BIOMG 1350 Notes Week 9
BIOMG 1350 Notes Week 9 BIOMG 1350
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This 14 page Class Notes was uploaded by genehan on Saturday September 3, 2016. The Class Notes belongs to BIOMG 1350 at Cornell University taught by Garcia-Garcia, M; Huffaker, T in Fall 2015. Since its upload, it has received 5 views. For similar materials see Introductory Biology: Cell and Developmental Biology in Molecular Biology and Genetics at Cornell University.
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Date Created: 09/03/16
BIOMG 1350 Professor Bretscher & GarciaGarcia Spring 2016 Week 9: Lecture 1 of 2 Monday, March 21, 2016 Lecture Title: Transitioning from cells to embryos – Meiosis and Mendelian inheritance Lecture Keywords: sexual reproduction, gametes, fertilization, zygote, haploid, germ cells, somatic cells, bivalent, synaptonemal complex, chiasmata, uniparental inheritance, homunculus, characters/traits, variants/alleles, F1 generation, dominant/recessive variant, hereditary factors, genotypes, Mendel’s First Law of Segregation, Punnett Square, homozygotes, heterozygotes, phenotypes, Mendel’s Second Law of Independent Assortment 1. Mitosis allows cells to proliferate by producing genetically identical cells. a. Mitosis contributes to asexual reproduction in unicellular and some multicellular organisms and growth in multicellular organisms. 2. Sexual reproduction involves specialized cells called gametes - eggs and sperm - that fuse through fertilization to form a zygote (fertilized egg) that will eventually develop into an embryo (2n = 46 chromosomes). a. This produces a new individual genetically distinct from both parents. This is advantageous in evolution because it contributes to genetic variety and the ability to adapt to new environments. b. Gametes are haploid, meaning they contain 1 copy of each chromosome. c. Diploid germ cells, localized in the gonads, are the only cells that can choose between meiosis and mitosis and germ cells generate gametes. d. The rest of the cells are called somatic cells and are diploid. 3. Mitosis vs Meiosis a. Both start with DNA duplication. b. Meiosis has 2 rounds of cell division but only one round of DNA duplication and generates 4 haploid gametes, while in mitosis, two diploid daughter cells identical to parent cells are formed. c. Both have similar stages – prophase, metaphase, anaphase, telophase, cytokinesis. (Meiosis I, meiosis II, and mitosis) d. The organization of chromosomes at the microtubule spindle is dramatically different. In mitosis, chromosomes line up independently at the metaphase plate whereas in meiosis I, homologous chromosomes line up as a pair at the metaphase plate. i. Before DNA replication, there is one maternal chromosome and one paternal chromosome (2n=2). ii. After DNA replication, each chromosome is formed by two sister chromatids bound by cohesins. iii. After chromosome condensation, homologous chromosomes form bound by a centromere. iv. The first meiotic division separates homologous chromosomes and in meiosis II, sister chromatids separate, just like mitosis. e. In prophase I, homologous chromosomes pair up during prophase I and form a bivalent, involving DNA sequence recognition between homologous chromosomes and proteins of the synaptonemal complex. f. Meiotic recombination or cross-over also takes place during prophase I. i. There is a change of genetic material between the two chromosomes. ii. This increases the genetic diversity by recombination and chiasmata is important because it maintains homologous chromosomes together during meiosis I. 1. Chiasmata are the points of exchange or “cross over.” iii. The formation of chiasmata is promoted by the repairing of double-strand breaks in DNA by using the adjacent sister chromatid from the homologous chromosome. iv. Every chromosomes undergoes 2-3 cross-overs in meiosis I. 4. Iclicker question – The diagram below most likely represents the chromosomes of a cell at anaphase of – a. Meiosis I b. Meiosis II – this represents sister chromatids being separated, and this takes place during meiosis II. c. Mitosis – also a valid answer d. An impossible situation 5. Before Mendel – most people believed in uniparental inheritance. a. People thought sperm had a tiny “homunculus” that would eventually grow into a new human being, since many sons resembled their fathers. b. The women role was to be a nurturing environment to allow the human being inside to grow. 6. Instead of using other animals for his experiments, Mendel used peas, which had many advantages. a. Pea breeding produces a lot of progeny, and made his data keeping very abundant. b. Peas had many different variables that were easy to study and identify. i. Pea shape, pea color, flower color, pod shape, pod color, plant height c. Mendel focused on 2 characters or traits, such as pea shape and pea color, that were present in the plants in two variants or alleles, such as green or yellow and round or wrinkled. 7. Mendel cross-fertilized true breeding yellow and true breeding green plants and this resulted in 100% yellow-seeded pea plants for the F1 generation (Filial I). He realized that yellow had a dominant variant over green. a. Then, he took the F1 generation offspring and self-fertilized them. This resulted in a 3:1 ratio of yellow to green pea plants. He called the green, a recessive variant. b. Mendel proposed that inheritance of a trait is determined by hereditary factors (now known as genes) that were present in two copies in each parent and segregate independently and combine randomly during fertilization. c. True breeding yellow plants had genotypes (combination of variants/alleles) of YY, true breeding green plants have yy, and the hybrid F1 generation consisted of Yy. d. This was known as Mendel’s First Law of Segregation i. A Punnett Square was used to determine different combinations of possible gametes that could arise from two individuals. 8. Homozygotes have two identical alleles (YY or yy) and heterozygotes have different alleles (Yy). a. Phenotypes are the physical traits of the plants, the particular variant/allele that is shown by a particular individual (green or yellow). 9. Mendel’s second law of independent assortment. a. Each trait segregates independently. b. The F2 generation is generated in a 9:3:3:1 ratio. 10. After Mendel, they discovered the mechanisms (chromosomes) that resulted in the findings that Mendel had discovered. 11. Iclicker question – The diagram below most likely represents the chromosomes of a cell at anaphase of a. Meiosis I b. Meiosis II c. Mitosis d. An impossible situation – different color sister chromatids imply that they belong to different homologous chromosomes. A maternal and paternal chromatid can not form a chromosome and therefore cannot be separated in any anaphase situation. However, pieces of a maternal chromatid could be separated from a paternal chromatid if a recombination event has taken place. 