Understanding Meiosis and Genetic Variation in Organisms
Meiosis forms variable gametes in organisms, leading to increased genetic variation through the production of haploid gametes by meiosis I and II. Key concepts include homologous chromosomes, crossing over, independent assortment, and linked genes. Terminology such as homologous pairs and human chromosome genes are also discussed. Explore the fascinating world of meiosis and its role in evolution and genetic diversity.
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Meiosis forms variable gametes CfE Advanced Higher Biology Unit 2: Organisms and Evolution
2bii Meiosis forms variable gametes Contents and number of slides for each topic 1. 2. 3. 4. 5. Terminology (4) Sexual life cycle of animals (1) Meiosis an overview (1) Meiosis I (4) Meiosis II (2) 6. 7. 8. 9. 10. Other sexual life cycles (3) After meiosis (1) Linkage maps (5) Independent assortment (2) Producing variation (1)
SQA mandatory key information Increased variation through the production of haploid gametes by meiosis meiosis I, meiosis II, gamete mother cell, chromosome, chromatid, homologous pairs, crossing over, chiasmata, independent assortment, linked genes and frequency of recombination. In many organisms, gametes are formed directly from the cells produced by meiosis. In other groups, mitosis may occur after meiosis to form a haploid organism; gametes form later by differentiation.
Key concepts Homologous chromosomes are pairs of chromosomes of the same size, same centromere position and with the same genes at the same loci. Each homologous chromosome is inherited from a different parent; therefore the alleles of the genes of homologous chromosomes may be different. Crossing over occurs at chiasmata during meiosis I. This process shuffles sections of DNA between the homologous pairs allowing the recombination of alleles to occur. Genes on the same chromosome are said to be linked. Correlation of the distance between linked genes and their frequency of recombination. Independent assortment occurs as a result of meiosis I with homologous chromosomes being separated irrespective of their maternal and paternal origin.
Terminology Homologous chromosomes A human body cell has 46 chromosomes in its nucleus. There are two copies of each of 23 types of chromosomes. The two chromosomes in a pair are called homologous chromosomes and they have: the same length the same centromere position genes for the same characteristics at the same loci(Latin: places ).
Terminology A homologous pair A closer look at the genes on human chromosome 12 Potassium channel Centromere Myosin One chain of collagen Chromosome 12 has between 1200 and 1400 genes. Genes that are found on the same chromosome are called linked genes. Phenylalanine hydroxylase
Terminology Homologous pairs can be different Homologous pairs are found because one copy of each chromosome has come from the female parent and the other copy has come from the male parent. Because they are inherited from different parents, the alleles of the genes on each homologous chromosome may be different. From female parent From male parent
Terminology Sets of chromosomes A single set of chromosomes comes from the female parent in her gametes. Another single set of chromosomes comes from the male parent in his gametes. Each gamete cell has a single set of chromosomes and is haploid. A cell with a full double set of homologous chromosomes is diploid. Haploid female gamete (1 set) Haploid male gamete (1 set) Diploid cell (2 sets) YouTube: Diploid and haploid (1:55 min)
Sexual life cycle of animals Most of the life cycle is spent as a diploid multicellular organism. Meiosis produces genetically variable haploid cells which develop into gametes. During fertilisation, gametes fuse their haploid nuclei to produce a diploid cell. Mitosis then produces genetically identical diploid cells to make a multicellular organism.
Meiosis an overview Meiosis reduces the number of chromosomes. Meiosis is not a cycle. Diploid gamete mother cell Meiosis I Homologous chromosomes separate Meiosis II Sister chromatids separate Haploid cells Chromosomes duplicate Pair of homologous chromosomes Sister chromatids
Meiosis I Pairing of homologous chromosomes During interphase, the homologous chromosomes duplicate so each is now made up of two sister chromatids. This is still a diploid cell, with two sets of homologous chromosomes (though it has four sets of genetic information). At the start of meiosis I, homologous chromosomes pair up so that they are aligned gene by gene. Protein strands form a complex to link the sister chromatids and the homologous pairs to form a bivalent. Sister chromatids Bivalent
Meiosis I Crossing over A chiasma(Greek: cross mark ) forms at a random position between the homologous pairs. Human chromosomes usually have two or three chiasmata. (Chiasma = singular. Chiasmata = plural.) Chiasmata never form between sister chromatids. Chiasma Chiasmata allow the shuffling of sections of DNA between homologous chromosomes, a process called crossing over. Crossing over leads to the recombination of alleles, and so helps to increase variation in the gametes. YouTube: Recombination (3:40 min)
Meiosis I Alignment on the metaphase plate The protein complex between all the chromatids breaks down. The centromeres still hold the sister chromatids together. Chiasmata still hold the homologous pair together so they can be aligned. The nuclear membrane breaks down. Centrosome Spindle fibres Centrosomes send out microtubules to connect with kinetochores which lie beside each centromere. The microtubules form spindle fibres linking across the cell. The homologous chromosomes align in the centre of the cell.
Meiosis I Separating homologous chromosomes The microtubules of the spindle fibres begin to shorten. The microtubules pull on the kinetochores so the homologous chromosomes separate to opposite ends of the cell. The chromosomes group in each end of the cell and a nuclear membrane forms around them. Cytokinesis separates the two cells. The sister chromatids are no longer identical due to the crossing over.
