Transcript Slide 1
Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes Topic 3: Genetics 3.3 Meiosis Nature of Science Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8) Understandings 3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei. 3.3.U2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes. 3.3.U3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. 3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. 3.3.U5 Orientation of pairs of homologous chromosomes prior to separation is random. 3.3.U6 Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number. 3.3.U7 Crossing over and random orientation promotes genetic variation. 3.3.U8 Fusion of gametes from different parents promotes genetic variation Applications and Skills 3.3.A1 Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities. 3.3.A2 Application: Studies showing age of parents influences chances of nondisjunction 3.3.A3 Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. 3.3.S1 Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. Key Terms Homologous Chromosomes The nucleus of normal human body cells consist of 46 chromosomes or 23 pairs of chromosomes (2 of each chromosome). This is referred to as the diploid number for humans (2n). Gametes, sex cells, only have one set of chromosomes (23). This is referred to as the haploid number for humans (n). In diploid cells, each pair of chromosomes have the same genes, arranged in the same sequence (loci), but they do not necessarily have the same alleles of all of the genes. They are therefore not identical but instead are homologous, Homologous chromosomes. “Homologous chromosomes have the same genes as each other, in the same sequence and location, but not necessarily the same allele of those genes.” Reductive Division The number of chromosomes in a cell can be reduced from diploid to haploid by the process of Meiosis. Meiosis is described as a Reductive Division of a diploid nucleus to form haploid nuclei. Organisms that reproduce sexually have to halve their chromosome number at some stage in their life cycle because the fusion of gametes during fertilisation doubles it again (restores the diploid number). Meiosis Meiosis is a type of cell division which: Results in the production of gametes (sex cells) Occurs in germ cells in the gonads - diploid Four gametes are produced from every germ cell Each gamete has half the number of chromosomes as the original parent cell – haploid Meiosis involves TWO divisions: Meiosis I Meiosis II In sexual reproducing species, haploid cells must be formed by meiosis before fertilisation to ensure the diploid number of chromosomes in offspring is obtained. Fertilisation Male germ cell in testis (diploid) 46 Female germ cell in ovaries (diploid) 46 Meiosis Meiosis Sperm-Gamete (haploid) 23 Egg-Gamete (haploid) 23 Fertilisation zygote 46 (diploid) Mitosis Embryo 46 Mitosis Foetus 46 Meiosis Meiosis I Homologous chromosomes pair up. They are called a bivalent. Non-sister chromatids cross over at points called chiasmata. They may exchange genetic material – crossing over. Homologous pairs line up at equator. Maternal and paternal chromosomes of each pair line up independently of other pairs – independent assortment. Homologous chromosomes separate and move towards opposite poles. Two new cells form, each with half the original chromosome number. Meiosis II New spindle apparatus forms. Chromosomes line up at the equator in a single line. Centromeres divide and sister chromatids move towards opposite poles. Each cells divides, resulting in a total of four haploid cells. Each cell formed is genetically unique due to crossing over and independent assortment. Meiosis I Meiosis II Ref: Advanced Biology, Kent Meiosis – Gamete Production Meiosis is the name given to a specialised for of cell division which produces the gametes. In animals this process occurs in organs called the Gonads. In mature human females, eggs are produced by a process called Oogenesis in the Ovaries. In mature human males, millions of sperm are produced daily by a process called spermatogenesis in the testes. Meiosis – Gamete Production Ref: Biology Key Ideas Mendel’s Law of Segregation When gametes are produced, each gamete must receive a full complement of genes. For this reason, the factors/alleles must separate so that only one factor/allele is present in each gamete. Mendel’s Law of Segregation states: “The characteristics of a diploid organism are determined by alleles which occur in pairs. Of a pair of such alleles, only one can be carried in a single gamete” Thus each gamete receives one complete set of alleles, and hence chromosomes: ie: 23 chromosomes. The two alleles of a gene are located on homologous chromosomes which move to opposite poles, causing segregation. Meiosis and Variation Meiosis gives rise to genetic variation. Variety in gametes is produced by how the bivalents line up on the equator during Metaphase I. Ref: Biology Key Ideas Meiosis and Variation You can see that for different arrangements of chromosomes, you get different gametes formed. 