Lecture 20 Flashcards
What is clonal reproduction?
Clonal reproduction is asexual reproduction which leads to an exact clone of the parent.
What clonal reproduction do single celled organisms use and give examples of organisms like this. What about multicellular plants and animals?
Single cells organisms like amoeba, paramecium, yeasts and algae reproduce via binary fission while multicellular plants and animals can also use vegetative means like runners, bulbs, tubers, rhizomes. Organisms like this are hydra viridissima, anemone, sponges and starfish.
What are the stages in the sexual cycle? What parts are diploid in humans? Which are haploid in humans?
Prior to meiosis cells in the gonads are diploid (2n), they then undergo meiosis to form four haploid gametes (n) from the original one, after fertilisation (when the two gametes fuse) the cells form a diploid zygote.
Why can gametes not be produced by mitosis?
gametes produced would be 2n, leading to embryos which are 4n. Hence a process is needed to half the number of chromosomes in the gametes.
What are the two main phases of meiosis?
Meiosis I and meiosis II.
What stage occurs prior to meiosis I and what occurs in meiosis I?
Before meiosis starts the cell must undergo interphase in order to double the amount of DNA, next is prophase I
Prophase I: Chromosomes become visible and and centrosomes form spindle as the nucleus breaks down, the homologous chromosomes pair (forming tetrads) and exchange segments (crossing over), crossing over is done by joining at chiasmata and exchanging segments.
Metaphase I: homologous chromosomes line up along the metaphase plate held by spindles with the homologous chromosomes on opposite sides.
Anaphase I: the spindles seperate the homologous chromosomes to different ends, ensuring one complete set of homologous chromosomes on both sides and sister chromatids remain attached.
Meiosis I finishes with telophase 1 where the cell seperates into two daughter cells and then cytokinesis.
What stages occur in meiosis II? What is it identical to and what is the difference?
Prophase II, metaphase II, anaphase II and telophase II, finished by cytokinesis. It is identical to mitosis except that it finishes with haploid cells rather than diploid cells.
Summarise the differences between mitosis and meiosis I.
Mitosis: Chromosomes align independently of the other chromosomes and have no chiasmata, centromeres meet on the metaphase plate and the chromatids disjoin, leaving the two produced cells diploid from the parent cell.
Meiosis I: has homologous chromosomes synapsing at the metaphase plate, joined at the chiasmata, chiasmata meet on the metaphase plates and the chromosomes disjoin, producing two haploid cells from one diploid.
Compare oogonia and spermatogonia.
spermatogonia firstly produces a primary spermatocyte which then experiences meiosis, leading firstly to two secondary spermatocytes in meiosis I and then 4 spermatids after meiosis II, producing tens of millions in a day.
Oogonia produces the initial primary oocytes while only a fetus (500,000 at six months), prophase I of meiosis is then suspended until ovulation, at which point a secondary oocyte is formed as well as the 1st polar body, metaphase II of meiosis II is then halted until fertilization, at which point one ovum is produced and three polar bodies. The three polar bodies are essentially useless, only the ovum matters.
What is the primary benefit of sexual reproduction compared to asexual reproduction and vice versa?
Sexual reproduction heavily increases variation in offspring (even without crossing over it is 2^n where n is the number of haploid chromosomes in a diploid individual). Asexual reproduction leads to much faster reproduction rate.
How does crossing over increase variation?
Adds more randomness to gene assortment in the gametes by mixing the maternal and paternal genes.
What does the variation created by sexual reproduction help with?
Changing environment, spatially variable environment (climate) and reduces sibling competition.
