Questions #9 Flashcards
October 24 - October 31 (47 cards)
What is the exception to the third postulate of cell theory: all cells arise by division?
Fertilization requires the union of 2 cells to make one.
Why is sexual reproduction so important for the evolutionary success of eukaryotes?
It increases the genetic diversity.
Why is meiosis necessary?
Reduces chromosome number in gametes.
What are gem-line cells, and what are somatic cells?
gem-line cells=2n cells that will undergo meiosis to produce gametes somatic=non reproductive cells.
What is synapsis, and what is crossing over?
- synapse happens at prophase of meiosis I. It refers to the homologous chromosomes coming close together.
- crossing over trades DNA between homologous and the result is genetic recombination and variation.
What is genetic recombination
the production of offspring with combinations of traits differing from either parent
What are chiasmata?
Sites where crossing over has occurred.
What holds the tetrads together during metaphase I?
chiasmata
Why does independent assortment occur?
One homologue comes from the mother and one from the father. When homologous chromosomes separate in meiosis I chance determines which direction the mother or father homologue go.
How is the chromosome number halved during meiosis?
tetrads line up at metaphase 1 and the microtubules attach to the kinetochores on the outside and pull them apart
What is the stage between divisions 1 and 2 of meiosis?
interkinesis
Why is division 2 necessary?
to separate sister chromatids
What are the four main differences between meiosis and mitosis?
- synapsis and crossing over in prophase 1
- tetrads of chromatids compared of homologous pairs joined by chiasmata on metaphase 1 plate
- separations of homologs rather than chromatids at anaphase 1
- interkinesis: no S between meiosis 1 and meiosis 2
How is genetic variation introduced by sexual reproduction?
- random fertilization
- crossing over
- independent assortment
What is parthenogenesis?
unfertilized eggs grow into an individual
ex. male bees in 1N (haploid)
in domestic turkeys viable offspring are sometimes produced from unfertilized eggs. these offspring are diploid, like their mother, what defect in meiosis could give rise to diploid eggs
offspring from unfertilized eggs are diploid because homologues fail to separate at Meiosis 1 (nondisjunction)
Down syndrome in humans is caused by having 3 copies of chromosome 21. What defect in meiosis could cause Down’s syndrome?
caused by inheritance of third number 21 chromosome, failed to separate
What are the current hypotheses for origin of sex?
(enormous amount of genetic variation permits rapid adaptation)
- certain kinds of DNA damage can only be repaired using homologous chromosome as template (only way for a haploid to get a homologue is to fuse with another haploid)
- contagion-viral infection; can rapidly spread if fuses with related cell and cross over
- get ride of parasites
What are the current hypotheses for the advantage of being diploid?
1.have a spare good copy of each gene and a spare bad copy of each gene
“red queen”- good to store bad genes until environment changes to where they are useful
2. Miller’s ratchet: diploids can lose bad genes more easily than haploids can
What
What was the major difference between Mendel’s model and earlier notions of inheritance?
first quantitative study
What is a model and what is a model organism?
model=comparison standard
model organism=used to infer things about all organisms
What is a hybrid?
results from the mating of dissimilar parents, offspring are different from the parents
What are the five assumptions of Mendel’s model of inheritance?
- parents transmit genes that provide information about characters
- each individual contains 2 alleles for each character
- not all copies of a gene are identical
- alleles from each parent segregate during gamete production
- presence of a gene doesn’t assure that you’ll see its expression
What are alleles?
Different forms of a genes
