Lectures 4-6 (Meiosis And Mitosis) Flashcards
Homologous chromosomes
Matched chromosomes. Among two homologs of a chromosome, one is paternal the other is maternal. Homologs carry the same genes, but may have different alleles.
Chromatin
DNA and protein
Sister chromatids
These compose a chromosome when it replicates
Centromere
Holds together sister chromatids
Meta centric chromosome
Centromere located near the middle
Acrocentric chromosome
Centromere is toward one end or the other.
G1 (gap 1)
Cell growth, preparation for DNA replication
S (synthesis)
Chromosome duplication including DNA replication
G2(gap2)
Cell growth, preparation for mitosis
M(mitosis and cytokinesis)
Nuclear division and segregation if chromatids, cell division
G0
Cells that are not actively growing or dividing
Prometaphase
Nuclear envelope breaks down. Kinetochore micro tubules attach to centromeres. Complex of micro tubules from centrosome forms the mitotic spindle.
Prophase
Chromosomes condense dramatically. Sister chromatids are attached. Centrosomes move apart toward opposite sides of the nucleus. Nucleoli, sites of ribosome production, begin to disappear.
Metaphase
Chromosome align in the metaphase plate at equator of the cell. Sister chromatids are attached but kinetochore to opposite poles of the mitotic spindle.
Anaphase
Centromeric connections between sister chromatids are severed. Kinetochore micro tubules shorten.
Telophase
Each chromatid is now considered a chromosome. Chromosomes decondense. A nuclear envelope forms around each set of chromosomes. Nucleoli reappear. Spindle micro tubules disperse.
Cytokinesis
Initiated during anaphase. Completed after telophase. Cytoplasm divided between two daughter cells. Results in two daughter cells with identical genetic information.
Animals vs plants in cytokinesis
In animals a contractile ring forms and forms a cleavage furrow, cell is cleaved in two. In plants a cell plate forms near the equator of cell. Starts in the middle and grows outward. Cell is divided into two. W
Syncytium
Multinucleate cell formed during mitosis without cytokinesis. Happens in early drosophila development. Sperm cell precursors of many animals
Somatic Cells
Most of the cells of the body. Are either in G0 phase (quiescence) or going through the regular cell cycle. Mutations not passed on.
Germ Cells
Precursors to gametes. Go through meiosis to produce haploid gametes. Mutations that arise in these cells may be passed to the next generation. (germline mutations)
Meiosis I
Homologus chromosomes segregate into separate daughter cells. Sister chromatids stay together. Creates two haploid cells.
Meiosis II
Sister chromatids of each chromosome segregate into separate daughter cells. Creates four haploid daughter cells; each chromosome is just one chromatid.
Prophase I
Chromosomes shorten, homologs pair, crossing over occurs, chromatids fully condense. Subdivided into 5 subphases: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis
Leptotene
Chromosomes begin to condense. Centrosomes begin to move to opposite sides of the nucleus.
Zygotene
Chromosomes enter synapsis: homologues pair. Synaptonemal complex forms
Pachytene
Synapsis complete, meiotic recombination (crossing over) occurs: non sister chromatids of homologs exchange genetic material
Diplotene
Synaptonemal complex dissolves. Crossover points appear as chiasmata; holds non sister chromatids together. Meiotic arrest occurs in many species at this point.
Diakinesis
Chromatids condense fully. Nuclear envelope breaks down, meiotic spindle begins to form. Non-sister chromatids of homologs remain attached at chiasmata.
Synaptonemal Complex
Specialized proteins that hold homologs together. Starts in zygotene and is fully complete by pachytene. Recombination nodules along the complex forms for crossing over between non-sister chromatids.
Metaphase I
Tetrads (homologous chromosomes) align along the metaphase plate. Each chromosome of a pair attaches to microtubules from one pole. Homologs are attached to opposite poles. Sister chromatids attached to the same pole.
Anaphase I
Homologous chromosomes are pulled to opposite poles. Centromere of each chromosome does not divide. Chiasmata migrate off ends of chromatids.
Telophase I
Nuclear envelope forms. Haploid cells. Chromosomes are composed of two sister chromatids. Cytokinesis of cells occurs after telophase I
Interkinesis
Similar to interphase but generally shorter. No chromosome duplication. Chromosomes may decondense depending on the species.
Prophase II
Chromosomes re-condense if they decondensed during interkinesis. Centrosomes move toward poles of nucleus. Nuclear envelope breaks down.
Metaphase II
Chromosomes align on the metaphase plate. Sister chromatids of each chromosome attach to spindle microtubules from centrosome attach to spindle microtubules from centrosomes on opposite poles.
Anaphase II
Centromeres are severed. Sister chromatids of each chromosome pulled to opposite poles.
Telophase II
Chromosomes begin to decondense. Nuclear envelopes form around each set of chromosomes. Chromosomes now each composed of one chromatid.
Cytokinesis (Meiosis)
Cytoplasm of each cell is divided to produce two daughter cells. 4 haploid cells.
Anuploidy
Abnormal number of chromosomes for one set of homologs
Independent assortment of unlinked genes.
Orientation of each set of homologs on metaphase plate of meiosis I is independent of orientation of other sets of homologs. Different relative orientations of different sets of homologs can yield different combinations of alleles of unlinked genes.
Meiotic Recombination
Some gametes carry chromosomes with a mix of maternal and paternal alleles of different genes. Each of the four gametes produced in a meiosis will be unique.
Oogenesis in Humans
Oogonia (diploid germ cells in ovaries of female embryos) divide by mitosis during fetal stage and enter meiosis I to become primary Oocytes. Arrest in diplotene until after birth occurs. At puberty, one oocyte per month resumes meiosis. At ovulation, meiosis 1 produces a secondary oocyte and first polar body. Secondary ooccyte arrests in metaphase of meiosis II. If oocyte is fertilized, meiosis II is completed. Secondary oocyte produces a mature ovum and second polar body. First polar body usually doesn’t divide.
Spermatogenesis
Spermatogonia (diploid germ cells found in testes) divide by mitosis throughout lifespan of individual. After puberty, spermatagonia undergo mitosis to produce primary spermatocytes. Primary spermatocytes undergo meiosis to produce two secondary spermatocytes. Secondary spermatocytes undergo meiosis II to produce two spermatids. Spermatids mature to become sperm. Equal number of X and Y sperm are produced.
Overview of Meiosis I
Homologous chromosomes segregate into separate daughter cells. Sister chromatid stay together. Two haploid cells are created.
Overview of Meiosis II
Sister chromatids of each chromosome segregate into separate daughter cells. Creates four haploid cells. Each chromosomes is now one chromatid.
Meiosis and Mendel’s Law of Segregation
Mendel’s law of segregation states that alleles of a gene segregate into separate gametes. This occurs as a result of segregation of homologous chromosomes in Anaphase I of meiosis. Each gamete only gets one homolog of each pair and therefore only one allele of each gene.
Meiosis and Mendel’s Law of Independent Assortment.
This applies to unlinked genes. Results from the fact that different homologous pairs of chromosomes can line up in different orientations on the metaphase plate in meiosis I
Hemizygous
Offspring has only one allele of a gene. Generally X-linked genes in males.
