Numerical Chromosomal Abnormalities I Flashcards
the chromosome content is reduced from __ to __ in the first meiotic division, and from __ to __ in the second meiotic division, where n is the euploid number of chromosomes
4n
2n
2n
n
Two key differences between mitosis and meiosis are that
1) paternally- and maternally-derived homologous chromosomes pair at the onset of meiosis (prophase I), whereas the two homologs segregate independently in mitosis
2) reciprocal recombination events between maternal and paternal sister chromatids generate chiasmata (physical linkages) between homologs in meiosis; in contrast, recombination between homologs is rare during mitosis
Meiotic prophase I
- Maternal and paternal homologs of each chromosome become paired, forming “bivalents”
- Formation of the synaptonemal complex
- Reciprocal recombination events occurring at this stage generate chiasmata
- On average, 2-3 crossovers occur on each chromosome, resulting in genetic reassortment between chromosomes.
- The synaptonemal complex disassembles at the end of prophase I
- Bivalents are therefore held together only by chiasmata.
Formation of bivalents requires
the formation of a proteinaceous structure called the synaptonemal complex, which promotes inter-homolog interactions
Chiasmata
physical crossover links between homologs
Bivalents
structures formed when homologs of each chromosome become paired or synapsed along their entire lengths
__________ is the most error-prone step of the process, and chromosome nondisjunction at this stage is the most frequent mutational mechanism in humans
Meiosis I
Meiosis I is the most error-prone step of the process, and __________ at this stage is the most frequent mutational mechanism in humans
chromosome nondisjunction
Meiosis II is very much like a __________ division
mitotic
chromosomes undergo a second round of __________ in meiosis II without an intervening round of __________
segregation
DNA replication
Genetic consequences of meiosis (3)
A. reduction in chromosome number from diploid to haploid
B. random segregation of homologous chromosomes, giving ~8x106 (or 223 ; 2 homologs for each of 23 chromosomes) different possibilities
C. random shuffling of genetic material due to crossover events, resulting in a vast increase in genetic variability from the above estimate
Mitosis (6)
A. one round of chromosome segregation, resulting in daughter cells identical in chromosomal content to the parental cell
B. DNA replication precedes each round of chromosome segregation
C. no pairing of homologous chromosomes
D. infrequent recombination
E. centromeres on paired sister chromatids segregate at each anaphase
F. occurs in somatic cells and in germ line precursor cells prior to entry into meiosis
Meiosis (5)
A. two rounds of chromosome segregation without an intervening round of DNA replication
B. parental cells must be diploid and the chromosome number is halved in the resultant cells
C. requires the pairing of homologous chromosomes and recombination for its successful completion
D. centromeres on paired sister chromatids divide only at anaphase II in a normal meiosis
E. occurs only in the germ line
Metacentric
the centromere is located in the middle of the chromosome, such that the two chromosome arms are approximately equal in length
Submetacentric
the centromere is slightly removed from the center
