Meiosis Sexual Reproduction Intelligent Design BIO 11 by Owen Borville August 26, 2024
Reproduction is an essential process of all living organisms. Many unicellular organisms and a few multicellular organisms can reproduce through asexual reproduction.
Sexual reproduction is the method of reproduction of most multicellular organisms, where parents produce two haploid cells that fuse together to form a diploid cell. Haploid cells are produced by a process of cell division called meiosis. Sexual reproduction involves both meiosis and fertilization, and produces variation in offspring.
Sexual reproduction requires the union of two specialized cells called gametes, each of which contains one set of chromosomes. When gametes unite, they form a zygote, or fertilized egg that contains two sets of chromosomes. Cells containing one set of chromosomes are called haploid and cells containing two sets of chromosomes are called diploid.
In sexual reproduction, the number of chromosome sets must be reduced by half. Most organisms are diploid with two sets of chromosomes.
Somatic cells are all cells of the organism except gametes or reproductive cells and each somatic cell contains a nucleus with two copies of each chromosome, called homologous chromosomes, which are matched pairs containing the same genes in identical locations along their lengths. Therefore diploid organisms inherit one copy of each homologous chromosome from each genetic parent or contributor.
Meiosis vs Mitosis: Meiosis is the nuclear division that forms haploid cells from diploid cells, and meiosis uses many of the same mechanisms as mitosis. Mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. Meiosis is divided into two rounds: Meiosis I and Meiosis II.
Meiosis I
Meiosis occurs after the interphase of G1, S, and G2 phases just like in mitosis. G1 (first gap) is focused on cell growth. During the S phase, the cell copies or replicates the DNA of the chromosomes. In G2 (second gap), the cell undergoes final preparations for meiosis. In the S phase the two copies are sister chromatids held together at the centromere by cohesin proteins, and are held together until anaphase II.
During the Interphase S Phase, chromosomes are duplicated. The resulting sister chromatids are held together at the centromere. The centrosomes are also duplicated.
Meiosis I begins with Prophase I, where chromosomes condense, and the nuclear envelope fragments. Homologous chromosomes bind firmly together along their length, forming a tetrad. Chiasmata form between non-sister chromatids. Crossing over occurs at the chiasmata. Spindle fibers emerge from the centrosomes.
In Prometaphase I, homologous chromosomes are attached to spindle microtublules at the fused kinetochore shared by the sister chromatids. Chromosomes continue to condense, and the nuclear envelope completely disappears.
In Metaphase I, homologous chromosomes randomly assemble at the metaphase plate, where they have been maneuvered into place by the microtubules.
In Anaphase I, spindle microtubules pull the homologous chromosomes apart. The sister chromatids are still attached at the centromere.
In Telophase I and Cytokinesis, sister chromatids arrive at the poles of the cell and begin to decondense. A nuclear envelope forms around each nucleus and the cytoplasm is divided by a cleavage furrow. The result is two haploid cells. Each cell contains one duplicated copy of each homologous chromosome pair.
Meiosis II begins with Prophase II, where sister chromatids condense. A new spindle begins to form. The nuclear envelope starts to fragment.
Prometaphase II begins when the nuclear envelope disappears, and the spindle fibers engage the individual kinetochores on the sister chromatids.
Metaphase II occurs when sister chromatids line up at the metaphase plate.
Anaphase II occurs when sister chromatids are pulled apart by the shortening of the kinetochore microtubules. Non-kinetochore microtubules lengthen the cell.
Telophase II and Cytokinesis begin when chromosomes arrive at the poles of the cell and decondense. The nuclear envelopes surround the four nuclei. Cleavage furrows divide the two cells into four haploid cells.
Comparing Meiosis and Mitosis
In both meiosis and mitosis, DNA synthesis occurs in the S phase of Interphase.
In meiosis, synapsis of homologous chromosomes occur during prophase I, but does not occur in mitosis.
In meiosis, crossover occurs during prophase I, but does not occur in mitosis.
In meiosis, homologous chromosomes line up at the metaphase plate during metaphase I, but does not occur in mitosis.
In meiosis, sister chromatids line up at the metaphase plate during metaphase II, and during metaphase of mitosis.
In meiosis has four haploid cells are produced at the end of meiosis II, while two diploid cells are made at the end of mitosis.
Life Cycles of Sexually Reproducing Organisms: Three Types
Most animals have a diploid-dominant life cycle strategy in which the diploid stage is the most pronounced stage, and only the haploid cells produced by the organism are the gametes. There is no multicellular-haploid life stage. Fusion of the gametes produces a fertilized egg cell, or zygote. The zygote will go through many cycles of mitosis to produce multicellular offspring.
The haploid-dominant life cycle is used by most fungi and algae in which the multicellular body of the organism is haploid. Specialized haploid cells from two individuals join to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore creates a multicellular haploid organism by mitosis.
The third type of life cycle is used by some algae and all plants is a blend of the haploid-dominant and diploid-dominant life cycles. This life cycle is an alternation of generations and have both haploid and diploid multicellular organisms as part of their life cycle.
