of meiosis Meiosis occurs in a series of stages, as illustrated in Figure 4.5 (overleaf), which result in the production of four cells. Whereas mitosis, used to replace or repair cells, is achieved with one cell division, meiosis involves two divisions the first reduces the number of chromosomes by half and the second produces four gametes each containing the haploid number of chromosomes. Exactly the same terms are used for the names of the stages, but since meiosis involves two divisions, the phases are numbered I and II. The first division is very similar to mitosis and the second division is exactly the same as mitosis. 4 GENETICS 1 71

2 Meiosis I Meiosis II 1 Prophase I 5 Prophase II nuclear envelope breaks up as in mitosis crossing over of chromatids may occur homologous chromosomes pair up to form a bivalent nuclear envelope and nucleolus disperse centrioles replicate and move to opposite poles of the cell Bivalent showing crossing over that may occur: centromere chromatids may break and may reconnect to another chromatid At the end of prophase 1, a spindle is formed. 2 Metaphase I (showing crossing over of long chromatids) bivalents line up across equator of spindle, attached by centromeres chiasma point where crossing over occurs (plural, chiasmata) one or more chiasmata may form, anywhere along length 6 Metaphase II chromosomes line up separately across equator of spindle 3 Anaphase I Centromeres do not divide, unlike mitosis. Whole chromosomes move towards opposite ends of spindle, centromeres first, pulled by microtubules. 4 Telophase I spindle formed as in mitosis nuclear envelope re-forming nucleolus re-forming cytokinesis as mitosis 7 Anaphase II centromeres divide and spindle microtubules pull the chromatids to opposite poles 8 Telophase II remains of spindle chromosomes have reached poles of spindle telophase II as mitosis telophase but four haploid daughter cells formed Figure 4.5 The stages of meiosis in an animal cell. Note that the cells are shown with just two homologous pairs of chromosomes to make it easier to understand the process. 72

3 Prophase I The chromosomes, which have replicated during interphase, now supercoil. Each one consists of two sister chromatids joined by the centromere. The homologous pairs of chromosomes line up side by side. Although the genes carried by each chromosome pair are identical, the alleles may not be. Exchange of genetic material between the pair can occur at this point. Sister chromatids may become entangled, break and rejoin so that alleles are exchanged between them during a process called crossing over. New combinations of alleles are formed and genetic variety in the resulting gametes increases. The final step in prophase I is the formation of spindle microtubules and the breakdown of the nuclear envelope. Metaphase I Chromosomes line up on the equator at the centre of the cell. Each one attaches by a centromere to the spindle microtubules. The alignment of the chromosomes is random so that maternal and paternal chromosomes can appear on either side of one another on the equator. This also increases the genetic variety in the gametes. Anaphase I The microtubules now contract towards opposite poles. The pairs of sister chromatids remain together but the homologous pairs are separated. This is the reduction division where the chromosome number is halved from diploid to haploid. Telophase I Now spindles break down and a new nuclear envelope forms. Cytokinesis follows and the cell splits into two cells, each containing only one chromosome of each homologous pair. Each chromosome, however, still consists of two sister chromatids at this point. The second division of meiosis now follows to separate the two sister chromatids. Prophase II In each of the two cells resulting from meiosis I, new spindle microtubules start to form, the chromosomes recoil and the nuclear envelope begins to break down. Metaphase II The nuclear envelope is broken down and individual chromosomes line up on the equator of each cell. Spindle fibres from opposite ends of the cell attach to each chromatid at the centromere. Anaphase II Sister chromatids are separated as the centromere splits and spindle fibres pull the chromatids to opposite ends of the cell. Telophase II Nuclear envelopes form around the four new haploid nuclei and the chromosomes now uncoil. A second cytokinesis occurs, resulting in four cells. In each homologous pair, one chromosome is a maternal chromosome and the other a paternal chromosome. After crossing over, the chromatids recombine to produce new and unique combinations of alleles. This is called recombination. 4 GENETICS 1 73

