Lecture 8: Meiosis Flashcards
What is Asexual reproduction and Sexual Reproduction?
-Asexual Reproduction: Defined as Reproduction without the fusion of sperm and egg (Examples include binary fission in bacteria, fragmentation in starfish, parthenogenesis in whiptail lizards)
-Sexual Reproduction: Involves Meiosis which is the production of gametes and the union of two gametes (generatres genetic diversity)
What are gametes, zygotes and fertilization?
-Gametes are reproductive cells
-Zygote are fertilized eggs
-Fertilization is the union of sperm and eggs
What are diploid cells? 2n
-Cells with two complete sets of chromosomes, one from each parent.
- In humans, somatic cells are diploid with 46 chromosomes (23 pairs).
-They are involved in growth, repair, and maintenance of tissues. Examples include human skin, muscle, and nerve cells.
-reproduce by mitosis
What are haploid cells? n
-cells with only one set of chromosomes (half diploid cells)
-In humans, gametes (sperm and eggs) are haploid with 23 chromosomes.
-Their purpose is Sexual reproduction.
-reproduced by meiosis
What is meiosis (reduction division), why is it important, and how many cell divisions occur?
-A type of cell division that reduces chromosome number by half, producing four haploid cells from one diploid cell.
- It ensures genetic diversity and produces gametes for sexual reproduction.
-Two divisions: Meiosis I and Meiosis II.
What happens during meiosis l?
Involves the Separation of homologous chromosomes, resulting in 2 daughter cells (n). The stages of Meiosis I are Prophase I, Metaphase I, Anaphase I, and Telophase I.
What is meiosis ll?
Involves the Separation of sister chromatids, resulting in 4 daughter cells (n). The stages of Meiosis II are Prophase II, Metaphase II, Anaphase II, and Telophase II. Basically meiosis l happens again so the products are doubled.
What happens in prophase l?
This is the most complex and longest phase. Chromatin condenses into visible chromosomes, and homologous chromosomes (one from each parent) pair up through a process called synapsis. These pairs, known as tetrads or bivalents, are held together by a protein structure called the synaptonemal complex. Crossing over occurs at points called chiasmata, where segments of chromatids are exchanged between homologous chromosomes, increasing genetic diversity. The nuclear envelope breaks down, spindle fibers form from centrosomes, and the chromosomes attach to them via kinetochores.
What happens during metaphase l?
The tetrads align along the metaphase plate in the center of the cell. Each homologous pair is oriented independently of the others, a process known as independent assortment, which contributes to genetic variation.
What happens during anaphase l?
Spindle fibers separate the homologous chromosome in each tetrad and pull them to the opposite poles of the cell breaking chiasmata Unlike mitosis, sister chromatids remain attached at the centromere. This phase reduces the chromosome number from diploid (2n) to haploid (n).
What happens during telophase l and cytokinesis?
The chromosomes reach opposite poles. The nuclear envelope may briefly reform, and the spindle fibers disassemble. Cytokinesis divides the cell into two haploid daughter cells, each with half the original number of chromosomes, but each chromosome still consists of two sister chromatids.
What happens during meiosis ll?
-Meiosis II: the 2 new nuclei, the 2 genetically different haploid daughter cells will separate into 4 haploid daughter cells
-DNA IS NOT REPLICATED
What is different about Meiosis II compared to Meiosis I?
-Meiosis I: Homologous chromosomes separate, crossing over occurs (Prophase I), reduces diploid → haploid, forms 2 haploid cells with duplicated chromosomes.
-Meiosis II: Sister chromatids separate, no crossing over, keeps haploid number, results in 4 genetically unique haploid cells (like mitosis).
What is synapsis, synaptonemal complex, and recombination nodules?
-Synapsis: The process of pairing of
homologous chromosomes
* Synaptonemal complex: A lattice of proteins that help the homologous chromosomes to pair up
* Recombination nodules: Later chiasmata attach via cohesion proteins
What are centromere?
where sister chromatids attach via cohesion proteins.
What are kinetochore?
Where the microtubules attach.
What two processes in meiosis contribute to genetic diversity?
Crossing over and Independent assortment
What is crossing over? What is it important for? What else is required for it to occur?
Two non-sister chromatids of homologous chromosomes exchange segments of genetic information. This swaps genes between the chromosomes, creating new combinations of alleles in gametes, which increases genetic diversity in offspring.
-crossing over is important for new gene combination, increase genetic variation in gametes and drives evolution.
- The formation of a Bivalent (tetrad) and Chiasma (plural, chiasmata) are crucial for this process.
What bivalent (tetrad)
the group of 4 sister chromatids in paired homologous chromosomes.
What is chiasma?
holding the bivalent together after the synaptonemal complex disappears at the end of prophase. Chiasmata are the points where homologous chromosomes exchange genetic material during crossing over in Prophase I of meiosis, increasing genetic diversity.
What is independent assortment?
The random arrangement of tetrads at the metaphase plate, maternal chromosome can face either pole and paternal chromosomes can face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads. The possible combinations of maternal and paternal chromosomes in the daughter cells is equal to 2^n.
What Random fertilization?
The Union of 2 gametes during fertilization further amplifies genetic diversity. Any one sperm can fertilize any egg resulting in increase genetic diversity.
How does meiosis l create genetically diverse gametes?
meiosis I creates genetically diverse gametes in two ways. First, during prophase I, crossover events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make their way into the gametes.
What happens in prophase ll?
In each haploid daughter cell, the nuclear envelope breaks down again, and spindle fibers reform. Importantly, no DNA replication or crossing over occurs at this stage.
What happens in metaphase ll?
Chromosomes (each with two sister chromatids) align along the metaphase plate. Spindle fibers attach to the kinetochores of each chromatid.
What happens in anaphase ll?
The centromeres split, and sister chromatids are pulled to opposite poles. Now each chromatid is considered an individual chromosome.
What happens in telophase ll and cytokinesis?
Nuclear envelopes reform around each set of chromosomes. Spindle fibers disappear, chromosomes decondense, and cytokinesis follows, producing four genetically unique haploid daughter cells, each containing one set of unreplicated chromosomes.
How does random fertilization happen?
This happens because the sperm and egg are both haploid cells with 23 chromosomes, and when they combine, they form a diploid zygote with 46 chromosomes.
How does the number of possible gamete combinations from random fertilization play a role in genetic diversity?
The number of possible gamete combinations plays a big role in this. Since each parent can make about 2²³ different gametes, the number of possible combinations in the offspring is 2²³ × 2²³, which equals over 70 trillion different zygote combinations.
