Ch.19 - Sex Reprod & Genetics Flashcards
19A: Benefits of Sex 19B: Meiosis & Fertilization 19C: Mendel & Laws of Inheritance 19D: Genetics as Experimental Tool
Crossing-over is facilitated by the formation of a _________ complex.
Crossing-over is facilitated by the formation of a synaptonemal complex.
- As duplicated homologs pair, this elaborate protein complex helps to hold bivalent t/g and align homologs so that strand exchange can readily occur b/w non-sister chromatids.
- Ea chromatid in a duplicated homolog can form a xover w either (or both) chromatids fr other chromo in bivalent.
- Synaptonemal complex also helps space out the crossover events that take place along each chromosome.
Describe how crossing-over is facilitated by the formation of a synaptonemal complex.
As duplicated homologs pair, this elaborate protein complex helps to hold bivalent t/g and align homologs so that strand exchange can readily occur b/w non-sister chromatids.
Ea chromatid in a duplicated homolog can form a xover w either (or both) chromatids fr other chromo in bivalent.
Synaptonemal complex also helps space out the crossover events that take place along each chromosome.
As duplicated homologs pair, this elaborate protein complex helps to hold bivalent t/g and align homologs so that strand exchange can readily occur b/w non-sister chromatids.
Ea chromatid in a duplicated homolog can form a xover w either (or both) chromatids fr other chromo in bivalent.
Synaptonemal complex also helps space out the crossover events that take place along each chromosome.
Each ________ corresponds to a crossover b/w two non-sister chromatids
Each chiasma corresponds to a crossover b/w two non-sister chromatids.
- Most bivalents contain > 1 chiasma → multiple xovers occur b/ow homologs.
- In human oocytes (cells that give rise to egg) an avg of 2-3 xover events occur w/i ea bivalent.
Besides providing a major source of genetic variation in sexually reproducing species, what other purpose do crossovers serve?
By holding homologs t/g during prophase I, chiasmata help ensure the mat/paternal homologs will segregate correctly at first meiotic division.
- Chiasmata help position/stabilize bivalents at metaphase plate.
- Recall: cohesin proteins keep sister chromatids glued t/g along length → suddenly degraded at start of anaphase I → recombined homologs can be separated.
Which protein must be rapidly degraded at start of anaphase I in order for recombined homologs to be separated?
Cohesin proteins keep sister chromatids glued t/g along length → suddenly degraded at start of anaphase I → recombined homologs can be separated.
- If arms didn’t come apart, duplicated mat/paternal homologs would remain tethered t/g at chiasmata.
T/F: kinetochore mtubs of sister chromatids point in same direction.
True
Kinetochore mtubs of sister chromatids point in same direction.
- Kinetochores of sister chromatids function as a unit.
T/F: Orientation of each bivalent at the moment of capture (by kinetochore mtub) is completely random.
True
Orientation of each bivalent at the moment of capture is completely random → assortment of maternal and paternal chromosomes is random as well.
T/F:
The frequency of chromo mis-segregation during production of human gametes is remarkably high.
True
The frequency of chromosome mis-segregation during the production of human gametes is remarkably high, partic in females: nondisjunction occurs in ~10% of meioses in human oocytes → eggs contain wrong # of chromos (aneuploidy).
- Aneuploidy occurs less often in human sperm, perhaps bc sperm dev is subjected to more stringent quality control, e.g. checkpoint mechanism is activated → meiosis arrests → apoptosis.
Describe the process of fertilization.
Sperm attracted to ovulated egg by chemical signals released by both egg/supporting cells that surround it.
Sperm finds egg → migrates thru protective layer of cells and then bind/tunnel thru egg coat (zona pellucida) → bind/fuse underlying egg pmem.
Many sperm may bind an egg, but only one normally fuses w egg pmem and introduces its DNA into egg cytoplasm; ensures fertilized egg (zygote) will contain two, and only two, sets of chromos, i.e. diploid.
- The first, successful sperm triggers release of wave of Ca2+ in egg cytoplasm → triggers secretion of enzymes that cause “hardening” of zona pellucida → prevents “runner up” sperm fr penetrating egg.
- Ca2+ wave also helps trigger egg dev.
Fertilization is not complete until the two haploid nuclei (pronuclei) come t/g and combine chromos into single diploid nucleus.
- Pronuclei fuse → diploid cell begins to divide, forming ball of cells that—thru repeated rounds of CD and differentiation—will give rise to an embryo.
Mutations that reduce or eliminate the activity of a gene are called __________ mutations, and are usually _______ (recessive/dominant).
Mutations that reduce or eliminate the activity of a gene are called loss-of-function mutations, and are usually recessive.
- An org in wh both alleles of a gene bear loss-of function mutations will typ display an abnormal phenotype.
- By contrast, the heterozygote, wh possesses one mutant allele and one normal, “wild-type” allele, typ makes enough active gene product to function normally and retain a normal phenotype.
- Thus loss-of-function mutations are typ recessive, bc—for most genes— ↓ normal amount of gene product by 50% has little impact.
Mutations that ↑ activity of a gene or its product, or result in the gene being expressed in inapprop circumstances, are called ___________ mutations and are typ ________ (recessive/dominant).
Mutations that ↑ activity of a gene or its product, or result in the gene being expressed in inapprop circumstances, are called gain-of-function mutations and are typ dominant.
- E.g. a Ras mutation.
A _________ typically involves examining many thousands of mutagenized individuals to find the few that show a specific altered phenotype of interest.
A genetic screen typically involves examining many thousands of mutagenized individuals to find the few that show a specific altered phenotype of interest.
- E.g. To search for genes involved in cell metabolism, one might screen mutagenized bacterial or yeast cells to pick out those that have lost ability to grow in absence of partic AA/other nutrient.
One way to study lethal mutations in haploid organisms makes use of ___________, in which the protein product of the mutant gene is only defective under certain conditions.
One way to study lethal mutations in haploid organisms makes use of conditional mutants, in which the protein product of the mutant gene is only defective under certain conditions.
- E.g. mutants that are temp-sensitive: protein functions normally w/i certain range of temps (permissive temp) but inactivated by shift to a non-permissive temp → abnormal phenotype can be switched on/off.
What does a complementation test reveal?
A complementation test reveals whether two mutations are in the same gene.
- If mutations are recessive and cause a loss of function, a complementation test can reveal whether they affect same/diff genes.
- Simple test: homozygous individual for one recessive mutation is mated w homozygous individual for another mutation → if two mutations affect same gene, then offpsring will show mutant phenotype, bc they carry only defective copies of target gene.
When two genome sequence variants coexist in the population and are both common, the variants are called __________.
When two genome sequence variants coexist in the population and are both common, the variants are called polymorphisms.
The majority of polymorphisms are due to the substitution of a single nucleotide, called ________________.
The majority of polymorphisms are due to the substitution of a single nucleotide, called single-nucleotide polymorphisms or SNPs.