12. Exceptions to Mendel’s second law a. Recombination influences allele segregation. b. Alleles for two different traits/genes that are located close together in the same chromosome will likely segregate together and not independently. c. For a series of genes located on chromosomes, some will have several crossover events whereas some crossovers will occur within a set of genes. BIOMG 1350 Professor Bretscher & GarciaGarcia Spring 2016 Week 9: Lecture 2 of 2 Wednesday, March 23, 2016 Lecture Title: From one cell to an organism Lecture Keywords: zygote, morula, blastula, embryonic axes, gastrula, gastrulation, germ layers, organogenesis, developmental genetics, wildtype, spontaneous and induced mutations, reverse genetics, forward genetics, labeling, cell fate 1. While cells are dividing, cells become specialized to do specific functions. When and how? Development takes place graduallt a. Zygote (fertilized egg), Morula, blastula, gastrula, late embryo, baby 2. Stages of embryonic development I a. Zygote is the fertilized egg and it starts cleaving through mitosis. b. Morula resembles berries. All the cells they have are identical to each other. c. These two are also known as early cleavages (mitosis). The cells divide very quickly and the size does not increase but more and more cells are formed within the same surface area. d. Blastula has morphological differences. Cells are specified in this stage. The embryonic axes are formed at this time that become the scaffold for embryonic tissue to build upon. 3. Stages of embryonic development II a. Gastrula – all the body axes are determined. Cells have a general idea of what they need to become. b. Gastrulation takes place and this is the formation of the germ layers – ectoderm, mesoderm, and endoderm. c. In the late embryo, organogenesis is the formation of definitive organs and the longest process during development. 4. Post embryonic development a. Baby undergoes adulthood. b. Cognition, locomotion, immune system development, vision, sexual maturity, and aging are just some events that occur. 5. To study development, everything is in our genome. a. Example, monozygotic twins resemble each other. b. We have between 25,000-30,000 genes and many of the genes are required for cells but not all are required for different functions in development. c. Finding genes that control development involves developmental genetics, which is the study of mutants. i. Normal fly (wild-type) has a specific normal gene. If you generate a mutation in some gene, some abnormalities in phenotype can be seen in these flies, such as having 4 wings and a duplicated structure. Thus, the gene disrupted by this mutation is required for the development of the fly body plan. 6. Iclicker question – Which one of the following terms used by Mendel is equivalent to the term “mutation” a. Trait – equivalent to character b. Inheritance factor – equivalent to gene c. Character – equivalent to trait d. Allele – mutations and alleles are alternative versions of DNA for a given gene. e. Phenotype – the observable character or trait 7. Spontaneous and induced mutations a. Spontaneous mutations arise spontaneously in nature in model organisms or in humans (ex. Polydactyly – extraneous digits such as toes) b. Induced mutations are experimentally introduced in model organisms. i. Reverse genetics uses genetic engineering and targets a particular gene of interest. Geneticist generates mutations in an interesting gene and then test if the mutation produces a phenotype (ex. Mouse knock out models) ii. Forward genetics uses a mutagen, such as radiation or chemicals, and this generates random mutations. We don’t know which genes are mutated and allows researchers to look deeper to search for what is happening. Geneticists select mutants with interesting phenotypes and try to identify the genes disrupted. 8. Before the genome era, studying development involved observing and manipulating embryos. a. Frog embryos are large and easy to study. Thus, experimental embryology involves manipulating the embryo. b. Labeling embryos shows cell fate i. Using GFP, cells in the embryo were stained where you can find morphological cues to trace it and look afterwards to see what those cells become. ii. Fate is obtained progressively during development. 1. If one of the two cells at the 2-cell stages is labeled, only half of the organism shows the labeled cell. (similarly in the 32-cell stage, the labeled cells only show up in one half of the organism.) iii. Fate maps showed three fundamental cell types 1. Endoderm – gut, liver lungs 2. Mesoderm – skeleton, muscle, kidney, heart, blood 3. Ectoderm – skin, nervous system 4. They discovered development is reproducible from organism to organism. c. Manipulating embryos i. At the two-cell stage, the two cells of the frog have been determined to a specific fate. ii. Wilhem Roux experiment – what would happen if he killed the cells that give rise to half of the organism. 1. He found that half the embryo died, but the other half developed normally. iii. Hands Driesch discovered that cells can change their fate. 1. – 9. Iclicker question – What did Hans Driesch observe when he separated the two cells of a sea urchin embryo after the first cell division? What would be your response if you had done the additional experiment, where you labeled one of the two cells with a fluorescent dye and obtained the result shown? a. Both cells survive and develop as perfectly normal larva b. Both cells survive and develop as smaller larvae that have an otherwise normal body – different organisms do different things with development (in humans, this would have resulted in twins) – Development is plastic. c. Both cells survive and develop into larvae that are missing half of their bodies d. One of the cell dies, the other one develops into a perfectly normal larva e. Both cells die, nothing develops 10. Cut-paste transplants revels determination a. If cells were determined, then they were independent. They will do something completely different than their surrounding cells. b. If cells did not acquire its fate, then transplanting it would result in a change of cell fate. c. The intermediate state is whether the cells is specified or committed, where cells had started to specialize at the time of transplantation, but have not been determined yet. The fate of committed or specified cells can change in transplantation experiments.
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