4 Comparing mitosis and meiosis Table 4.2 summarises the differences between cell division by mitosis and by meiosis. Mitosis occurs to replace and repair cells chromosomes line up individually on the spindle microtubules produces two cells with the same number of chromosomes as the original cell, the diploid number the two daughter cells are genetically identical Meiosis occurs in gamete formation chromosomes line up in homologous pairs on the spindle microtubules at metaphase I produces four haploid cells the four daughter cells are usually genetically different Table 4.2 Some differences between mitosis and meiosis. 7 In which phase or phases of meiosis do the following events occur? a homologous chromosomes separate b the nuclear envelope breaks down c the centromere splits d the sister chromatids line up on the equator e the chromosomes uncoil f reduction division occurs Non-disjunction Non-disjunction is a failure of homologous pairs of chromosomes to separate properly during meiosis. It results in gametes that contain either one too few or one too many chromosomes. Those with too few seldom survive, but in some cases a gamete with an extra chromosome does survive and after fertilisation produces a zygote with three chromosomes of one type, as shown in Figure 4.6. This is called a trisomy. Trisomy in chromosome 21 results in the human condition known as Down s syndrome (Figure 4.7). A gamete, usually the female one, receives 24 chromosomes instead of 23 and a baby with 47 instead of the usual 46 chromosomes in each cell is born. Karyotyping Chromosomes have unique banding patterns that are revealed if they are stained with specific dyes during prophase. Each chromosome is a characteristic length and has its centromere at a fixed place, and each one has a homologous partner. In a karyogram, chromosomes are stained and photographed. The image is then manipulated to arrange the chromosomes in order of their size, as shown in Figure 4.8. Karyograms indicate the sex of an individual because they show the sex chromosomes, 74

5 abnormal cell with no copy of this chromosome meiosis abnormal cell with two copies of this chromosome zygote has three copies of this chomosome fertilisation abnormal egg cell normal sperm cell Figure 4.6 Non-disjunction at anaphase II of meiosis. Non-disjunction can also occur at anaphase I. and they are also used in prenatal diagnosis to check for chromosome abnormalities. In the procedure called karyotyping, cells from an unborn child are collected in one of two ways chorionic villus sampling (CVS) or amniocentesis (page 76). The cells are grown in the laboratory and a karyogram is prepared. This is checked for extra or missing chromosomes. The procedure is normally used when there is concern about potential chromosome abnormalities for example, if the mother is over the age of 35 years. Down s syndrome, which is more common in babies of older mothers, can be detected using this method. Figure 4.7 People with Down s syndrome have characteristic physical features. Figure 4.8 Karyograms for a person with a normal chromosome complement, and for a person with trisomy 21 (Down s syndrome). 4 GENETICS 1 75

6 Prenatal screening Obtaining fetal cells for karyotyping by amniocentesis involves taking a sample of amniotic fluid from the mother between weeks 14 and 16 of her pregnancy. Chorionic villus sampling (CVS) involves taking a sample of cells from the chorionic villi, which are the fine projections of the placenta embedded in the lining of the uterus. This can be done 8 10 weeks into the pregnancy. Both methods carry a small risk of damaging the fetus or even causing a miscarriage. Once the results of the test are known, the parents may be offered the option to terminate the pregnancy if abnormalities are discovered. The test may reveal an abnormality but it cannot give any indication about the likely severity of the condition. Questions to consider 1 Karyotyping is a procedure involving medical and ethical decisions. Who should make the decision to carry out the procedure the parents or health care officials? How important are legal and religious arguments? 2 Both procedures carry the risk of a miscarriage. How can this potential risk to the unborn child be balanced with the parents desire for information? What safeguards should be in place when the karyotyping procedure is used? 3 Does the information that can be obtained from the karyogram outweigh the risk to the unborn child? 4 If the karyogram indicates a genetic abnormality, should the parents be permitted to consider a termination of the pregnancy? amniocentesis at weeks of pregnancy chorionic villus sampling at 8 10 weeks of pregnancy uterus placenta uterus needle chorionic villi catheter fetal cells in amniotic fluid vagina 8 Define the term homologous chromosomes. 9 Meiosis is a reduction division. What does this mean? 10 How many cells are formed when meiosis in one parent cell is completed? 11 What is a trisomy? 12 State one example of non-disjunction in humans. 13 State two characteristics of chromosomes that are used to identify them when karyotyping. 14 The soybean has 40 chromosomes in its root cells. What is the name given to this number of chromosomes? 76