2 chromosomes produces 4 combinations – 2n. Humans have 23 pairs of chromosomes – 223. That is 8,388,608 possible combinations. If you double that, because of each gamete, the total possible combinations is over 64 trillion. Non-disjunctions Non-disjunctions are a form of chromosome mutation. They occur when homologous chromosomes fail to separate properly during meiosis. An extra chromosome is drawn to on pole, producing gametes with an extra chromosome and gametes with one less chromosome. This is referred to as Trisomy. Down’s syndrome is an example of trisomy 21. Down’s syndrome people have an extra chromosome 21. A Non-disjunction Leading to Down’s Syndrome Ref: Advanced Biology, Kent Down’s Syndrome A Down’s syndrome boy. A Karyotype of a Down’s Syndrome Ref: Advanced Biology, Kent boy Karyotyping A complete set of chromosomes is called a karyotype. Each chromosome has genes specific for that chromosome making it identifiable. Karyotyping is arranging the chromosomes in pairs according to their size and structure. The chromosomes are arranged depending upon: Their length The position of their centromere Karyotyping can be used to detect chromosome aberrations in foetuses. eg: An amniocentesis to check for Downs syndrome (47 Chromosomes) Karyotyping Male Karyotype Female Karyotype Obtaining Cells for Karyotyping Cells can be collected from an unborn baby by; Chorionic villus sampling Taking cells from the fingerlike projections of the placenta Amniocentesis Using aneedle to extract amniotic fluid from around the foetus which contains some of the baby’s cells The cells are then grown and a karyotype is performed. From the karyotype the gender of the baby can be deduced and also any chromosomal abnormalities can be detected. Risk Factors Advancing maternal age. A woman's chances of giving birth to a child with Down syndrome increase with age because older eggs have a greater risk of improper chromosome division. By age 35, a woman's risk of conceiving a child with Down syndrome is about 1 in 350. By age 40, the risk is about 1 in 100, By age 45, the risk is about 1 in 30. However, most children with Down syndrome are born to women under age 35 because younger women have far more babies. Having had one child with Down syndrome. Typically, a woman who has one child with Down syndrome has about a 1 in 100 chance of having another child with Down syndrome. Being carriers of the genetic translocation for Down syndrome. Both men and women can pass the genetic translocation for Down syndrome on to their children. Mechanism for Non disjunction PhD Thesis: https://www.academia.edu/1032658/Genetic_mecha nisms_of_nondisjunction_in_humans A model system for increased meiotic nondisjunction in older oocytes. Jeffreys CA1, Burrage PS, Bickel SE. Abstract For at least 5% of all clinically recognized human pregnancies, meiotic segregation errors give rise to zygotes with the wrong number of chromosomes. Although most aneuploid fetuses perish in utero, trisomy in liveborns is the leading cause of mental retardation. A large percentage of human trisomies originate from segregation errors during female meiosis I; such errors increase in frequency with maternal age. Despite the clinical importance of age-dependent nondisjunction in humans, the underlying mechanisms remain largely unexplained. Efforts to recapitulate age-dependent nondisjunction in a mammalian experimental system have so far been unsuccessful. Here we provide evidence that Drosophila is an excellent model organism for investigating how oocyte aging contributes to meiotic nondisjunction. As in human oocytes, nonexchange homologs and bivalents with a single distal crossover in Drosophila oocytes are most susceptible to spontaneous nondisjunction during meiosis I. We show that in a sensitized genetic background in which sister chromatid cohesion is compromised, nonrecombinant X chromosomes become vulnerable to meiotic nondisjunction as Drosophila oocytes age. Our data indicate that the backup pathway that normally ensures proper segregation of achiasmate chromosomes deteriorates as Drosophila oocytes age and