Reproduction is an essential process of all living organisms. Many unicellular organisms and a few multicellular organisms can reproduce through asexual reproduction.
Sexual reproduction is the method of reproduction of most multicellular organisms, where parents produce two haploid cells that fuse together to form a diploid cell. Haploid cells are produced by a process of cell division called meiosis. Sexual reproduction involves both meiosis and fertilization, and produces variation in offspring.
Sexual reproduction requires the union of two specialized cells called gametes, each of which contains one set of chromosomes. When gametes unite, they form a zygote, or fertilized egg that contains two sets of chromosomes. Cells containing one set of chromosomes are called haploid and cells containing two sets of chromosomes are called diploid.
In sexual reproduction, the number of chromosome sets must be reduced by half. Most organisms are diploid with two sets of chromosomes.
Somatic cells are all cells of the organism except gametes or reproductive cells and each somatic cell contains a nucleus with two copies of each chromosome, called homologous chromosomes, which are matched pairs containing the same genes in identical locations along their lengths. Therefore diploid organisms inherit one copy of each homologous chromosome from each genetic parent or contributor.
Meiosis vs Mitosis: Meiosis is the nuclear division that forms haploid cells from diploid cells, and meiosis uses many of the same mechanisms as mitosis. Mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. Meiosis is divided into two rounds: Meiosis I and Meiosis II.
Meiosis I
Meiosis occurs after the interphase of G1, S, and G2 phases just like in mitosis. G1 (first gap) is focused on cell growth. During the S phase, the cell copies or replicates the DNA of the chromosomes. In G2 (second gap), the cell undergoes final preparations for meiosis. In the S phase the two copies are sister chromatids held together at the centromere by cohesin proteins, and are held together until anaphase II.
During the Interphase S Phase, chromosomes are duplicated. The resulting sister chromatids are held together at the centromere. The centrosomes are also duplicated.
Meiosis I begins with Prophase I, where chromosomes condense, and the nuclear envelope fragments. Homologous chromosomes bind firmly together along their length, forming a tetrad. Chiasmata form between non-sister chromatids. Crossing over occurs at the chiasmata. Spindle fibers emerge from the centrosomes.
In Prometaphase I, homologous chromosomes are attached to spindle microtublules at the fused kinetochore shared by the sister chromatids. Chromosomes continue to condense, and the nuclear envelope completely disappears.
In Metaphase I, homologous chromosomes randomly assemble at the metaphase plate, where they have been maneuvered into place by the microtubules.
In Anaphase I, spindle microtubules pull the homologous chromosomes apart. The sister chromatids are still attached at the centromere.
In Telophase I and Cytokinesis, sister chromatids arrive at the poles of the cell and begin to decondense. A nuclear envelope forms around each nucleus and the cytoplasm is divided by a cleavage furrow. The result is two haploid cells. Each cell contains one duplicated copy of each homologous chromosome pair.
Meiosis II begins with Prophase II, where sister chromatids condense. A new spindle begins to form. The nuclear envelope starts to fragment.
Prometaphase II begins when the nuclear envelope disappears, and the spindle fibers engage the individual kinetochores on the sister chromatids.
Metaphase II occurs when sister chromatids line up at the metaphase plate.
Anaphase II occurs when sister chromatids are pulled apart by the shortening of the kinetochore microtubules. Non-kinetochore microtubules lengthen the cell.
Telophase II and Cytokinesis begin when chromosomes arrive at the poles of the cell and decondense. The nuclear envelopes surround the four nuclei. Cleavage furrows divide the two cells into four haploid cells.
Comparing Meiosis and Mitosis
In both meiosis and mitosis, DNA synthesis occurs in the S phase of Interphase.
In meiosis, synapsis of homologous chromosomes occur during prophase I, but does not occur in mitosis.
In meiosis, crossover occurs during prophase I, but does not occur in mitosis.
In meiosis, homologous chromosomes line up at the metaphase plate during metaphase I, but does not occur in mitosis.
In meiosis, sister chromatids line up at the metaphase plate during metaphase II, and during metaphase of mitosis.
In meiosis has four haploid cells are produced at the end of meiosis II, while two diploid cells are made at the end of mitosis.
Life Cycles of Sexually Reproducing Organisms: Three Types
Most animals have a diploid-dominant life cycle strategy in which the diploid stage is the most pronounced stage, and only the haploid cells produced by the organism are the gametes. There is no multicellular-haploid life stage. Fusion of the gametes produces a fertilized egg cell, or zygote. The zygote will go through many cycles of mitosis to produce multicellular offspring.
The haploid-dominant life cycle is used by most fungi and algae in which the multicellular body of the organism is haploid. Specialized haploid cells from two individuals join to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore creates a multicellular haploid organism by mitosis.
The third type of life cycle is used by some algae and all plants is a blend of the haploid-dominant and diploid-dominant life cycles. This life cycle is an alternation of generations and have both haploid and diploid multicellular organisms as part of their life cycle.