Meiosis II Alignment on the metaphase plate Each cell is haploid, with one copy of each homologous chromosome (though it has two sets of genetic information). The nuclear membrane breaks down again. Centrosomes again send out microtubules and bind to the kinetochores of each sister chromatid. The chromosomes align in the centre of the cells.
Meiosis II Separating sister chromatids The protein complex between the centromeres breaks down. The microtubules of the spindle fibres begin to shorten. The microtubules pull on the kinetochores so the sister chromatids separate to opposite ends of the cell. After being separated, sister chromatids are called chromosomes. The new chromosomes group in each end of the cell and a nuclear membrane forms around them. Cytokinesis separates the two cells.
After meiosis Haploid cells become gametes Meiosis produces four genetically different haploid cells. Each cell has one copy of every homologous chromosome. In human males, each cell develops to form a sperm cell. In human females, it is more complex: meiosis I occurs in the last 3 months before birth only one of the cells develops further after an egg cell is released from the ovary it will not undergo meiosis II until a sperm nucleus has entered the nucleus of only one of the new cells will fuse with the sperm nucleus.
Watch: animation of meiosis http://www.cellsalive.com/meiosis.htm
Linkage maps Linked genes stay together Purple eye (r) and black body (g) are two alleles found on chromosome 2 of Drosophila melanogaster. Red eyes (R) and grey body (G) are the dominant alleles. Crossing RRGG with rrgg. The genes are linked so all the offspring inherit one chromosome with R G and the other with r g . What is the phenotype of the offspring? RRGG rrgg Gametes R G Gametes r g Offspring R G r g
Linkage maps Linked genes can recombine The offspring have red eyes and grey bodies (see left). Crossing these flies with rrgg (see right) produces four offspring phenotypes (shown below). Red eye Grey body Purple eye Black body Almost all the offspring look like the parents. A few of the offspring show recombinant phenotypes. Red eye Grey body Purple eye Black body Red eye Black body Purple eye Grey body
Linkage maps Recombinants are the result of crossing over Red eye Grey body Purple eye Black body R G r g r g r g Gametes r g Number of offspring Phenotype R G r g r g r g Most gametes Red eye Grey body R G 113 Purple eye Black body r g 122 Only 15 of the 250 offspring are recombinants. R g r g Red eye Black body R g 9 r G r g Purple eye Grey body A few gametes show recombination So recombination frequency is 6%. r G 6 Total = 250
Linkage maps Data for other linked genes Repeating these types of crosses for other genes on chromosome 2 gives different recombination frequencies. Genes used in the crosses Recombination frequency (%) Purple eye v. Black body 6 Purple eye v. Lobe eye 17 Vestigial wing v. Lobe eye 5 Black body v. Lobe eye 23 Chiasmata formation occurs at random positions along the chromosomes. What does a small recombination frequency suggest about the position of the two genes?
Linkage maps How to map the genes The recombination frequency for linked genes correlates with the distance between the loci of the genes on the chromosome. Genes used in the crosses Recombination frequency (%) Purple eye v. Black body 6 Purple eye v. Lobe eye 17 Vestigial wing v. Lobe eye 5 Black body v. Lobe eye 23 Lobe eye Black body Purple eye Vestigial wing 6 5 17 23 What recombination frequency would be predicted for crosses using purple eye v. vestigial wing? Animation: Discovery of linkage maps Animation: Discovery of linkage maps
Independent assortment Alignment during Meiosis I All diploid organisms have more than one homologous pair of chromosomes. Homologous pairs align in the centre of the cell. The orientation of the homologous chromosomes is irrespective of their maternal or paternal origin. Even with just three pairs of homologous chromosomes, there are four possible alignments.
Independent assortment Chromosome combinations in gametes At meiosis I, homologous pairs are separated irrespective of the maternal or paternal origin of the chromosome. This leads to variation in the combinations of chromosomes found in the haploid cells at the end of meiosis II. With three pairs of chromosomes, there are 23 = 8 combinations. In humans, with 23 pairs, there are 223= 8 388 308 combinations and crossing over shuffles pieces between chromosomes!
Producing variation Meiosis and sexual reproduction Meiosis produces haploid cells that are genetically variable. Sexual reproduction uses two haploid gamete cells to make a new diploid organism. Allows shuffling of sections of DNA between homologous chromosomes Allows many combinations of chromosomes of maternal and paternal origin in the gametes Brings genetic information from two different parents together in one organism Sexual reproduction Independent assortment Crossing over Two human parents can produce offspring with more than 70 million million combinations of chromosomes without considering the effects of crossing over. We are all unique!
Other sexual life cycles Comparing the three types Animals Fungi & protists Plants
Other sexual life cycles Plants In mosses and ferns, mitosis occurs after meiosis and so produces a large multicellular haploid organism with differentiated cells. Gametes are formed later by differentiation of haploid cells. In higher plants, a tiny male haploid organism is held within a pollen grain and a tiny female haploid organism is in the ovule. Fern image: Olegivvit / Wikimedia
Other sexual life cycles Fungi and most protists Again, mitosis occurs after meiosis and produces a unicellular or multicellular haploid organism. Gametes form later by the differentiation of the haploid cells. The diploid zygote goes straight into meiosis to form gametes. The malarial parasite (Plasmodium spp.) has this type of lifecycle.
