Chapter 13 - Meiosis and Sexual Life Cycles PART II Flashcards
What proteins hold sister chromatids together?
After interphase, sister chromatids are held together by proteins called cohesins
What is the synaptonemal complex?
A zipper like structure that holds one homolog tightly to the other (during Prophase I)
What is synapsis? What takes place during this association?
The non sister chromatids are broken at precisely corresponding points; DNA breaks are closed up so that each broken end is joined to the other corresponding segment of the non sister chromatid; thus a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vise versa
During synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the non sister chromatid, producing crossovers
What is the chiasmata (chiasma - singular)?
The point of attachment where crossovers have occurred show up as chiasmata;
After the synaptonemal complex disassembles, the homologs move slightly apart from each other but remain attached because of sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome
What is the reductional division?
Meiosis I is called the reductional division because it reduces the number of chromosome sets from two (diploid) to one (haploid)
What is the equational division?
During meiosis II, sister chromatids separate, producing haploid daughter cells
Compare mitosis and meiosis.
Mitosis
- Conserves the number of chromosomes sets, producing cells that are genetically identical to the parent cell
- Occurs in both diploid and haploid cells
- (DNA replication) - occurs during interphase before duplication begins
- (# of divisions) - one, including prophase, prometaphase, metaphase, anaphase, and telophase
- (Synapsis of homologous chromosomes) - does not occur
- (# of daughter cells and genetic composition) - two, each genetically identical to the parent cell, with the same number of chromosomes
- (Role in the animal or plant body) - enables multicellular animal or plant (gametophyte or sporophyte) to arise from a single cell; produces cells for growth, repair, and in some species, asexual reproduction; produces gametes in the gametophyte plant
Meiosis
- Reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
- Can only occur in diploid cells
- (DNA replication) - occurs during interphase before meiosis I begins
- (# of divisions) - two, each including prophase, metaphase, anaphase, and telophase
- (Synapsis of homologous chromosomes) - occurs during prophase I along with crossing over between non sister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion
- (# of daughter cells and genetic composition) - four, each haploid (n); genetically different from the parent cell and from each other
- (Role in the animal or plant body) - Produces gametes (in animals) or spores (in the sporophyte plant); reduces number of chromosome sets by half and introduces genetic variability among the gametes or spores
What three events are unique to meiosis and also occur in meiosis I?
- Synapsis and crossing over in prophase I - homologous chromosomes physically connect and exchange genetic material
- Homologous pairs (tetrads) at the metaphase plate
- Separation of homologs during anaphase I
Where / when are cohesins cleaved during mitosis and meiosis?
- In mitosis, cohesins are cleaved at the end of metaphase
- In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)
*Sister chromatid cohesion allows sister chromatids to stay together through meiosis I
What are the different versions of an organism’s genes?
Alleles; reshuffling of alleles during sexual reproduction produces genetic variation
What are mutations?
Changes in an organisms DNA; the original source of genetic diversity; create different versions of genes called alleles
What are three mechanisms that contribute to genetic variation?
- Independent assortment of chromosomes
- Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
- In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other
- The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
- For humans (n = 23), there are more than 8 million (2^23) possible combinations of chromosomes - Crossing over
- Produces recombinant chromosomes, which combine DNA inherited from each parent
- Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
- In humans an average of one to three crossover events occurs per chromosome - Random fertilization
- Any sperm can fuse with any ovum
- The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
*The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
What are recombinant chromosomes?
Individual chromosomes that carry genes derived from two different parents
What is the evolutionary significance of genetic variation within populations?
- Natural selection results in the accumulation of genetic variations favored by the environment
- Sexual reproduction contributes to the genetic variation in a population, which originates from mutations
- Animals that always reproduce asexually are quite rare
