Introduction to cell reproduction
Introduction to cell reproduction
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Date Created: 01/28/16
CH13 Sperm: male reproductive cell. Egg: female reproductive cell. Fertilization was first observed in large translucent sea urchins. This observation was published in 1876. From this document arose the question: “ how do the chromosomes from the egg and sperm combine and the offspring has the same number of chromosomes as the parents. August Weismann proposed, in 1837, that during the formation of the gametes there was a type of cell division halving the number of chromosomes. Gametes: reproductive cell; such as sperm or egg. Meiosis: nuclear division that leads to a halving of chromosome numbers. Each organism has a characteristic number of chrosomes, chromosomes have a distinct size and shape. Sex chromosomes: X and Y. Autosomes: non-sex chromosomes. Homologous: chromosomes that are the same size and shape Gene: section of DNA that influences some hereditary trait in an individual. Allele: different version of the same gene. Homologous chromosomes carry the same genes, but each homolog may carry a different allele. Karyotype: number and type of chromosomes. Diploid: two versions of each chromosome Haploid: on version of each chromosome “n” stands for the number of distinct number of chromosomes in a given cell and is called a haploid number. - Example. In humans n=23 To indicate the number of complete chromosome sets observed a number is placed before the “n” . - Example. 3n Ploidy: combination of numbers of sets and n is termed Maternal chromosomes: come from mom Paternal chromosomes: come from dad Humans are diploid; 2n=46. Haploid cells are simply labeled “n” because they only have one set of chromosomes Polyploidy: 3 or more of each type of chromosome in a cell. Sister chromatids: contain identical copies of DNA double helix in a un- replicated chromosome. Remaining joined during most of meiosis. Un-replicated chromosomes are never called chromatids. Meiosis consist of two cell divisions. During meiosis I the homologs in each chromosome pair separate from each other. One homolog goes to each daughter cell at random. At the end of meiosis I each daughter call has one of each type of chromosome instead of two; thus half as many chromosomes than the parent chromosome had. Meaning during meiosis I 2n becomes n; from a diploid two haploids are produced. During meiosis II sister chromatids from each chromosome separate. A sister chromatid (daughter chromosome) goes to one daughter cell and the other to another daughter cell. The cell started in meiosis II had one of each type of chromosome was still un- replicated ( meaning still consisting of two sister chromatids). Cells produced by meiosis II have one of each type of chromosome but chromosomes are no longer replicated. Chromosome movement during meiosis I and II are coordinated by microtubules of the spindle apparatus that is attached to kinetochores located at the centromere of each chromosome. Meiosis: reduction division Gametogenesis: process in which eventually form sperm and eggs when two gametes fuse during fertilization; a full complement of chromosomes is restored. Zygote: cell that results from fertilization (diploid) 1. Uncondensed chromosomes replicate in parent cell. 2. Early prophase I: chromosomes condense; spindle apparatus forms; nuclear envelope begins to break down. Synapsis of homologous chromosomes (structure resulting from synapsis is called tetrat). 3. Late prophase I: chiasmata (cross over points) visible, often multiple to increase genetic diversity; nuclear envelope broken down. 4. Metaphase I: migration of bivalents to metaphase plate is complete. 5. Anaphase: homologs separate and begin to move to opposite poles of the spindle apparatus. 6. Telophase I and cytokinesis: chromosomes move to opposite pole of spindle apparatus; spindle apparatus disassembles and the cell divides into two cells. End of meiosis I 1. Prophase II: spindle apparatus forms (no synthesis). 2. Metaphase II: chromosomes line up at the middle of the spindle apparatus- the metaphase plate (no crossing over). 3. Anaphase II: sister chromatids separate begin moving in opposite poles of the spindle apparatus.( no homologous sister chromatids in meiosis II). 4. Telophase II and Cytokinesis: chromosomes move to opposite poles of the spindle apparatus. Spindle apparatus disassembles Synapsis: chromosome pair come together Bivalent: structure result from synapsis Non-sister chromatids: chromosomes from different homologs. Chiasmata: non-sister chromatid which for a “x” structure Crossing over: reciprocal changes between different homologs create non- sister chromatids that have both paternal and maternal segments. Metaphase plate: region in the middle of the spindle apparatus Cytokinesis: division of cytoplasm Sister chromatids are held together by cohesion and condensation in prophase I. breaking of a homologous pair initiating crossing over between non-sister chromatids. Synaptonemal: complex proteins that hold two homologs tightly together At a chiasma non-sister chromatids from each homolog have been broken at the same point and attached to each other. Paternal and maternal chromosomes exchanged Meiosis happens only in sexual reproduction. Asexual reproduction: any way of producing an offspring that does not involve fusion of gametes. Mitosis: exact replicas of parent chromosome. Sexual reproduction: producing an offspring through the production and fusion on gametes.
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