provide an intriguing paradigm for certain classes of age-dependent meiotic nondisjunction in humans. Advances in the genetic aspects linking folate metabolism to the maternal risk of birth of a child with Down syndrome F Coppedè In 1999, it was first hypothesised that maternal polymorphisms of genes involved in folate metabolism might represent maternal risk factors for the birth of a child with Down syndrome. Several research articles have been produced worldwide to address that question, and recent meta-analyses of the literature suggest that at least two polymorphisms, namely MTHFR c.677C>T and MTRR c.66A>G, are associated with increased maternal risk for trisomy 21. Moreover, there is indication for an additive contribution of variants in folate pathway genes to the maternal risk for having a birth with Down syndrome. In addition, lack of folate supplementation at peri-conception, in combination with genetic polymorphisms of folate pathway genes, might represent maternal risk factors for congenital heart defects in the child with Down syndrome. The aim of this critical review was to discuss advances in genetic aspects linking folate metabolism to the maternal risk of giving birth to a child with Down syndrome. Conclusion Despite encouraging results, several factors such as ethnicity, age, dietary habits, and many others, could modulate those interactions and we are still far away from a complete understanding of the relationship between folate metabolism and chromosome 21 non-disjunction. Meiosis Summary of Meiosis: Meiosis involves two divisions. One cell or nucleus divides to for four cells or nuclei. The chromosome number is halved, from diploid to haploid. An almost infinite amount of genetic variety is produced as a result of crossing over in Prophase I and the random orientation of bivalents in Metaphase I. Genetic Variation in Meiosis Meiosis results in almost infinite genetic variety of gametes. This comes about because of: Crossing over in Prophase I. Random Orientation in Metaphase I. Crossing Over In Prophase I, homologous chromosomes, each consisting of two identical chromatids, lie adjacent to each other – they pair up. This is called a synapsis. The pair of chromosomes is referred to as a bivalent. At this stage corresponding sections of non-sister chromatids may touch (cross over). This point is called a chiasma (chiasmata – plural). Sections of the chromosomes are swapped between the non-sister chromatids. This produces recombinant chromosomes. This process is called Crossing-over. Crossing over increases the genetic variability of the offspring by altering the combination of genes on the gametes formed. Crossing Over Ref: Year 12 Biology Biozone Crossing Over Ref: Year 12 Biology Biozone Crossing Over Ref: Biology Key Ideas Independent Assortment of Chromosomes During meiosis, the homologous chromosomes line up along the centre of the cell. Each member of each pair will be arranged towards the centre of the cell in random order. Each member arranges independently of the other chromosomes. This is called Independent Assortment of Chromosomes. Independent Assortment increases variation. Random Orientation of Chromosomes Ref: Biology Key Ideas Random Orientation in Humans In human cells there are 23 pairs of homologous chromosomes. The possible number of combinations is 223 or about 8 million. This is for one of you parents and the figure is about the same for the other parent. Multiplying these two together gives about 64 trillion different arrangements of chromosomes in the offspring. Recombination Recombination is the reassortment of genes or characters into different combinations from those of the parents. Recombination occurs for: Linked genes: Genes that occur on the same chromosome. Occurs by crossing over. Unlinked Genes: Genes that occur on separate chromosomes. Occurs by random orientation (Independent Assortment) Independent Assortment Mendel devised a number of laws of genetics. His second law was the law of Independent Assortment. This means that when gametes are formed, each allele of a gene is selected independently of any other gene. This is the result of the random orientation of chromosomes during Metaphase I of Meiosis. Thus independent assortment increases variation in meiosis. Assortment Independent Mendel’s second law, his Law of Independent Assortment can be stated as: “Alleles of genes located on different chromosomes assort independently of one another.” or “Either pair of alleles of a gene is equally likely to be inherited with either of another pair of alleles of a different gene.”