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Biology Chapter 12.docx

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McMaster University
Lovaye Kajiura

Biology Chapter 12: Meiosis Bernard Ho October 22, 2010 Introduction to Meiosis − During sexual reproduction, a male reproductive cell (sperm) and a female reproductive cell (egg), unite to form a new individual − Process of uniting sperm and egg is called fertilization − Number of chromosomes is constant from cell to cell within a multicellular organism − Chromosome number is also the same in the parent and daughter cells of mitosis − During the formation of gametes, haploid reproductive cells that have one set of chromosomes such as sperm and eggs, there must be a distinctive type of cell division that leads to a reduction in chromosome number − Meiosis is nuclear division that leads to halving of chromosome number − Chromosomes are composed of DNA and proteins and carry genes − Sex chromosomes determine the sex of an individual − Non-sex chromosomes are known as autosomes − In humans, there are 24 chromosomes per cell, 12 pairs − Two types of sex chromosomes exist in humans , X and Y − Two chromosomes of each type are homologous chromosomes − Later work showed that homologous chromosomes are similar not only in size and shape, but also in content − Homologous chromosomes carry the same genes − A gene is a section of DNA that influences one or more hereditary traits in an individual o Made up of DNA (nucleic acids) located on chromosomes o Have specific sequence of nucleotides (monomers) o Most genes program cells to synthesize proteins o The actions of these proteins produce the organism’s inherited traits − A trait is a characteristic − The versions of a gene found on homologous chromosomes may differ − Biologists use the term allele to denote different versions of the same gene − Thus, homologous chromosomes carry the same genes, but each homolog may contain different alleles − A karyotype is the display of an individual’s chromosomes that is organized in terms of chromosome number, size and type − Karyotypes are useful for genetic screening to identify specific chromosomal defects in their number, size, and type − Karyotyping techniques o The first step is to obtain a sample of cells from the individual being studied o Next is to grow the cells in cultures and when the cultures are dividing rapidly, they are treated with colchicine, which stops mitosis at metaphase by disrupting the formation of the mitotic spindle o At this stage the chromosomes are relatively easy to study because they are condensed and consist of sister chromatids o Chromosomes of colchicine treated cells are then stained and examined with light microscope o Researchers can distinguish condensed chromosomes by size, position of centromere and by striping or banding patterns that appear in response to some stains o Subtler differences among chromosomes are apparent when a higher- resolution technique for karyotyping called spectral karyotyping (SKY) or chromosome paint is used o The “painting” is done with fluorescent dyes that are attached to short DNA molecules o The dyed pieces of DNA bind to particular regions of particular chromosomes o By using a combination of dyes, technicians can give each pair of homologous chromosomes a distinctive suite of colours − Banding patterns o Locus is the position of a gene along a chromosome (or along a DNA double helix) − Organisms such as humans are called diploid because they have to versions of each type of chromosome − Diploid organisms have two alleles of each gene, one on each of the homologous pairs of chromosomes − Organisms such as bacteria are called haploid because their cells contain just one of each type of chromosome − Haploid organisms do not contain homologous chromosomes, they have just one allele of each gene − By convention, the letter “n” stands for the number of distinct types of chromosomes in a given cell and is called the haploid number − Diploid cells have a haploid number, which indicates the number of different types of chromosomes present (n, 2n, 3n etc.) − The combination of a number and “n” is termed the cell’s ploidy − Since humans are diploids, they are designated 2n − However, human cells contain 23 distinct types of chromosomes so n = 23, therefore in humans 2n = 46 − Figure 12.2  Vocabulary How Meiosis Occurs − Cells replicate their chromosomes before undergoing meiosis (interphase) − When chromosomal replication is complete, each chromosome consists of two identical sister chromatids − Sister chromatids contain the same genetic information and are physically joined at a portion of the chromosome called the centromere − An unreplicated chromosome consists of a single DNA molecule with its associated proteins, while a replicated chromosome consists of two sister chromatids − However, both unreplicated and replicated chromosomes are considered single chromosomes, even though the replicated chromosome comprises two sister chromatids − Meiosis consists of two cell divisions, meiosis I and meiosis II − Meiosis I o During meiosis I, the homologs in each chromosome pair separate from each other o One homolog goes to one daughter cell, the other homolog goes to the other daughter cell o Diploid (2n) parent cell produces two haploid (n) daughter cells o Phases of meiosis I  Early prophase I • Chromosomes condense • Spindle apparatus forms • Nuclear envelope begins to disappear • Homologous chromosome pairs come together in a process called synapsis • Structure that results is called tetrad • A tetrad consists of two homologous chromosomes, with each homolog consisting of two sister chromatids • Chromatids from separate homologs are called non-sister chromatids  Late prophase I • Non-sister chromatids begin to separate at many points along their length • They stay joined at certain locations and look as if they cross over one another • Each crossover forms an X-shaped structure called a chiasma • Chromatids involved in chiasma formation are homologous, but not sisters • According to a hypothesis from Thomas Hunt Morgan, paternal and maternal chromatids break and rejoin at each chiasma, producing chromatids that have both paternal and maternal segments • Morgan called this process of chromosome exchange crossing over  Metaphase I • Tetrads are moved to a region called the metaphase plate by spindle fibres • Each tetrad moves to the metaphase plate independently of other tetrads • Alignment of maternal and paternal homologs from each chromosome is random  Anaphase I • Homologous chromosomes in each tetrad separate and begin moving to opposite sides of the cell • Disjunctional segregation occurs  Telophase I • Homologs finish moving to opposite sides of the cell  When meiosis I is complete, cytokinesis occurs and two haploid daughter cells form o End result of meiosis I is that one chromosome of each homologous pair is distributed to a different daughter cell (a reduction division has occurred) o The daughter cells of meiosis I are haploid o Sister chromatids remain attached in each chromosome, meaning haploid daughter cells produced still contain replicated chromosomes − To determine number of chromosomes, count number of centromeres − Meiosis II o Sister chromatids from each chromosome separate o Cells produced by meiosis II also have one of each type of chromosome, but now they are unreplicated o Phases of meiosis II  No further chromosome replication occurs between meiosis I and II  Prophase II • Spindle apparatus forms • If nuclear envelope formed at end of meiosis I, it breaks apart  Metaphase II • Replicated chromosomes, consisting of two sister chromatids are lined up at the metaphase plate  Anaphase II • Sister chromatids separate at centromeres • Unreplicated chromosomes that result begin moving to opposite sides of the cell • Disjunctional segregation occurs  Telophase II • Chromosomes finish moving to opposite sides of the cell • A nuclear envelope forms around each haploid set of chromosomes  When meiosis II is complete, cytokinesis occurs and each cell divides to form two daughter cells  Process results in four haploid cells with unreplicated chromosomes − The four haploid cells eventually go on to form egg cells or sperm cells via gametogenesis − When two gametes fuse during fertilization, a full complement of chromosomes is restored − The cell that results from fertilization is diploid and is called a zygote − In this way, each diploid individual receives both a haploid chromosome set from its mother and a haploid set from its father − A closer look at key events in prophase I o When homologs synapse, two pairs of non-sister chromatids are brought close together and are held there by a network of proteins called the synaptonemal complex o During the process of crossing over, a complex of proteins cuts the chromosomes and then reattaches segments from homologs o At each point where crossing over occurs, the non-sister chromatids from each homolog get physically broken at the same point and attached to each other o As a result, segments of maternal and paternal chromosomes are swapped − Evidence of crossing over (RWE) o Creighton and McClintock’s Experiment with Zea Mays (corn)  Strain 1 • Long chromosome 9 with knob, kernels are coloured and waxy  Strain 2 • Short chromosome 9 with no knob, kernels are colourless and starchy  Strain 1 crossed with strain 2 • Chiasmata form between non-sister chromatids  Some offspring have • Long chromosome 9 with no knob, kernels are colourless and waxy • Short chromosome 9 with knob, kernels are coloured and starchy • Same as parental type  Conclusion • Physical exchange of DNA (knob) took place, displayed in phenotypes • Crossing over has occured The Consequences of Meiosis − Due to independent shuffling of maternal and paternal chromosomes and crossing over during meiosis I, chromosomes in gametes are different from chromosomes in parental cells − Subsequently, fertilization brings haploid sets of chromosomes from a mother and father together to form a diploid offspring − The chromosome complement of this offspring is unlike that of either parent, it is a random combination of genetic material from each parent − This change in chromosomal complement is crucial − Chances in chromosome configuration occur only during sexual reproduction, not during asexual reproduction
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